Quantum Entanglement and time travel

[vincentm]

I'm not buying this for reasons of paradoxes, but Brian Greene is saying that time travel backwards is possible.

Despite years of debate, scientists still haven't completely ruled out the possibility of going back in time. "Many physicists have a gut feeling that time travel to the past is not possible," said Columbia University theoretical physicist Brian Greene. "But many of us, including me, are impressed that nobody's been able to prove that."

Source

what do you guys think?

 

[selfAdjoint]

This is a way of expressingit for maximum gee-whiz value (why am I not surprised?), but consider it's only another facet of this point: QM contains an irreversable componennt, which appears in the wave function formalism as collapse, but also appears in other formulations. And nobody has ever been able to prove that's a necessary component. Seems that all current formulations of QM, to say nothing of all those wild and crazy interpretations, are incomplete.

 

[complexPHILOSOPHY]

I was under the impression that when particles 'communicate' or interact nonlocally, that no information is being transferred. If this is definitely true, would this have any bearing on a QM theory of time travel?

Also, the arrow of time seems to irreversibly flow forwards, however, what the hell do I know.

 

[mgelfan]

I was under the impression that when particles 'communicate' or interact nonlocally, that no information is being transferred. If this is definitely true, would this have any bearing on a QM theory of time travel?

The physical nature of what's being 'measured' by detectors (or emitted by emitters) in quantum experiments is unknown. That is, nobody knows what's being transferred from emitters to detectors (or if it is also being transferred from detectors to emitters, or from detectors to detectors, or whatever). To call it 'nonlocal' (or 'local' for that matter) is probably not a good idea. But, if you must call it something, then 'acausal' and 'alocal' would seem to be fitting candidates (since qm is all about correlations between various sorts of events -- both emission and detection, and various combinations thereof). As far as can be determined, there is no ftl interaction between spatially separated detectors, filters, emitters, etc. in quantum experiments -- even though there are some ways to sort of 'back into' the idea that there is.

Is there a quantum theory of time travel??

Also, the arrow of time seems to irreversibly flow forwards, however, what the hell do I know.

I think that most physicists would agree with the idea that the arrow of time flows irreversibly forward. When people like Brian Greene talk about the possibility of backward time travel, and ftl or instantaneous causation at a distance, and quantum weirdness, etc., it should be taken with great skepticism.

 

[mgelfan]

I'm not buying this for reasons of paradoxes, but Brian Greene is saying that time travel backwards is possible.

Quote:
Despite years of debate, scientists still haven't completely ruled out the possibility of going back in time. "Many physicists have a gut feeling that time travel to the past is not possible," said Columbia University theoretical physicist Brian Greene. "But many of us, including me, are impressed that nobody's been able to prove that."


Source

what do you guys think?

There's nothing to prove. Backward time travel is a meaningless idea. Think about it. What is Greene talking about? Do you think it makes any sense to entertain the idea that the motions of some region of the universe for some interval can somehow be rewound and rerun like you would do with a vhs tape or a dvd?

 

[JesseM]

I'm not buying this for reasons of paradoxes, but Brian Greene is saying that time travel backwards is possible.



Source

what do you guys think?

Although the article conflates Greene's comments with Cramer's retrocausality experiment, my guess is that Greene wasn't talking about quantum physics at all, but rather about general relativity, which does theoretically allow time travel in certain unusual circumstances, like in the neighborhood of a traversable wormhole (though it is quite possible that when quantum effects are taken into account, these loopholes will be closed).

 

[JesseM]

There's nothing to prove. Backward time travel is a meaningless idea. Think about it. What is Greene talking about? Do you think it makes any sense to entertain the idea that the motions of some region of the universe for some interval can somehow be rewound and rerun like you would do with a vhs tape or a dvd?

Backwards time travel has nothing to do with "rewinding" anything, it has to do with a worldline that loops around and revisits a portion of spacetime it's already crossed through. It's important to think of these things in terms of relativity's view of spacetime as a 4-dimensional continuum in which past, present and future events all coexist, rather than the intuitive view that there is a single objective "present" and that things in the past have "ceased to exist" or that things in the future "don't yet exist".

 

[Demystifier]

Backwards time travel has nothing to do with "rewinding" anything, it has to do with a worldline that loops around and revisits a portion of spacetime it's already crossed through. It's important to think of these things in terms of relativity's view of spacetime as a 4-dimensional continuum in which past, present and future events all coexist, rather than the intuitive view that there is a single objective "present" and that things in the past have "ceased to exist" or that things in the future "don't yet exist".

That is exactly my point too.

 

[mgelfan]

Backwards time travel has nothing to do with "rewinding" anything, it has to do with a worldline that loops around and revisits a portion of spacetime it's already crossed through.

A geometric interpretation of relativity theory is one way of looking at it. But it doesn't provide a physical understanding of why backward time travel is a rather silly idea.

Define the universe (or some region thereof) as some set of objects. Any particular configuration of the set of objects is a time of the set of objects.

If the universe is expanding, then it is physically impossible for any universal scale configuration to be reproduced. But the capability to reconfigure very large scale configurations of objects is what would be needed in order to 'revisit' those configurations of objects (or, iow, travel backwards in time).

Revisiting the past would require rewinding a configuration of objects in the sense that it would involve a repositioning of those objects -- and even if the universe isn't expanding, it would still be an impossible task.

It's important to think of these things in terms of relativity's view of spacetime as a 4-dimensional continuum in which past, present and future events all coexist, rather than the intuitive view that there is a single objective "present" and that things in the past have "ceased to exist" or that things in the future "don't yet exist".

[/QUOTE]
I disagree. The intuitive view is better for understanding some things.

If you want to translate some data from one reference frame to another, then, yes, the definitions and conventions of relativity theory facilitate this in an unambiguous manner.

But, if you want to understand why backward time travel is a nonsensical idea, then using notions of a four-dimensional spacetime, etc., is not the most promising way to proceed.

 

[Chipset]

http://www.physics.uconn.edu/~mallett/main/time_travel.htm

 

[George Jones]

I'm going to expand on JesseM's comments by reposting stuff that I've posted elsewhere.

Going backwards in time means going into the past while traveling forwards in time.

Let p be an event in spacetime, Event q is in the (chronological) past of p if there exists a future-directed timelilke curve from q to p.

Suppose that event p is on the worldline of an observer, and that there is an event q is in the past of p such that a future-directed timelike curve from p to q. Then, it is possible for an observer to travel into his own past.

Joining the future-directed timelike curve form p to to q with the future-directed timelike curve from q to p, shows that this is completely equivalent to the existence of a closed timelike curve.

Its certainly allowed by general relativity, as there are numerous solutions to Einsten's equations that have closed timelike curves.

How does one deal with the paradoxes associated with time travel? Also as mentioned (in another thread), Matt Visser has written http://arxiv.org/PS_cache/gr-qc/pdf/0204/0204022.pdf" about this. He talks about four possibilies:

1. Radically rerwite physics from the ground up;

2. Permit time travel, but also invoke consistency constraints;

3. Quantum physics intervenes to prevent time travel;

4. the Boring Physics Conjecture, where we assume (until forced not) that our particular universe is globally hyperbolic, and thus doesn't have closed timelike curves.

In the past 4. was often assumed, but since global hyperbolicity is a very strong global condition and Einstein's equations are (local) differential equations, many physicists have moved to 2. and 3. Stephen Hawking likes 3., for example, and has formulated the Chronology Protection Conjecture, "It seems that there is a Chronology Protection Agency which prevents the appearance of closed timelike curves and so makes the universe safe for historians."

This roughly states that near a chronology horizon (horizon at which spacetime becomes causally ill-behaved), expectation values of stress-energy tensors for quantum fields blow up, thus preventing (by wall-of-fire barriers) physical objects from crossing chronology horizons. There seems to be some semi-classical evidence for this conjecture, but a http://arxiv.org/PS_cache/gr-qc/pdf/9603/9603012.pdf" by Kay, Radzikowski, and Wald muddies the picture a bit. Their analysis shows that the semi-classical stress-energy tensor is ill-defined, but not necessarily infinite, at a chronology horizon.

This may be just an indication that the semi-classical theory breaks down at chronology horizons, and that full quantum gravity is needed for definitive predictions.

 

[Anonym]

selfAdjoint:” This is a way of expressingit for maximum gee-whiz value (why am I not surprised?), but consider it's only another facet of this point: QM contains an irreversable componennt, which appears in the wave function formalism as collapse, but also appears in other formulations. And nobody has ever been able to prove that's a necessary component. Seems that all current formulations of QM, to say nothing of all those wild and crazy interpretations, are incomplete.”

Seems that all current formulations of relativistic QM…
May be Careful know the answer?

 

[vanesch]

I'm going to expand on JesseM's comments by reposting stuff that I've posted elsewhere.

Going backwards in time means going into the past while traveling forwards in time.

Let p be an event in spacetime, Event q is in the (chronological) past of p if there exists a future-directed timelilke curve from q to p.

Suppose that event p is on the worldline of an observer, and that there is an event q is in the past of p such that a future-directed timelike curve from p to q. Then, it is possible for an observer to travel into his own past.

Joining the future-directed timelike curve form p to to q with the future-directed timelike curve from q to p, shows that this is completely equivalent to the existence of a closed timelike curve.

Its certainly allowed by general relativity, as there are numerous solutions to Einsten's equations that have closed timelike curves.

How does one deal with the paradoxes associated with time travel?

I don't think that, in a strictly GR universe, there is any problem with CTC. Indeed, given that in a strict GR universe, all time evolution is deterministic, then "on the second round" one cannot make any "different decisions" than the first time one went by a certain event. This would imply, for instance, that there cannot be any different "memory" state "the second time around". The (deterministic) decisions will be identically the same. In other words, if you meet your grand-dad 100 years ago along such a curve, you must be in such a state that you don't know specifically that it is your granddad, and that you will do anything else than you "did the first time around".
The local state of a local spacelike foliation on a CTC cannot be different as a function of the "loop number" and hence, things must evolve in such a way that there is no paradox.

 

[George Jones]

I don't think that, in a strictly GR universe, there is any problem with CTC. Indeed, given that in a strict GR universe, all time evolution is deterministic,

Any spacetime that has CTCs is not globally hyperbolic, and so does not possesses a Cauchy surface necessary for the complete specification of initial data - initial-value problems are not well-posed in spacetimes that have closed timelike curves.

 

[JesseM]

A geometric interpretation of relativity theory is one way of looking at it. But it doesn't provide a physical understanding of why backward time travel is a rather silly idea.

Define the universe (or some region thereof) as some set of objects. Any particular configuration of the set of objects is a time of the set of objects.

If the universe is expanding, then it is physically impossible for any universal scale configuration to be reproduced. But the capability to reconfigure very large scale configurations of objects is what would be needed in order to 'revisit' those configurations of objects (or, iow, travel backwards in time).

Revisiting the past would require rewinding a configuration of objects in the sense that it would involve a repositioning of those objects -- and even if the universe isn't expanding, it would still be an impossible task.

I think you're still not understanding how the geometrical view sheds light on closed timelike curves in GR--nothing is being rewound or repositioned! To see how the geometric "block time" view works, imagine spacetime as a literal block of ice, with some pieces of string embedded in it to represent worldlines. Now imagine slicing this block up into a stack of very thin cross-sections, like slicing meat at a deli counter. Each cross-section of the block will contain cross-sections of all the strings, which will just look like dots embedded in a 2D sheet. If we were to take pictures of each cross-section in succession, and then run them together as frames in a movie, we'd see the dots moving around over time, corresponding to particles moving around in space.

Now, time travel in GR does not mean that the configuration of dots in the movie must return to a copy of their configuration in an earlier frame of the movie. Equivalently, it does not mean that a later cross-section of the ice looks identical to an earlier cross-section. Instead, returning to thinking about the whole block of ice before it was sliced into sections, a CTC should be thought of as a piece of string that loops around and intersects an "earlier" part of itself. From our perspective viewing the ice as a whole, nothing is changing, it's just a static configuration of strings embedded in the ice with one of them happening to form a loop. You could even imagine the block of ice being cone-shaped, so that successive cross-sections would be larger and larger, representing the expansion of space; contrary to what you suggested above, there is no notion of a past state having to be recreated when the universe is larger, since again, it's just a string which loops around and revisits a section of the cone closer to the tip where the cross-section is smaller.

Similarly, if you can vaguely imagine standing outside spacetime as a whole, it would just look like a static curved 4D surface with various worldlines embedded in it, and CTCs would just be worldlines that form a loop. This picture really only makes sense in terms of the "block time" view, thinking in terms of the view that time "really flows" will just get you confused.

I disagree. The intuitive view is better for understanding some things.

Well, the intuitive view has caused you to misunderstand the idea of CTCs in GR, so at least in this situation it doesn't seem very helpful.

But, if you want to understand why backward time travel is a nonsensical idea, then using notions of a four-dimensional spacetime, etc., is not the most promising way to proceed.

Backwards time travel might be problematic for other reasons, but it's definitely allowed in GR (though a theory of quantum gravity may change this), and your arguments for why it's nonsensical don't work, for the reasons I tried to explain above.

 

[JesseM]

Any spacetime that has CTCs is not globally hyperbolic, and so does not possesses a Cauchy surface necessary for the complete specification of initial data - initial-value problems are not well-posed in spacetimes that have closed timelike curves.

Something I wondered about--if a spacetime is "not globally hyberbolic", does that automatically imply it contain CTCs, or are there examples of spacetimes with no CTCs that are not globally hyberbolic for other reasons? (maybe a spacetime could have a naked singularity but no CTCs?) And my understanding is that if a spacetime is globally hyperbolic, that means it can be "foliated" into a series of spacelike hypersurfaces, while a spacetime that's not globally hyperbolic can't be, is that right?

 

[George Jones]

Something I wondered about--if a spacetime is "not globally hyberbolic", does that automatically imply it contain CTCs,

No, see your own answer below!

or are there examples of spacetimes with no CTCs that are not globally hyberbolic for other reasons? (maybe a spacetime could have a naked singularity but no CTCs?)

Right. For example, consider Minkowski spacetime with the positive x-axis (of some inertial frame) removed. This spacetime is not globally hyperbolic, but it contains no CTCs.

A spacetime that has CTCs has not only a Cauchy horizon, but also a chronology horizon.

And my understanding is that if a spacetime is globally hyperbolic, that means it can be "foliated" into a series of spacelike hypersurfaces

Yes, a globally hyperbolic spacetime M can be written as a product TxS of "time" and "space".

while a spacetime that's not globally hyperbolic can't be

I'm not sure about this.

 

[Careful]

I'm not sure about this.

Suppose (M,g_{ab}) is foliated by spatial hypersurfaces \Sigma_t and that M can be mapped to \Sigma \times R then it is easy to see that any past and future inextendible timelike curve will cross \Sigma_t exactly once ; hence the result. As a rule of thumb, you do not want CTC's expecially not in the asymptotically observable region, but they do occur already for simple systems such as rotating rods as far as I remember (Will Bonnor has done lots of work on that).

 

[RandallB]

Moving forward or backwards in time could easily be considered IF you think of time as a River with a meander developing into an Oxbow Lake. As you go down the flow of time on your riverboat as it goes into or out of the meander that is becoming an Oxbow you could swing across the developing cutoff point (Call this the ‘wormhole’ to deposit yourself into the flow of time (the river current) greatly separated from the position the riverboat in that flow.

BUT here’s the problem – there is no reason to expect the river boat to still in that part of the river, it only exists in the time and place you left it. The riverboat (representing your world and universe as you knew it) is lost, as you are now floating in a new part of the stream of time.

In a similar fashion in our 3D world if you hold a 2x4 in your hand a see that it is 2” in z and 4” in y experience has shown us there is no reason to believe we will find the same to be true about the 2x4 at any position in x. Or that the 2x4 will still even exist at any position in x as in could end leaving a void or allowing something else, like a brick (or some other world/universe) to take that position.

 

[vanesch]

Any spacetime that has CTCs is not globally hyperbolic, and so does not possesses a Cauchy surface necessary for the complete specification of initial data - initial-value problems are not well-posed in spacetimes that have closed timelike curves.

Yes, that's why I said "locally" (in a sufficiently small environment so that there's a "local" foliation, using, for instance, the eigentime parameter of the CTC around a small piece of it). Imagine a CTC, for instance, which takes about 400 years (eigentime). Over a few minutes (and probably even several years), we can have a local foliation around the curve which looks "normal enough". However, over a period of 400 years, it is of course not possible.
What you would, for instance, have, I presume, is that if a clock was sent on a CTC, that it would not have a memory state that allowed it to register its eigentime in such a way that it could find out "how many loops" it had executed, because when it "came by event P", then the neighbourhood of event P (which can be locally foliated in order to determine the dynamics and the memory state of the clock) will each time determine exactly the SAME memory contents of the clock. So it "cannot remember" its own past far enough in the past to know its "previous passage by event P", simply because its memory state is part of the local environment of P. So no matter how it "locally evolved" on its curve, it will be in a state, around P, such as not to remember its previous passage through P. Or am I wrong ?
I think that the "paradox" of CTC (killing our grandpa when he was 9 years old) appears because we implicitly allow for our memories, decisions and so on, NOT to live on a deterministic block universe, and hence allow ourselves erroneously to "be in a different memory state" the second time we come by the same event on a CTC. But our memory states being fixed by the neighbourhood of that event just as well, we won't know it, that we came by the second time.

 

[JesseM]

BUT here’s the problem – there is no reason to expect the river boat to still in that part of the river, it only exists in the time and place you left it.

That's just a problem with your analogy, not with the idea of time travel itself. From a perspective viewing spacetime as a whole, there is nothing moving "along" the worldlines, they just exist--your analogy would be improved by making the river frozen, and removing the boat altogether.

 

[RandallB]

That's just a problem with your analogy, not with the idea of time travel itself. From a perspective viewing spacetime as a whole, there is nothing moving "along" the worldlines, they just exist--your analogy would be improved by making the river frozen, and removing the boat altogether.

I don’t see how. You need to be on the riverboat just as you must be on the world you now know. If you step off it there is no reason to expect a duplicate to exist in another (prior or future) time position.

Put it this way, if you wanted to visit one of great(great) grandparents at say age 10, 100 years ago by transporting yourself to a “time” of t = -100. There is no reason to expect to find them at that time, nor the world they lived in. They were only at that time, while they were age 10; as they are now at an age of 110 they only exist (returned to dust or whatever) in our current time. And the world that once was in that time, now only exists aged by 100 years in our current time.
Who, what, or if there is even a world (riverboat) for you to find now passing though that point in time as it ages is speculation. But there is no reason to think that by getting back to that point in time you can expect to find any part of, or anything like, our current world as being anywhere in that time. Just as there is no reason to expect a 2x4, to extend to infinity in both directions of length in our spatial dimensions.

I’m not saying, this is what GR 4D says, just that it is a viable way to look at time travel. To use GR with it would require using the 5D version of GR with time defined in the extra time dimension of GR and time as we know it defined as the aging we experience, progressing at different rates based on relativity rules.

The GR Theory does not require the riverboat (world) to exist at both points of time simultaneously as we perceive time.

 

[JesseM]

I don’t see how. You need to be on the riverboat just as you must be on the world you now know. If you step off it there is no reason to expect a duplicate to exist in another (prior or future) time position.

But if the river represents your worldline, there isn't a "you" separate from your worldline, "you" at a particular moment are just a cross-section of the worldline at that moment. And if the river doesn't represent your worldline, what does it represent?

Put it this way, if you wanted to visit one of great(great) grandparents at say age 10, 100 years ago by transporting yourself to a “time” of t = -100. There is no reason to expect to find them at that time, nor the world they lived in. They were only at that time, while they were age 10; as they are now at an age of 110 they only exist (returned to dust or whatever) in our current time. And the world that once was in that time, now only exists aged by 100 years in our current time.

But you're assuming some sort of universal "now" which is constantly flowing forward, when in the block time view "now" is just a term that's relative to whoever's speaking, like "here" (this is what philosophers call the A series vs. the B series view of time). Again, just picture a static 4-dimensional object representing spacetime (like the block of ice with strings embedded in it from my earlier post), without any objective "now" moving along it; your 10-year-old great-grandparent is one location on the object, your 110-year-old great-grandparent is on a different location, they're both just cross-sections of this 4D worm that is the great-grandparent's worldline. If you say "my great-grandfather is now 110", in the block time view I can just understand that to mean that the cross-section of spacetime that includes the cross-section of your worldline that's saying those words also includes a cross-section of your great-grandfather's worldline that's 110 years old, but it's not as if that cross-section is "now" in some universal objective sense, or that the other cross-sections aren't equally real.

Of course "block time" is partly a philosophical view rather than a statement about physics, but I think the relativity of simultaneity does make the idea of a universal objective "now" a lot more unappealing. If your great-grandfather long ago emigrated to another star system, and there's one inertial frame where December 9, 2006 on Earth is simultaneous with the event of him being 110, but another inertial frame where December 9, 2006 on Earth is simultaneous with him being 111, do you think there is any objective truth about how old he really is "now"?

I’m not saying, this is what GR 4D says, just that it is a viable way to look at time travel.

It may be logically viable, but I think it contradicts GR--the theory of general relativity would have to be wrong for your idea of time travel, where the past would "no longer be there" even if you had a wormhole to loop back to it, to be right.

To use GR with it would require using the 5D version of GR with time defined in the extra time dimension of GR and time as we know it defined as the aging we experience, progressing at different rates based on relativity rules.

Maybe your ideas about time travel would require such a modification of GR, but GR itself certainly does allow time travel, and no extra time dimension is needed because, again, there is nothing "moving along" worldlines corresponding to an objective now (the riverboat in your metaphor), worldlines just exist in the "static" 4D manifold of spacetime. In this view, time travel is just a worldline that loops around and passes near an earlier part of itself, and a CTC is just a worldline that forms a closed loop in spacetime.

 

[mgelfan]

I think you're still not understanding how the geometrical view sheds light on closed timelike curves in GR--nothing is being rewound or repositioned!

To see how the geometric "block time" view works, imagine spacetime as a literal block of ice, with some pieces of string embedded in it to represent worldlines. Now imagine slicing this block up into a stack of very thin cross-sections, like slicing meat at a deli counter. Each cross-section of the block will contain cross-sections of all the strings, which will just look like dots embedded in a 2D sheet. If we were to take pictures of each cross-section in succession, and then run them together as frames in a movie, we'd see the dots moving around over time, corresponding to particles moving around in space.

Now, time travel in GR does not mean that the configuration of dots in the movie must return to a copy of their configuration in an earlier frame of the movie. Equivalently, it does not mean that a later cross-section of the ice looks identical to an earlier cross-section. Instead, returning to thinking about the whole block of ice before it was sliced into sections, a CTC should be thought of as a piece of string that loops around and intersects an "earlier" part of itself. From our perspective viewing the ice as a whole, nothing is changing, it's just a static configuration of strings embedded in the ice with one of them happening to form a loop. You could even imagine the block of ice being cone-shaped, so that successive cross-sections would be larger and larger, representing the expansion of space; contrary to what you suggested above, there is no notion of a past state having to be recreated when the universe is larger, since again, it's just a string which loops around and revisits a section of the cone closer to the tip where the cross-section is smaller.

Similarly, if you can vaguely imagine standing outside spacetime as a whole, it would just look like a static curved 4D surface with various worldlines embedded in it, and CTCs would just be worldlines that form a loop. This picture really only makes sense in terms of the "block time" view, thinking in terms of the view that time "really flows" will just get you confused. Well, the intuitive view has caused you to misunderstand the idea of CTCs in GR, so at least in this situation it doesn't seem very helpful. Backwards time travel might be problematic for other reasons, but it's definitely allowed in GR (though a theory of quantum gravity may change this), and your arguments for why it's nonsensical don't work, for the reasons I tried to explain above.

While being necessary to do calculations that can be completed in a reasonably timely manner, the geometrical view is an oversimplification of the physical reality. As such, it can lead to absurdities -- and backward time travel is one of those absurdities.

Backward time travel is allowed in GR. But GR is a oversimplification of the physical reality. Thinking in terms of static curved 4D surfaces, the geometrical block time view of the universe, and closed timelike curves might seduce you into thinking that backward time travel (vis GR geometry) is actually physically meaningful. But it isn't.

Again, if you want to revisit the configuration(s) of objects that are the physical reality of London on December 8, 1950, between 8 and 9 pm, then you're going to have to reproduce the configuration(s) of objects that are the physical reality of London on December 8, 1950, between 8 and 9 pm -- because physically those configurations of objects no longer exist.

 

[JesseM]

While being necessary to do calculations that can be completed in a reasonably timely manner, the geometrical view is an oversimplification of the physical reality. As such, it can lead to absurdities -- and backward time travel is one of those absurdities.

Backward time travel is allowed in GR. But GR is a oversimplification of the physical reality. Thinking in terms of static curved 4D surfaces, the geometrical block time view of the universe, and closed timelike curves might seduce you into thinking that backward time travel (vis GR geometry) is actually physically meaningful. But it isn't.

Again, if you want to revisit the configuration(s) of objects that are the physical reality of London on December 8, 1950, between 8 and 9 pm, then you're going to have to reproduce the configuration(s) of objects that are the physical reality of London on December 8, 1950, between 8 and 9 pm -- because physically those configurations of objects no longer exist.

Why? You're just making assertions here, not giving any rational arguments as to why the geometrical view of time, where past events have not "ceased to exist" in any objective sense, but are just in a different temporal "location" than my own, is illogical or impossible. If I spent my whole life on a train moving west, and was never able to return east to locations the train had already passed through, I might believe that that all locations east of my present location had "ceased to exist", but this would be an unfounded assumption. The geometric view may seem strange or counterintuitive to you, but common-sense intuitions are typically a poor guide to scientific truths.

Also, are you familiar with the "relativity of simultaneity" in relativity, and if so do you reject it and believe instead there must be a single true definition of what events are happening "now" throughout the universe? If I'm in a distant star system, then in one reference frame the event of Dec. 9 2006 on Earth may be simultaneous with me being aged 29 in that star system, while in another frame the event of Dec. 9 2006 on Earth may be simultaneous with me being aged 30. Do you think there must be a single objective truth to whether the 29-year-old me "still exists" or has "ceased to exist" when the date on Earth is Dec. 9 2006?

 

[Careful]

Why? You're just making assertions here, not giving any rational arguments as to why the geometrical view of time, where past events have not "ceased to exist" in any objective sense, but are just in a different temporal "location" than my own, is illogical or impossible.

Since when do we ask ourselves in science wat is impossible ? Usually, nothing is impossible, like the following statements :
``in 3700 years, the Earth will be populated by zombies''
``quantum mechanics survives until 2150''
``there exists no time, only space''
and so on.

If I spent my whole life on a train moving west, and was never able to return east to locations the train had already passed through, I might believe that that all locations east of my present location had "ceased to exist", but this would be an unfounded assumption.

You make the common mistake to forget that there is an arrow of time while there is no such thing as an arrow of space.

The geometric view may seem strange or counterintuitive to you, but common-sense intuitions are typically a poor guide to scientific truths.

Again, you show severe misconceptions about the ``meaning of scientific theories''. In contrast to religion, truth does not exist in science in an absolute sense. Science is about finding a theory matching observations while containing a minimal number of assumptions which are not captured by our common sense perception of the world.

Also, are you familiar with the "relativity of simultaneity" in relativity, and if so do you reject it and believe instead there must be a single true definition of what events are happening "now" throughout the universe?

It is also a common mistake to say that relativity forbids a ``now''. There is nothing wrong with picking a global coordinate system and calling the time coordinate t ``now''. True, such t is not a Dirac observable, but nobody says that the entire description of reality needs to be based upon what we observe. On the contrary, it occurs to me that hidden variables of that kind are necessary; moreover they do not conflict relativity, they merely complement it.

 

[JesseM]

Since when do we ask ourselves in science wat is impossible ? Usually, nothing is impossible, like the following statements

I wasn't saying anything was impossible, I was just reacting to mgelfan's claim that it was. Do you agree with mgelfan that GR's prediction of time travel can be ruled out a priori because it is not "physically meaningful"?

You make the common mistake to forget that there is an arrow of time while there is no such thing as an arrow of space.

The arrow of time is generally thought to be a consequence of low-entropy initial conditions rather than fundamental physics (aside from CP violations in some weak interactions, which are not thought to have anything to do with the normal arrows of time we observe in our ordinary experience). In any case, if you think the arrow of time has any bearing on the issue of whether the geometric view of time is correct or not, I'd like to see your argument for the relevance. Even in a universe whose fundamental laws made it impossible to reconstruct the past from the present, that wouldn't provide justification for thinking the past "does not exist" in some objective sense, and the laws of this universe might even allow for worldlines which form CTCs. Likewise, even in a hypothetical universe whose laws of physics lacked spatial translation symmetry, where there was some sort of "arrow of space", that wouldn't somehow make it justified for a traveler moving in a particular spatial direction to believe that everything behind him had ceased to exist.

Again, you show severe misconceptions about the ``meaning of scientific theories''. In contrast to religion, truth does not exist in science in an absolute sense.

And where in my post did I say or imply it did? I certainly haven't claimed the geometric view as an absolute truth, I'm reacting to mgelfan's seeming complete certainty that it's wrong. Again, are you agreeing with mgelfan's claim that we can rule out CTCs a priori without even needing to do any experiments, or with his implied view that there must be a single correct definition of simultaneity?

It is also a common mistake to say that relativity forbids a ``now''. There is nothing wrong with picking a global coordinate system and calling the time coordinate t ``now''. True, such t is not a Dirac observable, but nobody says that the entire description of reality needs to be based upon what we observe. On the contrary, it occurs to me that hidden variables of that kind are necessary; moreover they do not conflict relativity, they merely complement it.

I did not in fact say that relativity forbids a "now", I specifically pointed out that the "moving now vs. block time" debate was a philosophical one, but that the relativity of simultaneity makes the "moving now" view more unappealing:

Of course "block time" is partly a philosophical view rather than a statement about physics, but I think the relativity of simultaneity does make the idea of a universal objective "now" a lot more unappealing.

I agree it is in theory possible there is a single preferred definition of simulataneity but that it cannot be physically distinguished from any other frame's definition of simultaneity in any observable way, even in principle (one could call this a "metaphysically preferred reference frame", a phrase I've used in posts on other threads like the bottom of post #58 here). Similarly, you are also free to believe that there are invisible, immaterial blue dragons sitting on the shoulder of every human being on earth, but that they have no physical effects whatsoever so we can't observe them. But these sorts of beliefs are "unappealing" to most people because they hope that ultimate reality would not be so radically at odds with what can be observed (most people reject solipsism for a similar reason, even though it's logically possible that it's correct).

 

[mgelfan]

Why? You're just making assertions here, not giving any rational arguments as to why the geometrical view of time, where past events have not "ceased to exist" in any objective sense, but are just in a different temporal "location" than my own, is illogical or impossible.

Why? Because, as far as is known the objective constituents of the universe are continually in motion relative to each other. (ie., each object in the universe is changing position relative to most other objects in the universe).
The sequence of configurations of objects that was the reality of 1950's London is no longer a part of our physical universe.

The GR model serves as a calculational tool. But if it's an understanding of the physical reasons pertaining to the possibility of backward time travel, then the model won't necessarily provide that -- and, in my opinion, using the model in this way does lead to a rather absurd view of reality, and lots of unnecessary confusion about what's possible and what's not.

If I spent my whole life on a train moving west, and was never able to return east to locations the train had already passed through, I might believe that that all locations east of my present location had "ceased to exist", but this would be an unfounded assumption. The geometric view may seem strange or counterintuitive to you, but common-sense intuitions are typically a poor guide to scientific truths.

Common sense intuitions are the foundation of physical science. The geometric view that leads you to believe that 1950's London still exists is an extension of the kinematics of special relativity. 1950's London still exists only as the light from that era, which is still traveling to distant regions of the universe.

Now, don't we know that when we receive the light from distant star systems that we are seeing those systems as they used to be but no longer are?

Also, are you familiar with the "relativity of simultaneity" in relativity, and if so do you reject it and believe instead there must be a single true definition of what events are happening "now" throughout the universe?

The relativity of simultaneity is a consequence of the definitions and conventions adopted in SR so that an unambiguous kinematics could be developed.

For observation and translation of data it's necessary to 'stay inside the SR box', so to speak, so that we're all on the same page regarding the physical meaning of experiments -- because this information isn't transferred instantaneously between reference frames. Light is how we get our information, and light travels at a finite speed.

However, imagine yourself able to see the universe as a single evolving entity -- like watching your clock or watch. The universe (or some necessarily very large region thereof) as a clock, and some configuration thereof, is what we're talking about when we're considering the possibility of time travel.

If I'm in a distant star system, then in one reference frame the event of Dec. 9 2006 on Earth may be simultaneous with me being aged 29 in that star system, while in another frame the event of Dec. 9 2006 on Earth may be simultaneous with me being aged 30. Do you think there must be a single objective truth to whether the 29-year-old me "still exists" or has "ceased to exist" when the date on Earth is Dec. 9 2006?

The attosecond that (according to your local, very accurate, timepiece) you became 29 is a unique and transitory event.

Yes, the light from this event will require different travel times to reach different places. So what? Does that mean that the event that you experienced is happening over and over again -- that it, in effect, continually exists because the light from it still exists?

 

[Careful]

I wasn't saying anything was impossible, I was just reacting to mgelfan's claim that it was. Do you agree with mgelfan that GR's prediction of time travel can be ruled out a priori because it is not "physically meaningful"?

Yes, with a certitude of 99 percent. By the way, I would not even call it a prediction of GR, it is just how you wish to interpret GR (in the canonical picture I would only get globally hyperbolic spacetimes).

The arrow of time is generally thought to be a consequence of low-entropy initial conditions rather than fundamental physics (aside from CP violations in some weak interactions, which are not thought to have anything to do with the normal arrows of time we observe in our ordinary experience).

Come on, an arrow of time is something local, entropy not (I could play the same game you do and say there is nothing in GR which says that the arrow of time needs to be a globally well defined and nonvanishing vectorfield.). By the way, all these arguments are circular: when you speak about entropy of the universe, you speak about entropy of the spatial universe, but in order to speak about that, you need to have a notion of simultaneity. Now, you know as well as I do, that in general no such canonical notion can be found based upon the dynamical content of the theory (geometric invariants, preffered timelike vectorfields generated by the matter content and so on). What is usually done in practice is to adopt the approximate Friedmann notion of simultaneity, but there is no intrinsic meaning to that (the universe simply isn't perfectly homogeneous and isotropic).

But as far as I am concerned there is no problem with an ``arrow of time'', ontological time is just the coordinate t of a Minkowski frame and this one runs from minus infinity to plus infinity.

In any case, if you think the arrow of time has any bearing on the issue of whether the geometric view of time is correct or not, I'd like to see your argument for the relevance.

The geometric view has nothing to do with the arrow of time as far as I am concerned. GR says that gravitation is mediated by a field whose equations of motion are determined by and determine the matter content of the universe. Now, these equations happen to be generally covariant, but that does not imply that there is no preferred coordinate system in nature. I would say there is: it is determined by demanding that in the limit for the matter fields and coupling constants to zero, special relativity is recovered.

Even in a universe whose fundamental laws made it impossible to reconstruct the past from the present, that wouldn't provide justification for thinking the past "does not exist" in some objective sense, and the laws of this universe might even allow for worldlines which form CTCs. Likewise, even in a hypothetical universe whose laws of physics lacked spatial translation symmetry, where there was some sort of "arrow of space", that wouldn't somehow make it justified for a traveler moving in a particular spatial direction to believe that everything behind him had ceased to exist.

Of course, and if and if and if ... Look, nobody is contesting that what you say is correct in a mathematical sense; but you haven't given one shred of evidence so far why we should accept something that far removed from our experience. Usually, people only accept such things temporarily because it appears to be absolutely necessary for the consistency of the theory in absence of good ideas for a better alternative. You on the other hand, just merely seem to glorify the mere possibility !

And where in my post did I say or imply it did?

You clearly stated that common sense intuitions are poor guides to scientific truths : (a) you assume hereby that scientific truth exists (b) you mistakenly degrade intuition.

I certainly haven't claimed the geometric view as an absolute truth, I'm reacting to mgelfan's seeming complete certainty that it's wrong. Again, are you agreeing with mgelfan's claim that we can rule out CTCs a priori without even needing to do any experiments, or with his implied view that there must be a single correct definition of simultaneity?

Again, with 99 percent probability, yes. In a physicist's language, that equals absolute certainty.

I did not in fact say that relativity forbids a "now", I specifically pointed out that the "moving now vs. block time" debate was a philosophical one, but that the relativity of simultaneity makes the "moving now" view more unappealing: I agree it is in theory possible there is a single preferred definition of simulataneity but that it cannot be physically distinguished from any other frame's definition of simultaneity in any observable way, even in principle (one could call this a "metaphysically preferred reference frame", a phrase I've used in posts on other threads like the bottom of post #58 here).

Nope, relativity cannot be correct on all energy scales at least if you believe in locality (mind : not relativistic causality !), realism and particles as carriers of force fields. So that invalidates the rest you try to argue here

Similarly, you are also free to believe that there are invisible, immaterial blue dragons sitting on the shoulder of every human being on earth, but that they have no physical effects whatsoever so we can't observe them. But these sorts of beliefs are "unappealing" to most people because they hope that ultimate reality would not be so radically at odds with what can be observed (most people reject solipsism for a similar reason, even though it's logically possible that it's correct).

Ah, and who is assuming physics here which is at odds with observation ??
At least, my reasoning is based upon a general consequence of three physical assumptions and not upon some remote possibility in some interpretation of GR which is in direct conflict with all observations so far. It seems to me there is a world of difference between these two (and actually, I disagree that we wouldn't observe off shell particles, this is just a useful approximation)!

 

[JesseM]

I wasn't saying anything was impossible, I was just reacting to mgelfan's claim that it was. Do you agree with mgelfan that GR's prediction of time travel can be ruled out a priori because it is not "physically meaningful"?

Yes, with a certitude of 99 percent.

On what basis? Just your personal intuitions, or some type of scientific argument? Of course a lot of physicists predict that a theory of quantum gravity will not allow time travel based on semiclassical arguments, but mgelfan is not based on arguments about quantum physics at all.

Come on, an arrow of time is something local, entropy not

On the contrary, every physicist I have seen talking about the "arrow of time" issue (Hawking, Penrose, Greene, etc.) discusses thermodynamic irreversibility as one of the main arrows of time, and most other arrows (like the electromagnetic arrow or the psychological arrow) are understood as consequences of the thermodynamic arrow. What arrow of time were you thinking of, just the CP violations?

By the way, all these arguments are circular: when you speak about entropy of the universe, you speak about entropy of the spatial universe, but in order to speak about that, you need to have a notion of simultaneity.

I don't see why--you should be able to use any foliation, I don't think there'd be any where entropy would be higher in spacelike surfaces closer in time to the big bang than farther from it (although I think there are some problems with defining a notion of gravitational entropy...that didn't stop the physicists I mentioned above from saying the thermodynamic arrow is a consequence of the lower entropy close to the big bang, though). Anyway, as you say, it's pretty standard when discussing cosmological issues to use a foliation where the universe is as close to homogeneous in each surface as possible.

The geometric view has nothing to do with the arrow of time as far as I am concerned.

Well, why did you bring it up then? I had said 'If I spent my whole life on a train moving west, and was never able to return east to locations the train had already passed through, I might believe that that all locations east of my present location had "ceased to exist", but this would be an unfounded assumption.' and you responded 'You make the common mistake to forget that there is an arrow of time while there is no such thing as an arrow of space.', but I still don't see why my point is mistaken and how the existence of an arrow of time and nonexistence of an arrow of space has the slightest bearing on that point.

Even in a universe whose fundamental laws made it impossible to reconstruct the past from the present, that wouldn't provide justification for thinking the past "does not exist" in some objective sense, and the laws of this universe might even allow for worldlines which form CTCs. Likewise, even in a hypothetical universe whose laws of physics lacked spatial translation symmetry, where there was some sort of "arrow of space", that wouldn't somehow make it justified for a traveler moving in a particular spatial direction to believe that everything behind him had ceased to exist.

Of course, and if and if and if ...

That response is entirely free of content--are you saying that my what-ifs are not relevant? I think they are relevant to refuting your claim that somehow the existence of an arrow of time and nonexistence of an arrow of space invalidates my analogy of the guy on the train moving west. If the analogy would still work fine in a universe which did have an arrow of space, that shows the arrow of time/arrow of space issue is not relevant to judging the value of the analogy.

Look, nobody is contesting that what you say is correct in a mathematical sense; but you haven't given one shred of evidence so far why we should accept something that far removed from our experience.

First of all, you keep acting like I am trying to make a definite case here, ignoring the fact that it is mgelfan who is claiming total certainty about the invalidity of the geometric view, I'm just saying there's no justification for rejecting it out of hand, not saying anyone must accept it.

Second, I don't really understand what you mean when you say the geometric view is "far removed from our experience". Granted we can't see spacetime as a 4-dimensional object, but I'm treating "the geometric view" as basically synonymous with what philosophers call the B-series view of time, which just says that terms like past, present and future are relational, like "here", rather than absolute. And there is nothing in my experience that tells me me that these sorts of temporal terms have any meaning outside of their relation to my own experiences--for example, to me "I am looking at my computer monitor now" just means that my visual experience of the moniter and my mental experience of these thoughts are part of the same moment of subjective experience. There is certainly nothing in my experience that tells me that past events have ceased to exist in some universal objective way, any more than anything in my experience tells me that my apartment ceases to exist when I go outside.

Usually, people only accept such things temporarily because it appears to be absolutely necessary for the consistency of the theory in absence of good ideas for a better alternative. You on the other hand, just merely seem to glorify the mere possibility !

I disagree, most modern philosophers would probably say the B series view of time inherently makes more sense than the A series, independently of any physical arguments (McTaggart, who invented the terms, advocated the B series back in 1908, probably too early to have been influenced much by special relativity). And to me the A series view has always seemed inherently a bit incoherent or at least fuzzily-defined--what can it mean to say that the present is "moving" unless we have something like a second time dimension, for example?

You clearly stated that common sense intuitions are poor guides to scientific truths : (a) you assume hereby that scientific truth exists (b) you mistakenly degrade intuition.

You're reading way too much into my casual use of the phrase "scientific truths". I could have used a phrase like "statements about the universe which we can never absolutely know to be true or false, but which science gives us a strong basis for thinking are likely to be true", but that would have been a bit convoluted. And would you really object strongly if someone referred in conversation to evolution as a "scientific truth", for example? That is certainly a theory that goes against human common-sense--it's a lot easier for a child to understand the explanation that some complex organized structure was "made" by someone than it is for them to understand that it arose by random mutation and natural selection, and even evolutionary biologists tend to use teleological shorthand, talking about the "purpose" of a given adaptation.

I would say that quite a lot of the things that our best theories say about the world go against common-sense intuition, from relativity's claim that it would be impossible to accelerate anything past the speed of light or that a twin taking a relativistic voyage away from Earth and back would return younger than his twin on earth, to claims in QM like the one that you can't measure a particle's position and momentum simultaneously or just about anything related to the double-slit experiment. And more advanced physics, like curved spacetime of general relativity or the very abstract mathematics that goes into making predictions in quantum field theory, is also far from common sense intuitions about how the physical world works. I've seen a number of very good physicists talking about how common sense should not be trusted, as Einstein's quote that "Common sense is the collection of prejudices acquired by age eighteen", or Feynman's discussion of intuitive mechanical models vs. abstract mathematics in "The Relation of Mathematics to Physics" in the book "The Character of Physical Law", where he says things like:

But up to today, from the time of Newton, no one has invented another theoretical description of the mathematical machinery behind this law [the law of gravity] which does not either say the same thing over again, or make the mathematics harder, or predict some wrong phenomena. So there is no model of the theory of gravity today, other than the mathematical form.

If this were the only law of this character it would be interesting and rather annoying. But what turns out to be true is that the more we investigate, the more laws we find, and the deeper we penetrate nature, the more this disease persists. Every one of our laws is a purely mathematical statement in rather complex and abstruse mathematics.

...[A] question is whether, when trying to guess new laws, we should use seat-of-the-pants feelings and philosophical principles--'I don't like the minimum principle', or 'I do like the minimum principle', 'I don't like action at a distance', or 'I do like action at a distance'. To what extent do models help? It is interesting that very often models do help, and most physics teachers try to teach how to use models and to get a good physical feel for how things are going to work out. But it always turns out that the greatest discoveries abstract away from the model and the model never does any good. Maxwell's discovery of electrodynamics was made with a lot of imaginary wheels and idlers in space. But when you get rid of all the idlers and things in space the thing is O.K. Dirac discovered the correct laws for relativity quantum mechanics simply by guessing the equation. The method of guessing the equation seems to be a pretty effective way of guessing new laws. This shows again that mathematics is a deep way of expressing nature, and any attempt to express nature in philosophical principles, or in seat-of-the-pants mechanical feelings, is not an efficient way.

So would you say Einstein and Feynman were misguided in their attitude towards the role of common-sense intuitions in science?

Again, with 99 percent probability, yes. In a physicist's language, that equals absolute certainty.

But are you claiming "99 percent probability" based purely on physical arguments, or based on personal intuitions and philosophical convictions? A physicist hopefully would not claim "absolute certainty" about some opinion of his whose basis had nothing to do with scientific arguments, like an opinion about politics or something.

And if you're basing this on physical arguments, then what are those arguments, specifically? Do you think a physicist like Kip Thorne is incompetent for not agreeing we should totally discount the possibility of CTCs? Also, are you equally confident about the wrongness of the "block time" view in general as you are about the impossibility of CTCs?

I did not in fact say that relativity forbids a "now", I specifically pointed out that the "moving now vs. block time" debate was a philosophical one, but that the relativity of simultaneity makes the "moving now" view more unappealing: I agree it is in theory possible there is a single preferred definition of simulataneity but that it cannot be physically distinguished from any other frame's definition of simultaneity in any observable way, even in principle (one could call this a "metaphysically preferred reference frame", a phrase I've used in posts on other threads like the bottom of post #58 here).

Nope, relativity cannot be correct on all energy scales at least if you believe in locality (mind : not relativistic causality !), realism and particles as carriers of force fields. So that invalidates the rest you try to argue here

What do you mean by "realism", exactly? Are you talking about a hidden variables interpretation, or would you count something like the MWI as a form of "realism"? If the former, it hardly invalidates what I said, since I was describing as "unappealing" any view of physics which postulates fundamentally unobservable entities akin to my "invisible immaterial blue dragons", and indeed it is true that most physicists find hidden-variables interpretations of QM to be unappealing (and those that find them appealing usually have some hope that a future hidden-variables theory will actually have new experimental consequences, rather than being in-principle undetectable like Bohmian mechanics).

And if you weren't talking about hidden variables, then when you say "relativity cannot be correct", which aspect of relativity are you talking about? The only aspect of relativity that enters into my argument about the unappealing idea of "a metaphysically preferred frame" is local Lorentz-invariance, and as far as I know there are very few physicists who think this aspect of relativity will end up being invalidated and that there will be a single physically preferred frame in a given local region.

Similarly, you are also free to believe that there are invisible, immaterial blue dragons sitting on the shoulder of every human being on earth, but that they have no physical effects whatsoever so we can't observe them. But these sorts of beliefs are "unappealing" to most people because they hope that ultimate reality would not be so radically at odds with what can be observed (most people reject solipsism for a similar reason, even though it's logically possible that it's correct).

Ah, and who is assuming physics here which is at odds with observation ??

Not me, I have certainly never "assumed" that CTCs are possible (in fact I rather doubt they'll turn out to be), and on the topic of the relativity of simultaneity, I've only said that 'the relativity of simultaneity does make the idea of a universal objective "now" a lot more unappealing', this statement does not assume that it is impossible the relativity of simultaneity might be invalidated someday. But as long as observations do continue to uphold the relativity of simultaneity, then any philosophical theory of a single objective "now" must be at odds with observation, since by definition any observation that showed one local definition of simultaneity to be physically preferred over others would violate the relativity of simultaneity.

At least, my reasoning is based upon a general consequence of three physical assumptions and not upon some remote possibility in some interpretation of GR which is in direct conflict with all observations so far.

Although CTCs are indeed very speculative, I thought I was pretty clear that my views on the "unappealingness" of the "single objective now" view were also based on the relativity of simultaneity, and most physicists would probably consider a violation of local Lorentz-invariance to be a rather remote possibility itself. And once again, my main point here is not really to make a positive argument for the geometric view of spacetime in the first place, it's mainly to argue against mgelfan's notion that we should reject the geometric view out-of-hand. I don't see why you should object to this, unless you are completely closeminded about the mere possibility that the geometric/B-series view of time could be correct.

 

[Careful]

On what basis? Just your personal intuitions, or some type of scientific argument? Of course a lot of physicists predict that a theory of quantum gravity will not allow time travel based on semiclassical arguments, but mgelfan is not based on arguments about quantum physics at all.

No personal intuitions, just the mere fact that nobody has observed it yet in human recorded history. Is that not enough for you ?! I don't need quantum mechanics to understand that.

On the contrary, every physicist I have seen talking about the "arrow of time" issue (Hawking, Penrose, Greene, etc.) discusses thermodynamic irreversibility as one of the main arrows of time, and most other arrows (like the electromagnetic arrow or the psychological arrow) are understood as consequences of the thermodynamic arrow.

The thermodynamic arrow is an imaginary non-local concept which has no meaning for the fundamental laws of physics. It is just statistics, that's all there is to it. For compact dynamical systems, Poincare's theorem shows us that the second law fails (for a short period of time that is), this is what recurrency time means. Obviously, our perception of dynamics and hence ``time'' is related to the behavior of many particle systems, but that is just an emergent thing.

I don't see why--you should be able to use any foliation, I don't think there'd be any where entropy would be higher in spacelike surfaces closer in time to the big bang than farther from it (although I think there are some problems with defining a notion of gravitational entropy...that didn't stop the physicists I mentioned above from saying the thermodynamic arrow is a consequence of the lower entropy close to the big bang, though).

Right, and Penrose still works with Newtonian models : actually I gave this some thought myself, defining gravitational entropy is extremely difficult to say the very least.

Anyway, as you say, it's pretty standard when discussing cosmological issues to use a foliation where the universe is as close to homogeneous in each surface as possible. Well, why did you bring it up then? I had said 'If I spent my whole life on a train moving west, and was never able to return east to locations the train had already passed through, I might believe that that all locations east of my present location had "ceased to exist", but this would be an unfounded assumption.' and you responded 'You make the common mistake to forget that there is an arrow of time while there is no such thing as an arrow of space.', but I still don't see why my point is mistaken and how the existence of an arrow of time and nonexistence of an arrow of space has the slightest bearing on that point.

I meant exactly the same as mgelfan : history does not exist anymore in a physical sense, one cannot return to the past. That is the nontrivial difference between time and space. Your construction moreover assumes you are the single observer having experiences about the world.

Second, I don't really understand what you mean when you say the geometric view is "far removed from our experience". Granted we can't see spacetime as a 4-dimensional object, but I'm treating "the geometric view" as basically synonymous with what philosophers call the B-series view of time, which just says that terms like past, present and future are relational, like "here", rather than absolute.

If you go back to my post, you might notice that I meant that CTC's are far removed from our experience.

There is certainly nothing in my experience that tells me that past events have ceased to exist in some universal objective way, any more than anything in my experience tells me that my apartment ceases to exist when I go outside.

I don't understand what the latter has to do with the former but fine. And of course your experience tells you that your past events have ceased to exist.

I disagree, most modern philosophers would probably say the B series view of time inherently makes more sense than the A series, independently of any physical arguments (McTaggart, who invented the terms, advocated the B series back in 1908, probably too early to have been influenced much by special relativity).

Do you know of any modern philosopher who has any serious impact on physics ?

And to me the A series view has always seemed inherently a bit incoherent or at least fuzzily-defined--what can it mean to say that the present is "moving" unless we have something like a second time dimension, for example?

What do we need the second time dimension for (not for eigentime anyway) ?

And would you really object strongly if someone referred in conversation to evolution as a "scientific truth", for example? That is certainly a theory that goes against human common-sense--it's a lot easier for a child to understand the explanation that some complex organized structure was "made" by someone than it is for them to understand that it arose by random mutation and natural selection, and even evolutionary biologists tend to use teleological shorthand, talking about the "purpose" of a given adaptation.

Huh, I always found natural selection pretty obvious. Although, the way the educational system is going does make me doubt about the meaning of the word evolution though.

I would say that quite a lot of the things that our best theories say about the world go against common-sense intuition, from relativity's claim that it would be impossible to accelerate anything past the speed of light or that a twin taking a relativistic voyage away from Earth and back would return younger than his twin on earth, to claims in QM like the one that you can't measure a particle's position and momentum simultaneously or just about anything related to the double-slit experiment.

So (a) who says relativity is correct under all circumstances (for example see the de Broglie mass problem for first quantized complex KG fields) (b) of course you can measure a single particle's position and momentum with ``arbitrary'' accuracy, QM does not claim any such thing like you say (c) I do not find the double slit mysterious.

And more advanced physics, like curved spacetime of general relativity or the very abstract mathematics that goes into making predictions in quantum field theory, is also far from common sense intuitions about how the physical world works.

Wrong, QFT does not tell how the word works, it gives at best some approximation to the statistics of outcomes of repeated experiments.

I've seen a number of very good physicists talking about how common sense should not be trusted, as Einstein's quote that "Common sense is the collection of prejudices acquired by age eighteen", or Feynman's discussion of intuitive mechanical models vs. abstract mathematics in "The Relation of Mathematics to Physics" in the book "The Character of Physical Law", where he says things like: So would you say Einstein and Feynman were misguided in their attitude towards the role of common-sense intuitions in science?

Now, you are talking nonsense. Common sense should always be measured against experimental facts, I could easily turn this around and ask you whether the same Einstein was too prejudiced when he was attacking QM or whether Dirac and Feynman lost their mental powers when they too, started looking for alternatives ? (Dirac at the age of 35 by the way) Moreover, Feynman had the greatest respect for how Einstein discovered GR, and how do you think Albert did this ? All this is just small talk which greatly depends upon the succes of some method in physics of that particular time.

But are you claiming "99 percent probability" based purely on physical arguments, or based on personal intuitions and philosophical convictions? A physicist hopefully would not claim "absolute certainty" about some opinion of his whose basis had nothing to do with scientific arguments, like an opinion about politics or something.

Rubbish, I guess you are not a physicist. Do you really think that we are interested in what has a remote chance to be possible or not ?! A physicist is interested in getting serious evidence that such travesty cannot be avoided. You are actually talking here about how many angels can sit on the head of a pin and you are moreover convinced that this is somehow worthwhile talking about.

And if you're basing this on physical arguments, then what are those arguments, specifically? Do you think a physicist like Kip Thorne is incompetent for not agreeing we should totally discount the possibility of CTCs?

It has nothing to do with ``incompetence'', neither do I know of his personal reasons to say so. Nor do I find such line of argumentation interesting, you would do much better if you were to actually give more detail to why he believes this to be true. This is the second time you try to use authority in your arguments, do you actually have a further point ? I could equally say, do you believe 't Hooft is an idiot for claiming that CTC's do not make sense ?

What do you mean by "realism", exactly? Are you talking about a hidden variables interpretation, or would you count something like the MWI as a form of "realism"?

Realism means that there is an objective dynamics underlying our observations, that is ``things exist and move according to definite laws''. And I do not care about your estetical arguments, there are actually very good physical arguments as to why some unobserved things should be real (and I briefly gave some of them already). Moreover, the entire game of quantum gravity is about unobserved Planck scale degrees of freedom, likewise is string theory about unobserved high energy phenomena. So perhaps you are going to tell to all these scientists now that they are doing unappealing things ?

And if you weren't talking about hidden variables, then when you say "relativity cannot be correct", which aspect of relativity are you talking about? The only aspect of relativity that enters into my argument about the unappealing idea of "a metaphysically preferred frame" is local Lorentz-invariance, and as far as I know there are very few physicists who think this aspect of relativity will end up being invalidated and that there will be a single physically preferred frame in a given local region.

Yep I meant local lorentz invariance. Note that my conclusion is a logical one and again your social arguments are basically irrelevant and incorrect at least what the quantum gravity community is concerned.

But as long as observations do continue to uphold the relativity of simultaneity, then any philosophical theory of a single objective "now" must be at odds with observation, since by definition any observation that showed one local definition of simultaneity to be physically preferred over others would violate the relativity of simultaneity.

Of course this is all wrong. It is entirely possible to construct theories with a preferred frame which recover Lorentz invariance at some coarse grained level. Hence you must assume all ``fundamental particles'' to be collective excitations, but (again) this is exactly what quantum gravity is about.

 

[JesseM]

No personal intuitions, just the mere fact that nobody has observed it yet in human recorded history. Is that not enough for you ?! I don't need quantum mechanics to understand that.

But that sort of argument has no place in science. Nobody has observed the Higgs particle yet in human recorded history, but that's hardly a rational reason to feel 99% certain they don't exist.

The thermodynamic arrow is an imaginary non-local concept which has no meaning for the fundamental laws of physics.

I agree it is not based on fundamental laws, my only point was that it is standard terminology to refer to it as an "arrow of time", and in fact that is what physicists are usually talking about when they used this phrase. You jumped all over me for asking if you were talking about the thermodynamic arrow of time when you used the phrase "arrow of time", but there was nothing unreasonable about this question. You really seem strangely hyper-aggressive to me about virtually every little comment I make, what's your damage?

I meant exactly the same as mgelfan : history does not exist anymore in a physical sense, one cannot return to the past.

The issue of whether it is possible to return to the past is logically distinct from the issue of whether history "exists any more". As I said, I personally doubt CTCs will turn out to be possible, but I still think there are a number of good reasons for doubting the "moving now" view, from philosophical arguments to the relativity of simultaneity. And as always, I'm not taking the definite stand that one must accept the geometric view, just saying there is no scientific basis for being totally certain it's false. Do you deny that this issue is one on which "reasonable people can disagree"? If so, then what is your scientific evidence or logical argument that clearly shows the "moving now" view is clearly true and the geometric view clearly false?

Your construction moreover assumes you are the single observer having experiences about the world.

And what am I exactly, if not a single observer having experiences about the world?

If you go back to my post, you might notice that I meant that CTC's are far removed from our experience.

No, that wasn't clear at all from your original post, you just said "Look, nobody is contesting that what you say is correct in a mathematical sense; but you haven't given one shred of evidence so far why we should accept something that far removed from our experience." And the comment of mine immediately before that which you were responding to mentioned both the question of CTCs and the broader question of whether the past "ceases to exist" or not.

There is certainly nothing in my experience that tells me that past events have ceased to exist in some universal objective way, any more than anything in my experience tells me that my apartment ceases to exist when I go outside.

I don't understand what the latter has to do with the former but fine. And of course your experience tells you that your past events have ceased to exist.

How does it do that, exactly? It just tells me I can't physically visit the past, that it's inaccessible to me. But inaccessibility is not evidence of nonexistence, that was the whole point of my analogy about the guy on the train which is always moving west, he shouldn't conclude that just because he can never return to points eastward of himself they don't exist.

Do you know of any modern philosopher who has any serious impact on physics ?

Do you claim that the issue of the existence or nonexistence of the past is purely a question of physics rather than ontology or some other area of philosophy? If so, can you propose an experiment that would settle the issue?

What do we need the second time dimension for (not for eigentime anyway) ?

Because the "moving now" view pictures the present moving forward in time, like a pointer moving along a timeline. But we don't have any concept of "movement" without time, so it seems like you need meta-time to make sense of this, like "at an earlier meta-time, the pointer was pointing at 2003, while at a later meta-time it had moved to 2004". If you try to use the original time dimension to describe the "movement" of the present, then you get statements like "in 2003 the present was at 2003, in 2004 the present was at 2004", which does not seem any different from the relational B-series view of time. (You can find a similar argument at the start of chapter of 11 of David Deutsch's 'Fabric of Reality'.)

Huh, I always found natural selection pretty obvious.

And it is obvious once you study the evidence and consider the arguments, but the "common sense" of a person totally ignorant of this evidence and arguments would more likely tell them that complex purposeful structures were designed by someone, this is after all the conclusion that virtually everyone around the world came to before Darwin and modern science came along.

So (a) who says relativity is correct under all circumstances (for example see the de Broglie mass problem for first quantized complex KG fields)

Haven't studies quantum field theory so I'm not familiar with that. I do know that QFT is supposed to show Lorentz-symmetry though, and of course there may be problems with QFT that would have to be resolved by some future unified theory, but few physicists seem to think such a theory would involve re-introducing a preferred local frame, and anyway there's no reason to expect future theories will be more "common-sensical" than present ones.

(b) of course you can measure a single particle's position and momentum with ``arbitrary'' accuracy

Independently you can measure either arbitrarily accurately, but I specifically said "claims in QM like the one that you can't measure a particle's position and momentum simultaneously". If you disagree with this, how would you describe the implications of the uncertainty principle for measurements of position and momentum?

I do not find the double slit mysterious.

Maybe not, but do you think the results of the experiment are common-sensical? Most people's common sense would tell them the particle must have gone through either one slit or the other, I think. Of course you can adopt a hidden-variables interpretation where this is still true, but only at the expense of introducing other constructs which defy common sense, like the Bohmian "pilot wave" which guides the particle's path differently based on instantaneous knowledge of whether the other slit is open or closed.

Wrong, QFT does not tell how the word works, it gives at best some approximation to the statistics of outcomes of repeated experiments.

My point is that there is no common-sense picture of why this very abstruse mathematical procedure would be the correct one to predict what will happen in a given experiment.

I've seen a number of very good physicists talking about how common sense should not be trusted, as Einstein's quote that "Common sense is the collection of prejudices acquired by age eighteen", or Feynman's discussion of intuitive mechanical models vs. abstract mathematics in "The Relation of Mathematics to Physics" in the book "The Character of Physical Law", where he says things like: So would you say Einstein and Feynman were misguided in their attitude towards the role of common-sense intuitions in science?

Now, you are talking nonsense.

No, I'm just quoting Einstein and Feynman. If the quotes are nonsense, then it's they who you should accuse of talking nonsense, not me.

Common sense should always be measured against experimental facts

Of course it should, what's your point? My point is that experimental facts usually go against whatever prediction you might have made based on "common sense" before knowing the results of the experiment.

I could easily turn this around and ask you whether the same Einstein was too prejudiced when he was attacking QM or whether Dirac and Feynman lost their mental powers when they too, started looking for alternatives ?

I don't think these physicists were looking for alternatives based primarily on "common sense" though.

(Dirac at the age of 35 by the way) Moreover, Feynman had the greatest respect for how Einstein discovered GR, and how do you think Albert did this ?

Not by using the sort of common sense "mechanical intuitions" that Feynman was talking about, but by much more abstract principles of symmetry and mathematical elegance like the equivalence principle.

All this is just small talk which greatly depends upon the succes of some method in physics of that particular time.

I have no problem with saying the question of common sense in science is one on which reasonable people can disagree, but you are attacking me like I have said something crazy or obviously ignorant.

But are you claiming "99 percent probability" based purely on physical arguments, or based on personal intuitions and philosophical convictions? A physicist hopefully would not claim "absolute certainty" about some opinion of his whose basis had nothing to do with scientific arguments, like an opinion about politics or something.

Rubbish, I guess you are not a physicist.

What argument above is rubbish, the one that a physicist would not claim absolute certainty about something that has nothing to do with scientific arguments, such as politics?

Do you really think that we are interested in what has a remote chance to be possible or not ?! A physicist is interested in getting serious evidence that such travesty cannot be avoided.

I have no idea what your point here is, it sounds kind of like you're just venting at me. Of course it's true that physicists would not generally be too interested in possibilities which scientific evidence shows are almost guaranteed to be wrong, but the issue being debated here is what is your basis for claiming CTCs or the geometric view of time are almost certainly wrong, whether they are in fact based on "scientific evidence or arguments" (if so, then for god's sake present them) or just based on your personal emotional feelings or philosophical convictions. And if you do have such convincing evidence, perhaps you should write it up in a paper and change the minds of all those physicists who think the question of CTCs is an interesting open issue, or that the geometric view of time makes more sense than the "moving now" view. If tell me "I guess you are not a physicist" for saying such things, would you also question the competence of all the physicists who feel the same way about either of these issues?

You are actually talking here about how many angels can sit on the head of a pin and you are moreover convinced that this is somehow worthwhile talking about.

More venting? I'm just responding to the posts of mgelfan which claims it is self-evidently wrong to even consider the possibilities of CTCs or the geometric point of view, as well as your posts which seem to claim I am self-evidently wrong about, well, just about everything I say. If you don't think it's not "worthwhile talking about" this stuff, then perhaps you should stop.

And if you're basing this on physical arguments, then what are those arguments, specifically? Do you think a physicist like Kip Thorne is incompetent for not agreeing we should totally discount the possibility of CTCs?

It has nothing to do with ``incompetence'', neither do I know of his personal reasons to say so. Nor do I find such line of argumentation interesting, you would do much better if you were to actually give more detail to why he believes this to be true.

Presumably Thorne thinks we shouldn't discount the possibility because the standard interpretation of GR does allow CTCs, and there are no obvious arguments from other areas of physics that demonstrate they should be impossible. If you think there are such arguments, then again, it would save us both a lot of pointless argument if you would actually present them in detail.

This is the second time you try to use authority in your arguments, do you actually have a further point ?

Yes, arguments from authority are perfectly relevant when you act like I'm some sort of crackpot for an attitude that is well within the mainstream of modern physics (and please note once again that I would guess CTCs will more likely than not turn out to be impossible), and when you make comments like "Rubbish, I guess you are not a physicist."

I could equally say, do you believe 't Hooft is an idiot for claiming that CTC's do not make sense ?

Does he in fact claim total certainty that they are impossible, or does he just favor the idea that they will not turn out to be possible in a theory of quantum gravity? Can you cite a source where he talks about this issue so I can see his exact comments?

Realism means that there is an objective dynamics underlying our observations, that is ``things exist and move according to definite laws''. And I do not care about your estetical arguments, there are actually very good physical arguments as to why some unobserved things should be real (and I briefly gave some of them already). Moreover, the entire game of quantum gravity is about unobserved Planck scale degrees of freedom, likewise is string theory about unobserved high energy phenomena. So perhaps you are going to tell to all these scientists now that they are doing unappealing things ?

I would hope you'd understand the essential difference between postulating phenomena which are too difficult for us to observe because they would require extremely high energies or some other conditions we can't attain with modern technology, and postulating phenomena which would be in principle impossible to observe according to the theory itself, and thus have absolutely no effect on any observable feature of the universe anywhere.

Yep I meant local lorentz invariance. Note that my conclusion is a logical one and again your social arguments are basically irrelevant and incorrect at least what the quantum gravity community is concerned.

Again, the social arguments are hardly irrelevant when you act like I'm talking crazy for citing the relativity of simultaneity as a reason to find the "moving present" view of time unappealing (this argument depends on whether or not the relativity of simultaneity is likely to be violated by a future theory, and since I'm no quantum gravity expert it makes sense for me to look at the collective hunches of the physics community). As far as the quantum gravity community is concerned, string theory does not suggest a locally preferred frame, does it? And in loop quantum gravity some favor "doubly special relativity", I'm not sure if this could be said to violate Lorentz-invariance but I'm pretty sure it does not introduce a preferred frame. Who are the physicists who consider an aether-like approach with a single preferred frame to be the most promising approach to quantum gravity?

But as long as observations do continue to uphold the relativity of simultaneity, then any philosophical theory of a single objective "now" must be at odds with observation, since by definition any observation that showed one local definition of simultaneity to be physically preferred over others would violate the relativity of simultaneity.

Of course this is all wrong. It is entirely possible to construct theories with a preferred frame which recover Lorentz invariance at some coarse grained level. Hence you must assume all ``fundamental particles'' to be collective excitations, but (again) this is exactly what quantum gravity is about.

Again, my understanding is that very few approaches to quantum gravity introduce an aether-like preferred frame with a preferred definition of simultaneity, regardless of whether they'd be said to preserve "Lorentz invariance". My argument is solely about the implications of the relativity of simultaneity, any other aspects of Lorentz-invariance wouldn't be relevant.

 

[Careful]

But that sort of argument has no place in science. Nobody has observed the Higgs particle yet in human recorded history, but that's hardly a rational reason to feel 99% certain they don't exist.

I hope you see the difference between the impact of a more or less necessary prediction of the empirically most successful ``theory'' in physics (that is the standard model) so far, and a possibility in some abstruse interpretation of GR.

You really seem strangely hyper-aggressive to me about virtually every little comment I make, what's your damage?

I hate discussions about how many angels can sit on the head of a pin, there is nothing interesting about it, such as:

The issue of whether it is possible to return to the past is logically distinct from the issue of whether history "exists any more".

now, my little brain tells me that if you cannot go back to the past, there is no ground to claim its existence (empirical verification, you know), even though these issues are logically distinct (but I thought you disliked dragons sitting on ... ).

If so, then what is your scientific evidence or logical argument that clearly shows the "moving now" view is clearly true and the geometric view clearly false?

If I speak to my neighbor, then he or she is having a conscious participation in the conversation, so there must exist a now spacelike to me at the moment I spoke when this person was ``living''. Can I prove that my neighbor simply isn't a zombie copy of the real conscious neighbor ? No, I can't but I don't care, I am not going to invent blue dragons unless there are some deep motivations for doing so.

But inaccessibility is not evidence of nonexistence, that was the whole point of my analogy about the guy on the train which is always moving west, he shouldn't conclude that just because he can never return to points eastward of himself they don't exist.

You still do not grasp my comment. Of course ``space'' exists in an objective sense; how do I know it : lifelong experience plus assuming that everyone in the universe has an equally democratic position as I have. New York is still there even when I do not hear about it.

If so, can you propose an experiment that would settle the issue?

Never read Popper he ??

And it is obvious once you study the evidence and consider the arguments, but the "common sense" of a person totally ignorant of this evidence and arguments would more likely tell them that complex purposeful structures were designed by someone, this is after all the conclusion that virtually everyone around the world came to before Darwin and modern science came along.

Really, so Darwin must have been meeting plenty of people like you informing him about the common accepted truisms, pretty annoying he?

Haven't studies quantum field theory so I'm not familiar with that. I do know that QFT is supposed to show Lorentz-symmetry though, and of course there may be problems with QFT that would have to be resolved by some future unified theory, but few physicists seem to think such a theory would involve re-introducing a preferred local frame, and anyway there's no reason to expect future theories will be more "common-sensical" than present ones.

What I told you in the previous message is that both can happily coexist together, it simply requires looking differently at our existing theories.

Independently you can measure either arbitrarily accurately, but I specifically said "claims in QM like the one that you can't measure a particle's position and momentum simultaneously". If you disagree with this, how would you describe the implications of the uncertainty principle for measurements of position and momentum?

The postion/momentum uncertainty is just a consequence of Fourier analysis and has nothing to do with single events. For states with positive Wigner density for example, I can set up a fully classical interpretation of single events.

Maybe not, but do you think the results of the experiment are common-sensical?

Yes, they can be understood by common sense (in either particles exist all the time, follow definite paths and so on).

Most people's common sense would tell them the particle must have gone through either one slit or the other, I think. Of course you can adopt a hidden-variables interpretation where this is still true, but only at the expense of introducing other constructs which defy common sense, like the Bohmian "pilot wave" which guides the particle's path differently based on instantaneous knowledge of whether the other slit is open or closed.

Of course the particle goes trough one slit and, no, I don't need action at a distance.

No, I'm just quoting Einstein and Feynman. If the quotes are nonsense, then it's they who you should accuse of talking nonsense, not me.

I never said the quotes are nonsense, but that your perception of them is; as well as that such things are said in times that this particular method goes well. A thread on the relativity of quotes would be a blessing for many.

Of course it should, what's your point? My point is that experimental facts usually go against whatever prediction you might have made based on "common sense" before knowing the results of the experiment. I don't think these physicists were looking for alternatives based primarily on "common sense" though.

It is just that these people's common sense was better developped. Are you telling me now that by common sense you mean the simplistic phrase ``prejudices because of belief'' ?? I explicitely stated that common sense should pass experiment, and indeed, I believe experiment still allows for it (in either, a world in which things really exist all the time and measurement is just another interaction).

Of course it's true that physicists would not generally be too interested in possibilities which scientific evidence are almost guaranteed to be wrong, but the issue being debated here is what is your basis for claiming CTCs or the geometric view are almost certainly wrong, whether they are in fact based on "scientific evidence or arguments" (if so, then for god's sake present them) or just based on your personal emotional feelings or philosophical convictions.

I replied to this already in my second mail : I said that I have no troubles with CTC's when doing GR the canonical way, so GR isn't telling me at all that they exist. I do not have to disprove their existence, the lack of observation does that for me.

If tell me "I guess you are not a physicist" for saying such things, would you also question the competence of all the physicists who feel the same way about either of these issues?

No, my ``I gues you are not a physicist'' clearly deals with the fact that your way of discussing is like that of a hyper axiomatic mathematician or a philosopher.

Presumably Thorne thinks we shouldn't discount the possibility because the standard interpretation of GR does allow CTCs, and there are no obvious arguments from other areas of physics that demonstrate they should be impossible.

First of all, I contest the ``standard'' interpretation says such thing. Second you constantly use negative arguments, give us a positive reason why we should even remotely consider it. What can it explain, what interesting experiment can be done ,etc ??

Does he in fact claim total certainty that they are impossible, or does he just favor the idea that they will not turn out to be possible in a theory of quantum gravity? Can you cite a source where he talks about this issue?

He simply says that CTC's make no sense, period. You can find this in the book ``quo vadis quantum mechanics''.

I would hope you'd understand the essential difference postulating phenomena which are too difficult for us to observe because they would require extremely high energies or some other conditions we can't attain with modern technology, and postulating phenomena which would be in principle impossible to observe according to the theory itself, and thus have absolutely no effect on any observable feature of the universe anywhere.

Of course I do, it are in some sense your dragons you like to put on my shoulder : nobody says that what you postulate to be impossible to measure today, could not be measured tomorrow. In physics postulates are only there for our comprehension of nature, no real physicist takes an absolutist point of view towards them. That is why it is difficult to speak to (intelligent) mathematicians and philosophers who read physics books from time to time.

Again, the social arguments are hardly irrelevant when you act like I'm talking crazy for citing the relativity of simultaneity as a reason to find the "moving present" view of time unappealing (this argument depends on whether or not the relativity of simultaneity is likely to be violated by a future theory, and since I'm no quantum gravity expert it makes sense for me to look at the collective hunches of the physics community).

The hunches of the physics community are like melted butter on bread; very spread out and rather thin in these times !

As far as the quantum gravity community is concerned, string theory does not suggest a locally preferred frame, does it?

No, but it did not get much further either yet, did it ?

And in loop quantum gravity some favor "doubly special relativity", I'm not sure if this could be said to violate Lorentz-invariance but I'm pretty sure it does not introduce a preferred frame.

True it does not introduce a preferred frame, but it predicts severe deviations from Lorentz symmetry at high energies. Moreover, I can deform any theory with a preferred frame into one without it, so basically I do not bother about it.

Who are the physicists who consider an aether-like approach with a single preferred frame to be the most promising approach to quantum gravity? Again, my understanding is that very few approaches to quantum gravity introduce an aether-like preferred frame with a preferred definition of simultaneity, regardless of whether they'd be said to preserve "Lorentz invariance".

I guess Bill Unruh would qualify as such person. Concerning those approaches with deformed Lorentz groups and so on, the effect it produces on the physics is as good as introducing some preferred frame. If the description of reality needs to be at odds with local Lorentz invariance at very high energies, then you have two natural ways to go : (a) introduce a preferred frame (b) look for another local spacetime symmetry group. The advantage of (a) is that you do not have a problem with time operators, there is a very clean ontology and so on...

Actually, you cannot discuss this topic without taking into account all difficulties of quantum gravity. Perhaps you should consider that relativity of simultaneity is incorrect at the deepest level, but that the effective dynamical laws are so that this is a collective phenomenon. Look I am not saying that all of this is ``true'', what I do know for sure is that a naive unification of GR and QM (apart from severe technical difficulties) leads to a picture of the world which is immensely far removed from observation. At such point, you should consider different roads and please do not talk to me about MWI, it is still a mystery to me how this gets published.

 

[JesseM]

I hope you see the difference between the impact of a more or less necessary prediction of the empirically most successful ``theory'' in physics (that is the standard model) so far, and a possibility in some abstruse interpretation of GR.

I don't think most physicists would agree that a spacetime that satisfies the Einstein field equations everywhere requires an "abstruse interpretation of GR", but even if I grant this difference for the sake of argument, your notion that "if we haven't seen it, we should be 99% certain it doesn't exist" is still silly either way. If you'd prefer another example that doesn't involve an empirically successful theory like the standard model, take the example of theories of quantum gravity which all lack empirical verification, would you therefore say that we should have complete certainty in the nonexistence of any new phenomena which is part of such a theory (strings, loops, discrete space, whatever)? It is one thing to be skeptical and point out there is as of yet no evidence for these things, it's another to act as though there is some scientific basis for the definite claim that they don't exist. Absence of evidence is not evidence of absence, unless a theory specifically predicts we should have seen some phenomena that we didn't see in an experiment we have already done, but clearly the situations where CTCs would arise in GR are not ones we are able to create in a lab or even observe through a telescope.

I hate discussions about how many angels can sit on the head of a pin, there is nothing interesting about it

If you feel that way, you're free to just bow out of the discussion, instead of continuing to argue but then being angry at me for responding to your arguments.

such as:

The issue of whether it is possible to return to the past is logically distinct from the issue of whether history "exists any more".

now, my little brain tells me that if you cannot go back to the past, there is no ground to claim its existence (empirical verification, you know), even though these issues are logically distinct (but I thought you disliked dragons sitting on ... ).

Would you also say there must not be anything beyond the horizon of the observable universe, or that a person who falls into a black hole should believe the external universe ceases to exist as soon as he crosses the event horizon? The difference I see between this and the dragon is that the dragon is not based on an existing testable theory, while the examples above are just based on extending theories known to work in your own region into regions you can't interact with, and postulating that things continue to exist in this region anyway.

If so, then what is your scientific evidence or logical argument that clearly shows the "moving now" view is clearly true and the geometric view clearly false

If I speak to my neighbor, then he or she is having a conscious participation in the conversation, so there must exist a now spacelike to me at the moment I spoke when this person was ``living''. Can I prove that my neighbor simply isn't a zombie copy of the real conscious neighbor ? No, I can't but I don't care, I am not going to invent blue dragons unless there are some deep motivations for doing so.

I don't understand how this is supposed to be a counterargument, since the block time view certainly doesn't involve a belief that moments on your neighbor's worldline at a spacelike distance are "zombies" either, it would say every moment on your neighbor's worldline is real and conscious.

And again, I am not trying to argue that the block time view is definitely correct, just that there are no overwhelmingly clear reasons to dismiss it.

But inaccessibility is not evidence of nonexistence, that was the whole point of my analogy about the guy on the train which is always moving west, he shouldn't conclude that just because he can never return to points eastward of himself they don't exist.

You still do not grasp my comment. Of course ``space'' exists in an objective sense; how do I know it : lifelong observation. New York is still there even when I do not hear about it.

Yes, because you can go back and revisit it. Hence the analogy of the guy on the train who always moves west, and is unable to return eastward.

If so, can you propose an experiment that would settle the issue?

Never read Popper he ??

Once again you're reading a casual phrase in a very uncharitable way. Of course my question above can be treated as equivalent to "can you propose an experiment that would falsify the block-time view?" If there are no experiments that can shed light on block time vs. the moving now either way, then obviously it's a philosophical issue rather than a scientific one.

Really, so Darwin must have been meeting pleanty of people like you informing him about the common accepted truisms, pretty annoying he?

I assume you're talking about the "common accepted truisms" of other scientists? The difference is, if I was talking to someone who said something like "I've got this theory in the works that can settle the issue of CTCs (or block time), but it's still in the works and not ready to present to the public" then I wouldn't try to discourage them by quoting the views of the physics community in general. But you aren't doing that, as far as I can see, you're saying that somehow it is rational to take a definitive view on these issues already, without the need for any new theories or evidence.

Haven't studies quantum field theory so I'm not familiar with that. I do know that QFT is supposed to show Lorentz-symmetry though, and of course there may be problems with QFT that would have to be resolved by some future unified theory, but few physicists seem to think such a theory would involve re-introducing a preferred local frame, and anyway there's no reason to expect future theories will be more "common-sensical" than present ones.

What I told you in the previous message is that both can happily coexist together, it simply requires looking differently at our existing theories.

But if this type of "looking differently" involves postulating new entities which are impossible to observe in principle, then I stand by my comments about the "unappealing" nature of such ideas. On the other hand, if you're making a suggestion about what a future testable theory of quantum gravity might say, then sure, I accept that if such a theory was successful it would change things somewhat, I'm just saying that such speculations are not a basis for dismissing block time or the relativity of simultaneity now, and that most physicists would bet against the idea that a theory of quantum gravity will take this form.

The postion/momentum uncertainty is just a consequence of Fourier analysis and has nothing to do with single events.

It does if you want to relate the wavefunction to actual experimental results, in which case you must use the Born rule in which you take the amplitude squared to represent a probability distribution for finding a given value (or range of values) for the position and momentum on a given measurement.

Most people's common sense would tell them the particle must have gone through either one slit or the other, I think. Of course you can adopt a hidden-variables interpretation where this is still true, but only at the expense of introducing other constructs which defy common sense, like the Bohmian "pilot wave" which guides the particle's path differently based on instantaneous knowledge of whether the other slit is open or closed.

Of course the particle goes trough one slit and, no, I don't need action at a distance.

Then which interpretation are you using, if not the Bohmian one with the pilot wave? And if you look at a large set of trials in which the particle went through the right slit, and in half the trials the left slit is covered and in the other half the left slit was open, you will see a very different statistical distribution of particles on the screen in the two halves--how do you explain this, without the particle having some sort of at-a-distance "knowledge" of what's happening at the left slit as it passes through the right slit?

I never said the quotes are nonsense, but that your perception of them is;

And which perception is that? You don't agree that both quotes are denigrating common sense, especially Einstein's?

It is just that these people's common sense was better developped. Are you telling me now that by common sense you mean the simplistic phrase ``prejudices because of belief'' ??

No, I meant intuitions based on our experiences of how things work with the ordinary objects we interact with in everyday life. For example, this is the basis for the idea that a theory should have a "mechanism" which explains everything in terms of little classical parts interacting with each other by touch (this was how Feynman was using 'mechanism' in the quote I posted, the context was a discussion of 'mechanical' theories of gravity like pushing gravity[/url]). This is basically the way most dictionaries define common sense, like this one which defines it as "Sound judgment not based on specialized knowledge; native good judgment." If a certain view is only formed once people have a lot of specialized understanding of experimental results or theoretical arguments, then I would say that view is not a common-sense one.

I replied to this already in my second mail : I said that I have no troubles with CTC's when doing GR the canonical way, so GR isn't telling me at all that they exist. I do not have to disprove their existence, the lack of observation does that for me.

And again, "lack of observation = certainty of nonexistence" is a terrible argument scientifically.

No, my ``I gues you are not a physicist'' clearly deals with the fact that your way of discussing is like that of a hyper axiomatic mathematician or a philosopher.

What specifically are you referring to? A hardheaded nonphilosophical physicist would not claim with certainty that the past ceases to exist, she would simply dismiss the whole question of the "existence" or "nonexistence" of the past as a pointless philosophical one (and would probably dismiss questions about the interpretation of QM, including hidden-variables interpretations, on the same grounds). And again, if you think "a physicist" would not favor block time I can show you quite a lot of examples of physicists that do, and if you think "a physicist" would dismiss CTCs with certainty I can show you quite a lot of examples of physicists that don't.
[quore=Careful]First of all, I contest the ``standard'' interpretation says such thing.[/quote] OK, I don't have enough knowledge of the average views of the physics community to be sure you're wrong, but certainly the idea that any continuous spacetime which satisfies the field equations of GR everywhere should be considered a valid "solution" in GR is a common interpretation (especially if we're talking about asymptotically flat spacetimes like one that contains a wormhole could be).

Second you constantly use negative arguments, give us a positive reason why we should even remotely consider it. What can it explain, what interesting experiment can be done ,etc ??

I use negative arguments because I am reacting to claims of religious-like certainty on these issues. And we should "consider it" simply because it is a prediction of a theory that is consistent with all known observations, and the only reason to dismiss something in science is because of evidence against it (such as a theory that makes predictions which are contradicted by experiment), not because there is no evidence one way or another. The question of why physicists find it interesting to think about CTCs is separate, I guess partly there's just a general interest in probing extreme cases of existing theories, and partly it may be because arguments for "chronology protection" usually involve quantum effects so thinking about whether and how nature prevents CTCs could give some new ideas about quantum gravity.

He simply says that CTC's make no sense, period. You can find this in the book ``quo vadis quantum mechanics''.

Thanks, I'll see if I can find that.

I would hope you'd understand the essential difference postulating phenomena which are too difficult for us to observe because they would require extremely high energies or some other conditions we can't attain with modern technology, and postulating phenomena which would be in principle impossible to observe according to the theory itself, and thus have absolutely no effect on any observable feature of the universe anywhere.

Of course I do, it are in some sense your dragons you like to put on my shoulder.

OK, so when you were advocating hidden-variables approaches you were thinking in terms of some future fundamental theory which would incorporate them in an experimentally-testable way, rather than existing hidden-variables interpretations which are in principle impossible to distinguish from other interpretations?

And in loop quantum gravity some favor "doubly special relativity", I'm not sure if this could be said to violate Lorentz-invariance but I'm pretty sure it does not introduce a preferred frame.

True it does not introduce a preferred frame, but it predicts severe deviations from Lorentz symmetry at high energies.

But the only issue which is relevant here is whether, if such a theory were true, there would be any experimental reason for a preferred definition of simultaneity. My comment about the moving now view being "unappealing" was based only on the relativity of simultaneity, other aspects relating to whether Lorentz-symmetry is broken or not don't matter.

Moreover, I can deform any theory with a preferred frame into one without it, so basically I do not bother about it.

I'm not familiar with this idea, but are you talking about some purely mathematical "deformation"? Again, the key issue I'm talking about is whether there could be any experimental evidence (even in principle) that would cause us to prefer one definition of simultaneity, if not then we're back to blue dragons and a "metaphysically preferred" definition of simultaneity which has no observable consequences whatsoever. If you are advocating the possibility of a theory which give a preferred definition of simultaneity that could at least in principle be determined by experiment (does Unruh advocate this sort of theory?), then yes, I agree this would nullify the point about relativity making the "moving now" view unappealing.

 

[Careful]

I don't think most physicists would agree that a spacetime that satisfies the Einstein field equations everywhere requires an "abstruse interpretation of GR"

I did not say that; I said that in the canonical picture CTC's do not occur.

but even if I grant this difference for the sake of argument, your notion that "if we haven't seen it, we should be 99% certain it doesn't exist" is still silly either way. If you'd prefer another example that doesn't involve an empirically successful theory like the standard model, take the example of theories of quantum gravity which all lack empirical verification, would you therefore say that we should have complete certainty in the nonexistence of any new phenomena which is part of such a theory (strings, loops, discrete space, whatever)?

Of course not, but all these theories should count the number of assumptions which cannot be decided upon by any reasonable test in the very end. You simply introduce CTC's without any further deeper motivation : again you compare two things which are not to be compared, I want evidence for their necessity ! That is what you refuse to give.

Would you also say there must not be anything beyond the horizon of the observable universe, or that a person who falls into a black hole should believe the external universe ceases to exist as soon as he crosses the event horizon?

Again, this has nothing to do with our issue. I know I have to take into account data beyond my horizon and I know someone dissapearing beyond the event horizon of a black hole will still live (for some time ) because (a) extrapolation is normal in science, without it we could not make any predictions (b) the Horizon of a black hole is as calm as a beach on a sunny day.

The difference I see between this and the dragon is that the dragon is not based on an existing testable theory, while the examples above are just based on extending theories known to work in your own region into regions you can't interact with, and postulating that things continue to exist in this region anyway.

And exactly the same applies to the type of hidden variable theories I am talking about.

it would say every moment on your neighbor's worldline is real and conscious.

That does not make any sense.

Yes, because you can go back and revisit it. Hence the analogy of the guy on the train who always moves west, and is unable to return eastward.

Nope even when I would never see it again, I would still believe it exists assuming that no atomic bomb or desease of some kind destroyed it, but the latter has to do with my lack of knowledge about the dynamics in NY, there is nothing fundamental to it.

I assume you're talking about the "common accepted truisms" of other scientists?

Nope, almost no reasonable scientist on a high level position would speak in terms of truisms; but he would for sure demand theoretical evidence.

e difference is, if I was talking to someone who said something like "I've got this theory in the works that can settle the issue of CTCs (or block time), but it's still in the works and not ready to present to the public" then I wouldn't try to discourage them by quoting the views of the physics community in general. But you aren't doing that, as far as I can see, you're saying that somehow it is rational to take a definitive view on these issues already, without the need for any new theories or evidence.

You are looking at it from the wrong perspective. The only thing which counts is whether you would put a PhD student on this for his doctoral thesis (assuming you did have that possibility). That is what makes it rational to dismiss it.

But if this type of "looking differently" involves postulating new entities which are impossible to observe in principle, then I stand by my comments about the "unappealing" nature of such ideas.

Again, (a) the same goes for quantum gravity approaches (b) nobody says these hidden variables need to be impossible to observe in principle ! We cannot observe them yet, or we simply have mistaken something else, period.

It does if you want to relate the wavefunction to actual experimental results, in which case you must use the Born rule in which you take the amplitude squared to represent a probability distribution for finding a given value (or range of values) for the position and momentum on a given measurement.

Nobody says you need to commit such stupidity.

And if you look at a large set of trials in which the particle went through the right slit, and in half the trials the left slit is covered and in the other half the left slit was open, you will see a very different statistical distribution of particles on the screen in the two halves--how do you explain this, without the particle having some sort of at-a-distance "knowledge" of what's happening at the left slit as it passes through the right slit?

Of course the particle knows about the slits but this does not conflict local physics.

No, I meant intuitions based on our experiences of how things work with the ordinary objects we interact with in everyday life. For example, this is the basis for the idea that a theory should have a "mechanism" which explains everything in terms of little classical parts interacting with each other by touch (this was how Feynman was using 'mechanism' in the quote I posted, the context was a discussion of 'mechanical' theories of gravity like pushing gravity[/url]).

But QED for example is a theory where little tiny particles carry information (at least when you do it correctly). If you call this classical or quantum is all semantics, I prefer to call it classical since it captures the deterministic, local aspect.

This is basically the way most dictionaries define common sense, like this one which defines it as "Sound judgment not based on specialized knowledge; native good judgment." If a certain view is only formed once people have a lot of specialized understanding of experimental results or theoretical arguments, then I would say that view is not a common-sense one.

I call it common sense, since it is the least removed from experience. For example, would YOU call Einstein's protest against QM an act of good judgement based upon understanding of experimental results or not ?! Or are you going to claim that he did not understand the double slit experiment ? On the other hand would you say that some interpretations some theories attach to experiments (like QM) testify of sound judgement ?

And again, "lack of observation = certainty of nonexistence" is a terrible argument scientifically.

I did say : lack of observation + no compelling theoretical reason to take it seriously. Do not twist my words, there is a world of difference in this +.

A hardheaded nonphilosophical physicist would not claim with certainty that the past ceases to exist, she would simply dismiss the whole question of the "existence" or "nonexistence" of the past as a pointless philosophical one (and would probably dismiss questions about the interpretation of QM, including hidden-variables interpretations, on the same grounds).

Well you know, it is more polite to say that only the question is meaningless, not that the mere fact that the question is asked in such way is nonsense.

And again, if you think "a physicist" would not favor block time I can show you quite a lot of examples of physicists that do, and if you think "a physicist" would dismiss CTCs with certainty I can show you quite a lot of examples of physicists that don't.

Sure, and that should be impressive ?! I know physicists who still think that topology change is a crucial piece in the puzzle for quantum gravity, despite of the fact that there is no evidence for it and that it is notoriously difficult to include it without getting even worse divergencies in your path integral. So what ?!

OK, I don't have enough knowledge of the average views of the physics community to be sure you're wrong, but certainly the idea that any continuous spacetime which satisfies the field equations of GR everywhere should be considered a valid "solution" in GR is a common interpretation (especially if we're talking about asymptotically flat spacetimes like one that contains a wormhole could be).

Of course not, GR gives rise to spacetimes which can be safely considered to be entirely unphysical.

And we should "consider it" simply because it is a prediction of a theory that is consistent with all known observations, and the only reason to dismiss something in science is because of evidence against it

Of course not, most things are simply dismissed because they are simply not very likely.

The rest of your comments I largely disagree with, if you can find a deterministic theory with some preferred frame which unifies gravity and QM then you are done, whether this frame is observable or not. I personally don't think it is, but I have other physical reasons to want it in my theory (which I gave already). Moreover, keeping time as you do it leads to even more excess bagage than I have in my description of reality. You basically need for every particle an independent time parameter to write out your eigentime operators, the latter parameters are needed in your theory but they have no meaning (moreover, you need to make sure that all these parameters run in the same direction and so on)! It seems to me much better to take one coordinate time t, declare it ``event time'' if you want to and build up your physics from that.

Careful

 

[vanesch]

Of course not, GR gives rise to spacetimes which can be safely considered to be entirely unphysical.

Well, they surely aren't unphysical in a toy universe where these laws are supposed to hold, no ? After all, that's all what CTC considerations are about: "what if" questions in a toy universe where GR is strictly true, and where moreover they occur in the specific solution corresponding to that toy universe.

This has nothing to do with the question whether:
a) GR is strictly true in our universe (probably not, as any theory we know about)
b) even so, whether the specific solution, in that case, that corresponds to our universe, contains CTCs.

 

[JesseM]

I don't think most physicists would agree that a spacetime that satisfies the Einstein field equations everywhere requires an "abstruse interpretation of GR"

I did not say that; I said that in the canonical picture CTC's do not occur.

From my limited understanding of this stuff, the "canonical picture" is some kind of Hamiltonian version of GR which physicists can use when they want to try to quantize GR, but I don't think this Hamiltonian version is always used when physicists are dealing with questions about ordinary classical GR, is it? Do you deny that there are examples of spacetimes which satisfy the field equations locally everywhere and which also contain CTCs? If not, then you are saying that believing such a spacetime is possible requies an "abstruse interpretation of GR".

but even if I grant this difference for the sake of argument, your notion that "if we haven't seen it, we should be 99% certain it doesn't exist" is still silly either way. If you'd prefer another example that doesn't involve an empirically successful theory like the standard model, take the example of theories of quantum gravity which all lack empirical verification, would you therefore say that we should have complete certainty in the nonexistence of any new phenomena which is part of such a theory (strings, loops, discrete space, whatever)?

Of course not, but all these theories should count the number of assumptions which cannot be decided upon by any reasonable test in the very end.

Even if there is no "reasonable" way to test certain assumptions (perhaps it would require an accelerator which could attain the Planck energy to test it), that is not a basis for automatically judging the assumption to be false, it just means you should be undecided about it since you don't have any evidence one way or another.

You simply introduce CTC's without any further deeper motivation

The motivation is that they appear in spacetimes that satisfy the equations of GR everywhere, regardless of whether or not they appear in the Hamiltonian version of GR. I wish you would stop attacking me for statements that are completely within the mainstream of physics, if you think there is no motivation to even consider CTCs then you should be arguing that with people like Hawking and Thorne, not some guy like me on the internet who's just going by what he's read from such physicists.

again you compare two things which are not to be compared, I want evidence for their necessity !

What do you mean by "necessity"? Do you dismiss any idea that is not "necessary", even if it is perfectly compatible with existing physics and there is no evidence against it? Again, that's a completely unscientific attitude! You may not find CTCs interesting and therefore have no interest in thinking about them yourself, but that's not the same as claiming certainty they don't exist.

Would you also say there must not be anything beyond the horizon of the observable universe, or that a person who falls into a black hole should believe the external universe ceases to exist as soon as he crosses the event horizon?

Again, this has nothing to do with our issue.

Sure it does, because just like believing the world exists beyond your horizons is simply a matter of extrapolating existing theories rather than inventing new ones as in the case of the blue dragons, so it is also true that believing past times exist does not require any new theories (relativity already puts all of spacetime on equal footing as part of a single 4D manifold), although I am not claiming that this is quite as trivial an extrapolation as the horizon case. Still, it was you who compared the belief in the existence of past times to the blue dragon example, I was just explaining why I don't think they're the same.

The difference I see between this and the dragon is that the dragon is not based on an existing testable theory, while the examples above are just based on extending theories known to work in your own region into regions you can't interact with, and postulating that things continue to exist in this region anyway.

And exactly the same applies to the type of hidden variable theories I am talking about.

What existing theory are you extrapolating that leads you to believe in hidden variables? Not orthodox QM or QFT, surely?

it would say every moment on your neighbor's worldline is real and conscious.

That does not make any sense.

Why not? I see nothing illogical about the idea.

Yes, because you can go back and revisit it. Hence the analogy of the guy on the train who always moves west, and is unable to return eastward.

Nope even when I would never see it again, I would still believe it exists

Yes, of course you would because you have experience of moving back and forth in space, and would still think of it as possible even if you were stuck in a situation where you couldn't. But the guy in my thought experiment has spent his whole life stuck to the same part of the train, with no idea that any sort of spatial motion is possible aside from the westward motion of the train.

I assume you're talking about the "common accepted truisms" of other scientists?

Nope, almost no reasonable scientist on a high level position would speak in terms of truisms

That's not what I was asking, I wasn't sure what "common accepted truisms" you were talking about when you said "so Darwin must have been meeting pleanty of people like you informing him about the common accepted truisms, pretty annoying he?" I was trying to clarify if the statements of mine that you characterized as "common accepted truisms" that you found "annoying" were my statements about what the majority of scientists think is likely to be true, or something else.

The difference is, if I was talking to someone who said something like "I've got this theory in the works that can settle the issue of CTCs (or block time), but it's still in the works and not ready to present to the public" then I wouldn't try to discourage them by quoting the views of the physics community in general. But you aren't doing that, as far as I can see, you're saying that somehow it is rational to take a definitive view on these issues already, without the need for any new theories or evidence.

You are looking at it from the wrong perspective. The only thing which counts is whether you would put a PhD student on this for his doctoral thesis (assuming you did have that possibility). That is what makes it rational to dismiss it.

Nonsense, a good physicist would never say that because they personally did not find a possibility interesting or promising enough to work on it or assign others to work on it, that is grounds for them to rationally dismiss it. Hunches about which novel ideas are likely to pan out and which are likely to be dead ends cannot be treated as equivalent to rational beliefs about what we can judge to be true or false on the basis of the evidence we have now!

But if this type of "looking differently" involves postulating new entities which are impossible to observe in principle, then I stand by my comments about the "unappealing" nature of such ideas.

Again, (a) the same goes for quantum gravity approaches

Which quantum gravity approaches postulate entities which would be impossible to observe in principle, even if we had access to things like Planck-energy particle accelerators?

(b) nobody says these hidden variables need to be impossible to observe in principle !

Bohmian mechanics does, for example. Its predictions don't differ in any way from those made by other interpretations like the Copenhagen interpretation.

It does if you want to relate the wavefunction to actual experimental results, in which case you must use the Born rule in which you take the amplitude squared to represent a probability distribution for finding a given value (or range of values) for the position and momentum on a given measurement.

Nobody says you need to commit such stupidity.

Uh, every introductory textbook on quantum mechanics does. How do you connect the theoretical wavefunction to the results of actual experiments without interpreting the wavefunction in terms of probabilities?

Of course the particle knows about the slits but this does not conflict local physics.

So what informs it about the other slit, if not something like the Bohmian pilot wave? In any case, to have particles "know" about things at huge distances from them itself goes against common-sense classical intuitions, which is what we were talking about originally. Plus you can talk about cases like the EPR experiment, where Bell's theorem shows each particles knowledge of the other must violate any local hidden-variables theory you could come up with.

But QED for example is a theory where little tiny particles carry information (at least when you do it correctly).

I have not studies QED formally, but if you're talking about something like a Feynman diagram, then I don't think this fits with the common-sense idea of explaining physics in terms of a bunch of definite interactions, since you have to do a sum over many diagrams and can't say that any of them represent what "really happened", and anyway these diagrams are just understood as visual representations of terms in a perturbation series, if you had a nonperturbative approach they wouldn't come up at all.

I call it common sense, since it is the least removed from experience. For example, would YOU call Einstein's protest against QM an act of good judgement based upon understanding of experimental results or not ?!

No, but Einstein did not argue dogmatically about this, it was just his hunch that QM was not a complete description, and that one could find an underlying hidden variables theory which reproduced its successful predictions. It's not bad judgement to have hunches about the future direction of physics, if it was then everyone working on one particular approach to quantum gravity over others would be guilty of bad judgement.

On the other hand would you say that some interpretations some theories attach to experiments (like QM) testify of sound judgement ?

I'm not sure exactly what you're asking, but I think it would be bad judgement to claim dogmatically that one interpretation was definitely correct while the others were definitely wrong, if they all predict the same experimental results.

And again, "lack of observation = certainty of nonexistence" is a terrible argument scientifically.

I did say : lack of observation + no compelling theoretical reason to take it seriously. Do not twist my words, there is a world of difference in this +.

My "certainty of nonexistence" comment was referring to this exchange:

Again, are you agreeing with mgelfan's claim that we can rule out CTCs a priori without even needing to do any experiments

Again, with 99 percent probability, yes. In a physicist's language, that equals absolute certainty.

So, I don't think I was "twisting your words" at all.

(edit: never mind, rereading this I realized you weren't objecting to my 'certainty of nonexistence' comment, just to the fact that I left out the part about 'no compelling theoretical reason to take it seriously'. Of course this doesn't change the fact that this is a totally unscientific attitude, since the mere fact that our present theories don't give us a compelling reason to believe it exists is no argument for a phenomenon's nonexistence, assuming it's totally compatible with current theories, or is even predicted by certain formulations of current theories as with GR and CTCs. If someone suggested protons might be made up of smaller particles back in the 1930s, would you claim absolute confidence they weren't based on the lack of observation + no compelling theoretical reasons argument?)

Anyway, what do you mean by "no compelling theoretical reason to take it seriously"? Does "take it seriously" just mean "devoting your own time and energy to exploring it", or does it simply mean "saying there is insufficient evidence to dismiss it as a possibility"? Because there are plenty of theories a given theory might be "taken seriously" in the second sense but not the first (for example, a string theorist might not totally dismiss the possibility that loop quantum gravity could be correct, but might at the same time have no interest in exploring it themselves).

A hardheaded nonphilosophical physicist would not claim with certainty that the past ceases to exist, she would simply dismiss the whole question of the "existence" or "nonexistence" of the past as a pointless philosophical one (and would probably dismiss questions about the interpretation of QM, including hidden-variables interpretations, on the same grounds)

Well you know, it is more polite to say that only the question is meaningless, not that the mere fact that the question is asked in such way is nonsense.

Either way, this practical physicist would make no claim that one of the two (block time vs. the moving present) is correct and the other is incorrect, that's all I was saying.

And again, if you think "a physicist" would not favor block time I can show you quite a lot of examples of physicists that do, and if you think "a physicist" would dismiss CTCs with certainty I can show you quite a lot of examples of physicists that don't.

Sure, and that should be impressive ?!

No, I wasn't trying to "impress" you, I was just countering your comment that 'my ``I gues you are not a physicist'' clearly deals with the fact that your way of discussing is like that of a hyper axiomatic mathematician or a philosopher.' If you admit there are plenty of physicists (perhaps even the majority, in the case of block time) who are so foolish as to question claims which you, in your great wisdom, see as totally obvious and with no room for doubt, then maybe you should tone down your rhetoric about my being unphysicist-like in my arguments.

Of course not, GR gives rise to spacetimes which can be safely considered to be entirely unphysical.

And on what basis is this "safe", especially in the case of asymptotically flat spacetimes? Would you say there is widespread agreement among physicists that these spacetimes are unphysical?

And we should "consider it" simply because it is a prediction of a theory that is consistent with all known observations, and the only reason to dismiss something in science is because of evidence against it

Of course not, most things are simply dismissed because they are simply not very likely.

Again, you're conflating "dismissing" something in the sense of having a hunch that it's very unlikely to pan out (many physicists would probably feel this way about MOND, for example) and "dismissing" something in the sense of thinking there is a rational basis for making the positive claim that it can almost certainly be ruled out based on evidence we already have (at least until some of the most recent evidence for dark matter clouds, I think few physicists would have dismissed MOND as a possibility in this sense).

The rest of your comments I largely disagree with, if you can find a deterministic theory with some preferred frame which unifies gravity and QM then you are done, whether this frame is observable or not.

I'm skeptical you could think up a theory that would be a) testable, b) would have a preferred definition of simultaneity that the theory itself says can never be determined experimentally even in principle, and c) cannot be trivially modified into a new theory which makes the same experimental predictions but involves no preferred definition of simultaneity.

Moreover, keeping time as you do it leads to even more excess bagage than I have in my description of reality. You basically need for every particle an independent time parameter to write out your eigentime operators, the latter parameters are needed in your theory

What comments of mine are you referring to as a "theory", and why would I need "eigentime operators"? The block time view does not lead to any predictions which differ from the moving now view, so it doesn't make sense that it would force you to introduce new operators.

 

[Careful]

Well, they surely aren't unphysical in a toy universe where these laws are supposed to hold, no ?

Again, that depends upon how far you are willing to consider GR.

After all, that's all what CTC considerations are about: "what if" questions in a toy universe where GR is strictly true, and where moreover they occur in the specific solution corresponding to that toy universe.

Do you think this is an interesting question from a physical point of view ? :zzz: I thought this was physicsforums, not take your wish for reality teaclub.

 

[tehno]

I'm not buying this for reasons of paradoxes, but Brian Greene is saying that time travel backwards is possible.



Source

what do you guys think?

I ruled this out long time ago

 

[RandallB]

Maybe your ideas about time travel would require such a modification of GR, but GR itself certainly does allow time travel, and no extra time dimension is needed because, again, there is nothing "moving along" worldlines corresponding to an objective now (the riverboat in your metaphor), worldlines just exist in the "static" 4D manifold of spacetime. In this view, time travel is just a worldline that loops around and passes near an earlier part of itself, and a CTC is just a worldline that forms a closed loop in spacetime.

I don't agree with the 4D of GR (or 5D) as being a manifold of "spacetime", spacetime is just a view of SR as seen from our perspective in a 3D world not a GR one. The 4D world of GR is much more complex than simple "spacetime".

For example, given a world line as you describe it, that you may follow though a lifetime, I see nothing in GR that requires that what you "are" at age 5 remain at whatever location on that line while you progress on that line to an age of 20. If by following the rules of GR you were able to get back to the coordinates in as many dimensions as needed that correspond to where you were when you were only 5 years old, I see nothing in GR that requires that any version of you still exist at that complex point let alone a version that is still 5 years old. To require such a thing would demand that our perception or observation of time passing (aging) is something fundamental of significant. That is much too ego-centric to be considered scientific; it is nearly a demand that our time be treated a preferred time. GR as currently understood stands much more in opposition to such a preferred view of reality.

I will leave it that IMO if GR does allow us to revisit a point in time in the past, it, GR ,would not require that our history still exist at that point in the form of real particles with mass that we could interact with. That is simply demanding much more it, than what GR as a theory requires.

 

[Careful]

From my limited understanding of this stuff, the "canonical picture" is some kind of Hamiltonian version of GR which physicists can use when they want to try to quantize GR ... Even if there is no "reasonable" way to test certain assumptions (perhaps it would require an accelerator which could attain the Planck energy to test it), that is not a basis for automatically judging the assumption to be false, it just means you should be undecided about it since you don't have any evidence one way or another.

First, go and study the Hamiltonian version (then try tro criticize my attitude). Second, an idea which has neither any compelling reason for its existence, nor can be reasonably decided upon by experiment, is simply not worthwile considering. That is a very common attitude.

The motivation is that they appear in spacetimes that satisfy the equations of GR everywhere, regardless of whether or not they appear in the Hamiltonian version of GR. I wish you would stop attacking me for statements that are completely within the mainstream of physics, if you think there is no motivation to even consider CTCs then you should be arguing that with people like Hawking and Thorne, not some guy like me on the internet who's just going by what he's read from such physicists.

Three comments :
(a) these statements are not really mainstream (most physicists would simply stick with globally hyperbolic universes)
(b) if you proclaim some views, YOU need to defend them, regardless of whether your opinion is backed up by some physics pope or not.
(c) If you are really interested : yes I do consider many of Hawking ideas about quantum gravity as ``nonsense''.

What do you mean by "necessity"? Do you dismiss any idea that is not "necessary", even if it is perfectly compatible with existing physics and there is no evidence against it? Again, that's a completely unscientific attitude!

First it is debatable whether CTC's are compatible with existing physics (according to me it even goes against GR, since you basically abandon determinism). Second, an idea is only interesting (read : not dismissed) when it adds value to our understanding of the universe, CTC's don't do any such thing.

Sure it does, because just like believing the world exists beyond your horizons is simply a matter of extrapolating existing theories rather than inventing new ones as in the case of the blue dragons, so it is also true that believing past times exist does not require any new theories (relativity already puts all of spacetime on equal footing as part of a single 4D manifold), although I am not claiming that this is quite as trivial an extrapolation as the horizon case.

So, if you understand that there is a world of difference between the initial data extrapolation and the existence of past times, why don't you finally provide us ONE good reason why it should be considered.

What existing theory are you extrapolating that leads you to believe in hidden variables? Not orthodox QM or QFT, surely?

Maxwell theory and QM rather naturally lead to hidden variable theories if you demand locality, realism and particle nature of interactions.

I see nothing illogical about the idea. Yes, of course you would because you have experience of moving back and forth in space, and would still think of it as possible even if you were stuck in a situation where you couldn't. But the guy in my thought experiment has spent his whole life stuck to the same part of the train, with no idea that any sort of spatial motion is possible aside from the westward motion of the train.

Fine, so what has that to do with our universe ? You are making some irrelevant thought experiment and blame me not to accept it as an ``indication'' for the possibility of existence of past events.

That's not what I was asking, I wasn't sure what "common accepted truisms" you were talking about when you said "so Darwin must have been meeting pleanty of people like you informing him about the common accepted truisms, pretty annoying he?" I was trying to clarify if the statements of mine that you characterized as "common accepted truisms" that you found "annoying" were my statements about what the majority of scientists think is likely to be true, or something else.

Ok, how do you know that what you tell are majority statements ? Do you believe that Hawking points of view represent some large portion of scientific opinion: I have to dissapoint you, they don't. Neither do Penrose's, nor Smolin's. Again, social comments.

Nonsense, a good physicist would never say that because they personally did not find a possibility interesting or promising enough to work on it or assign others to work on it, that is grounds for them to rationally dismiss it.

A really good physicist allows for any thesis subject he deems worthwile. Moreover, you twist my words again, I basically suggested that the thesis subjects of a good physicist reflect what he deems scientifically sound, I never asserted that he would say the ``rest'' isn't ! But basically it boils down to that.

If you admit there are plenty of physicists (perhaps even the majority, in the case of block time) who are so foolish as to question claims which you, in your great wisdom, see as totally obvious and with no room for doubt, then maybe you should tone down your rhetoric about my being unphysicist-like in my arguments.

Almost no one takes CTC's seriously (and yes, I know that), and I am pretty sure that the overwhelming majority goes against block time (apart from some relativists) too. Where do you get these impressions ?

And on what basis is this "safe", especially in the case of asymptotically flat spacetimes? Would you say there is widespread agreement among physicists that these spacetimes are unphysical?

Now, you are getting really low : as everyone knows asymptotically flat (or de Sitter) universes are widely studied and interesting for various reasons, although our universe isn't asymptotically flat. An example of an unphysical universe is the Godel universe, I doubt if someone takes that seriously.

Really, if your only problem with my attitude is that I consider many arguments against an idea as a rational basis basis for dismissing it, then let's quit the ``discussion''.

Ohw, I noticed I did not react upon your comments concerning QM (and QED). Let me give you some advice, if you talk to someone who has been studying physics fulltime for 14 years and who came to conclusions which are somewhat unorthodox, then the last thing you do is to say ``oh, I never studied this, but I have heard that the standard opinion is such and such, are you sure ?´´. Not only do you express a great deal of ignorance in this way :

(a) you are not familiar with the Hamiltonian form of GR, but still you act as if you were a GR expert
(b) you never studied QFT, but still you proclaim that such and such interpretation is meaningful (read : correct)
(c) you think all solutions to the einstein equations are legitimate (never heard of the weak energy condition I guess, neither about the Godel universe)

but you are telling to physicists how physicists behave. Look, the best you get when you speak at dinner about CTC's is a smile or a sigh or perhaps some joke about kiddo's from the future.

Ah, I still react to one though, the last few sentences of your post where you claim that the block spacetime view does not lead to any new physics, so no new operators. Classically, there is no difference but quantum mechanically there is since you basically do not take any time gauge. Go and study some work on quantum gravity with point particles and you will see what I mean. Actually, it is already sufficient to study a quantum theory of relativistic interacting particles without choosing a time gauge in order to get this point.

 

[vanesch]

Again, that depends upon how far you are willing to consider GR.

All the way, of course. That's the point of the exercise: what does a theory really tell us if it is 100% to be taken seriously (which it maybe never is, of course, but you don't know that for sure either...).

Do you think this is an interesting question from a physical point of view ? :zzz: I thought this was physicsforums, not take your wish for reality teaclub.

Physics is exactly that, no ? Distilling "principles" from observations, and then taking the consequences of those principles to the extreme, because they now replaced our "common sense". "Common sense" being yet another "principle" which we used to take 100% for true, before we knew any better.

So, yes, it is a potentially interesting question to ask what exactly does GR tell us about the possibility of CTC, independent of whether we think them to be actually possible in "reality" (whatever that is) or not, and this for a different reason: we intuitively think that there is some "paradox" associated with CTC (like killing your grandpa), and to see how a theory such as GR deals with that.

 

[Careful]

All the way, of course. That's the point of the exercise: what does a theory really tell us if it is 100% to be taken seriously (which it maybe never is, of course, but you don't know that for sure either...).

It is not that trivial, most of these ugly spacetimes come from nonunique, nontrivial pasting procedures. Like I said, someone who regards GR as a deterministic theory has to look at it from the initial value (in either globally hyperbolic) perspective.

Physics is exactly that, no ? Distilling "principles" from observations, and then taking the consequences of those principles to the extreme, because they now replaced our "common sense". "Common sense" being yet another "principle" which we used to take 100% for true, before we knew any better.

Same comment here, if you take Einsteins equations and look for globally well posed initial value problems then no such travesty arises. Many people just read too many science fiction books (and then they proclaim fiction = science).

 

[vanesch]

It is not that trivial, most of these ugly spacetimes come from nonunique, nontrivial pasting procedures. Like I said, someone who regards GR as a deterministic theory has to look at it from the initial value (in either globally hyperbolic) perspective.

Why ? You are entering here a specific requirement, that of global determinism, but why should that have to hold ? Why is it not possible to conceive a structure which doesn't have that global determinism (but is deterministic nevertheless because of local laws!) ? It is only that the "initial conditions" have now a slightly more complicated topological structure (with a bad choice of name, namely "initial" which is meaningless), and can't just be given on a single spacelike hypersurface: the specifications on some set of points on CTC is then also part of the "initial conditions". Of course, all that makes only sense in the original view on GR, which is a block universe. If you plug a Newtonian view onto it, then of course you are restricting a bit more the class of manifolds which can be acceptable spacetimes. But these are extra requirements (which may very well hold for our universe !) which are not part of the initial specification of GR.

 

[Demystifier]

Somebody said that there is a time arrow, but that there is no space arrow.
However, the fundamental equations of motion we know certainly allow for solutions with a space arrow, i.e., states in which local entropy density (or some other measure of disorder) increases, say, from the left to the right. Moreover, if matter was moving faster than light (which is NOT forbidden by relativity itself), then we would probably live in a universe with a space arrow, i.e., in a universe in which the direction of the entropy increase is a spacelike vector. In such a universe, we probably could not remember events on the right, so we would perceive them as if they not happened yet. The events on the right would be called "future" and events on the left would be called "past".
Apparently, nature has not chosen such a solution, at least not in our part of the universe. But, as far as we know, there is nothing fundamental about it.

 

[Careful]

Why ? You are entering here a specific requirement, that of global determinism, but why should that have to hold ?

Good old Albert certainly intended a determinstic universe !

Why is it not possible to conceive a structure which doesn't have that global determinism (but is deterministic nevertheless because of local laws!) ? It is only that the "initial conditions" have now a slightly more complicated topological structure (with a bad choice of name, namely "initial" which is meaningless),

Sure, everything is possible, so now you explain us why we don't observe closed timelike curves if you know there are many more ``universes'' like that, than globally hyperbolic ones. And no, the Einstein Hilbert action does not surpress topologies which allow for closed timelike curves (like a dougnut in 1+1 dimensions). And, oh yes, a tiny question, how are you going to define quantum mechanics on such universe ?

 

[Careful]

Somebody said that there is a time arrow, but that there is no space arrow.
However, the fundamental equations of motion we know certainly allow for solutions with a space arrow, i.e., states in which local entropy density (or some other measure of disorder) increases, say, from the left to the right.

Sure, but that was hardly the issue ! The question was whether the arrow of time is something which needs to be fundamentally build in or not. Then, you come and tell us that there exist situations where an arrow of space arises dynamically. Moreover, if matter were allowed to move out of the lightcone in a measurable way, then - euh forgive me this stupid question - what would be the purpose of introducing a causal lightcone in the first place ?

 

[vanesch]

Sure, everything is possible, so now you explain us why we don't observe closed timelike curves if you know there are many more ``universes'' like that, than globally hyperbolic ones.

How would you know that there are CTC's ? Imagine they loop over 20 billion years. How would you distinguish on a "local" patch, a piece of CTC from a piece of globally hyperbolic universe ? After all, locally, CTC's don't "look" any different than "non-CTC's" and you wouldn't notice...
That's what I claimed, earlier: an observer on a CTC would never know he's on a CTC (and certainly not how many "loops" he underwent already, if his memory is part of the same spacetime manifold as the one containing the CTCs, and hence will not allow for a "special memory state" indicating he "already came by". As such, CTC's won't give rise to all the sci-fi paradoxes of time travel.

 

[Careful]

How would you know that there are CTC's ? Imagine they loop over 20 billion years. How would you distinguish on a "local" patch, a piece of CTC from a piece of globally hyperbolic universe ?

You seem to underestimate global problems. By the way, you miss the point of my previous comment : if you allow for CTC's in general you have to show that they do not occur on timescales of seconds, minutes, days. Certainly one would notice that if one assumes memory indeed not to influence the physical state ( which I guess you as a ``consciousness fan'' do not protest against ). If, on the other hand your memory is wiped away, then you would be in trouble with the observation that since millions of years a rectilinear evolution occurs : every minut/second of eigentime our information increases.

That's what I claimed, earlier: an observer on a CTC would never know he's on a CTC (and certainly not how many "loops" he underwent already, if his memory is part of the same spacetime manifold as the one containing the CTCs, and hence will not allow for a "special memory state" indicating he "already came by". As such, CTC's won't give rise to all the sci-fi paradoxes of time travel.

Yep, see the contradiction with experience you arrive then above.

But much more interesting is the topic of quantum mechanics in such universe.

 

[mgelfan]

I just noticed the time arrow - space arrow thing a couple of messages back. It would seem to me that if there is a discernable arrow of time such as isotropic expansion on both local and universal scales, then might not this also be called the arrow of space (ie., the universe is expanding outwardly, omnidirectionally from its point(s) of origin -- just like a disturbance in air or water or other familiar local media)? For that matter, how could there be an arrow of time without (at least in some sense) an arrow of space? But I digress.

What I really wanted to do in this message was to try to further clarify for JesseM why I don't believe that relativity theory is the best tool to determine the possibility of backward time travel, and also briefly mention why quantum theory (at least an inference regarding its physical meaning) might have something to offer in considering this issue.

Acceleration, such as a gravitational field, produces real physical changes in objects -- and in accumulative indexing machines like clocks these changes can be made especially evident. Throw enough accelerations in there and you've got worldlines going every which way (maybe even backward in time, eh?) ... and all of this sort of confounds the issue, I think.

I need a more general definition of time than relativity theory offers, or a more comprehensive (an all-encompassing, an absolute?) clock. The universe will be my clock, and the arrangement of all the objects in the universe is the time of this clock.
(Ok, this is a bit 'out there', but cosmologists adopt this sort of hypothetical 'god's eye' view all the time, don't they? And, for the purpose of our analysis, it does simplify things a bit.)
I assume that the universe is finite in extent and that within its boundaries there exist a finite, but perhaps changing, number of ponderable bodies. I assume that the universal configuration is not just changing, but evolving -- and that no two universal configurations are the same. (If I had taken 3D snapshots of the universe every second from its beginning to now, then each snapshot would reveal a unique configuration, a unique time, of the universe.

I'm interested in traveling from the current universal epoch (I'm using epoch here to mean some continguous sequence of configurations, a proper subset of the set of all universal configurations.) to a prior one. I want to move, say, a bazillion configurations back to the epoch when the Wright brothers were making their first successful flight(s).

Now, maybe here we can appeal to quantum theory which might lead me to believe that I am a part of, a product of, and intimately entwined with the evolution of the universe ... that I can no more be lifted out of this epoch and deposited in a previous one than a quantum of light can be separated from the experimental preparation that produced it.

Ok, maybe that's not such a great analogy. So, I'll just restate my original objection to using the block universe model to determine the possibility of backward time travel. An overwhelming amount of observational evidence tells me that the real universe is not a block universe, and so I've assumed that the universe is evolving. This is important because an evolving universe precludes backward time travel. The universe of today is not the universe of the Wright brothers' time. Things changed position, things were created, things were destroyed, a bit of general expansion ... you get the idea. I can't revisit the Wright brothers' time, because it simply doesn't exist any more. Our universe has evolved into a somewhat different collection and configuration of objects than what characterized the Wright brothers' time. Doesn't this view seem more akin to what is observed than a block universe where all configurations that ever were or ever will be exist right now (I have no idea what that might mean, physically), and for all eternity, and through which we travel ... what? ... more or less independently ... I don't know? Anyway, the 4D representation makes it difficult to say which aspects of the model correspond to the physical world and which don't, and how. I prefer to use a more realistic model to flesh out the idea of backward time travel, because I'm not calculating anything here, but rather just trying to get some sense of the logic of backward time travel ... and my conclusion is that there's no logic to it, that it's a silly idea.

If you think the block universe model is better for this purpose, then I would be glad to learn your reasons.