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 Einstein'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 Einstein'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.