Quantum Entanglement and time travel

[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.

 

[Careful]

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)?

No, because the expansion is not only isotropic but also homogeneous. If you inflate a balloon there is no way to tell in which direction it is expanding globally. If you put a bug on it (an operation which distroys homogeneity) you can define an arrow of space locally relative to the bug (in a closed universe, you get into trouble). But again, such concept would be emergent and not fundamental, you believe the existence of a universal clock to be of fundamental nature.

 

[mgelfan]

No, because the expansion is not only isotropic but also homogeneous. If you inflate a balloon there is no way to tell in which direction it is expanding globally. If you put a bug on it (an operation which distroys homogeneity) you can define an arrow of space locally relative to the bug (in a closed universe, you get into trouble). But again, such concept would be emergent and not fundamental, you believe the existence of a universal clock to be of fundamental nature.

I'm not sure I'm following you here. If something is expanding globally then doesn't that mean it's expanding ... everywhere? So, in that case, wouldn't the expansion be omnidirectional, and therefore the arrow of space is outward from every point?

Isn't the fundamental motion of the universe isotropic expansion? If not, then what?

In other words, there is an arrow of time precisely because there is an arrow of space. Or no?

 

[Careful]

I'm not sure I'm following you here. If something is expanding globally then doesn't that mean it's expanding ... everywhere? So, in that case, wouldn't the expansion be omnidirectional, and therefore the arrow of space is outward from every point?

Definition of an arrow field : a function from the points of the (spatial) manifold to its tangent space. As you might know a function can have only one image. The arrow of time means that there exists a globally well defined timelike vectorfield such that the motion of every particle is timelike and has positive projection on the field (for signature +---). In your example there is no direction in which space moves as seen from any point, space just moves outwards from it. So, you might wonder whether expansion somehow relates to an arrow of time. No, it doesn't, a Friedmann universe which recollapses again also contains an arrow of time, likewise does a Schwarzschild universe which neither expands nor contracts.

 

[RandallB]

Good old Albert certainly intended a determinstic universe !

No I don’t think that is fair.
Einstein expected a Determinate Local reality; determinate is not the same thing as deterministic. Determinate can be best understood by his expectation of a hidden not yet known variable that must exist as part of each of the two entities used in EPR type experiments. These HUV are determined, fixed, and unchanging parts of the photons, or particles. This “determinate” character of the variable is established at the creation of the photon or particle, and would remain only until it sufficiently interacted with something, such as a detector.

Expecting, even finding, such a variable would not require any ability to acquire sufficient knowledge to predict precisely what that and other variables will be prior to the generation of the “entangled pair”. Nor does it demand the possibility accruing enough information to precisely forecast exactly what kind of interaction and results the two separate photons / particles futures.
That kind of expectation of our current state as being predetermined from our past and our future is uncontrollable in a predestined future of a “Deterministic Universe” is not at all what Einstein implied.

Even though to date the evidence has not found such a hidden variable, and indicates Einstein was wrong to demand that such a variable even exists, that’s fine. But it is not fair to stick him with claiming a deterministic universe, that is not the same as his stubborn demand for Local Realism.

 

[Careful]

No I don’t think that is fair.
Einstein expected a Determinate Local reality; determinate is not the same thing as deterministic. Determinate can be best understood by his expectation of a hidden not yet known variable that must exist as part of each of the two entities used in EPR type experiments. These HUV are determined, fixed, and unchanging parts of the photons, or particles. This “determinate” character of the variable is established at the creation of the photon or particle, and would remain only until it sufficiently interacted with something, such as a detector.

Expecting, even finding, such a variable would not require any ability to acquire sufficient knowledge to predict precisely what that and other variables will be prior to the generation of the “entangled pair”. Nor does it demand the possibility accruing enough information to precisely forecast exactly what kind of interaction and results the two separate photons / particles futures.
That kind of expectation of our current state as being predetermined from our past and our future is uncontrollable in a predestined future of a “Deterministic Universe” is not at all what Einstein implied.

Even though to date the evidence has not found such a hidden variable, and indicates Einstein was wrong to demand that such a variable even exists, that’s fine. But it is not fair to stick him with claiming a deterministic universe, that is not the same as his stubborn demand for Local Realism.

Ok, can you give me a source for that ?! And how is this compatible with his sentence ``God does not play dice ?''. I mean, you seem to suggest for example that he would content himself with a locally stochastic universe, this also seems to be at odds with the following citation from Chapter 1 of Holland's Quantum Theory of Motion, p. 13 :

"In his (i.e., Einstein's) view, the indeterministic aspect of quantum
mechanics follows from the failure to provide a complete description
and not because it is an intrinsic characteristic of matter. In a
letter to Schrodinger in 1950 he says (Prizbram, 1967, p. 40) it seems
certain to me that the fundamentally statistical character of the
theory is simply a consequence of the incompleteness of the
description. In Einstein's programme, resolving the difficulty of
describing a determinate reality entails constructing a causal
(determinist) description, because he felt that this is a basic
requirement of a complete physical theory. (cf. Fine (1986, p. 103)).
That is, in the process of making microphysics determinate, it would
cease to be intrinsically statistical."

Anyway, I am sure he held many different opinions at different times, it was also debated on http://www.lepp.cornell.edu/spr/2001-12/msg0037413.html

 

[RandallB]

Ok, can you give me a source for that ?! And how is this compatible with his sentence ``God does not play dice ?''.

I do not know how to provide a source to establish something that Einstein did not say.
What he did say; “God does not play dice'' and authors descriptions of things as “indeterministic” when the item being considered only need be indeterminate to make the point in the discussion, are addressing the Non-Local vs. Local issue. Not the idea that we live in a predetermined life and world.

I don’t understand your use of “stochastic”.
Einstein believed that the results of an EPR should not be dependent upon an uncertain probability but a determinate HV, that is Local Realism.
If you have a source that is specific in showing how Einstein ever extended Local Realism to a deterministic predestined universe, I’d like to see it.

 

[Careful]

Ok, can you give me a source for that ?! And how is this compatible with his sentence ``God does not play dice ?''.

I do not know how to provide a source to establish something that Einstein did not say.

I was clearly asking for a source to establish your claim for a determinate instead of deterministic universe.

I don’t understand your use of “stochastic”.
Einstein believed that the results of an EPR should not be dependent upon an uncertain probability but a determinate HV, that is Local Realism.
If you have a source that is specific in showing how Einstein ever extended Local Realism to a deterministic predestined universe, I’d like to see it.

Well, by stochastic here I meant something like a random walk, a stochastic field theory (particle coupled to a fundamentally stochastic field) or so. I just provided you with a reference, where his use of the word statistical strongly suggests a deterministic universe.

 

[vanesch]

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.

I don't see why ? We're discussing the theoretical possibility of CTC's as allowed in principle by Einstein's local equations of GR. Now, there are variations on GR which would not allow for them, I understand, but that doesn't mean that the original view of a spacetime manifold + fields which respects everywhere the Einstein equations cannot allow for them. Whether this view is an appropriate description of *our universe* is to be seen. But also, it is not because the fundamental equations of GR allow in principle for CTCs, that they have to occur - and moreover occur in all kinds of flavors - in our specific universe, and "near" to us. The usual reason to reject CTCs is because of the "paradoxes" that they would generate, but I'm trying to argue that if you take GR seriously all the way, that these paradoxes do not occur, even on CTCs.

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 ).

Of course memory influences the physical state: it is part of it ! You never understood exactly what I meant with those consciousness things, it is not some kind of ghostly figure floating around in ectoplasm world living his life of his own (and maybe even with little wings and eating sweet deserts with golden spoons)! A conscious observer has no "hidden memory" disconnected from physical reality (as would have, I take it, such a ghostly creature).

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.

Well, that's for sure something that cannot (and will not) happen along a CTC, and it is the essence of my argument: if you are "living on a CTC", then the laws physics along that line will have to induce such an evolution, that when you cross "again" the same event, that your memory state must be exactly as it was, the "first time" you crossed it. If all the other fields are also defined over the spacetime manifold, they must (because they are single-valued) take on exactly the same values too, and because they respect their evolution equations over the manifold too, this evolution must come about "naturally" along the CTC. Along a CTC, there can't be anything else but a "whiped-away" memory from the "previous passage". A creature living on a CTC will remember a part of the CTC, and call that "his past". The amount of information, along the CTC, must be a periodic function.

All this doesn't mean that CTCs have to exist in our world. Only, the often-cited argument that they can't exist because leading to paradoxes, is IMO, wrong. I don't think that if you take GR exactly litterally, that there is any form of paradox. And given that CTCs are a theoretical possibility in a certain way (the original way) of formulating GR, it is interesting to think about them up to a point.

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

Yes, but given that we don't even know how to do QM with even much simpler geometries, I think that one simply can't answer that question as of now.

 

[Jheriko]

This thread makes for a great read whilst bored at work. I did see a couple of things that bothered me though:

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.

This kind of 'logic' always bugs me so I will say my piece:

Not having observed something doesn't even qualify as good evidence that nobody has observed something... let alone being a valid way to discredit a theory. All of recorded human history isn't exactly a significant span of time either. All we can say is that during the course of our history we /think/ that nobody has observed X. All that is evidence for, is that nobody has observed X... and don't forget that history is selective and recorded by the few, so we don't know with any useful certainty if *anything* has never been observed, much less knowing that something does not exist.

It is somewhat akin to taking a bathtub full of Smarties, Skittles or (insert multicolored sweets here), picking out 5 or 6 with a biased selection process and then saying that that no blue sweets in your hand is evidence that there are no blue sweets in the bathtub...

This is probably part of why we develop self-consistent theories based on observations, it allows us to rapidly disprove them, constrain them and improve them by making small numbers of observations. This is why we talk of the lower-bound for the proton half-life for instance... even though nobody has observed a proton decay. Non-observation isn't useful in the same way, all it let's us do is speculate with a false sense of security.

So what you are saying is actually a personal intuition... or is, at least, motivated by experience or personal opinion. Either way it has no logical or theoretical value.

No I don’t think that is fair.
Einstein expected a Determinate Local reality; determinate is not the same thing as deterministic. Determinate can be best understood by his expectation of a hidden not yet known variable that ...

I have to disagree, I have read quite a few quotes from Einstein that imply that he believed that everything is pre-determined. More importantly what does a hidden variable theory have to do with determinism?

AFAIK, hidden variable theories are just about explaining the distributions from quantum mechanics in terms of a deterministic theory. e.g. If I fire a gun at a target there is a distribution to where the shots land, however, given sufficiently good instruments we can measure the causes of this distribution and predict the correct result given sufficient information and time to calculate, in practice however it is easier to just model the distribution. This is how I think Einstein viewed quantum mechanics and why he expected a hidden variable theory to provide a more accurate description of nature, and what's more if this was his opinion he didn't just believe in determinism.

I will see if I can find a specific quote I am thinking of, but Google is failing me

It went something like this, but with more elegant wording: "I find it easier to forgive people for their mistakes, and not to dwell on my own, since all of our actions are simply the results of the laws of nature".

Anyone know of the exact quote I am thinking of? If I find it I will edit this post...

 

[GUS]

Quote:
Originally Posted by JesseM
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.

Well that clears that up then