Quantum Entanglement and time travel

[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

 

[GUS]

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

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

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

yet its also supposedly impossible for nonlocality to occur yet we know it happens. Perhaps it is not the fact that we need to recreate past configurations but rather that these configurations of past , present and future all exist simoultaneously

 

[GUS]

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

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

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

Would that not imply in effect that our destiny is fixed and we can do nothing to change it ? What about the possibility of an infinite amount of different versions of the static bits of string ? (im trying to avoid the word timiline)

 

[RandallB]

RB: "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 ………

As I said HVT has nothing to do with the determinism of a deterministic theory of the universe, as you proceed to claim that it does. This was not Einstein’s view and twisting some obscure quote or two from him, into his believing that he had no choice or control of the achievements in his life won’t do. It will need to be direct and specific enough to counter his religion not of a individual Jewish God, but a religion of science, where humanistic free choice does allow for, and I suspect in his opinion call for, moral choices for ‘good’. An extended debate on Einstein’s Philosophy belongs in the Philosophy Forum. I just don’t think it fair to allow miss representations of it to go un-rebutted here. To the extent one can claim a deterministic universe as a legit theory (I do not) – it was not one accepted by Einstein.

 

[ueit]

yet its also supposedly impossible for nonlocality to occur yet we know it happens.

No, we don't know that. In fact, there are good reasons to think that non-locality (understood as an interaction taking place faster than c) is indeed forbidden in our universe.

 

[GUS]

No, we don't know that. In fact, there are good reasons to think that non-locality (understood as an interaction taking place faster than c) is indeed forbidden in our universe.

I thought Quantum entanglement was something we were pretty sure about ? Isnt quantum entanglement itself pretty much impossible from a common sense point of view ?

 

[RandallB]

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

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.

I never made a claim for a determinate universe! Just a determinate solution to EPR issues in that at the point of creation of two “entangled” particles determinate HV’s could provide a Local Realist solution to “entanglement” not a problematic (stochastic) one.
That’s why I couldn’t understand your use of stochastic in post #55 or what “a locally stochastic universe” could even mean. Einstein’s HVT takes the view that the problematic (stochastic) solutions to EPR are incomplete and deal with microphysics not the whole or even 'local' universe.

The quote you gave me (from Prizbram, not Einstein) refers to establishing a determinate explanation to a microphysics problem with very limited boundaries, not the question of a deterministic universe. That is an unfounded extrapolation to the large scope of the whole universe, unjustified by only any conclusions from a limited EPR test. Even if HV’s were to be proven as real, Determinate Microphysics by itself would not be able to establish a requirement for a Determinate/Deterministic universe.

 

[ueit]

I thought Quantum entanglement was something we were pretty sure about ? Isnt quantum entanglement itself pretty much impossible from a common sense point of view ?

Entanglement requires that there exist correlations between parts of a quantum system, regardless of the distance between the parts (as seen in the EPR experiments). Nothing is said, however, about the mechanism by which those correlation are enforced and there is no reason to believe this mechanism to be non-local.

 

[RandallB]

Entanglement requires that there exist correlations between parts of a quantum system, regardless of the distance between the parts (as seen in the EPR experiments). Nothing is said, however, about the mechanism by which those correlation are enforced and there is no reason to believe this mechanism to be non-local.

Every reputable experiment so far on EPR/entanglement indicates that only non-local solutions will work and strong imply that no truly Local HVT can be possible. So, there is a huge reason to believe this mechanism is non-local.

But there are several different ways to reach a non-local solution, including some that do not require physical interactions taking place faster than the speed of light. Some with a form of extra dimensional part of the items being tested remaining connected even as they separate in our three dimensions. So with so many alternatives like BM MWI etc. in addition to QM theories and no conclusive result on A correct one, just that it does seem to work “Non-Locally”, the issue is NOT something we are "pretty sure about" as GUS may have thought.

Although the evidence is strong for non-local, I would agree that there is no reason to stop those that wish to look for a Local solution if they wish. They just need to understand they are going against the majority view, and there is nothing automatically wrong with that. And they need to accept the risk of not making any real progress – and I do accept that risk myself.

 

[vanesch]

But there are several different ways to reach a non-local solution, including some that do not require physical interactions taking place faster than the speed of light. Some with a form of extra dimensional part of the items being tested remaining connected even as they separate in our three dimensions. So with so many alternatives like BM MWI etc. in addition to QM theories and no conclusive result on A correct one, just that it does seem to work “Non-Locally”, the issue is NOT something we are "pretty sure about" as GUS may have thought.

I hate to repeat this for the 100th time or so, but in MWI, there is no genuine non-locality in the EPR situation. In fact, MWI kills Bell's theorem for the simple reason that there is no unique outcome at Bob and Alice (which is of course a requirement in Bell's theorem). Given that both outcomes exist simultaneously, the observed correlations when the "results come together" are then nothing else but a local interference pattern of these different outcomes which only occur upon their coming together.
In other words, in this view, the "correlations between Alice and Bob" are not more surprising than the interference pattern of a lightbeam that has been split, got far away, and then came together again, to interfere.

 

[Careful]

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.

You miss my point again, when I spoke about universes with CTC's having a large probability (and saying that the action does not surpress them), I was clearly referring to problems you should have in quantum gravity where each universe is represented democratically at least if we take the Feynman path integral seriously. Moreover, each time you allow for extra variables in a physical theory, you have more and more a problem of finetuning initial data.

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).

Sure, I know this, I was merely teasing you. But admit it, it is kind of unbelievable that this wonderful consciousness of yours is so important in order for your brain to make observations, that it cannot do anything else but that !

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.

Sigh, my argument was that we measure information to increase in function of eigentime, so the mere ability to do that as well as the fact that it happens excludes CTC's by the mere argument you just repeated. The mere fact that we have authentic books dating before Jezus Christ means that there was no CTC going back to a later moment than that date.