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Thread: Time travel is a fantasy

  1. #1 Time travel is a fantasy 
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    You can beat an egg, but you canít beat a good movie. And there are some real crackers out there. The Terminator was a classic, with that remorseless skeletal T-800 that just wonít stop. Another cracker was 12 Monkeys, which featured a scary way to save the planet. Then thereís Men in Black3, Looper, and Lost in Space. Or Frequency, or Dťjŗ Vu, or Source Code. And if chick-flick is your thing thereís Kate and Leopold, The Lake House, and The Time Travellerís Wife. They all feature time travel, and I loved them all, and more besides. One of my special favourites was Timescape starring Jeff Daniels, where he and his other self hammered the church bell to sound the warning. Dang Ding Dang. Brilliant. I loved it even though I know that time travel is science fiction. It isn't science, it isn't physics. Instead it's fantasy.

    Yes, a fantasy. Time travel to the past is impossible. Not because of the grandfather paradox or the chronology protection conjecture. But because we donít even ďtravel forward through timeĒ. Thatís just a figure of speech. So is ďthe flow of timeĒ. If you open up a clock you donít see time flowing through it like itís some cosmic gas meter. You see little cogs and things, moving. In a grandfather clock thereís a pendulum swinging. In a quartz wristwatch thereís a crystal vibrating. In an atomic clock itís the hyperfine-transition microwaves. Moving. Clocks always feature some kind of regular cyclic motion, and they clock this up to show you some kind of cumulative display that you call ďthe timeĒ.

    You must have seen some movie or TV program where somebody has a device that can stop time. Did you notice that the gizmo doesnít so much stop time as stop motion? Itís the same kind of deal with bullet time. Itís not time going slower. Itís bullets going slower. Or just hold your hands up. See that gap between them? That's a space, and you can see itís there, even though you can't see the space itself. Now waggle those hands. That's motion, and you can see that too. But can you see time? No. Can you see time flowing? No. Can you see any actual travelling through time? No. Have you ever heard Hawking saying you can ďtravelĒ to the future on a superfast train? Imagine youíre on it. The train moves fast. Through space. And because it does, the local rate of motion in your body brain and clocks has to reduce. Itís called time dilation, but itís all down to motion really. It occurs because the maximum rate of motion is the speed of light. If that train could move at the speed of light, your local motion would be zero. If it wasnít, adding the train motion to your local motion would mean your total motion exceeds the speed of light. Thatís a no-no. Special relativity says no, and the wave nature of matter says no. Time dilation doesnít really mean you travel to the future faster. All it means is your local motion is reduced compared to mine, in line with Pythagoras's theorem:


    Public domain image by Mdd4696, see Wikipedia.

    You never were actually travelling through time. You were travelling through space. And whilst you were doing that I could have watched you every step of the way. You could have looked out the window and watched me every step of the way too. You didnít disappear up some time tunnel to start living your life in the middle of next week. You just fast-forwarded through the week. And when you step off the train itís me who meets you. I donít miss you by a week.

    Itís the same kind of thing with gravitational time dilation. It's not quite the same, but it's still simple. It can be idealised via the stasis box, which is kind of like the ultimate refrigerator. Yes, itís something out of science fiction too, just like time travel. But itís kind of fun to fight fire with fire. No motion of any kind occurs inside this stasis box. So when I put you inside, electromagnetic phenomena donít propagate. So you canít see, you canít hear, and you canít even think. Hence when I open the door a week later, to you itís like I opened the door just as soon as I closed it. And get this: you ďtravelledĒ to the future by not moving at all. Instead everything else moved. And all this motion wasnít through time, or spacetime, it was through space. Yes, the stasis box is science fiction, but donít forget, we can freeze embryos now. In the future maybe weíll be able to freeze an adult. Then you could ďtravelĒ to the future by stepping into a glorified freezer. But you arenít really travelling to the future. You arenít moving. Instead everything else is.

    So if there isnít really any travelling forward through time, how are you going to travel back in time? Arrange for the big guy in the sky to press his cosmic rewind button? Whilst leaving your memories intact? Donít think so. Besides, thatís not time travel. Thatís just one up from dragging the bar on a YouTube video. And despite what you may have heard about closed timelike curves, thatís not time travel either. Thatís just Groundhog Day, only you wouldnít know it. The thing is, spacetime is a mathematical model. It presents the dimensions of space along with a time dimension, depicting all times at once. Itís like you film something with a movie camera, then develop the film, then cut it up into individual frames and form them into a stack. And because of that, it is static. It isnít something you can move through. It isn't what space is.


    Spacetime depiction by John D Norton, see Einstein for everyone course HPS0410

    You cannot look up to the clear night sky and point to a world line or a light cone, because these things are abstractions, like spacetime is an abstraction. Yes, general relativity refers to curved spacetime, but only to give the equations of motion through inhomogeneous space. And the time dimension is in itself derived from motion, such as the motion of clocks, and the motion of light. So that time dimension just isnít the same as the dimensions of space. You can hop forward a metre, but you canít hop forward a second. And you canít hop backward a second either.

    You can read more about this stuff in A World Without Time: The Forgotten Legacy of Godel and Einstein. Itís a book featuring philosophy and relativity and what Einstein and Godel used to talk about in Princeton. It doesnít say time does not exist, itís more like time exists like heat exists. And just as you canít literally climb to a higher temperature, you canít literally travel forward in time. Or backwards. No way, no how.

    Bear all this in mind next time you see some celebrity physicists talking about time travel on the Discovery Channel. Like those time-travel movies, itís fantasy. Not physics. But a time travel movie can still be a great movie. There is no time travel, but there is no Santa Claus either, and you can still have a great Christmas.
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  2. #2  
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    I know that. However matter-energy transporters, replicators and warp drive.... just kidding. Chuckle chuckle.
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    Uh, the science of Star Trek. This isn't a bad little article though:

    NASA - The Science of Star Trek

    Note that it says here's a list of the standard Star Trek features, roughly in order of increasing scientific incredibility. And time travel is at the bottom, along with wormholes. But then follow my Hawking link in the OP, and you find yourself reading How to build a time machine by Stephen Hawking. And lo, what you see is this: All you need is a wormhole, the Large Hadron Collider or a rocket that goes really, really fast. Amazingly, Hawking doesn't understand time, he doesn't understand that time travel is a fantasy. And because he doesn't understand time, he doesn't understand spacetime. It doesn't stop there. I started this thread because Markus and I got talking about black holes and Hawking radiation, and about what looks like an irreconcilable problem. It isn't.
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  4. #4  
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    Quote Originally Posted by Farsight View Post
    And lo, what you see is this: All you need is a wormhole, the Large Hadron Collider or a rocket that goes really, really fast
    Yes, and you also see this : "Given enough power and advanced technology, perhaps a giant wormhole could even be constructed in space. I'm not saying it can be done, but if it could be, it would be a truly remarkable device"

    Amazingly, Hawking doesn't understand time, he doesn't understand that time travel is a fantasy.
    I'd say he understands it pretty well - the article is a pop-sci article, and he is merely hypothesizing. I would, however, agree that he is a little too optimistic about the whole thing, because the entire concept is based on a purely classical theory - personally, I would conjecture that Einstein-Rosen bridges will either not feature at all in a full model of QG ( remember that the domain of applicability of GR is limited ), or will always be hidden behind event horizons. Same goes for CTCs and artificial constructs such as Krasnikov tubes.

    To cut a long story short - I don't think we will ever be able to travel in time, and least not in the way depicted in Star Trek. It's still a good show, though
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    I really don't think he understands time, Markus. Cross my heart and hope to die. The moot point is that a clock "clocks up" some kind of regular cyclical local motion. It doesn't actually "measure the flow of time". Our world is modelled using spacetime, and that model works. But the map is not the territory. Our world is one of space and motion, and equations of motion. And there is nothing you can do to undo that motion. You can't literally travel through a cumulative measure of local motion, just as you can't literally climb to a higher temperature. Temperature being a measure of average motion. Or average kinetic energy if you prefer, see wiki. So Hawking's Chronology Protection Conjecture is unnecessary.

    Quote Originally Posted by Markus Hanke
    I would conjecture that Einstein-Rosen bridges will either not feature at all in a full model of QG ( remember that the domain of applicability of GR is limited ), or will always be hidden behind event horizons. Same goes for CTCs and artificial constructs such as Krasnikov tubes.
    Sounds good to me. That's like what I was saying about non-real solutions and how to recognise them.

    Quote Originally Posted by Markus Hanke
    To cut a long story short - I don't think we will ever be able to travel in time, and least not in the way depicted in Star Trek. It's still a good show, though.
    Yep. I particularly liked the 2009 movie. However I'm not a fan of Doctor Who. Even when I was a kid, it would be getting all exciting towards the climax, and I'd be saying Hey what's the rush? You've got a time machine haven't you?
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  6. #6  
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    Yep. I particularly liked the 2009 movie.
    Funny thing, because none of the Star Trek movies really "did it" for me. Personally I really like the Star Trek:TGN television series - not because it is in any way realistic, but because I grew up with it

    Our world is modelled using spacetime, and that model works.
    Yes, Farsight, I agree. And for me personally, this is what physics does - it makes models about aspects of the world around us. A model is much like a machine - you input a specific scenario and problem, and the model will process these boundary conditions and outputs a prediction. You then go and compare that prediction to empirical data - if it holds, we have ourselves a good model, to be further applied to more scenarios. If it almost holds, we have a convenient approximation. If it fails, it gets rejected and/or reworked. I don't think physics can tell us "the truth" ( whatever that means ! ), it can only model what we perceive as reality. In this context GR is a very good approximation, but it is a model that is clearly limited, because it excludes the entire domain of quantum physics; hence it is doomed to fail in the final stages of processes such as gravitational collapse. Anything that is derived from these processes - such as the existence of CTCs in Kerr space-time - is to be regarded with a healthy amount of suspicion.

    Our world is one of space and motion, and equations of motion.
    That view of reality certainly has validity in its own right ( after all Newtonian mechanics works this way, and it's a pretty good approximation ! ), but GR pursues a different approach. The concept of space-time does away with "motion" in the classical sense, and replaces it with static world lines; the equations of motion then become the geodesic equations. As you say above, this approach works really well. I agree with you though that it isn't ( and can't be ) the final word on the matter, since in itself it doesn't explain why time "works" only one way, unlike the spatial dimensions. That doesn't make GR wrong, it is only a symptom of the fact that it is incomplete.

    I am somewhat biased towards the approach taken by Causal Dynamical Triangulations to the issue of causality and time, simply because it is very elegant and straightforward, at least conceptually. At the moment though I would only consider CDT a hypothesis ( albeit a very interesting one ), since there are too many loose ends to be tied up - it might turn out to be a dead end, who knows.
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    Yes, a model is something like a machine. You build it, you tinker with it, you test it, you fix it. But sometimes it just breaks down and you have to scrap it.

    Quote Originally Posted by Markus Hanke
    I don't think physics can tell us "the truth" ( whatever that means!), it can only model what we perceive as reality.
    I think it can tell us the truth. The truth is out there. And some aspects of that truth are hidden in plain sight. To see it you have to take a step back and adopt the God's eye view. The big-picture view. You look at two frames at once.

    Quote Originally Posted by Markus Hanke
    In this context GR is a very good approximation, but it is a model that is clearly limited, because it excludes the entire domain of quantum physics;
    That's true enough.

    Quote Originally Posted by Markus Hanke
    hence it is doomed to fail in the final stages of processes such as gravitational collapse.
    I disagree again I'm afraid. IMHO it only fails when people mistake non-real solutions for real solutions.

    Quote Originally Posted by Markus Hanke
    Anything that is derived from these processes - such as the existence of CTCs in Kerr space-time - is to be regarded with a healthy amount of suspicion.
    But I agree with that. And then some.

    Quote Originally Posted by Markus Hanke
    That view of reality certainly has validity in its own right ( after all Newtonian mechanics works this way, and it's a pretty good approximation ! ), but GR pursues a different approach.
    That's what people say. But when you read The Foundation of the General Theory of Relativity, Einstein does talk about the equations of motion. The wikisource version is searchable. Search on motion.

    Quote Originally Posted by Markus Hanke
    The concept of space-time does away with "motion" in the classical sense, and replaces it with static world lines; the equations of motion then become the geodesic equations. As you say above, this approach works really well.
    It works, but the map is not the territory. The static model is a machine for working out the motion of light or particles. Or Mercury.

    Quote Originally Posted by Markus Hanke
    I agree with you though that it isn't ( and can't be ) the final word on the matter, since in itself it doesn't explain why time "works" only one way, unlike the spatial dimensions. That doesn't make GR wrong, it is only a symptom of the fact that it is incomplete.
    I think it's more final and more complete than you do. You don't need GR to explain anything about time. You just open up a clock. And once you know about A World without Time: The Forgotten Legacy of Godel and Einstein you've got it on Einstein's authority too. That's more than enough.

    Quote Originally Posted by Markus Hanke
    I am somewhat biased towards the approach taken by Causal Dynamical Triangulations to the issue of causality and time, simply because it is very elegant and straightforward, at least conceptually. At the moment though I would only consider CDT a hypothesis ( albeit a very interesting one ), since there are too many loose ends to be tied up - it might turn out to be a dead end, who knows.
    I got as far as how the spacetime fabric itself evolves. Spacetime is static.
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  8. #8  
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    Quote Originally Posted by Farsight View Post
    The big-picture view. You look at two frames at once.
    Yes, that's the whole purpose of GR; remember when I mentioned earlier on that GR globally relates observers to one another ? This is what I meant, and that global picture is your "God's eye view".

    That's true enough.
    I disagree again I'm afraid.
    That's a contradiction. You can't agree that GR is purely classical, but then disagree that it fails to describe the final stages of gravitational collapse, which is a process where quantum effects cannot be neglected.

    The static model is a machine for working out the motion of light or particles. Or Mercury.
    In the context of GR, Mercury becomes a static twisted helix in static space-time ( the twist is just the perihelion precession ). Each time-like slice of the helix corresponds to one instant in time of Mercury on its orbital path through space. Same goes for any other particle or body. These approaches aren't mutually exclusive, they are just different.

    I think it's more final and more complete than you do
    Hmm...only last year I would have probably agreed with you. But since then I have studied some possible extensions and generalisations of GR, and a few of those are very viable models that simply cannot be ruled out by currently available empirical data, and make some interesting predictions. The most notable one is probably Einstein-Cartan gravity.

    All I am saying is that we must entertain the possibility that GR is not the final word on the matter, based on the fact that there are other possibilities.

    You don't need GR to explain anything about time.
    It is not the purpose of GR to explain the nature of time; GR is a model of geometrodynamics only, i.e. it only provides us with answers as to how the geometry of space-time is related to its energy-momentum content. It cannot tell us anything about the nature or internal workings of space-time itself, or where it comes from, or why we use space-time in the first place. The existence of space-time is a postulate, it doesn't follow from the model. We are going to need something more general and fundamental to explain why ( as opposed to how ) space-time works as a model of gravity, and why time works the way it does in the observable universe. This is why I briefly mentioned CDT, as it provides a very elegant explanation ( I'm not saying it is right, though ).

    I got as far as how the spacetime fabric itself evolves.
    Did I use these words ? I can't find them in my text.
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  9. #9  
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    I forgot to mention something - while both spacetime and "motion in space" have their places in our understanding of the world, there is an important difference between these points of view so far as gravity is concerned.

    In general, the mathematical object that completely describes curvature on a manifold is the Riemann tensor; its trace forms the Ricci tensor ( which is what appears in the Einstein equations ), and the remaining trace-free part forms another rank-4 tensor called the Weyl tensor ( which doesn't appear in the field equations ). Very simply put, the Ricci tensor tells us how the volume of a ball of test particles evolves over time, whereas the Weyl tensor is a measure of how the shape of that ball changes due to tidal gravity; it is an expression of what influence distant sources of gravitation have, so it represents the effects of gravitational radiation.

    The issue now is that in three dimensions, the Weyl tensor is always automatically zero - hence, in a model of gravity that works only in space ( i.e. in 3 dimensions ) but not in space-time, there can be no gravitational radiation.

    This just as an aside.
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    Your aside noted, Markus. I wouldn't say gravity only works in space but not spacetime. That doesn't sound right. That sounds like throwing motion out of the window, and like I was saying on the other thread, I think motion is very important.

    Quote Originally Posted by Markus Hanke
    Yes, that's the whole purpose of GR; remember when I mentioned earlier on that GR globally relates observers to one another ? This is what I meant, and that global picture is your "God's eye view".
    I root for relativity. I think it's the Cinderella of modern physics. But despite the "coordinate independence" I would say that GR tells you how you see the world, "in your reference frame". It doesn't say look at these two guys alongside one another in their different frames. It doesn't give the God's eye view.

    Quote Originally Posted by Markus Hanke
    I disagree again I'm afraid. That's a contradiction. You can't agree that GR is purely classical, but then disagree that it fails to describe the final stages of gravitational collapse, which is a process where quantum effects cannot be neglected.
    I can because I think quantum effects are "purely classical" too. No spooky action at a distance, and no magick.

    Quote Originally Posted by Markus Hanke
    In the context of GR, Mercury becomes a static twisted helix in static space-time ( the twist is just the perihelion precession ). Each time-like slice of the helix corresponds to one instant in time of Mercury on its orbital path through space. Same goes for any other particle or body. These approaches aren't mutually exclusive, they are just different.
    No probs.

    Quote Originally Posted by Markus Hanke
    Hmm...only last year I would have probably agreed with you. But since then I have studied some possible extensions and generalisations of GR, and a few of those are very viable models that simply cannot be ruled out by currently available empirical data, and make some interesting predictions. The most notable one is probably Einstein-Cartan gravity.
    Again OK noted. But see avoidance of singularites? in Wikipedia. It says this:

    "According to general relativity, the gravitational collapse of a sufficiently compact mass forms a singular black hole. In the EinsteinĖCartan theory, instead, the collapse reaches a bounce and forms a regular Einstein-Rosen bridge (wormhole) with a new, growing universe on the other side of the event horizon".

    IMHO the first sentence isn't true. And whilst I'm forever quoting Einstein, I'm really not keen on wormholes.

    Quote Originally Posted by Markus Hanke
    All I am saying is that we must entertain the possibility that GR is not the final word on the matter, based on the fact that there are other possibilities.
    OK.

    Quote Originally Posted by Markus Hanke
    GR is a model of geometrodynamics only, i.e. it only provides us with answers as to how the geometry of space-time is related to its energy-momentum content. It cannot tell us anything about the nature or internal workings of space-time itself, or where it comes from, or why we use space-time in the first place. The existence of space-time is a postulate, it doesn't follow from the model. We are going to need something more general and fundamental to explain why ( as opposed to how ) space-time works as a model of gravity, and why time works the way it does...
    Note that everything you've said above suggests that spacetime is a thing that's out there. And yet you don't think of space and energy in such terms.

    Quote Originally Posted by Markus Hanke
    Did I use these words ? I can't find them in my text.
    No. You didn't. They were in the Wikipedia article. Space evolves. Spacetime is "space at all times", so it doesn't.
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  11. #11  
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    Quote Originally Posted by Farsight View Post
    Your aside noted, Markus. I wouldn't say gravity only works in space but not spacetime.
    Is this what my post seems to imply ? If so I apologise, because I meant to point out the exact opposite - gravity works fine in space-time, but not in space. You can't just eliminate time and expect everything to still work.

    But despite the "coordinate independence" I would say that GR tells you how you see the world, "in your reference frame".
    It does, but it goes above and beyond that and also tells you how different observers are related to one another. That is where the covariance comes in.

    I can because I think quantum effects are "purely classical" too. No spooky action at a distance, and no magick.
    There's certainly no magic, no.
    This is a matter of convention though - classical physics is meant to be everything that doesn't account for quantum effects ( whatever their nature ).

    IMHO the first sentence isn't true. And whilst I'm forever quoting Einstein, I'm really not keen on wormholes.
    In the absence of a countermechanism to stop the collapse, the singularity seems to me to be unavoidable in classical GR. In ECG, there cannot be a singularity due to the presence of torsion, which leads to fermions becoming spatially extended objects.

    I have no issue with wormholes as such, especially not the non-traversable kind. The thing is that the Einstein equations constrain only the local geometry, but not the global topology of space-time. Both multiply and singly connected space-times are hence allowed under classical GR. Whether this is still the case in a consistent model of quantum gravity is a different issue, though.

    Note that everything you've said above suggests that spacetime is a thing that's out there
    Really ? I don't think of it as a "thing". Space-time is just the set of all events, and its geometry and topology is how these events are related to one another.

    No. You didn't. They were in the Wikipedia article. Space evolves. Spacetime is "space at all times", so it doesn't.
    I think what is meant here is that space-time emerges from the more fundamental set of 4-simplexes, a system which is in itself not spatio-temporal in nature - the word "evolve" is a rather unfortunate choice of term, and not technically correct. The initial state in CDT is a superposition of pure quantum states of geometric 4-simplexes, without any notions whatsoever of space and time. Only when these simplexes are joined in certain ways ( i.e. certain states of the phase space of the system are realised ) that a manifold with spatial and temporal dimensions emerges. Interestingly, this manifold is anything but trivial.

    To be honest, I really struggle to explain this with adequate terminology, because it is so far removed from classical notions of our everyday world. Here is where English not being my first language really gets the better of me.
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    Sorry to be slow replying Markus. It was my birthday then the weather was sunny.

    Quote Originally Posted by Markus Hanke
    Is this what my post seems to imply ? If so I apologise, because I meant to point out the exact opposite - gravity works fine in space-time, but not in space. You can't just eliminate time and expect everything to still work.
    No, I think there was some confusion somewhere. I didn't mean to suggest you eliminate time altogether. I meant to say you shift your interpretation a little and put more emphasis on motion.

    Quote Originally Posted by Markus Hanke
    It does, but it goes above and beyond that and also tells you how different observers are related to one another. That is where the covariance comes in.
    I'm glad you mentioned that. The big-picture issue is essentially this: covariance can be mistaken for no variance.

    Quote Originally Posted by Markus Hanke
    There's certainly no magic, no. This is a matter of convention though - classical physics is meant to be everything that doesn't account for quantum effects ( whatever their nature ).
    OK noted.

    Quote Originally Posted by Markus Hanke
    In the absence of a countermechanism to stop the collapse, the singularity seems to me to be unavoidable in classical GR. In ECG, there cannot be a singularity due to the presence of torsion, which leads to fermions becoming spatially extended objects.
    They're spatially extended in QFT. Or should be. It's quantum field theory, not quantum point-particle theory. Everything is fields and waves.

    Quote Originally Posted by Markus Hanke
    I have no issue with wormholes as such, especially not the non-traversable kind. The thing is that the Einstein equations constrain only the local geometry, but not the global topology of space-time. Both multiply and singly connected space-times are hence allowed under classical GR. Whether this is still the case in a consistent model of quantum gravity is a different issue, though.
    That brings us back to what I was saying about how to recognise a non-real solution. See Schwarzschild wormholes? They're also called EinsteinĖRosen bridges. I have an issue with them. And to justify that I have to justify the nature of black holes. And to justify that I have to get you to think about why the light doesn't get out.

    Quote Originally Posted by Markus Hanke
    Really? I don't think of it as a "thing". Space-time is just the set of all events, and its geometry and topology is how these events are related to one another.
    Me too.

    Quote Originally Posted by Markus Hanke
    I think what is meant here is that space-time emerges from the more fundamental set of 4-simplexes, a system which is in itself not spatio-temporal in nature - the word "evolve" is a rather unfortunate choice of term, and not technically correct. The initial state in CDT is a superposition of pure quantum states of geometric 4-simplexes, without any notions whatsoever of space and time. Only when these simplexes are joined in certain ways ( i.e. certain states of the phase space of the system are realised ) that a manifold with spatial and temporal dimensions emerges. Interestingly, this manifold is anything but trivial. To be honest, I really struggle to explain this with adequate terminology, because it is so far removed from classical notions of our everyday world. Here is where English not being my first language really gets the better of me.
    To be honest Markus, I think you'd struggle to explain it in any language.
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    Quote Originally Posted by Farsight View Post
    then the weather was sunny.
    Not where I am, it wasn't

    No, I think there was some confusion somewhere. I didn't mean to suggest you eliminate time altogether. I meant to say you shift your interpretation a little and put more emphasis on motion.
    Ok. Note that GR does not "eliminate" motion in any sense of the word, it just makes no reference to it. Space-time models all events at all times, and then lets us "slice it up" in any way we desire. The way we label the slices then just becomes our time coordinate, and allows us to recover the classical concept of motion. The difference to Newtonian mechanics is that this "slicing" and "labelling" is arbitrary. Of course, the interesting question still remains as to why we perceive a smooth, well-ordered and futured-oriented succession of slices ( moments in time ), but GR has nothing to say on this. It is just a classical model of a static space-time. To explain what we so carelessly call "flow of time" we need more than just GR alone.

    They're spatially extended in QFT. Or should be. It's quantum field theory, not quantum point-particle theory. Everything is fields and waves.
    True, but both GR and ECG are purely classical and make no reference to any quantum phenomena. One would thus expect particles to be point-like in the context of these classical theories, but that is not the case for fermions under ECG. Note also that torsion couples to spin, and would hence change the Dirac equation, making it non-linear.

    I have an issue with them.
    In what way ? So long as they are not traversable and/or hidden behind event horizons, they are really of not much physical consequence in the classical model.
    But regardless, the fact is that while all physical space-times are necessarily solutions of the Einstein equations, the reverse is not true - not all solutions to the field equations are necessarily physical.
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    Quote Originally Posted by Markus Hanke
    Ok. Note that GR does not "eliminate" motion in any sense of the word, it just makes no reference to it. Space-time models all events at all times, and then lets us "slice it up" in any way we desire. The way we label the slices then just becomes our time coordinate, and allows us to recover the classical concept of motion. The difference to Newtonian mechanics is that this "slicing" and "labelling" is arbitrary.
    All good stuff. Note though that you make your measurements of space and time using the motion of light, that your measurements change when you change your state of motion.

    Quote Originally Posted by Markus Hanke
    Of course, the interesting question still remains as to why we perceive a smooth, well-ordered and futured-oriented succession of slices ( moments in time )
    Because that's how the world is. Ours is a world of space and motion, not some static motionless block universe.

    Quote Originally Posted by Markus Hanke
    but GR has nothing to say on this. It is just a classical model of a static space-time. To explain what we so carelessly call "flow of time" we need more than just GR alone.
    Maybe. But I don't think we need some other theory altogether.

    Quote Originally Posted by Markus Hanke
    True, but both GR and ECG are purely classical and make no reference to any quantum phenomena. One would thus expect particles to be point-like in the context of these classical theories, but that is not the case for fermions under ECG. Note also that torsion couples to spin, and would hence change the Dirac equation, making it non-linear.
    I've never understood why an electron is supposed to be pointlike in classical theories. Waves are classical, you can diffract electrons, and what we call the Aharonov-Bohm effect was predicted by Ehrenberg and Siday in their 1949 "electron optics" paper. Stuff like this particle adventure image don't help IMHO.

    Quote Originally Posted by Markus Hanke
    In what way?
    If you had a wormhole you could time travel. Only you can't time travel, because time is a cumulative measure of local motion. So wormholes are a fantasy too.

    Quote Originally Posted by Markus Hanke
    So long as they are not traversable and/or hidden behind event horizons, they are really of not much physical consequence in the classical model.
    You might say that. But I'd say one fantasy is built on another, and the consequences are severe.

    Quote Originally Posted by Markus Hanke
    But regardless, the fact is that while all physical space-times are necessarily solutions of the Einstein equations, the reverse is not true - not all solutions to the field equations are necessarily physical.
    Now you're talking.
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    Quote Originally Posted by Farsight View Post
    I've never understood why an electron is supposed to be pointlike in classical theories.
    This is just an assumption that greatly simplifies a lot of the mathematics involved in field theories; it's not really meant to be a physical claim as such, it's more of an approximation method.

    If you had a wormhole you could time travel.
    That isn't necessarily true. Firstly, most types of wormholes aren't traversable. Secondly, a wormhole can connect distant spatial regions rather then the same region at different times. Thirdly, wormholes appear to always be behind event horizons, so there isn't any physical net effect anyway. The wormholes from Star Trek are a fantasy, that isn't how Einstein-Rosen bridges work.

    But regardless, in the real universe we won't find either Schwarzschild black holes nor Einstein-Rosen bridges, simply because all stellar objects have some amount of angular momentum. One would thus have to take a closer look at interior Kerr geometries ( not the exterior metric ), which are very complicated since they exhibit chaotic dynamics. I am not actually sure whether these can form stable wormholes, since there is no neat coordinate chart to represent such regions; the exterior Kerr metric is only good up to some way down from the horizon, but not in the vicinity of the ring singularity itself.
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    I have spoken to professional physicists who insist that the electron is "pointlike".

    Fair enough about the "isn't necessarily true" re wormholes and time travel. I was thinking of this sort of thing.

    Quote Originally Posted by Markus Hanke
    But regardless, in the real universe we won't find either Schwarzschild black holes nor Einstein-Rosen bridges, simply because all stellar objects have some amount of angular momentum.
    One for another day.

    Quote Originally Posted by Markus Hanke
    One would thus have to take a closer look at interior Kerr geometries ( not the exterior metric ), which are very complicated since they exhibit chaotic dynamics. I am not actually sure whether these can form stable wormholes, since there is no neat coordinate chart to represent such regions; the exterior Kerr metric is only good up to some way down from the horizon, but not in the vicinity of the ring singularity itself.
    As above. We're on the wrong thread for black holes. Let's come back to them at a later date.
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    Quote Originally Posted by Farsight View Post
    I have spoken to professional physicists who insist that the electron is "pointlike".
    It is point-like in the sense that no deep inelastic scattering experiment has ever been able to determine any internal structure for them. However, the energy available in those experiments is limited, and on theoretical grounds one could argue that a physically point-like entity must be a singularity. I find that unlikely in the case of electrons. Personally, I don't agree with the notion of elementary particles being physically point-like - I take it as a convenient approximation only to simplify our models. Ultimately we quite simply don't know what elementary particles are in a physical sense. It's a very interesting question though.
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    Quote Originally Posted by Markus Hanke
    It is point-like in the sense that no deep inelastic scattering experiment has ever been able to determine any internal structure for them. However, the energy available in those experiments is limited, and on theoretical grounds one could argue that a physically point-like entity must be a singularity. I find that unlikely in the case of electrons. Personally, I don't agree with the notion of elementary particles being physically point-like - I take it as a convenient approximation only to simplify our models.
    Nor do I. It's quantum field theory, not quantum point-particle theory. The electron "is" field. It has a standing-wave nature, you can diffract it. It isn't some billiard-ball thing that "has" a field. IMHO the scattering experiments are like hanging from a helicopter probing a whirlpool with a bargepole, and saying "Nope, I can't feel the billiard-ball, it must be really small".

    Quote Originally Posted by Markus Hanke
    Ultimately we quite simply don't know what elementary particles are"in a physical sense. It's a very interesting question though.
    Some of us have some ideas about the electron. But the standard model doesn't have much to say about it. I think this will change, but it will take time. I am reminded of geons, and how Wheeler picked the wrong confining force. Talking of Wheeler, he's referred to in A World without Time: The Forgotten Legacy of Godel and Einstein. He misunderstood the CTCs of GŲdel's universe, saying "one could live one's life over and over again". Remember how I mentioned Groundhog Day in the OP and said you wouldn't know it? Have a read of this from page 142:

    Quote Originally Posted by Palle Yourgrau
    "Wheeler, unfortunately has conflated a temporal circle with a cycle, precisely missing the force of GŲdel's conclusion that the possibility of closed, future-directed, timelike curves, ie time travel, proves that space-time is a space, not a time in the intuitive sense. Whereas a circle is a figure in space, a cycle is a journey undertaken along a circular path, one that can repeated, in Wheeler's words, "over and over again". Exactly how many times, one wants to ask Wheeler, is the journey supposed to be repeated? The question clearly cannot be answered, since the time traveller's journey is not over time, along the closed timelike curve: it is the curve itself. Just as one cannot ask of a circle how many times the points that constitute that figure have gone around, one cannot sensibly ask how often the time traveller in the GŲdel universe has made his or her trip."
    Wheeler got it wrong. And that isn't the only thing that's wrong. Because once you appreciate that there is no literal time flowing in a clock, you know that when a clock goes slower, it's because the thing inside it is going slower. It might be a pendulum, or a rocker and a spring, or a piezoelectric crystal. Or something else.

    Time for another thread.
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    Quote Originally Posted by Farsight View Post
    Wheeler got it wrong.
    He got the mathematics right - a CTC is just a world line that goes through an event, then continues on only to eventually return to the same event ( same in both location and time ). As such, a traveller would record a finite, well defined amount of proper time to travel between the same event over and over again, as in an infinite loop. This isn't time travel though, more like the exact opposite. Note also that there is no violation of causality.

    We are faced with the fact that CTCs are mathematically valid constructs, but I highly doubt that they physically exist.
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    You need to look at that "over and over again" some more Markus. Read the quote again. (Over and over again!) Remember you don't actually move along a worldline.

    I am confident that CTCs do not exist in any real or physical sense, because a world line maps out motion through space over time. There is no way you can move to undo all the motion that occurs around you. Even moving every atom back to where it was doesn't undo the motion that happened. And if CTCs do not exist in any real or physical sense, one has to query that "mathematically valid". The square root of 16 is 4. That's mathematically valid. And a carpet measuring 4m by 4m is physically valid. But there's another square root of 16, namely -4. That's mathematically valid. But a carpet measuring -4m by -4m isn't physically valid. It's a non-real solution. And IMHO when it comes to people and their understanding of relativity, there are other things like this.
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    Quote Originally Posted by Farsight View Post
    Remember you don't actually move along a worldline.
    I understand that, Farsight. And there is no need for me to read over it "again and again", because I can do the maths if needed. However, as things stand I personally consider CTCs mathematical artefacts only, so I am not too bothered by them at all. It's one of those things that I think will simply "go away" once quantum effects are included in the description of gravity.

    And if CTCs do not exist in any real or physical sense, one has to query that "mathematically valid".
    There isn't any problem with this, because the GR field equations only constrain the local geometry, but not the global topology. This is well known, and as such it is expected that not all mathematically valid solutions are necessarily physically real. To ensure that the space of all mathematically valid solutions coincides with all physically valid scenarios, you would need to have a "Theory of Everything" that accounts for all laws of physics in a common framework. We are not quite there yet.

    But a carpet measuring -4m by -4m isn't physically valid.
    Yes, we understand that, since f(x)=x^2 is obviously not a bijective function. To exclude such solutions, one needs to impose boundary conditions, such as x>0. The same holds true for GR - to specifically exclude things like CTCs, one would have to impose further conditions over and above the Einstein equations, to constrain the topology of your manifold. The issue is that such constrains would be entirely ad-hoc and arbitrary, unless they are motivated by physical principles. This is one of the reasons why it is important to investigate what happens in a consistent model of quantum gravity.
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    Quote Originally Posted by Markus Hanke
    understand that, Farsight. And there is no need for me to read over it "again and again", because I can do the maths if needed. However, as things stand I personally consider CTCs mathematical artefacts only, so I am not too bothered by them at all.
    Good.

    Quote Originally Posted by Markus Hanke
    It's one of those things that I think will simply "go away" once quantum effects are included in the description of gravity.
    I think they go away once you understand that time is derived from motion myself.

    Quote Originally Posted by Markus Hanke
    There isn't any problem with this, because the GR field equations only constrain the local geometry, but not the global topology. This is well known, and as such it is expected that not all mathematically valid solutions are necessarily physically real. To ensure that the space of all mathematically valid solutions coincides with all physically valid scenarios, you would need to have a "Theory of Everything" that accounts for all laws of physics in a common framework. We are not quite there yet.
    No. But I still maintain that you can spot where some mathematically valid solutions aren't physically real. I'm leading up to that for black holes. We've spoken about time and the the varying speed of light. With a bit more about gravity and why doesn't the light get out? I hope you'll really appreciate it in a visceral way.

    Quote Originally Posted by Markus Hanke
    Yes, we understand that, since f(x)=x^2 is obviously not a bijective function. To exclude such solutions, one needs to impose boundary conditions, such as x>0. The same holds true for GR - to specifically exclude things like CTCs, one would have to impose further conditions over and above the Einstein equations, to constrain the topology of your manifold. The issue is that such constrains would be entirely ad-hoc and arbitrary, unless they are motivated by physical principles.
    There is one. And it is a Lulu.

    Quote Originally Posted by Markus Hanke
    This is one of the reasons why it is important to investigate what happens in a consistent model of quantum gravity.
    I think an outline of that drops out neatly from all the stuff we've been talking about. Ah, so much to talk about.
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    Quote Originally Posted by Farsight View Post
    There is one. And it is a Lulu.
    What's that supposed to be ?
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    I'll tell you after I've said something about gravity. My aim is to present a chain of logic in simple steps. The idea is that all of the steps are totally reasonable and free from flaws, but you end up in a very different place to where you're at now.
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