Cancer Posted January 4, 2010 Report Share Posted January 4, 2010 Re: Instantaneous Communications plus Time Dilation Equals ??? Is the train car catching up with the light' date=' or not?[/quote'] Absolute motion cannot be detected. That is the first fundamental assertion of relativity. The speed of light is a constant in all frames of reference. That is the second fundamental assumption. What you're doing, realizing it or not, is insisting upon defining one reference frame as being absolute, and putting all motions ... train, light, and observer ... into that. And that is a fundamental and incorrigible mistake. The very idea of "catching up with the light" makes no sense if you are in the frame of reference of the train car itself. In that frame, the car is not moving. It can't catch up to anything because in that frame the car is not moving. If you choose to observe this sequence of events from a location which does not move with the train car, then the idea of "catching up" does make some sense, since the car has a velocity along its long axis and your photon detectors are at the ends of the moving car. Then, in such a frame, you can work out the sequence of times at which the light strikes the ends of the car, and decide, on the precise basis of what you mean as "catching up with the light", whether that has happened. (I put it that way because I can think of more than one possible interpretation of that "catching up with the light" and I don't want to impose a possible misinterpretation on my part into this.) But those two different frames are equally valid for assigning a sequence of events. There is no reason, from the Universe's point of view, to prefer one of those over the other. There is no absolute frame. That means, there is a frame in which the very question "Does it catch up with the light, or not?" is nonsensical. And from the point of view of the Universe, there is no reason to prefer a frame in which the question does make sense to this one, where the question doesn't make sense. That is a powerful suggestion that the question doesn't really make sense at all, and you are looking at the situation from a fundamentally confused outlook. Einstein was very careful to define precisely what he meant by simultaneity in section 1 of his special relativity paper. My guess is that you will find this definition overly restrictive, but as he goes on to show, that is the only such definition that is physically viable. If you try to extend it at all, you get into trouble. Quote Link to comment Share on other sites More sharing options...
Zeropoint Posted January 5, 2010 Report Share Posted January 5, 2010 Re: Instantaneous Communications plus Time Dilation Equals ??? How about this: we can tell you what order the events at each end "really" happen in, as soon as you give us a way to tell how the train is "really" moving, or which observer is "really" stationary. Quote Link to comment Share on other sites More sharing options...
Nyrath Posted January 5, 2010 Report Share Posted January 5, 2010 Re: Instantaneous Communications plus Time Dilation Equals ??? I am convined that the universe is objective' date=' so everything, IMO, has to work from there.[/quote'] Well, if you are convinced of the truth of what Einstein's relativity says is false, there is no way you will ever be reconciled with relativity. Which more or less means you are wasting your time in this thread. Quote Link to comment Share on other sites More sharing options...
Kristopher Posted January 5, 2010 Report Share Posted January 5, 2010 Re: Instantaneous Communications plus Time Dilation Equals ??? Well' date=' if you are convinced of the truth of what Einstein's relativity says is false, there is no way you will ever be reconciled with relativity. Which more or less means you are wasting your time in this thread.[/quote'] I'm not convinced it's false. I'm trying to reconcile objective reality with the experimentally demonstrated effects of relativity. Quote Link to comment Share on other sites More sharing options...
sinanju Posted January 5, 2010 Report Share Posted January 5, 2010 Re: Instantaneous Communications plus Time Dilation Equals ??? I'm not convinced it's false. I'm trying to reconcile objective reality with the experimentally demonstrated effects of relativity. Reality is objective, and reality says that the only thing we can say for sure is that light moves at lightspeed. Everything else--the mass, dimensions, and even the rate of time passing for any given object depends on your frame of reference. There is no single answer that is "objectively" right. Everyone in a given frame of reference can agree on figures, yes, but that's only because they're all in the same frame of reference. Observers in a different frame may see something entirely different, and their observations are correct too. Quote Link to comment Share on other sites More sharing options...
Zeropoint Posted January 5, 2010 Report Share Posted January 5, 2010 Re: Instantaneous Communications plus Time Dilation Equals ??? I'm trying to reconcile objective reality with the experimentally demonstrated effects of relativity. And therein lies your problem: some things, like whether something is moving or stationary, or the simultaneity (or order of occurrence) of events with a spacelike separation, DO NOT HAVE an objective reality. Quote Link to comment Share on other sites More sharing options...
Kristopher Posted January 5, 2010 Report Share Posted January 5, 2010 Re: Instantaneous Communications plus Time Dilation Equals ??? **shrug** Guess I'll have to keep looking. Quote Link to comment Share on other sites More sharing options...
Zeropoint Posted January 5, 2010 Report Share Posted January 5, 2010 Re: Instantaneous Communications plus Time Dilation Equals ??? No need to go looking any further; you HAVE the answer. You have, however, rejected it for reasons which seem to derive from a desire to have some sort of single concrete answer to "what's REALLY happening", despite the fact that we've known there isn't one for over a hundred years now. Quote Link to comment Share on other sites More sharing options...
Kristopher Posted January 5, 2010 Report Share Posted January 5, 2010 Re: Instantaneous Communications plus Time Dilation Equals ??? No need to go looking any further; you HAVE the answer. You have' date=' however, rejected it for reasons which seem to derive from a desire to have some sort of single concrete answer to "what's REALLY happening", despite the fact that we've known there isn't one for over a hundred years now.[/quote'] At least I don't believe in telepathy. Quote Link to comment Share on other sites More sharing options...
Zeropoint Posted January 5, 2010 Report Share Posted January 5, 2010 Re: Instantaneous Communications plus Time Dilation Equals ??? You keep accusing me of that. Prove me wrong; all it takes is telling us what your objection ACTUALLY IS. How are we supposed to get anywhere if you won't tell us where you're seeing a problem? Clearly state your objection to the relativistic model, admit to rejecting it for emotional/aesthetic reasons, or admit to being a troll. This is getting ridiculous. Quote Link to comment Share on other sites More sharing options...
Kristopher Posted January 5, 2010 Report Share Posted January 5, 2010 Re: Instantaneous Communications plus Time Dilation Equals ??? You're always allowed to walk away, if you're of that opinion of me. No one is forcing you to continue if the conversation is "rediculous". So far, all I've been told is that there is no answer to the basic question raised by the "train car" thought exercise. Quote Link to comment Share on other sites More sharing options...
Zeropoint Posted January 5, 2010 Report Share Posted January 5, 2010 Re: Instantaneous Communications plus Time Dilation Equals ??? So far, all I've been told is that there is no answer to the basic question raised by the "train car" thought exercise. Really? Because from our perspective, it looks like you've been answered quite adequately, but you are rejecting that answer and won't tell us WHY. Quote Link to comment Share on other sites More sharing options...
Kristopher Posted January 5, 2010 Report Share Posted January 5, 2010 Re: Instantaneous Communications plus Time Dilation Equals ??? Really? Because from our perspective' date=' it looks like you've been answered quite adequately, but you are rejecting that answer and won't tell us WHY.[/quote'] Because of terminology, it's a hard question to ask, because terms like "relative" are already taken up. I've tried to dig down to a question that doesn't deal with "observers" or "frames of reference" or "simultaneity", to find an actual answer that can be built back up from. No matter how far I dig, people just keep saying "it depends". So far, there hasn't been an answer regarding the actual relationship between the motion of the photons and the motion of the car -- the train car cannot be moving at both "c - c" and at "c - (c-x)". Quote Link to comment Share on other sites More sharing options...
Cancer Posted January 5, 2010 Report Share Posted January 5, 2010 Re: Instantaneous Communications plus Time Dilation Equals ??? So far' date=' there hasn't been an answer regarding the actual relationship between the motion of the photons and the motion of the car -- the train car [i']cannot[/i] be moving at both "c - c" and at "c - (c-x)". In all meaningful frames of reference (and "meaningful" is one that is not moving at c, that is, riding with a photon), the photons are always moving at c. Clocks slow down and measuring rods change length as you change reference frames in just such a way as to ensure that the velocity of a photon is c no matter what reference frame you're measuring in. Quote Link to comment Share on other sites More sharing options...
Kristopher Posted January 5, 2010 Report Share Posted January 5, 2010 Re: Instantaneous Communications plus Time Dilation Equals ??? In all meaningful frames of reference (and "meaningful" is one that is not moving at c' date=' that is, riding with a photon), the photons are [u']always[/u] moving at c. Clocks slow down and measuring rods change length as you change reference frames in just such a way as to ensure that the velocity of a photon is c no matter what reference frame you're measuring in. So everything is moving at "c - c"? EDIT: it was supposed to be a question, and not just based on Cancer's statement, but on the context of the discussion, and the question I was asking. Quote Link to comment Share on other sites More sharing options...
Cancer Posted January 5, 2010 Report Share Posted January 5, 2010 Re: Instantaneous Communications plus Time Dilation Equals ??? So everything is moving at "c - c"? No ... no matter how you are moving, you always see photons as moving at c ... AND so does everyone else, now matter how you observe them to be moving. Quote Link to comment Share on other sites More sharing options...
sinanju Posted January 5, 2010 Report Share Posted January 5, 2010 Re: Instantaneous Communications plus Time Dilation Equals ??? Because of terminology, it's a hard question to ask, because terms like "relative" are already taken up. I've tried to dig down to a question that doesn't deal with "observers" or "frames of reference" or "simultaneity", to find an actual answer that can be built back up from. Exactly. There ISN'T an answer that doesn't deal with observers and frames of reference. You're asking a question that has no answer, because it's based on incorrect assumptions. No matter how far I dig, people just keep saying "it depends". So far, there hasn't been an answer regarding the actual relationship between the motion of the photons and the motion of the car -- the train car cannot be moving at both "c - c" and at "c - (c-x)". It can and does. Whether the train car is moving a .9 C and an observer is stationary, or the train car is stationary and the observer is moving at .9 C, the results are the same. That's the whole point of relativity. The only absolute is the speed of light in ANY frame of reference. Time and space are relative; duration and physical dimensions can and will be perceived differently depending on your frame of reference. There is no absolute answer to "how fast is the train car going"; there is only an answer in a particular frame of reference. Quote Link to comment Share on other sites More sharing options...
Kristopher Posted January 5, 2010 Report Share Posted January 5, 2010 Re: Instantaneous Communications plus Time Dilation Equals ??? Not sure where to go with that, then. I never really had any doubts about the subject until this thread. I'm just going to drop it, there's no progress to be had. Quote Link to comment Share on other sites More sharing options...
austenandrews Posted January 6, 2010 Report Share Posted January 6, 2010 Re: Instantaneous Communications plus Time Dilation Equals ??? Here's a question for the group: Make a wormhole. Put one mouth at one end of the train car and the other mouth at the other end. Run the experiment. Where do the two beams of light meet? Quote Link to comment Share on other sites More sharing options...
ajackson Posted January 6, 2010 Report Share Posted January 6, 2010 Re: Instantaneous Communications plus Time Dilation Equals ??? Here's a question for the group: Make a wormhole. Put one mouth at one end of the train car and the other mouth at the other end. Run the experiment. Where do the two beams of light meet? Well, if the wormhole isn't involved in the experiment, it has no effect. If we assume the wormhole is involved (i.e. we have a single generator and send one beam through the wormhole), travel through the wormhole is instant with respect to the reference frame of the wormhole. Since the wormholes are in the train and moving at the same speed as the train, that is the same as the reference frame of the train, and the two beams of light meet in the middle of the car (all observers will agree on where they meet, though many observers will claim that transit through the wormhole took non-zero and possibly negative time). Quote Link to comment Share on other sites More sharing options...
austenandrews Posted January 6, 2010 Report Share Posted January 6, 2010 Re: Instantaneous Communications plus Time Dilation Equals ??? From the frame of reference of the train car, the two beams meet at the wormhole itself, right? (Assuming the interior of the wormhole is a trivial length.) Because in that frame of reference, the two beams strike the ends of the car simultaneously, and the wormhole mouths are at the ends. From the platform, though, the rear-facing beam hits its wormhole mouth before the front-facing beam. Do the beams therefore collide somewhere in the front half of the car? Quote Link to comment Share on other sites More sharing options...
Nyrath Posted January 6, 2010 Report Share Posted January 6, 2010 Re: Instantaneous Communications plus Time Dilation Equals ??? http://www.xkcd.com/675/ Quote Link to comment Share on other sites More sharing options...
austenandrews Posted January 6, 2010 Report Share Posted January 6, 2010 Re: Instantaneous Communications plus Time Dilation Equals ??? Cute, but it's an honest question. Quote Link to comment Share on other sites More sharing options...
ajackson Posted January 6, 2010 Report Share Posted January 6, 2010 Re: Instantaneous Communications plus Time Dilation Equals ??? From the frame of reference of the train car, the two beams meet at the wormhole itself, right? (Assuming the interior of the wormhole is a trivial length.) Because in that frame of reference, the two beams strike the ends of the car simultaneously, and the wormhole mouths are at the ends. From the platform, though, the rear-facing beam hits its wormhole mouth before the front-facing beam. Do the beams therefore collide somewhere in the front half of the car? I am quite uncertain how your experiment is supposed to be structured, but all observers will agree where the beams collide, they will just disagree on the length of time it takes to pass through the wormhole. Quote Link to comment Share on other sites More sharing options...
austenandrews Posted January 6, 2010 Report Share Posted January 6, 2010 Re: Instantaneous Communications plus Time Dilation Equals ??? I am quite uncertain how your experiment is supposed to be structured' date=' but all observers will agree where the beams collide, they will just disagree on the length of time it takes to pass through the wormhole.[/quote'] I believe that may be the right answer, I'm just unclear why it would be. What's unique about the wormhole? Quote Link to comment Share on other sites More sharing options...
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