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Whose frame is right ?

  1. Jun 24, 2005 #1
    whose frame is "right"?

    i have been reading The Elegant Universe and i have learned quite a bit.

    however, i dont understand how person A can say that they are still while B is the one who is moving and vice versa.

    I guess i understand how each person can say they are "still," but i dont understand what would happen to time for each person.

    for example: if A says B is moving, B's clock would be moving slower. But the same can apply B to A at the same time!
  2. jcsd
  3. Jun 24, 2005 #2


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    If they've never met, each can equally claim to be stationary. This works in all forms of relativity, not just Einstein's. Imagine playing table tennis on a train - as long as it isn't accelerating, it doesn't matter if its moving or stationary - you can still play the game just fine.

    Now, if the two people start off next to each other an one moves away, technically (sometimes you have to add some bizarre unseen forces) you can still call either one stationary, but it makes more sense (and the math is easier) if you call the accelerating one the one that is moving.
  4. Jun 24, 2005 #3
    You'll probably be surprised by this, but you are correct. A will make the claim that B's clock moves slower and B will make the claim that A's clock moves slower. And, oddly enough, both A and B are correct in these statements. Many people get confused by this same thing when first learning about relativity. The good news is that I can assure you as you continue to try to understand it, and read more and more explanations of it, you'll begin to be more comfortable with the idea, so don't give up. It'll click.

    The reason that both A and B can be correct is because of what's called the relativity of simultaneity. To sum it up, velocity not only affects the rate of the flow of time, but also the order of time. Some things that A claims happen simultaneously (the explosion of two stars, for instance), B will claim happen at different times (B will say that one explosion happens before the other). For a better explanation, google for "relativity of simultaneity". In particular, you may want to read Einstein's thought experiment involving a train and two lightning bolts (here's a link http://www.bartleby.com/173/9.html).

    For a better explanation of how this can make sense, read up on the "twins paradox". The twins paradox is the classic story of two twins, one who leaves Earth in a very fast space ship, and one who stays on Earth. Upon returning to Earth, the twin who left finds his brother has aged considerably more. There have been many detailed explanations of this on PF, so do a search (here's one to get you started https://www.physicsforums.com/showthread.php?t=77441&highlight=twins+paradox). Good luck.
    Last edited: Jun 24, 2005
  5. Jun 24, 2005 #4
    so lets say A is moving left at .25c and B is moving right at .75c, but neither know it.

    Both are obviously experiencing time dilation, and to an invisible stationary observer at the meeting point, B is aging slower than A.

    But lets say some physically freaky event occurs, such as both drop out of consciousness, and during that time, they assume the same constant speed going in the opposite direction (note: both think that they are still).

    1) what times will each person have when they make it back to the meeting point? i assume they will all have the same time, but why: A has obviously travelled slower

    2) if B were to call A with a light phone sometime on the divergence, will there be any sort of delay? i assume not, but why does the speed of light make up any sort of difference PERFECTLY?

    3) if B were to call A with a phone that travelled the same speed that B is going (.75c), what will happen with regards to everyone's time?
  6. Jun 24, 2005 #5
    I'm not sure I understand you. Are you saying initially A and B are moving away from each other, but both get knocked out and while they're unconscious both of their ships turn around and start heading toward each other? This seems to be what you're saying, so that's what I'm going to assume..

    WRONG! A has not "obviously travelled slower". This statement assumes a universal reference frame, but according to relativity, there is no such thing. It is true that relative to some observer where they meet, A could be traveling slower. It is also true that if A goes from traveling .25c away from this observer (we'll call him C), and turns around and starts moving at .25c toward C, while B goes from moving .75c away from C and turns around to move .75c toward C that B has accelerated more than A. Since B has accelerated more than A, then when they meet up at C, A will have aged more. According to the special theory of relativity, all three (A, B, and C) will be in agreeance that B is the one who has accelerated the most.

    Again, I'm not really sure what you mean, but yeah, there'd be a delay between when A said something and when B heard it, because the signal can only travel at the speed of light. This is why if you watch the news and they interview someone in a foreign country, there's a delay between what the interviewer asks and when the interviewee answers. The signal has to travel to and from a satellite before they can hear what each other have said. The signal travels very fast, but the delay is noticeable.

    Is this phone things something in The Elegant Universe? I've just now started reading the book myself, so I'm sorry I'm not familiar with whatever phones you're talking about. If B tried to pick up a telephone that was moving at a different speed than him, he'd have a hard time, so B would have to call A on a phone that was moving at his own speed, wouldn't he? I'm sorry, I just can't be of more help on this question. The important thing you need to note is there is no such thing as absolute speed. You can't make the claim that B is moving faster than A. That statement simply doesn't make sense. You can only claim that B is moving faster than A relative to some observer, like C.

    And everybody's frame is "right". There isn't such a thing as a "better" frame. The principle of relativity states that all frames are equivalent for the description of the laws of nature.
    Last edited: Jun 25, 2005
  7. Jun 25, 2005 #6


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    Jeremy, you might want to take a look at the diagrams I posted on this thread--I showed two rulers with clocks mounted along them moving alongside each other, and how each ruler sees the other ruler's distance-intervals shrunk and the other ruler's clocks slowed down, but because they define simultaneity differently they still manage to agree about all local events (like what a given clock on one ruler reads the moment it passes a given clock on the other ruler).
  8. Jun 25, 2005 #7
    that's not the problem of simultaneity. this problem can only be explained by general relativity law which we must include accereelation of speed into the consideration. if A is moving and B is standing still, it's certainly that A have to accerelate from 0m/s to a certain speed. tat's y we say acceleration is needed to produce force and do work. so, A, who has been accerelating, is doing work or transfering energy (e.g: kinectic energy & heat energy). that's why we can say A is moving, while B who never accelerate and tansfer any energy is standing still.
  9. Jun 25, 2005 #8

    Doc Al

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    OK. That means that there is a third frame, let's call it C, that A and B are moving with respect to. (By the way, both A and B will measure their relative speed as being about 0.84c.)

    Don't know what you mean by "experiencing time dilation", since everyone's clocks are working just fine. It is true that frame C will measure B's clocks to run more slowly than A's clocks.

    No, the clocks will not read the same times. Looking at things from the C frame is easiest, since that frame remained a single inertial frame throughout the exercise: According to frame C, when all three clocks reunite, B's clock will have recorded less time than A's clock. All three will agree on this.

    (Note: I assume that the situation is this: A and B separate with the given speeds with respect to C. At some instant--according to C--both A and B turn around and return to the starting point.)

    Of course there would be a delay. Light travels at a finite speed and requires time to travel a distance. From B's viewpoint, A is traveling away at 0.84c, so, with a bit of algebra, you can compute the time--according to B's clocks--it will take for a signal to reach A.

    I assume you mean a "phone" that used something that traveled at 0.75c with respect to B to convey the signal? The signal would never reach A, since A is traveling at 0.84c. (Until A turns around, of course.) I don't know what you mean by "what will happen with regards to everyone's time?". Why would anything happen to anyone's clocks?
  10. Jun 25, 2005 #9
    i guess i even misunderstood some of my questions...

    when i said "what will happen with regards to everyone's time," i meant that, if B asked A what time he had (on the phone), would the delay cause A and B's clocks to be the same or would they be different.

    thanks for all your input
  11. Jun 27, 2005 #10


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    When does B receive the signal--when he's still moving apart from A, or after they've both turned around and started heading towards each other? If it's when they're moving apart, and they started from the same position with both clocks synchronized, then B would see A's clock behind his, since in his frame A's clock is ticking more slowly than his and the signal took a while to get from A to him. If it's when they started heading back, then it really depends on the details of the situation, you need to put some numbers into your scenario to get the answer.
  12. Jun 27, 2005 #11
    The total measured distance between them would always be the same if the acceleration is uniform. Even if we knew B was accelerating there would be no way to convince him with out other things to compare A with respect to B. If B was accelerating and A was stationary they would have measured the same total distance.
    A sends a light pusle to B and B moves toward the light during the time it takes light to reach him. His reflection travels back to A at the same distance the light reflects off B. B sends a light pulse to A and B moves in the same direction as the light as it travels towards A. When the light reflects back towards B, B is still moving towards the light as it travels back. So the total measured distance is equal.
    A's measurements:

    B's measurements:

    They just know when they sent the light signal and when they received it back. So it doesn't matter who is accelerating if they do not feel the acceleration they can rightly say that they are stationary and the other is accelerating.
  13. Jun 28, 2005 #12
    When one observer is moving with respect to other , each of them would see that other's time is running slower.So whose frame should we believe in? . The simplest method to make out the 'true inertial frame' is that the 'person who feels the acceleration and surroundings' is to be believed.

    Taking the example of the twin-paradox , A stays on earth and B travels in spaceship and then comes back , A tells that B's clock is running slower , but same is true for B's view of A but B is the one who is feeling all the accelerations and will see the objects falling around when his spaceship takes a turn, his is the 'true inertial reference frame'
  14. Jun 28, 2005 #13


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    But you have no obligation to analyze things from A's inertial frame--you could equally well analyze the situation from the point of view of any one of an infinite number of other inertial frames which are in motion relative to A. So I think it would be better to just say that we have to analyze things from the perspective of some inertial frame (and because B accelerates, his frame is not inertial), not that any one inertial frame is more "true" than any other inertial frame.
  15. Jun 28, 2005 #14
    Agreed , ofcourse the physics laws would remain the same for both and other infinite observers, even though both everyonewould think that each other's time is running slower but they both would agree on almost every phenomena.

    I would like to quote the example of a cuboid box placed horizontally on ground such that front view tells us its length and side view, its width .We can see only one side at one time and cannot see one side and adjascent side simultaneously conserving the exact dimensions of the cuboid box, its just the matter of tilting your head in the right direction to see the other side.

    Same happens with space-time , both are inter-connected and are the same aspects of one single event (like the length and width are two different aspects of one single cuboid but different length's and width's would be recorded by different observers moving relative to each other), its just that one observer's brain would calculate the apparent dimensions of a stationary thing in less than a second but would never be able to visualise at the same instant what the same thing would look like when seen from stationary observer.
  16. Jun 28, 2005 #15
    the answer is their time would be different due to time dilation.
  17. Jul 1, 2005 #16
    That is a very good book. I suggest rereading it if you are not clear on a concept. Brian Greene does an excellent job of explaining, but that still doesn't mean everything will come to you on the first try.

    I also recommend reading "The Fabric of The Cosmos," which is also by Brian Greene.
  18. Jul 25, 2005 #17
    The principle of repropricity

    There is an answer to this question which involves repropicity. One observer must see his clock slow down and experience length contraction with respect to the other observer or reference frame. This is actually what happens in a gravitational field or the real universe, one frame is selected for validity over the other. The principal of repropicity is violated in a gravity field. General relativity or Riemann geometry or curvature actually answers this question in a very subtle way. One observer sees time dilation the other observer sees time contraction and length dilation! You must remember that special relativity is in some ways not applicable to the real universe that is why Einstein invented General relativity in the first place. General relativity applies to the real universe of curvature and gravity.
  19. Jul 25, 2005 #18
    Well paul - I don't think that is quite correct - SR applies to the real world just as does GR. What is at issue in SR is something that is explained in different ways by different authors - some demand that you can't get a real answer w/o using GR because after two clocks are brought in sync in the same frame - one of them must accelerate in order for their to be a relative velocity - but that doesn't mean you need GR - the acceleration is simply incidental to determining which one moved - and to this question, Einstein give the answer to how the clock rates would be different in his 1905 paper ...long before he invented (discovered) the principle of GR.
  20. Jul 26, 2005 #19


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    Uh, now I am really confused. How do I sort out acceleration's effect from velocity's effect on time?

    Suppose a girl named Astra is born on a spaceship in year 1906, shortly after Einstein publishes the theory of SR. A boy named Cosmo is born on the "same day" on another spaceship. At any point in time, the two ships are getting apart with constant "net" velocity v. Neither baby experiences any acceleration, ever.

    Which of the two is aging faster? Astra thinks she is at rest, but Cosmo is in motion so he must age slower. Cosmo thinks he is the one at rest, so Astra must age slower. Years later, when the two meet in cyberspace and exchange photos, who looks older, he or she?
  21. Jul 26, 2005 #20


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    There is insufficient information to solve the problem. But you do say that the spaceships are "moving apart" - and then you talk about "meeting" in cyberspace.

    The "meeting" picture does not jibe well with the physical reality of light-speed communications, where the propagation delays start out at some unspecified value, and increase as the couple moves apart. (If velocities are high, propagation delays can increase rapildy). It would be instructive, I think, if you would think about the problem of comparing ages over a communications channel where the propagation delays are large and increasing.

    My view of the "paradox" is very simple, BTW. Simultaneity is relative, so it is impossible for two observers to unambiguously compare clocks unless they are physically at the same location.
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