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Twin #1 is calling twin #2 in mid flight on instant-stellar phone

  1. Mar 29, 2007 #1
    I about halfway to understanding the twin paradox. I just don't get why time can speed up and slow down. So can someone tell me what would happen if twin #1 could call twin#2 on a special phone that would allow them to talk while twin #2 is speeding away from twin #1 at light speed.

    Why why why! Does time have to be experienced differently depending on how fast you are going?

  2. jcsd
  3. Mar 29, 2007 #2


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    it's the only way for both observers to experience the speed of propagation (E&M, light, gravity, any other "instantaneous" action) to be the same, even when they are moving relative to each other.

    it's how one observer views the other observer's clock. both observers think that time, as they experience it, is flowing normally. but when they see the other observer's clock, it appears to them to be ticking more slowly than their own clock.
    Last edited: Mar 30, 2007
  4. Mar 29, 2007 #3
    But wouldn't the twin in the spaceship see the eart twin's clock moving very quickly and the earth twin would see the spaceship twin's clock move very slowly?

    Is it worth understanding this concept? I feel lightyears away from getting the damn thing.
  5. Mar 29, 2007 #4


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    Give my animations at www.phy.syr.edu/courses/modules/LIGHTCONE/LightClock/
    a try. They were designed to address that very question... with a minimal amount of mathematics.
  6. Mar 29, 2007 #5


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    Physics doesn't really deal with "why" questions, it just tells you what happens.
    The twins don't actually move apart at light speed, just at some significant fraction of it, like 0.8c. And what do you mean by "special phone"? Do the signals have to travel through the phone at the speed of light, or are you imagining some sort of magic phone that allows the twins to communicate instantaneously? If the signals move at the speed of light, then see this section of the twin paradox FAQ for a discussion of what each twin sees if they are watching the other one using light-signals. And if you're thinking of instantaneous communication, it's important to understand that different frames in relativity define "simultaneity" differently, meaning that two events that happen at the "same time" in one frame happen at different times in another. So in my rest frame I might be turning 45 "at the same time" that you're turning 40, while in your rest frame you're turning 40 "at the same time" that I'm turning 36. So if you call me up on your 40th birthday on the instantaneous communicator, how old will I be? It depends on which frame the communicator is transmitting signals "instantaneously" in.
  7. Mar 29, 2007 #6
    Ok I'll give them a try. And what happens if the spaceship just goes round and round earth at the speed of light? When he lands will time have gone quicker on earth than on his spaceship? I really don't get that. Does that mean that the photons that photons around us could be hundreds of years old in a matter of years?
  8. Mar 29, 2007 #7
    Well in this case the question is simply wrong.
    Time neither speeds up nor slows down.
  9. Mar 29, 2007 #8
    I'm sorry "why does time appear to speed up and slow down?" is what I meant to say. It seems to do so when the twins compare clocks after the interstellar trip.
  10. Mar 29, 2007 #9
    It does not appear to speed up or slow down.

    Let me ask you this: if we both go by car from A to B, each of us taking a different road, would you conclude that if our odometers show a different distance that space must have contracted or expanded? No, right?

    Think about that, but then for time. :smile:
    Last edited: Mar 29, 2007
  11. Mar 29, 2007 #10
    Of course I can't disagree with your car example. What this tells me is that you are understanding something that I am not. What I understand by your example is that our cars have odometers and chronometers except that whereas the odometers mark the miles no matter how fast you go or which route you take, the chronometers don't always measure in the same way. Why is that?
  12. Mar 29, 2007 #11


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    Just as different paths through space have different lengths (as measured by an odometer, or by a tape measure placed along the paths, whatever), different paths through spacetime have different "proper times", ie the time as measured by a clock that travels along that path through spacetime. And just as a straight-line path between two points in space will always be shorter than any bent path between the two points, so a "straight" path between points in spacetime will always have a greater proper time than any "bent" path in SR--so the twin that maintains a constant velocity between the events of the two twins departing and the two twins reuining will always have aged more than the twin that changes velocity (moving away from the first twin and then turning around and returning).
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