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Simple questions on relativity

  1. Oct 22, 2005 #1
    Hi I'm new to the forum, I am 15 years old and currently taking my GCSEs. I have asked my physics teacher several questions which he has not answered so I was wondering if you would be willing to take the time to give me some answers.

    Firstly, lets use the twins A and B and suppose twin A travelled in a spaceship that was going at a speed whereby 1 year passed on that spaceship would mean 5 years passed on Earth.Twin B stays on Earth. Lets suppose that they had TVs on which they could watch the other twin. If twin A watched a movie which lasted 2 hours would it appear to be a 10 hour film if Twin B was watching Twin A watch it? If twin B watched a movie lasting 2 hours would it seem only 24 minutes in length to twin A, assuming he was watching Twin B watch it?

    Does going at the speed of light actually increase the length of life that a human could experience? Assuming the previous conditions, although it would appear to twin B that twin A would have lived for 50 years but only aged 10, would twin A only have experienced 10 years? Does this mean that if everyone started being accelerated at near the speed of light, nobody would really recognise it because we would have nothing else to compare it to?
    What would be the uses of being able to move at near the speed of light? The only one that I could think of would be someone who was curious about the future of the human race being able to slow down their time compared to the Earths to be able to see further into the future.

    If we were to move at the speed of light(I recognise it's impossible but for the sake of this question assume it's possible) would we have to eat, drink, sleep etc? I'm wondering about this because I'm assuming our biological clocks would stop completely.

    I apologise if these seem very simple questions but I would greatly appreciate
    any answers to these questions that have been puzzling me for some time.
  2. jcsd
  3. Oct 22, 2005 #2
    With the "twins," you're forgetting the time it would take for a TV signal to arrive from one twin to the other. Until one of them goes somewhere far, and gets back to the other one, there will be no difference.

    If you were moving rapidly, it would appear to you that I move slowly as you would appear to me, but in any case everything slows down (including neural activity) therefore you will not actually experience a longer time.

    While it's impossible to move at the speed of light, it's not impossible to stop time - on the event horizon of a black hole the time and space (in fact the radial component) co-ordinates of the metric swap, so time does literally stop, and if you could observe that you would literally see the end of time as all time would pass by in an instant outside the black hole.
  4. Oct 22, 2005 #3


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    Staff: Mentor

    In relativity, you have to distinguish between what the twins see which is affected by the time it takes light or radio/TV signals to travel between them, and what they observe, i.e. what they infer is actually happening to the other twin after correcting for the signal-propagation time.
    For analyzing your situation, you need to use not just the relativistic time-dilation equation, but instead the relativistic Doppler-effect equation, which includes both time-dilation and signal-propagation effects. See posting #3 in this linked thread which calculates the results for a very similar situation.
    Right. As far as twin A is concerned, he and everything else inside his spaceship behave completely normally. If his ship is traveling at constant velocity, he perceives absolutely no effects from his motion, so long as he doesn't look out the window or receive signals from home.
  5. Oct 22, 2005 #4


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    Science Advisor

    But it's impossible for anyone moving slower than light to stay still at the event horizon of a black hole, and if you just fall into a black hole, you don't see an infinite amount of time pass by in the outside universe as you cross the horizon.
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