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Special relativity time issues

  1. Dec 3, 2015 #1
    We know that clocks slow down if moving relative to something that's still and clocks speed up if still relative to something in motion. So, what if I was moving relAtive to something thats still and still, at the same time, relative to something that's in motion? How can time go quicker and slower simultaneously for me?
  2. jcsd
  3. Dec 3, 2015 #2
    No matter what velocity you are travelling at, time does not change for you.
    Your watch will still tick at one second per second and you won't get quickly older or start ageing less.
    Time dilation is an effect which observed by somebody watching you as you travel at a relativistic speed/
    THEY see you ageing more slowly then them, but to YOU, nothing appears to change.
  4. Dec 3, 2015 #3
    Sorry let me rephrase. There's somebody moving relative to me, so they see me aging faster, and there's someone still relative to me, so they see me aging slower. How can I be growing older faster and slower simultaneously?
  5. Dec 3, 2015 #4
    No. All that matters is relative motion. If you and your friend each have a clock in your hands, and you move relative to each other, you will each observe the other's clock running slow.

    You could say you're at rest and your friend is in motion. Or your friend is at rest and you're in motion. The two situations are equivalent.

    There's no way to distinguish between a state of rest and a state of uniform motion. That was Galileo's Principle of Relativity, Newton's First Law, and Einstein's First Postulate. It has survived despite all attempts to disprove it.
  6. Dec 3, 2015 #5
    Well I guess the only alternative to some kind of relativity principle has to be a proposal that there is some kind of absolute always present frame of reference.
    The problem with that though is that there is no evidence at all of there being any such unique frame.
    Last edited: Dec 3, 2015
  7. Dec 3, 2015 #6


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    You are basically asking how it is can be possible that clock A is running slow compared with clock B, but clock B is also running slow compared with clock A. To answer this question we have to be clear about exactly what we're saying when we say that one clock is running slower than another, and we have to understand the role of relativity of simultaneity. (If you are not familiar with relativity of simultaneity, stop right now and google for "Einstein train simultaneity" - it is impossible to make sense of special relativity otherwise).

    Suppose that we start with both clocks reading 12:00 noon at the same time. An observer at rest relative to clock A waits an hour, then checks the clocks again. Clock A reads 1:00 PM, of course, but suppose that at the same time that clock A reads 1:00 PM clock B reads 12:30 PM. We conclude that clock A is running faster than clock B. That's all there is to saying that one clock is running faster or slower than another - you see what they read at the same time, then repeat a little while later and compare the time between the two readings on the two clocks.

    But now we have to remember the relativity of simultaneity. Clock B read 12:30 PM at the same time that clock A read 1:00 PM according to the observer at rest relative to clock A; according to this observer the events "clock A read 1:00 PM" and "clock B read 12:30 PM" were simultaneous. However, because of the relativity of simultaneity, an observer at rest relative to clock B does not find that those two events were simultaneous. For him the event that happened at the same time as "clock B read 12:30 PM" is "clock A read 12:15" so he concludes that clock A is running slow.

    So what's going on here is that relativity of simultaneity allows the two observers to disagree about what clock A reads at the same time that clock B reads 12:30 PM, and that allows them both to correctly conclude that the other clock is the slower one.
  8. Dec 3, 2015 #7


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    It's not the whole story by any means, but let me rephrase your question:

    There's somebody bigger than me, so they see me as smaller than them, and there's someone smaller than me, so they see me bigger than them. How can I be smaller and bigger simultaneously?
  9. Dec 3, 2015 #8
    Right. There's a lack of evidence to support the notion of a preferred rest frame. But more importantly, there's tons of evidence to support the lack of a preferred rest frame.
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