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Question about time

  1. Mar 8, 2013 #1

    I think this question goes here...
    My question is about time. I have seen some documentaries and read some articles in the past but I still don't get it.
    Which is the time that is being measured when articles speak about space-time? Which devices are used to measure time?
    As almost everyone does, I carry a watch on my wrist, which is a mechanical device. If we could stop time, my intuition says that my watch will continue to work because we humans give the watch the meaning of being a instrument of measuring time, right?
    I don't understand the theoretical concept and the relationship between the time of space-time and the time we measure ourselves here on earth :(
    Does someone have a good link for dummies on this subject? I will be thankful! :)

    Thank you!
  2. jcsd
  3. Mar 8, 2013 #2


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    Yes, that is right. However, if time were to slow down for you and your watch, how would you know? Actually, you'd have to be traveling very fast and make a round trip before your wristwatch would show a different time than another one remaining stationary. But if you had a pair of very accurate and stable atomic clocks, you could tell the difference in how they keep track of time by moving one of them some distance away and bringing it back to the first one.

    Or you could see the difference in time simply by putting one at a different altitude here on earth. Are you aware that the atomic clocks at Greenwich England near sea level run slower than identical atomic clocks at Boulder Colorado at a higher altitude?
    I think this forum is a good place to learn. Just scroll down on the relativity page and find some threads that look interesting to you or click on the links at the bottom of this page.
  4. Mar 8, 2013 #3
    George, thanks for the explanation.
    Yes, if time slows down for me and my watch I would never notice, you are right. But, what seems odd to me is that when I travel very fast, my wristwatch also slows down... how does time know that it has to slow down my wristwatch? Or is it slowing all devices down, independently of what they are? It actually slows all things down organic and not?
    If this is true, thinking at it, it seems to me that the space where all things are in gets more and more denser or "viscose"

    I didn't knew about the atomic clocks of Colorado and Greenwich, but I saw something "similar" in a documentary with earth and satellite atomic clocks, that they differ, what I guess might be the same phenomenon?

    Thanks again!
  5. Mar 8, 2013 #4


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    I don't think you are still getting it. When you travel with your wristwatch, you DO NOT SEE your wristwatch slowing down. So you are contradicting your first statement there.

    What is slowing down, according to an observer in another frame, is NOT just your wristwatch. It is for your whole "frame". Your biological clock, the oscillation of atoms and molecules, etc. The time "coordinate frame" in your reference frame appears to be slower to that external observer. It would be exceedingly strange if this slowdown only affects certain devices and not others - what makes those devices so special anyway?

  6. Mar 8, 2013 #5


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    If one thing slows down, everything traveling with it slows down identically. It's a result of the Principle of Relativity.
    This has nothing to do with space getting more denser or viscose. The relationship is symmetrical. If you and I are traveling in space at a constant speed, we each will determine that the other ones clocks and time are effected identically compared to our own. That's the Principle of Relativity. If it were a result of something happening to space itself, then it wouldn't be symmetrical, would it?

    Or to put it another way, if you and I took off in spaceships at the same speed going in opposite directions, we would each determine that the other ones clock is affected identically to our own. Neither of us could blame space because we are both traveling through the same kind of space, just in opposite directions.
    There are two different phenomena that can cause the slowing of clocks, one is a difference in gravity and the other is relative speed. For the stationary clocks at different altitudes, it's just gravity. For satellite clocks it's both gravity and relative speed.
  7. Mar 8, 2013 #6
    Relativity is always about the other guy's clocks and rods, never your own. The observer in the other frame has his own ideas about what events are simultaneous, and it is these, together with his observations, which lead him to conclude that your watch must be running more slowly. And with your own notions (different from his) of what events are simultaneous, you'll conclude that it is his watch which is slowed.
  8. Mar 8, 2013 #7
    Thanks all for your answers.
    Now I'm starting to understand a bit more, I think. I didn't know that the complete frame slows down for an external observer. The information I had access to always talked about clocks and it left me asking always the same questions which were contradicting in my head.
    I will have a look at the links.
    Good to have this forum :)
    Have nice day.
  9. Mar 9, 2013 #8
    Hi Robertito
    I am also trying to get similar thing. I would request you to kindly visit my thread, 'can time really be slowed' in this very segment of relativity.
  10. Mar 9, 2013 #9
    I've noted your question, and sketched out a reply to the original thrust of it: is there an alternative 'process-hypothesis'? I think the big kids ran away with it.
  11. Mar 9, 2013 #10
    The time that is not in any way 'ambiguous' is the local time you carry with you...it's called 'proper time'..... like your wristwatch. Such a time proceeds at a steady rate regardless of your motion...but distant observers may see it vary according to their conditions.

    In one sense they are the same....'proper time'....if you are referring to local time. The sense in which they are in general different is...'time dilation' ..... two identical wristwatches, one moving fast and one stationary, when brought together and compared will in general show different elapsed times.
    You can read about those terms in Wikipedia and many other sources....

    Einstein figured out that the 'tick tock' of different but identical [perfect] time pieces will record different elapsed times even though each locally appears to proceed at at steady 'tick tock'. He figured out that two things can affect such a comparative passage of time...relative velocity and gravitational potential.

    So identical timepieces initially on, say earth, where they are perfectly synchronized and the second transported to the moon where it sits for a while and then returns to earth will record different elapsed times.
    Another example are GPS satellite clocks which tick in space at a different rate than terrestrial earth bound clocks of the system...and so the system must make frequent corrections to keep them synchronized.

    is a good basic to keep in mind....I always remember that MY CLOCK [locally, that I carry with me] always ticks at a steady rate.
    Last edited: Mar 9, 2013
  12. Mar 12, 2013 #11
    Dear Everyone!
    Thanks for all the answers. Now I think I get what's going on, even if I still have lots of additional questions that I will be posting over time.
    This subject is new for me and completely counter intuitive... On Saturday I saw a documentary on internet about the illusion of time that clarified some other aspects, but also added more mess to my mess :)
    Have a nice day.
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