Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How do we measure that two objects can have different time?

  1. Sep 20, 2016 #1
    How is this done in practice? I know that in GR the gravity is believed to curve not only space, but also time, and this prediction has been confirmed somehow in practice. How can we measure that? Because as far as i know, there is no device that actually measures time, but instead measures a mechanically created intervals (such as a pocket watch ticking) that we humans then use to measure the time we experience.

    Also what sounds very strange to my ear is, that if the time experienced by one object could differ from another object, then how would they still exist in same present moment? Shouldn't the slower running clock just dissappear from my present moment if it actually would be living in in slower running time for even a short period of time?
     
    Last edited: Sep 20, 2016
  2. jcsd
  3. Sep 20, 2016 #2

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    Something that cannot be measured is of no interest to physics. In physics time is what clocks measure.
     
  4. Sep 20, 2016 #3

    Dale

    Staff: Mentor

    The two basic ways are to either send a signal from one height to another and measure the frequency at both ends (eg Pound Rebka experiment) or to take identically constructed clocks and move one to a greater height and then bring them back to compare (eg Haelfe Keating experiment)
     
  5. Sep 20, 2016 #4
    How does the changed frequency of a signal refer to bending time instead of bending space for example? How can the both clocks still exist in same present moment if either of them has experienced slowed or accelerated time? Could the difference between the two clocks be explained by other physical phenomena? Cause as far as i know the basic idea of any clock is just to tic in an interval to enable us to measure our every-day time-experience, where the interval is created by either a computer or a clockwork which relies only on other physical phenomena such as speed/movement and chemical reactions, and therefore is not any way directly connected to the time as a dimension.
     
  6. Sep 20, 2016 #5

    russ_watters

    User Avatar

    Staff: Mentor

    None that have been identified and it is unlikely.
    Why not? All those processes take time, so clearly they can be used to measure time. More pointedly, do you have the same problem with length? Why not?
     
  7. Sep 21, 2016 #6

    Dale

    Staff: Mentor

    I can go through the math if you want, but I don't think you will find it particularly illuminating.

    The "same present moment" turns out to not be a measurable physical concept, but rather a matter of arbitrary convention. Whether or not they both exist in the same present moment can be defined differently by different people without changing any of the measurable physics.

    I understand what you are saying here, and if only one physical phenomenon slowed, or if different physical phenomena slowed by different amounts, then I would agree. But all physical phenomena slow by the exact same amount. What would be the difference between that and time slowing?
     
  8. Sep 21, 2016 #7
    "I can go through the math if you want, but I don't think you will find it particularly illuminating. " Nope, i don't know mathematics well so that wouldn't tell me much. I'm interrested just about the basic logic and how it's measured on all my questions i proposed here.

    "The "same present moment" turns out to not be a measurable physical concept, but rather a matter of arbitrary convention." I understand how this works when talking about two different people for example, but what i don't understand is how this could work if only one person takes the measurements. One person still experiences only one persent moment at a time, right?

    "But all physical phenomena slow by the exact same amount. What would be the difference between that and time slowing?" Don't know, but what comes to my mind is that all the other factors on a particular test, such as gravity and distance, should be the same too. Could this create what we see?
     
  9. Sep 21, 2016 #8

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    If some factor affects the rate of all processes in the same way, then we say that it affects time itself.
     
  10. Sep 21, 2016 #9

    Dale

    Staff: Mentor

    A person's experience is based entirely on the causal past, not the present. For example, if something is one light year away in my frame then my experience can only depend on its state one year ago, not its present state. There is no physical sense in which you experience the present.


    I am not aware of anyone who has produced such a theory.
     
  11. Sep 21, 2016 #10
    "There is no physical sense in which you experience the present." but thats only cause of my senses, which doesn't mean that i wouldn't exist in one certain present moment at the time.

    "I'm not aware of anyone..." could it still be possible, or is there some factor which suggests that this is not the case?
     
  12. Sep 21, 2016 #11
    How precise predictions the GR makes about the distortion seen in two clocks experiement, and how well does the result match to this prediction? Or the distortion seen in the frequency experiement?
     
  13. Sep 21, 2016 #12

    Dale

    Staff: Mentor

    No it is not only because of your senses. It is because of physical causality. All causal influences are limited to v≤c, whether it is one of your senses or not.

    The present moment, as you call it, is not something physical. It is merely an arbitrary convention. From what we can tell of the laws that govern the universe, the universe does not care at all about our conventions regarding the present. The universe cares about causality, not simultaneity.

    Don't read too much into my previous comment. To the best of my knowledge it is not possible. My comment only indicates that I am aware of the fact that I am not omniscient. Hopefully you don't expect people on an internet forum to answer your questions with omniscience.
     
  14. Sep 21, 2016 #13
    if you're not happy to be here, then don't be. i didn't ask you to appear in to this thread in first place.
     
  15. Sep 21, 2016 #14

    Dale

    Staff: Mentor

    As of 2010 (Chou, et al. Optical Clocks and Relativity. Science) we could detect gravitational time dilation factors of 4 10^-17. This was in agreement with the GR prediction for a difference in altitude of 33 cm. So the precision is astounding.
     
  16. Sep 21, 2016 #15

    russ_watters

    User Avatar

    Staff: Mentor

    The fact that different experiments measuring time with different types of processes all provide the same results strongly suggests it is not the case; they strongly suggest that it is indeed time that is the variable.
     
  17. Sep 21, 2016 #16

    Dale

    Staff: Mentor

    Huh? I certainly never said anything about my happiness. Please stop reading in to my comments more than I said.
     
  18. Sep 21, 2016 #17
    This was the kind of answer that i was searching.
     
  19. Sep 21, 2016 #18

    russ_watters

    User Avatar

    Staff: Mentor

    You'll have to excuse @Dale; getting questions like this one over and over despite the spectacular success and exquisitely thorough testing of Relativity over the course of a hundred years wears on us a bit. We've all let that frustration show through from time to time -- and his response was still helpful, even if you did detect a slight edge.
     
  20. Sep 21, 2016 #19

    Dale

    Staff: Mentor

    Hmm, I must be coming off cranky. I am a bit jet lagged, but didn't think it was obvious. I guess it is good that I am doing this instead of writing emails to customers or my boss.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: How do we measure that two objects can have different time?
  1. How we measure time (Replies: 11)

Loading...