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Time dilation in relativity theory

  1. Jun 9, 2015 #1
    Is time dilation an actual phenomenon or it is just an apparent change relative to some other frame of reference? If it is so then why the age of a person slows down and elongates actually?
     
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  3. Jun 9, 2015 #2

    bcrowell

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    Welcome to Physics Forums!

    "Actual" and "apparent," like "real," are words that have no generally agreed-upon meaning in physics.

    It sounds like you're referring to the twin paradox, which is not really a paradox. The twin paradox is an observable effect, and it has been confirmed in experiments such as the Hafele-Keating experiment: http://en.wikipedia.org/wiki/Hafele–Keating_experiment .
     
  4. Jun 9, 2015 #3

    phinds

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    I agree w/ Ben that you have to be careful using such terms because they SEEM to have precise definitions in English, but do not in physics, due to some subtleties that you are not yet aware of.

    As I believe you are using these terms, time dilation due to motion is "apparent" not real, but aging is a different process and is "real".

    That is, time dilation due to motion is something never experience by YOU but only by someone watching you from a frame in which you are moving. Difference in aging is due to taking different paths through space-time.

    You, right now as you read this, are MASSIVELY time dilated according to an "accelerated" particle at CERN.
     
  5. Jun 9, 2015 #4

    bcrowell

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    One way of thinking about this kind of thing that helps me a lot is the following analogy with surveying. When we talk about time dilation and length contraction, we're talking about the transformation from one set of coordinates to another. Coordinates are optional -- you can do physics without coordinates -- and coordinates are just arbitrary names for events. Although coordinates are arbitrary, the coordinates we have in mind here are a special set of coordinates called Minkowski coordinates. Minkowski coordinates are the result of a complicated surveying process, like surveying land. For example, you can construct Minkowski coordinates throughout a certain region of spacetime using clocks, radar, and radar reflectors.

    When we say that time is dilated, what we're doing is comparing the results of the surveying process carried out by observer A with the results of the surveying process carried out by observer B. A and her surveying equipment are in a different state of motion than B and his.
     
  6. Jun 9, 2015 #5

    phinds

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    And to stretch Ben's analogy (perhaps past the breaking point), differences in aging are a bit like the fact that the angle between two points will be different depending on where you are standing when you do the surveying. Different paths through space-time result in differing amounts of aging.
     
  7. Jun 10, 2015 #6
    The first direct measurement of time dilation was done as described here: https://en.wikipedia.org/wiki/Ives–Stilwell_experiment

    As you may get from the description, it is an actual phenomenon that has been measured. Classical physics postulates no time dilation and it leads to a different prediction of what will be observed.
     
  8. Jun 10, 2015 #7

    bcrowell

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    Ives-Stilwell experiments are some of the most high-precision tests of SR, but they are not direct tests of time dilation. I think it's a bit of a stretch even to say that they are indirect tests of time dilation. What you observe is the forward and backward longitudinal Doppler-shift factors. The discrepancy between these numbers and the values predicted by some nonrelativistic expression can be interpreted as a time-dilation effect, but that's very indirect, and it depends on what nonrelativistic expression you think is appropriate.

    If you want a more direct test from the same era as the original Ives-Stilwell, a better example would be cosmic ray muons, Rossi and Hall, 1941.
     
  9. Jun 11, 2015 #8
    Everything is a matter of interpretation. "Transverse Doppler" was the first positive test of time dilation (I wrote "direct" but I meant it in the sense of "positive", different from MMX), Also IMHO the test with cosmic ray muons was more direct and therefore more impressive.
     
    Last edited: Jun 11, 2015
  10. Jun 11, 2015 #9

    bcrowell

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    As far as I know, the first tests of the transverse Doppler effect were not until the 1960s: H. J. Hay et al, Phys. Rev. Lett. 4, 165 (1960); W. Kündig, Phys. Rev. 129, 2371 (1963).

    You will find people referring to Ives-Stilwell experiments as tests of the transverse Doppler effect, but that's misleading. In Ives-Stilwell experiments the Doppler shifts being measured are longitudinal. When they say that these experiments test the transverse Doppler effect, what they mean is that if you factor the effect somehow into a nonrelativistic factor and a relativistic factor, the relativistic factor can be interpreted as a time dilation factor. Since the transverse Doppler effect is purely a time-dilation effect, people will sometimes refer to this as a test of the transverse Doppler effect.
     
    Last edited: Jun 11, 2015
  11. Jun 11, 2015 #10
    Oops sorry that's very right - my memory failed me although in fact I knew it! :sorry:
    Ah yes, evidently my memory was "polluted" by such comments! Thanks for making me understand how this happened. :smile:
     
  12. Jun 11, 2015 #11

    bcrowell

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    I should have said that Hay and Kündig were just the earliest ones of which I was aware, or the earliest high-precision ones. It seems like there must have been observations from before 1960 that gave at least some low-precision test of the transverse Doppler shift.
     
  13. Jun 11, 2015 #12
    A introductory conceptual example I keep in mind is that of two observers looking at the same building: To the observer right next to the building it seems huge; to the distant observer, just a spec. Who is right? They both are. Different observers don't always measure things the same.

    There will always be two clocks involved in any time dilation scenario, whether gravitational or speed-based. The statement that 'time is dilating' always comes down to a statement about how one clock or process is proceeding more slowly than the other. Each ticks locally at the same fixed rate, but does not appear to do so from a distance.

    Relativity shows time is not constant. It varies between observers due to relative speed and/or differences in gravitational potential. As pHinds posted: Difference in aging is due to taking different paths through space-time. This means special relativity weakens the notion of absolute time that may appear to the casual observer as fixed; general relativity weakens the notion of absolute time even further.
     
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