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Looking for 'absolute still' with time dilation

  1. Feb 1, 2015 #1
    I looked through this thread here regarding how fast we're moving relative to the CMBR, but I wonder if it would be hypothetically possible to get an even better measurement while also looking for a decent value for 'absolute still' using the apparent asymmetry of time dilation/constriction.

    A hypothetical 'still' entity would appear to be moving rapidly to us, but if we were to attach a clock to it a comparison would reveal that we're moving and it is being more 'still' since its time would appear constricted by comparison yes?

    Say that we're traveling at an average direction x at our 'home' velocity (all velocities averaged together for the Earth). An object 'A' traveling in an opposite -x direction would be effectively 'slowing up', just like if you're on a freeway going 60 next to a car matching your speed and they put on their brakes they will appear to move 'backwards'.

    Then another object 'B' could move in different directions and speeds around object 'A' comparing clocks to look for an even more 'still' reference frame.

    Is this possible or do I have a fundamental misunderstanding of time dilation and relativity?
  2. jcsd
  3. Feb 1, 2015 #2

    Vanadium 50

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    You have a fundamental misunderstanding of time dilation and relativity. There is no "absolute still" (or, as it is usually called, an "absolute rest frame"). The frame where the CMBR is isotropic is no more and no less special than any other frame.
  4. Feb 1, 2015 #3


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    Going into more detail, all the observations of time dilation and length contraction are symmetrical: If Bob sees Alice moving w/r to Bob's rest frame, then he'll observe Alice's clocks as slowing down. At the same time, Alice sees Bob moving in her rest frame and, according to her, it's Bob's clocks that are slowing down.

    In your example, the first mistake was to assume that Earth has got some velocity x without specifying with respect to what reference frame. Next, you switched to Earth's rest frame and assumed it was special - that objects A and B moving with respect to Earth are time dilated and Earth as seen from A or B isn't.

    Furthermore, there is no such thing as time contraction in special relativity. Time of all moving objects passes more slowly (is dilated) than time in the observer's rest frame.

    There is a time acceleration effect in general relativity (due to gravity), but that's got nothing to do with motion.
  5. Feb 1, 2015 #4
    Dang, so there is no asymmetry here with the velocity in the way that I was thinking, and we can't tell 'who is moving fast' and 'who is moving slow' since there is no absolute rest frame.

    We could via dilation/contraction be able to tell who is in a substantial gravity well, but that's not particularly useful as this would usually be really obvious.

    What's worse is I found some threads that explained this after I posted, even though I had searched the forum. Dreadful.

    Thanks for your time guys.
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