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I have some understanding that a relationship exists between time and velocity. As velocity increases, time passes slower for the person in motion at any significant velocity more than those at rest or moving slowly in relationship to the one moving significantly faster.

The example often given is the astronaut moving at half light speed to a distant planet returns to find everyone he knew has aged more than he has. I am comfortable that time can change with velocity as well as with gravity. I've read that a photon emitted from the first star has been traveling for billions of years at the speed of light but has experienced no time passing.

My question is 'Why?' Why does this relationship exist between time and velocity. Why, for example, doesn't time move more quickly when it's hot and more slowly when it's cold? Why isn't time temperature dependent?

Why doesn't time pass more quickly or slowly depending upon perturbations in the quark field?

What gives velocity the power to affect time?

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  • $\begingroup$ Motion clearly affects spatial coordinates, as in Galilean boosts. It should not be too surprising that we’ve found it also affects the “other coordinate”, time. $\endgroup$
    – G. Smith
    Commented Jan 28, 2020 at 6:54
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    $\begingroup$ Time dilation is a direct mathematical result of Einstein's Theory of Special Relativity. $\endgroup$
    – Sam
    Commented Jan 28, 2020 at 6:54
  • $\begingroup$ The example with the (twin) astronaut shows, that not the speed is relevant for the difference in aging but the gravity resp. acceleration. If the astronaut is simply passing by with constant speed vector, for symmetry reason one can not say if the planet is moving or both with half speed in opposite directions. Only the acceleration breakes the symmetry, f.e. flying a 180 degree curve for returning to the planet via swing by technique in the strong gravity field of a big mass. $\endgroup$
    – xeeka
    Commented Jan 28, 2020 at 7:26
  • $\begingroup$ In string and other theories the spacetime has more dimensions. As a result, passing by a planet moving "straight" with constant speed vector (in our 4 dimensions) would result in coming back to the planet from the opposite direction, similar like running on a circle. Also in this case, the symmetry should disable any difference in aging. $\endgroup$
    – xeeka
    Commented Jan 28, 2020 at 7:36
  • $\begingroup$ Mainly due to light speed constant in all reference frames; en.wikipedia.org/wiki/… $\endgroup$
    – Adrian Howard
    Commented Jan 28, 2020 at 8:21

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According to relativity, your four-velocity vector basically is your time axis. However, projecting the time axis of another observer in relative motion onto your own will yield a greater instead of a smaller value due to the non-Euclidean nature of spacetime.

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  • $\begingroup$ I believe these answers are correct in explaining the result of velocity in relation to time, but I didn't see the 'why' this is true. Could be my fault but I'm looking an answer that something along these lines "Time is affected by velocity because the 'x' attribute of velocity affects the 'y' attribute of time." Much like passing through the Higgs field adds mass by .... $\endgroup$
    – Jim Buono
    Commented Jan 29, 2020 at 8:47
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    $\begingroup$ I don't know what more to tell you: Changes in velocity affect the relative flow of time because your velocity vector is your time axis. "If I tilt my coordinate system, why do the coordinates of a point change?" - "Because you tilted your coordinate system, and that's what doing so does" $\endgroup$
    – Christoph
    Commented Jan 29, 2020 at 9:17
  • $\begingroup$ I've read that the time axis and velocity axis are different. Are you saying the velocity vector and the time axis are the same? $\endgroup$
    – Jim Buono
    Commented Jan 30, 2020 at 5:44
  • $\begingroup$ Also, I've read that everything is already moving at the speed of light. How does velocity change this? $\endgroup$
    – Jim Buono
    Commented Jan 30, 2020 at 5:44
  • $\begingroup$ There's no 'the' time axis: Each observer has their own, given by their 4-velocity vector, measuring their 'eigentime'. Everything is moving at the speed of light because 4-velocities are normalized to c. 3-velocities only make sense after a space/time decomposition has been performed. They are given by the spatial part of 4-velocity, corrected for time-dilation. $\endgroup$
    – Christoph
    Commented Jan 30, 2020 at 11:28

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