# Time dilation or time relativity?

## Main Question or Discussion Point

Are these terms synonyms or do they actually mean different things?

I see both refer to the same phenomenon but it looks as if "time dilation" assumes the point of view of only one observer whereas "time relativity" or "relativity of time" seems to include all the observers. Still, most literature refers to this as "time dilation".

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time relativity = a generic term applying to all motion physics - Aristotlean, Galilean, Einsteinian, etc.

time dilation = the phenomenon of time "slowing down" IAW SR (Einsteinian.)

That's a guess...

stevmg

Consider the Earth as the center of a frame of reference. Have a star 10 ltyr away. If traveling at 0.6 c it would take you 10/0.6 = 16 2/3 years to get there in the Earth's time frame.

Gamma for this is 0.8, therefore the length contracted distance in the rocket ship's time frame is 8 ltyr.

The "time dilated" (the rocket's time frame) would be 13 1/3 years rather than the 16 2/3 years in Earth time.

That's time dilation. Notice that 8 lt-yr/13 1/3 yr still = 0.6 c

I suppose time relativity would be the same in the sense that most interpret scu a situation in Einsteinian or SR terms.

I bet you I got that all wrong...

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bcrowell
Staff Emeritus
The Lorentz transformation of time has the form $t'=(\ldots)t+(\ldots)x$. To me, "time dilation" has the connotation of referring to the fact that the coefficient of the first term on the right doesn't have its Galilean value of 1, while "relativity of simultaneity" focuses more on the fact that the second coefficient doesn't have its Galilean value of 0. I haven't heard "relativity of time" as much, so I wouldn't hazard a guess as to its connotations; it might be a convenient blanket term for all such relativistic effects.
The Lorentz transformation of time has the form $t'=(\ldots)t+(\ldots)x$. To me, "time dilation" has the connotation of referring to the fact that the coefficient of the first term on the right doesn't have its Galilean value of 1, while "relativity of simultaneity" focuses more on the fact that the second coefficient doesn't have its Galilean value of 0. I haven't heard "relativity of time" as much, so I wouldn't hazard a guess as to its connotations; it might be a convenient blanket term for all such relativistic effects.