# B A question of why time dilates and what is described by it

#### PeroK

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Wow. This totally confuses me. So I see your watch running slower than mine, and your see mine running slower than yours. What time are they each showing upon my return?
Appreciate the clarification, though still confused. How do the two clocks in relative motion "know" they can never reunite?
You can't really learn much by asking such questions based on no clear understanding of the scenario or the basis on which arguments are being made. Understanding time dilation, differential ageing and SR (Special Relativity) require you to sit down and take the concepts one at a time: reference frames, the constancy of the speed of light, clock synchronisation, time dilation for a clock moving in some reference frame etc.

Eventually, you can quite happily disentangle these apparent paradoxes, but you really need to begin at the beginning.

#### phinds

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You can't really learn much by asking such questions based on no clear understanding of the scenario or the basis on which arguments are being made. Understanding time dilation, differential ageing and SR (Special Relativity) require you to sit down and take the concepts one at a time...
Yeah. What he said.

#### Chris Miller

They do not know. Two clocks that are separating inertially, never to reunite behave in all ways identically to two clocks that are separating, and will at some future point be subject to accelerations so that they will eventually reunite.
Thanks, I've read the Twin Paradox link, still pretty fuzzy what with Doppler & acceleration effects all tossed into the thought experiment. (The Enterprise, I believe had some sort of "intertial dampeners" for instantaneous velocity which I'd want to apply to this obviously well traveled topic).

#### Orodruin

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The Enterprise, I believe had some sort of "intertial dampeners" for instantaneous velocity which I'd want to apply to this obviously well traveled topic)
You would do well to separate science fiction from actual science.

#### Chris Miller

You would do well to separate science fiction from actual science.
Wasn't suggesting it's "science" or doable, only trying to frame a thought experiment. Q: given I view your clock flashing slower, and you mine, whose (as in which of ours) does a 3rd frame's observer view as running slower?

#### Orodruin

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Wasn't suggesting it's "science" or doable, only trying to frame a thought experiment. Q: given I view your clock flashing slower, and you mine, whose does a 3rd frame's observer view as running slower?
And the answer is that it depends on the state of motion of the two of you relative to that observer.

#### Mister T

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To use a metaphore, one can describe the Earth's orbit using pure math describing it as an ellipses. But that math alone can't tell you why Earth moves that way.
Agreed. But let's see where this takes us ...

Pure math won't reveal the existance of gravity in this, instead, you need to expand your view and start looking at other aspects of Earth and other celestial bodies, namely the sun, or have some other insight to figure out gravity. In this, gravity is part of why Earth has that orbit and not some other orbit.
Gravity can be used to explain why the orbit is an ellipse by pointing to two particular properties of gravity. One, that the force is directed towards the sun and two that the strength of the force is inversely proportional to the square of the distance to the sun.

So in one sense something is accomplished because we've used math to explain how ellipses follow from gravity. But in another sense we are no further along because we are still left with your original query. Namely, why does gravity have that particular direction and that particular dependence on distance to the sun. And we can go on with another explanation that uses math to explain how that follows from something deeper. But we are then still left with wondering why the deeper thing is true.

Likewise, relativity is the math of the result, it tells us how to predict things, but it doesn't tell us why things happen that way, and that is what I'm wondering.
Special relativity is more than just math. It's a complete explanation of, for example, time dilation. And it explains, using math, how time dilation follows from the two postulates.

So if you want to know why time dilates it's because of the two postulates: equivalence of inertial reference frames and invariance of light speed.

You are of course still left wondering why all reference frames are equivalent and why the speed of light is the same in all reference frames. Those things can be explained in terms of deeper reasons, but that has nothing to do with special relativity itself. Special relativity makes no attempt at explaining why the postulates are true, it simply explains how things will behave given the validity of the postulates. and it's the experimental verification of those conclusions that gives us faith in the validity of those postulates.

There is no known experiment you can perform to distinguish between a state of rest and a state of uniform motion. That seems to be a fundamental feature of our universe and the fact that time dilates is a consequence.

#### phinds

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Wasn't suggesting it's "science" or doable, only trying to frame a thought experiment.
Yes, but the problem is that framing actual physics scenarios using fiction is likely to lead you to misunderstandings and wrong answers. Best to stick with reality.

"A question of why time dilates and what is described by it"

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