# What exactly is Time Dilation?

#### MostlyHarmless

So I know it has to do with reference frames and such, and I know that it is all about time relative to different observers, what I don't understand is how exactly is it that when you're moving at a speed close to 'c', you actually physically experience a different time interval than someone who is "motionless," as opposed to simply percieving a different time interval but physically undergoing the same time interval as the "motionless" person. And by physically expierence I mean aging.

I was reading something, I don't recall exactly what it said but it was talking about time slowing as you condense space, and it sparked this thought. From a certain perspective if you move across a space you're effectively shrinking or expanding the space by going faster or slower, respectively. So by traveling at such a great velocity you are "shrinking" the distance that you travel, from your perspective, so it takes a shorter amount of time than witnessed by the "motionless" bystander to travel that distance.

Is this a good/accurate way to think of time dilation? or am I completely off? And to clarify my credentials, I have none. So anything I think of idependently, that is I don't read it verbatim from something else, I assume wrong, this is no different. So if/when you tell me I'm wrong, please elaborate, I would very much like to know the answer to the original question, What exactly is Time Dilation?

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#### Mentz114

Gold Member
You should be aware that there is no 'absolute motion' measured against spacetime. The only measurable velocities are relative. So we can say that 'A is travelling at speed v with respect to B', and so on, but not just that 'A is travelling at speed v', which has no meaning in relativity.

Time dilation is a coordinate effect. If we use the Lorentz transformation to change coordinates between two frames, then the 'other' frame t coordinate is multiplied by γ, the relativistic factor. However, the clocks in both frames run normally and no effect is seen by either observer on their own clock. This leads to the confusing expression 'moving clocks run slower'. If Doppler measurements are made between frames, then a clock moving away appears to run more slowly, but an approaching clock will appear to run faster.

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#### phinds

Gold Member
try this:

www.phinds.com/time dilation [Broken]

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#### MostlyHarmless

Thank you both, I think I have what you two have described, but to be sure that I'm not starting from a misconception, I want to be sure that my interpretation is a valid one.

From a certain perspective if you move across a space you're effectively shrinking or expanding the space by going faster or slower, respectively. So by traveling at such a great velocity you are "shrinking" the distance that you travel, from your perspective, so it takes a shorter amount of time than witnessed by the "motionless" bystander to travel that distance.
So, in short, correct, or why not?

#### phinds

Gold Member
From a certain perspective if you move across a space you're effectively shrinking or expanding the space by going faster or slower, respectively.
Well, you are not actually doing ANYTHING to the space.

So by traveling at such a great velocity you are "shrinking" the distance that you travel, from your perspective, so it takes a shorter amount of time than witnessed by the "motionless" bystander to travel that distance.
Yes. It is the PERSPECTIVE that matters. From YOUR perspective, everything is perfectly normal. There is no time dilation and no length contraction, you're just going really fast. From OTHER perspectives, you are experiencing time dilation and length contraction.

#### MostlyHarmless

Ok, so I'm almost there then. I'll do some more reading and come back if I've got anymore questions. Thanks!

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