# I Help with understanding time dilation

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1. Nov 17, 2017

### Junkwargo

Hi, I've always had a doubt concerning time dilation, and today the curiosity got the best and I decided to ask people that know their stuff, you people. Maybe the question has been asked before, but I did a quick search and couldn't find it, sorry if that's the case. Well, here it goes.

As I understand, time dilation occurs by different velocities or positions relative to a gravitational field. In the case with velocity, I understand that if an observer has a clock, let's call it clock A, and there is also a second clock (clock B) moving relative to the observer, then clock B will register a slower passage of time than clock A. Now my question is this, if velocity is relative, why does clock B ticks slower, if from the point of view of clock B, it is clock A that's moving, and should therefore be the slower clock? Are both clocks slower? How does that work?

I'm not a physicist (biologist here!), so my understanding of these things is shallow at best. Any help would be greatly appreciated.

Last edited by a moderator: Nov 17, 2017
2. Nov 17, 2017

### phinds

Neither clock is actually slower. They both tick at the same rate of one second per second and they are both perceived by their moving counterpart as ticking more slowly that it. That is, if you can I are moving relative to each other then I see your clock ticking slower than mine and you see my clock ticking slower than yours whereas in each of our local frames, our clocks are both ticking at one second per second.

This HAS been discussed here, at last count, 8,746 times.

3. Nov 17, 2017

### Staff: Mentor

When I use the forum Search feature (at the right of the toolbar at the top of the page) to search for "time dilation" in this forum only, I get 4 pages of hits. I think you need to try again.

Time dilation due to different velocities occurs whether there is a gravitational field or not. Since that's the case you're asking about, I would leave out the gravitational field and just concentrate on the simple case of nonzero relative velocity. The answer @phinds gave addresses this case.

4. Nov 18, 2017

### Ibix

Imagine you are driving along a straight road at 30mph. Imagine another straight road crosses yours at angle $\theta$. Look out of your side window at a car also doing 30 on the other road. It'll fall behind because its speed in your forward direction is only $30\cos\theta$. But the driver of the other car can look out of her window at you, and will see you falling behind for the exact same reason.

Who is really falling behind? The question doesn’t make sense. The effect is entirely a result of disagreeing on what "moving forward with no sideways component" really means.

This is closely analogous to time dilation. Remember that time is a dimension in spacetime. Remember also that both clocks A and B can regard themselves as stationary - that is, moving in time but not space. Noticing that the other clock ticks slowly is like noticing that the mile-markers on the other road don't match up to the ones on your road. The mile-markers are the same distance apart but with different definitions of "going straight forward". In relativity the ticks of the clocks are the same time apart, but with different notions of "moving only in time".