# About time when you get closer to C

1. Jul 31, 2014

### sublime56

Ok so I read this on another site... "the Milky Way is part of the Local Group...The Local Group is calculated to move relative to the CMB at about 600 km/s (2,200,000 km/h)"

When you get into a 'spaceship' and travel say 50% the speed of light, then come back to earth, more time has passed for you than for those on earth.

My question is, does it matter what direction you travel in?

2. Jul 31, 2014

### Staff: Mentor

No, the direction you move doesn't matter. The amount of time dilation and time passed on Earth will be the same.

3. Jul 31, 2014

### Chronos

Time dilation is relative. If you have a high velocity relative to earth, earth clocks will appear to be slower than your pocket watch.

4. Jul 31, 2014

### Staff: Mentor

It's the other way around - more time has passed on earth than for you.

5. Jul 31, 2014

### haael

Direction doesn't matter and even speed doesn't matter. What matters is acceleration.

The amount of time that passed for you depends on the length of your path in time-space. The time space is isotropic, so the length doesn't depend on direction, and Lorentz-invariant, so the length doesn't depend on speed. However it is not invariant with regards to acceleration. So, by accelerating you change the tempo of your time passing.

6. Jul 31, 2014

### ghwellsjr

It really would help if you would provide a link to the other site. Is your second paragraph also a quote from it? Those two sentences don't seem related.

7. Jul 31, 2014

### ghwellsjr

No, acceleration doesn't matter, only speed according to an Inertial Reference Frame matters. Of course, an acceleration that changes the speed of an object indirectly matters but you can also have an acceleration that does not change the speed and that won't matter at all for that particular IRF. If you transform to another IRF, all the speeds can change but for a situation where you are comparing the times accumulated by two objects that start out colocated and end up colocated, it won't matter which IRF you use.

These are a very confusing set of comments. First off, what do you mean by "time space" and "time-space"? If you mean spacetime, why do you change the terminology?

8. Jul 31, 2014

### haael

There are infinite count of inertial reference frames moving relative to each other. You can have any speed you wish by selecting one of them.

The point is the rest frame of the spaceship is non-inertial.

OK, I will use "spacetime" from now on.

9. Jul 31, 2014

### ghwellsjr

Yes, and they are all equally valid and none is preferred over the others.

That's true but not significant. Inertial Reference Frames can handle non-inertial objects just fine and there is no standard way of constructing a non-inertial frame so if you want to use one, you will have to define how you're going to do it and when you get done, you will not have learned anything more about the scenario than if you had just used an IRF.

10. Aug 3, 2014

### sublime56

Sorry, time pass more slowly for the guy in the ship*