# Time slowing down for someone traveling just under the speed of light

1. Feb 12, 2013

### mycotheology

Lets say ship 1 is travelling just under the speed of light so time slows down for the crew and the age less quickly than the stationary crew in ship 2. Relative to the crew in ship 1, the crew of ship 2 are travelling at just under the speed of light, so time moves slower for them and they should be the ones aging more slowly. Which is it?

2. Feb 12, 2013

### Staff: Mentor

Re: Time slowing down for someone traveling just under the speed of li

It's both. Time and time dilation are relative, not absolute.

3. Feb 13, 2013

### Staff: Mentor

Re: Time slowing down for someone traveling just under the speed of li

The one that fired its engines, which you didn't specify.

4. Feb 13, 2013

### PAllen

Re: Time slowing down for someone traveling just under the speed of li

What if they both fired their engines. Anyway, it doesn't matter unless they get back together. There is no sense in which you can pick which one is 'really' slower if they just travel away from each other, possibly exchanging signals.

5. Feb 13, 2013

### harrylin

Re: Time slowing down for someone traveling just under the speed of li

Time dilation between inertial frames is mutual - according to each reference system it is the clock at rest in the other system that is ticking more slowly. This disagreement is due to disagreement about who is "in rest" and related things such as distant simultaneity.
See for example the elaborations here: http://en.wikipedia.org/wiki/Time_d...relative_velocity_symmetric_between_observers

6. Feb 13, 2013

### Staff: Mentor

Agreed.

7. Feb 13, 2013

### mycotheology

Re: Time slowing down for someone traveling just under the speed of li

I read that if a spaceship is traveling at a really high speed, lets say just under lightspeed, that the crew will age more slowly than us earthlings because time slows down when they are traveling at such a high speed. What doesn't make sense to me is that relative to the spaceships crew, its the earth that is traveling at an extremely high speed so its the earthlings clocks that slow down. Lets say the spaceship travels around in a circle and returns to earth 15 years later. Will the earthlings have aged more than the crew of the spaceship? From the spaceships perspective, it should be the earthlings that aged slower.

Isn't it irrelevant who fired the engine? I meant to imply that spaceship 1 fired the engine and took off at just under light speed relative to spaceship 2. The crew of spaceship 2 will observe that spaceship 1s clocks have slowed and the length of the spaceship has contracted. Whats confusing me is that relative to the ship 1 (the fast moving ship), ship 2 is the one thats traveling at just under lightspeed. So does time dilation occur for both ship 1 and ship 2? I'm a full on visual thinker so this is a real mind boggler for me. Trying to come up with a way to visualise this concept.

Last edited: Feb 13, 2013
8. Feb 13, 2013

### PAllen

Re: Time slowing down for someone traveling just under the speed of li

No, if the spaceship travels in a circle the situation is not symmetric. This becomes a case of differential aging (not symmetric) rather than time dilation. The spaceship traveling in a circle is not inertial, and cannot pretend they are at rest in an inertial frame and use the normal SR formulas. The spaceship traveling in a circle will age less.

9. Feb 13, 2013

### Staff: Mentor

Re: Time slowing down for someone traveling just under the speed of li

You might try the Usenet Physics FAQ entry on the twin paradox:

In particular, the Doppler Shift Analysis and the Spacetime Diagram Analysis.

The Doppler Shift Analysis focuses on what each twin actually sees, in the sense of the actual light signals each twin receives from the other; this might help in visualizing since it removes all the abstractions about "time dilation" and just focuses on the actual observations.

The Spacetime Diagram Analysis gives a sort of "God's eye view" of what is happening and how it all fits together globally.

10. Feb 13, 2013

### Staff: Mentor

That would mean that nothing happened to the rocket that fired its engine, but the other rocket that did nothing accelerated.

11. Feb 13, 2013

### mycotheology

Re: Time slowing down for someone traveling just under the speed of li

Ah, so its called the twin paradox. Thanks a lot. Hopefully those FAQs will resolve it for me. The wiki page looks good too.

12. Feb 13, 2013

### Staff: Mentor

Re: Time slowing down for someone traveling just under the speed of li

Be careful. There are two deceptively similar but interestingly different paradoxes.

1) The one your raised in your first post: When they're moving relative to one another, which one is "really" slow? The answer is "neither - they both see time passing more slowly for the other and there is no contradiction because of relativity of simultaneity".

2) The twin paradox: If they start out in the same place at the same time with the same age, travel for a while, then rejoin at a later time (which means that one of them has necessarily accelerated or free-fallen through a fairly exciting gravitational field) so that they can directly compare their age, which one is younger? The answer here is that one of them will be unambiguously younger.

13. Feb 13, 2013

### danR

Re: Time slowing down for someone traveling just under the speed of li

By whose clock(s)? Theirs, Earth's, or yours?

14. Feb 13, 2013

### Chronos

Re: Time slowing down for someone traveling just under the speed of li

See Misner, Charles W.; Thorne, Kip S.; Wheeler, John Archibald (1973), Gravitation, San Francisco: W. H. Freeman, ISBN 978-0-7167-0344-0.