# Need help understanding something. About light speed - Thanks!

1. Aug 5, 2010

### nukeman

Hey guys, this is my first post. I posted here a few time a while ago, but forgot my logins :)

I am no physics expert, just really enjoy learning about it when I can.

Ok, what I need help understanding is the theory that for example if I leave earth travelling at the speed of light, and I travel for a peroid of time, lets say a year, when I come back to earth, people will not be 1 year older, but 30 or so.

Now first, is that true?

Can someone explain this to me?

Thanks, hope its not a stupid question. Thanks very much!

2. Aug 5, 2010

### Petr Mugver

First of all, you cannot go at the spped of light, because you have a mass, and only massless particles can travel c.

But, suppose you go sumething like v = c/2 respect to your twin (if you have one), who is at rest in some inertial system. Then yes, when you come back you will be younger than your twin.. This is a phenomenon called dilation of time, and it is explained in every special relativity textbook.

3. Aug 5, 2010

### sophiecentaur

There is the additional problem / paradox that asks why, as the Earth is moving relative to you, why the same doesn't occur to the people on Earth - i.e. you all appear to be the same age to each other.
Relativity does 'explain it all' in the end, though.

4. Aug 5, 2010

### Petr Mugver

I pointed out that the twin must be in an inertial system, se there isn't any twin paradox. Yes, your twin moves relative to you, but you are not in an inertial system (because to go away and then come back you must have a 4-acceleration) so you can't make the reasonment of your twin, so you WILL actually be younger than him... by how much?
(assume you go at constant speed c/2, with direction that can vary)

5. Aug 5, 2010

### Dr Lots-o'watts

Don't expect to fully understand. It's not intuitive. It must simply be accepted that that is how nature works. It cannot be contradicted.

6. Aug 5, 2010

### sophiecentaur

No paradox there, of course, but there is the situation when two people happen to going past each other fast (no acceleration). Each one will the the other one ageing (i.e. observing the the ticking of their two identical clocks) at a different rate to himself- and both will see the same difference. That is a sort of paradox.

7. Aug 5, 2010

### Fredrik

Staff Emeritus
Only in the sense that it's a counterintuitive result, and the same can be said about the other thing. So you should either call both of them paradoxes, or neither of them, depending on whether you define "paradox" to mean "a counterintuitive result" or "a contradiction".

8. Aug 5, 2010

Yup

9. Aug 5, 2010

### bcrowell

Staff Emeritus
Just because something is counterintuitive, that doesn't mean it can't be understood.

No, it doesn't simply have to be accepted. There are more fundamental reasons for it, although to some extent it is a matter of taste what one considers to be more fundamental than what.