# The relation with time and light

1. Feb 26, 2010

### Ren313

hey everyone, Im a 16 year old student still in high school, i was just wondering if anyone can help me with this question. - what is the relation between time and light?

2. Feb 26, 2010

### Staff: Mentor

3. Feb 26, 2010

### jfy4

The famous clock on a train thought experiment is another example of light being a great time keeper. if you have two mirrors parallel to each other on a train moving at constant speed with a light beam between them, its a perfect clock because the speed of light is constant which is cool. So it can be a a time keeper.

also, if you ARE light, then it takes you no time at all to get from place to place which is sweet also. I mean takes time here in the classical sense because it doesn't take "time" for anything to take place, that's like saying "whats happening in the Andromeda galaxy right NOW? not a very good question cause there isnt background "TIME" like we thought before Einstein... time is related to light by the way we experience things!

4. Feb 26, 2010

### Fredrik

Staff Emeritus
jfy4, I don't agree with your second claim. I'll explain why by quoting myself...
Your first point is good though, and it can be expanded to cover the synchronization procedure mentioned in the post I linked to above. I'd say that that defines the most important connection between light and time.

5. Feb 26, 2010

### jfy4

Well put, and enlightening! The main point of my second paragraph was to show the elimination of the background time and to illuminate that the relationship Things and light have is through experience, not through time, which is a classical idea. Thank you for that tid-bit, it makes a lot of sense! Ill reconsider ever using that phrase again.

6. Feb 26, 2010

### Ren313

hey guys, thanks everybody your help! i think i need to learn some more math first lol. But is it true that light is the ultimate speed in the universe? and why is it if u travel faster than the speed of light that u're going back into the past?

7. Feb 26, 2010

### Fredrik

Staff Emeritus
That's ok. Plenty of time for that if you're only 16.

It's an invariant speed in the sense that you would measure its speed to be the same regardless of your velocity relative to the light source. This is an extremely counterintuitive result if you think about it, but it's not that strange when you have understood clock synchronization, simultaneity and spacetime diagrams. (If you want to understand special relativity before you learn the math, the best way...no the only way, is to learn spacetime diagrams).

Massive objects can't be accelerated to the speed of light. They are forever trapped with speeds <c, because the energy it would take to accelerate a massive object to speed v goes to infinity as v goes to c. (This of course also means that a massive object can't be accelerated to a speed >c).

It is conceivable that there exist particles that move faster than light. They're called "tachyons". A speed v>c doesn't imply that it reaches the destination before it leaves the point of departure. Even an infinite speed would only mean that it "gets there" at the same time as it "leaves here". But the funny thing about events that are farther apart in space than in time, $x_2-x_1>c(t_2-t_1)$, is that different observers disagree about which one of them was earlier. (The best way to see that is to learn about spacetime diagrams and what that synchronization procedure is all about). So if you send a tachyon from event A to event B, a person moving in the same direction as the tachyon will, if he's moving fast enough, consider B to be earlier than A. So to him, it would look like the tachyon was sent from B to A.

It's possible to show (see this post and the correction below) that tachyons must have some really strange properties if they exist, because otherwise we could use them to send messages to the past, which would lead to paradoxes.

8. Feb 26, 2010

### Ren313

thanks for ur help! do u have a website or a book that u can suggest to me for studying these things? cause i see that i still hv a lot of things to learn. and if u dont mind me asking, how many years of school did u study physics for? because u seem like an real professional on this, lol :)