# Why light?

1. Mar 6, 2009

### ONON

What about light is it that allows it to achieve a speed that is theoretically impossible for anything else to?

2. Mar 6, 2009

### Staff: Mentor

It has no mass.

3. Mar 6, 2009

### daytripper

Actually, I have a related question that I feel belongs here. Let's just say that you're a photon. Would you always and forever see every place that you've ever been or ever will? You would live out your life in an instant.

4. Mar 6, 2009

### ZikZak

Sorry, you can't be a photon. Photons don't have rest frames.

5. Mar 6, 2009

### daytripper

Well... then let's speak speculatively. Let's say you could be a photon. I am officially declaring that an axiom of my SR. haha. Would it follow?
I seem to always get caught up on some small detail in the scenario. I realize that this new axiom would probably create a contradiction somewhere but from your experience, what would you conclude about the perspective of the photon?

6. Mar 6, 2009

### confinement

Think about it this way, either the universe has a maximum speed or else it doesn't. Our universe does have a maximum speed, and light, as well as gravity and color glue, all travel at this speed because they are carried by particles with no mass.

7. Mar 6, 2009

### Staff: Mentor

If you are writing your own science fiction then you, as the author, are completely free to decide whatever you want.

8. Mar 6, 2009

### daytripper

good point. guess I'm getting a little overly zealous here. Thanks for the responses and time.

9. Mar 9, 2009

### ONON

what may i ask is 'color glue"? How are you defining the word "mass" when you suggest that a particle can exist without it?

10. Mar 9, 2009

### DeShark

I think the best way to think of mass is as a type of energy that an object has, which is not contained in its kinetic energy.

$$E^2 = (pc)^2 + (m c^2)^2$$

Thus, you see that the total energy is made up of momentum plus another kind of energy. We call this other energy mass. For a photon, m=0. This means that all of its energy is contained in its momentum. E = pc. Momentum in this case is a conserved (vector) quantity. Energy is a conserved (scalar) quantity.

For any particle, the momentum and velocity are related via

$$\vec{p}=\gamma m_0 \vec{v}$$

In this case, gamma is infinite, but m_0 is zero. So we have to be careful in taking the limits here.

Looked upon in a different way, suppose we have a particle with zero mass. Now E^2 = (pc)^2 + 0. In order for the zero mass particle to have any energy (an experimentally observed fact), it must have v=c.

11. Mar 9, 2009

### neopolitan

It's an essential ingredient in a rainbow.

12. Mar 9, 2009

### Naty1

I say "be a photon" if you like!!!!

If "thought experiments" were good enough to get Einstein started as a teenager...they are good enough for me....his "impractical" question was "What would light look like if I caught up with it?? How ridiculous!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Imagination and new ways of thinking can lead to breakthroughs (and a lot of false starts as well!!)

Photons don't "age" since they travel at the speed of light. You can draw many conclusions from that...not all make practical sense to me...for example, if photons are quanta of electromagnetic waves, do electromagnetic waves "age"??? Gravitons also travel at the speed of light,are massless, do they "age"...I guess not.

I believe mass particles have some theoretical life expectancy (is that right??) which is likely many orders of magnitude greater than the age of the universe...I'm guessing photons and gravitons last "forever"...

13. Mar 10, 2009

### Ich

It was Einstein who found that the answers to this question are nonsensical, which led him towards SR. SR reflects that insight, so why not spell it out?

14. Mar 10, 2009

### v2kkim

If I wrote a science fiction on 'if i am a photon', I 'd define the world as: a 2 dimensional simple world because I will loose 1 space dimension due to infinite contraction in light proceeding direction, and loose time due to frozen clock. So Nothing much to say, maybe that is why there is not such a SF story.

15. Mar 11, 2009

### cragar

Einstein invisioned riding next to a light beam when he was young.

16. Mar 11, 2009

### neopolitan

I guess if you can come up with a groundbreaking theory, you can get away with your youthful excesses.

The rest of you will be caned.

17. Mar 11, 2009

### Fredrik

Staff Emeritus

18. Mar 13, 2009

### UncertainOne

it isn't just light, all electromagnetic waves travel that fast (microwaves, x-rays, ect). This is why Maxwell is so crucial to the theory of relativity.

19. Mar 13, 2009

### neopolitan

A better question might be what is it about everything else which prevents travel at maximum speed? It seems to be mass. If you have mass you can't travel at lightspeed. If you don't have mass, then it seems you can.

It could be that having enough energy gives you mass (or possibly "mass-like qualities", if you include borderline cases like a neutrino).

It doesn't seem that being a particle is the issue, since photons act like particles - although it could be said that photons act like particles when interacting with other things, and it is possible that just then, when interacting with other thing, photons don't travel at c.

Anyway, the original question ONON asked "What about light is it that allows it to achieve a speed that is theoretically impossible for anything else to?" may have a bit of anthropic principle in it. If you don't start your model with any mention of light and build it up with other postulates, you will end up finding that something with certain characteristics would be able to travel at invariant speed. Those characteristics would be those which light exhibits. It's the way it is because that's the way it is.

cheers,

neopolitan

20. Mar 13, 2009

### cragar

you can ride next to a light beam they have managed to slow down light, slower than people run by passing it through an einstein-bose condensate.

21. Mar 14, 2009

### UncertainOne

i see, sorry i misread the question; i thought he said only light..