# Travel at the speed of light

1. Dec 18, 2003

### FUNKER

I was watching the movie K-PAX the other day and the dude from K-PAX (another planet obviously)was asked how he can travel at the speed of c because his mass would have to infinate, through Einstiens statements and he replied that Albert was not wrong but we just read the statement wrong, he said we cannot accelerate to the speed of light but what about objects alreday travelling at the speed of light? This got me thinking; any comments would be most welcome

2. Dec 18, 2003

### chroot

Staff Emeritus
Don't think too much. It's a movie.

- Warren

3. Dec 19, 2003

### hemmul

nothing!
consider a photon. It is already travelling at c. But can you stop a photon? :) the only thing you can do - either absorb it, or force to rotate around a black hole :) The guy from K-PAX was not photon...

by the way, just can not leave it:
not the mass of an object grows to infinity, but its energy is going to infinity, the results are obvious.

Of course, K-PAX is a good movie, but it's a movie

4. Dec 19, 2003

### hey.like

Again all light particles that is true

The other idea is . again all light particles , that is true but it is meanless. sorry as it.

5. Dec 21, 2003

### Andthensome

Did he mean the guy himself moving at the speed of light or his vehicle? If he meant his vehicle, that answer is obvious; warp spacetime around it. They are already working on faster than light travel at quite a few centers around the world and the main hurdle right now is to create enough energy to make a warp bubble around the ship. The basic concept is to compress space-time in front of you and expand it behind you, in essence standing still yet moving.

As to particles already travelling at the speed of light, I'm not sure what that has to do with a humanoid; not sure what he meant. But if he's not already travelling at the speed of light and if particles that are already travelling at the speed of light are not a part of him, then I have no idea what kind of crap he's spewing.

P.S. Hummel, you're wrong. It's the mass that grows to infinity and the amount of energy needed to propel it closer and closer to the speed of light becomes infinite.

Last edited: Dec 21, 2003
6. Dec 22, 2003

### hemmul

Then, let me ask, how do you define mass?

7. Dec 22, 2003

### hemmul

Well, you see, we are both right...
There is rest mass and relativistic mass.

it is described very good here.
Well, i beleive one should understand "sometimes", in the quote above, as a result of that this quantity is convinient in a very small area of physics.
I'try to explain why:
Consider the expression for the energy:
E=m*c^2/sqrt(1-v^2/c^2);
here m - is a rest mass.
You can surely rewrite this as
E=m*c^2;
with m denoting the relativistic mass.

But the problem (well not a real problem) arises when you try to derive the expression for the force from the momentum (p=E*v/c^2)...
Using the relativistic mass, the momentum is written as p=m*v - as in classical physics.
Deriving it, we receive the expression for the force, where relativistic gamma is in the power of 3/2 (if considering rest mass), and if we try to substitute a relativistic mass, we will not receive F=m*a...
Not very convinient? eh?
Moreover, it is difficult to give the relativistic mass a real physical interpretation. That's why in modern physics rest mass is preserved, and it is much more convinient to use
E=m*c^2*(relativistic gamma)
p=m*v*(relativistic gamma)
and so on...

But of course, i agree with you, if dealing with relativistic mass - it is being increased...
That's what i meant :)

Last edited by a moderator: Dec 22, 2003