# B Time and speed of a photon

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1. Feb 6, 2017

### shihab-kol

Someone asked me the other day that
a photon is travelling at c and he is also travelling at c (suppose)
then to him the photon is at rest and so it must have a mass

I could not answer him and so I need some help

2. Feb 6, 2017

### Staff: Mentor

That supposition is not physical. There is no frame in which the photon is at rest, since the photon travels at c in all inertial frames.

3. Feb 6, 2017

### shihab-kol

Please clarify on what you mean by "physical"
Why is it not possible?

4. Feb 6, 2017

### Staff: Mentor

Because the laws of physics do not allow for someone traveling at $c$.

5. Feb 6, 2017

### shihab-kol

But why?
I have not studied much of the relativity theory and so, am not able to grasp the thing.
Thanks

6. Feb 6, 2017

### maline

The reference frame of "someone travelling at c" is not one of the frames where the laws of physics can be described in a straightforward way. This is because such an observer does not have a "timelike world line"- in a sense, he is not moving forward through time at all.
Of course, that's also why massive beings like us can never reach c.

7. Feb 6, 2017

### shihab-kol

I understand, as t=0, the idea is not feasible .
Thank You.

8. Feb 6, 2017

### Staff: Mentor

The simple answer at a "B" level is that it you would need an infinite amount of energy to accelerate a massive body to the speed of light.

9. Feb 6, 2017

### shihab-kol

Okay, Thanks for the clarification.

10. Feb 6, 2017

### Ibix

The answer I find simplest is to say that relativity starts with the postulate that the speed of light is the same in all reference frames. So in "the rest frame of light" the light would be stationary and moving at c at the same time - which is contradictory. So you cannot describe the rest frame of light in relativity. And relativity keeps passing experimental tests, so it's basic postulates seem to be correct.

11. Feb 6, 2017

### Mister T

No matter how fast you chase after a beam of light, the beam recedes from you at the same speed. Thus you will never see it have any speed other than $c$. You certainly would not see it have a speed of zero.

From there, it's easy to deduce the fact that you can never have a speed of $c$.

12. Feb 6, 2017

### mikeyork

Another answer to your questioner is that he is assuming an (incorrect) Galilean transformation (as in Newtonian mechanics) for relative velocity rather than the (experimentally verified) Lorentz transformation.

13. Feb 7, 2017

### shihab-kol

Thank You to all of you for replying, I understand the idea now perfectly.

14. Feb 7, 2017

### Battlemage!

I think you can also look at it going "backwards" in the logic. That is, if the speed of light is the maximum speed limit any inertial reference frame will see, then it also has to be the same in every inertial reference frame, otherwise you'd get the speed of light as a + c in some frame, a >0.

15. Feb 8, 2017

Thanks