# Time and speed of a photon

• B

## Main Question or Discussion Point

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

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DrClaude
Mentor
a photon is travelling at c and he is also travelling at c (suppose)
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.

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

DrClaude
Mentor
Please clarify on what you mean by "physical"
Why is it not possible?
Because the laws of physics do not allow for someone traveling at ##c##.

Because the laws of physics do not allow for someone traveling at ##c##.
But why?
I have not studied much of the relativity theory and so, am not able to grasp the thing.
Thanks

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.

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

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

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.
Okay, Thanks for the clarification.

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.

Mister T
Gold Member
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
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##.

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.

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

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.

• SiennaTheGr8
Thanks