# Question from the FAQ on Rest Frame of a Photon

#### nitsuj

You are right that $k^{\mu }k_{\mu } = 0$ along a photon's world line so I don't understand what he means either.
[STRIKE]What does $k^{\mu }k_{\mu } = 0$ mean?[/STRIKE] nvm, i won't understand your reply

#### DonB

Do you understand how the concept of an inertial frame where light is at rest is logically self-contradictory?
I am familiar with the arguments, yes.

#### nitsuj

What does this even mean? To me, this sounds like you're saying photons can be described by something of the form $k^{\mu} = (0,0,0,T)$, but photons must be describable by null vectors as far as I know.
A fair amount of reading and a third attempt at rewording.

A photon is an elementary particle and is apparently (wiki) "treated" mathematically as a point particle which has no [physical] dimension.

#### Pengwuino

Gold Member
A fair amount of reading and a third attempt at rewording.

A photon is an elementary particle and is apparently (wiki) "treated" mathematically as a point particle which has no dimension.
Ok, with no disrespect intended, I suggest you retract your argument. Please don't try to piece together an argument based off things you don't understand on a wikipedia page. We're trying to explain something to someone and contributing statements that are nonsensical and vague will do far more harm than good.

#### nitsuj

Ok, with no disrespect intended, I suggest you retract your argument. Please don't try to piece together an argument based off things you don't understand on a wikipedia page. We're trying to explain something to someone and contributing statements that are nonsensical and vague will do far more harm than good.

sorry for the harm i caused

#### Dale

Mentor
I am familiar with the arguments, yes.
Good. Then is there anything remaining to discuss?

#### DonB

Good. Then is there anything remaining to discuss?
Maybe in due time. Plenty to just chew on for the time being.

#### harrylin

What you say here fits with my earlier quandary -- the faster you go, the shorter the time to cover the distance. Then when one crosses over the "light barrier", and v=c, time and distance have reduced down to zero.
Right -except that you meant "reaching the light barrier" (which cannot be reached...)
Thus the light beam leaves the star and is instantaneously here at earth for me to see.
I think that what you mean is wrong - and others already pointed this out to you.
The frozen clock that the photon uses (although that idea is nonsense) to observe the Earth, is *not* the clock that you use to observe the photon. Your "time" experience cannot be affected by a stuck clock that you don't use.
But others have shown me that that is not exactly what happens. Rather, once the light barrier has been crossed, [...]
Nothing to do with "crossing a light barrier".

In order to understand it, stick with nearly the speed of light as I suggested, and reflect on that. After you understand what goes on for that case, then you can extrapolate to the unattainable limit for a clock with v=c, as Einstein did.

Harald

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#### DonB

Right -except that you meant "reaching the light barrier" (which cannot be reached...)
No, actually I meant crossing the light barrier, but I made a shift (ref. to light that travels at c) that wasn't clear. (See my original posting of this statement.)

I think that what you mean is wrong - and others already pointed this out to you.
Yes, and that is what I said ("But others have shown me that that is not exactly what happens.")

The frozen clock that the photon uses (although that idea is nonsense) to observe the Earth...
Or non-clock, I think some would say.

Nothing to do with "crossing a light barrier".
Ah, but it does, within the discussion that I was pursuing. If we consider traveling in general, as we amp up the speed, we get closer and closer to c, and then as we near that peak we find that one thing (light -- the immediate context of my earlier post) has crossed that line -- has exceeded that barrier. That was my point.

#### harrylin

[..] If we consider traveling in general, as we amp up the speed, we get closer and closer to c, and then as we near that peak we find that one thing (light -- the immediate context of my earlier post) has crossed that line -- has exceeded that barrier. That was my point.
That point was either wrong or poorly formulated - and now it's clearer what that point was, so we're still making progress.

According to SR:
- light in vacuum always propagates at c; it doesn't "cross a line" or "exceed a barrier".
- Material objects (such as you) cannot reach the speed of light.

You could say that light always propagates at a speed that is the limit speed for material objects.

Also, you did not say that your "time" experience cannot be affected by a stuck clock that you don't use. You seemed to say the opposite...

Best,
Harald

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#### rede96

Since it will take ten minutes for the image of B to reach A and another ten minutes for the laser pulse to travel from A to the intersection, A needs to pull the trigger twenty minutes before it appears that B would arrive at the intersection, in other words when B is 33.333 miles before the intersection.
Thanks ghwellsjr.

So as Harold pointed out here:
In order to understand it, stick with nearly the speed of light as I suggested, and reflect on that. After you understand what goes on for that case, then you can extrapolate to the unattainable limit for a clock with v=c, as Einstein did.

Harald
I though I would try and do just that.

Imagine that instead of A firing a laser, A just decides to set off in his super fast ship and fires himself at B.

So ignoring acceleration for now, if A was capable of travelling at 0.9999986111095911 c, then A would feel that he covered the 10 light minutes in just 1 second.

A must also see B travel the 33.3333 miles B was away from the intersection in just one second too, as A will still hit B, but will just be a little off target.

However B would still feel like it took him 10 minutes.

So if we imagine just for a moment what a photon might experience from A's laser in the original experiment, once the photon was created, it would be instantaneously at B's target where it would be absorbed.

B, which was 33.333 miles from the intersection would also have to be instantaneously at the intersection for the photon to hit the target.

Although A and B still feel the passing of time as normal, for the photon, as everything happens instantaneously, there is nothing for the photon to experience. Which I guess is why we can't look at this problem from the frame of a photon. There is nothing to look at!

I know that the above is not strictly in keeping with the principles of SR, but I thought it might help address DrDon's questions.

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