# The Naked Scientists Forum

### Author Topic: Relative speed of photons - I'm puzzled  (Read 6466 times)

#### DoctorBeaver

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##### Relative speed of photons - I'm puzzled
« on: 17/11/2006 18:52:22 »
I've just replied to a thread about time dilation and something occurred to me. No matter how fast you travel, light will always appear to travel at 300,000kps(ish). Think about 3 photons travelling in the same direction 1 behind the other. Would the photon at the back percieve the 2 in front as travelling away from it at light speed? And would the 1 in front percieve the other 2 as falling behind at 300,000kps? But the poor little bugger in the middle would get horribly confused because both of the others would seem to be receding at 300,000kps (and that's faster than Neil's hair is receding!)

In the 2 twins case where 1 zips off on a journey at relativistic speeds, he will return having aged less than the twin who remains stationary. Therefore, from the perspective of the stationary twin time has passed more slowly for the twim who zipped off. The faster the moving twin travels, the less time will have passed for him relative to the other. Taking that to extremes, if the 1st twin had travelled at light speed then no time at all would have passed for him from the perspective of the 2nd twin; he would reappear an instant after leaving. Does this effect have something to do with my question about photons?

And this raises another question. If an object travelling at light speed appears to a stationary observer to travel any distance instantaneously, why does light take so long to reach us? Surely photons would appear to travel instantaneously too. WAAAAA my brain hurts again!

Any help appreciated.

#### ukmicky

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##### Re: Relative speed of photons - I'm puzzled
« Reply #1 on: 18/11/2006 00:19:12 »
I've just replied to a thread about time dilation and something occurred to me. No matter how fast you travel, light will always appear to travel at 300,000kps(ish). Think about 3 photons travelling in the same direction 1 behind the other. Would the photon at the back perceive the 2 in front as travelling away from it at light speed? And would the 1 in front perceive the other 2 as falling behind at 300,000kps? But the poor little bugger in the middle would get horribly confused because both of the others would seem to be receding at 300,000kps (and that's faster than Neil's hair is receding!)

In the 2 twins case where 1 zips off on a journey at relativistic speeds, he will return having aged less than the twin who remains stationary. Therefore, from the perspective of the stationary twin time has passed more slowly for the twin who zipped off. The faster the moving twin travels, the less time will have passed for him relative to the other. Taking that to extremes, if the 1st twin had travelled at light speed then no time at all would have passed for him from the perspective of the 2nd twin; he would reappear an instant after leaving. Does this effect have something to do with my question about photons?

And this raises another question. If an object travelling at light speed appears to a stationary observer to travel any distance instantaneously, why does light take so long to reach us? Surely photons would appear to travel instantaneously too. WAAAAA my brain hurts again!

Any help appreciated.
Hi Doc but isnt it the case that photons dont interact with spacetime like mass does and so the photon donsent experience any of the reletavistic effects like we do.

Also i could be wrong but i thought if the stat observer could somehow view an object moving at c from many thousands of LY away he would see it moving, its travel wouldn't be instantaneous if it was moving across his field of vision.
But what do i know , not much

Ps I corrected your typos for you because i'm a nice guy. [^]
« Last Edit: 18/11/2006 16:39:59 by ukmicky »

#### lightarrow

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##### Re: Relative speed of photons - I'm puzzled
« Reply #2 on: 18/11/2006 13:13:53 »
I've just replied to a thread about time dilation and something occurred to me. No matter how fast you travel, light will always appear to travel at 300,000kps(ish). Think about 3 photons travelling in the same direction 1 behind the other. Would the photon at the back percieve the 2 in front as travelling away from it at light speed? And would the 1 in front percieve the other 2 as falling behind at 300,000kps? But the poor little bugger in the middle would get horribly confused because both of the others would seem to be receding at 300,000kps (and that's faster than Neil's hair is receding!)

I'm sorry to tell you, but the reference frame of a photon doesn't exist.
You can say that light's speed is always c in every inertial ref frame, which speed is as fast as you want (with respect another inertial RF) but not exactly c.

However, you can determinate what happens in the ref frame of very fast moving objects, as near c as you want. In this case, you can find that, if in a ref frame two of those objects travel in the same direction and versus with exactly the same speed, then, in the ref frame of one of them, the other is stationary; if, instead, in one ref frame they have a slightly different speed, V1 and V2, and so their relative speed is V2-V1, in the ref frame of one of the two, the other is moving at a speed V, which is not V2-V1, and you can choose V2 and V1 so that V2-V1 ≈ 0 and V ≈ c!

Quote
And this raises another question. If an object travelling at light speed appears to a stationary observer to travel any distance instantaneously, why does light take so long to reach us? Surely photons would appear to travel instantaneously too
No!
1. An "object" travelling at exactly light speed can only be light.
2. An object, e.g. a starship travelling very near to light speed doesn't appear to travel a great distance to a stationary observer, but only to an observer inside the starship! To a stationary observer it appear to move at a finite speed ≈ c.

If you start a journey inside a starship moving very near to c with respect planeth earth, you, inside the starship will experience to explore the entire universe in a few seconds, but for people on earth, you DON'T!
(And, of course, when you come back, you discover that earth and solar system too, don't exist anymore, because many billions of years has passed there!).

« Last Edit: 18/11/2006 13:28:09 by lightarrow »

#### DoctorBeaver

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##### Re: Relative speed of photons - I'm puzzled
« Reply #3 on: 20/11/2006 14:35:49 »
Thank you Alberto, that explained it nicely.

#### lightarrow

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##### Re: Relative speed of photons - I'm puzzled
« Reply #4 on: 20/11/2006 14:42:17 »
Thank you Alberto, that explained it nicely.
You're welcome!

#### Soul Surfer

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##### Re: Relative speed of photons - I'm puzzled
« Reply #5 on: 26/11/2006 15:37:02 »
The other point about the three photons is that they dont "see" each other at all. Firstly photons do not interact with each other in the absence of matter.  Secondly for a photon time has stopped, the moment of its creation is exactly the same to it as the moment of destruction maybe millions of years (or a picosecond) later.

#### neilep

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##### Re: Relative speed of photons - I'm puzzled
« Reply #6 on: 26/11/2006 19:30:12 »
The other point about the three photons is that they dont "see" each other at all. Firstly photons do not interact with each other in the absence of matter.  Secondly for a photon time has stopped, the moment of its creation is exactly the same to it as the moment of destruction maybe millions of years (or a picosecond) later.

Does a photon actually get destroyed ?..or does it change into something else ?

#### Soul Surfer

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##### Re: Relative speed of photons - I'm puzzled
« Reply #7 on: 26/11/2006 23:27:20 »
Because of the conservation of energy it always gets changed int oanother form of energy.  It could for example change the orbital of an electron or make an atom move quicker (or slower)

#### lightarrow

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##### Re: Relative speed of photons - I'm puzzled
« Reply #8 on: 27/11/2006 08:30:04 »
...or create other particles, e.g. a pair electron-positron (hitting an atomic nucleus and if the photon's energy is greater than 1022 KeV).
« Last Edit: 27/11/2006 08:31:49 by lightarrow »

#### Soul Surfer

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##### Re: Relative speed of photons - I'm puzzled
« Reply #9 on: 27/11/2006 09:40:58 »
I didn't mention that because I was thinking of visible light and not just all electromagnetic radiation.  This is I think one of the most amazing features of the laws of physics,  that pure energy in the form of EM radiation can change into massive particles just by passing through a strong electromagnetic field gradient.  To me this shows clearly that everything must be built up out of space-time itself probably as some sort of complex vortexes.

#### thebrain13

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##### Re: Relative speed of photons - I'm puzzled
« Reply #10 on: 27/11/2006 17:40:37 »
Soul surfer do you have a good link for that?

#### Soul Surfer

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##### Re: Relative speed of photons - I'm puzzled
« Reply #11 on: 28/11/2006 18:33:50 »
Not quite sure what you want a link for brain.  Please can you be a bit more specific.

If you want to find out more about the conversion of gamma ray photons into particle antiparticle pairs google  "pair production" it will give you lots of references about the process.
« Last Edit: 28/11/2006 18:41:57 by Soul Surfer »

#### thebrain13

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##### Re: Relative speed of photons - I'm puzzled
« Reply #12 on: 29/11/2006 03:54:09 »
thats what I was talking about, thanks.

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##### Re: Relative speed of photons - I'm puzzled
« Reply #12 on: 29/11/2006 03:54:09 »