Speed of a photon relative to another photon

  • #1

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Say a beam of light is cast somewhere - are the photons transmitted at a speed equal relative to one another, or would one photon still perceive the one behind it as moving at light speed?
 

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  • #2
If you would see from the point of view of a photon...another photon moves at the speed of light relative to it....for a photon ...there is no conception of space and time
 
  • #3
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Say a beam of light is cast somewhere - are the photons transmitted at a speed equal relative to one another, or would one photon still perceive the one behind it as moving at light speed?
Photons ALWAYS travel at the speed of light. So, this answers your question, no ?

marlon
 
  • #4
pervect
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Photons don't actually experience time. Technically, this means that the Lorentz interval along the worldline of a photon is zero (null), while the Lorentz interval along the worldline of any massive object is timelike.

It is because the Lorentz interval along the worldline of a massive object is timelike that massive objects (such as human beings) exerience time, and paramaterize their worldlines by it.

While photons can paramaterize their worldline by an affine parameter, this affine parameter cannot really be considered to be time.
 
  • #5
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Photons ALWAYS travel at the speed of light.
Do they now?.. I thought you could slow down a photon if you push it through a space occupied by other particles. Don't photons move slower through the atmosphere, or through a glass prism?
 
  • #6
ZapperZ
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Do they now?.. I thought you could slow down a photon if you push it through a space occupied by other particles. Don't photons move slower through the atmosphere, or through a glass prism?
Please read the FAQ in the General Physics forum.

Zz.
 
  • #7
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That sort of makes sense, so the photon doesn't actually slow down, it just takes a longer route through the medium?

ps. minor spelling/grammar error on the FAQ
The emitted photon may encounter other lattice ions as it makes its way through the material and this accumulate the delay.
 
  • #8
Fredrik
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Say a beam of light is cast somewhere - are the photons transmitted at a speed equal relative to one another, or would one photon still perceive the one behind it as moving at light speed?
If you would see from the point of view of a photon...another photon moves at the speed of light relative to it....for a photon ...there is no conception of space and time
There no natural way to define the "photon's point of view". See my posts in this thread.

In particular, there's no inertial frame that has the world line of a photon as its time axis.

Photons ALWAYS travel at the speed of light. So, this answers your question, no ?
It doesn't. Photons have speed c in all inertial frames, but not in general coordinate systems.

Edit: I didn't realize how old this thread is.
 
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  • #9
if 2 photos are travelling, they would observe each other as 'stationary'.
if a being on one photon fired another photon, the 2nd photon would travel the same speed as the 1st photon i.e the 1st photon will observe the 2nd photon travelling at the same speed, and the 2nd photon observing the being on the 1st photon trAvelling together. of course, this situation is possible if the being on the 1st photon was made of mc².
 
  • #10
HallsofIvy
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if 2 photos are travelling, they would observe each other as 'stationary'.
if a being on one photon fired another photon, the 2nd photon would travel the same speed as the 1st photon i.e the 1st photon will observe the 2nd photon travelling at the same speed, and the 2nd photon observing the being on the 1st photon trAvelling together. of course, this situation is possible if the being on the 1st photon was made of mc².
First a photon cannot "observe" anything- as Fredrick said, it has NO "point of view".

Also, throughout this you talk about speeds without saying what they are relative to. Third, I have no idea what you mean by "made of mc2".
 
  • #11
By "observe" I meant any form or method of detection. Obviously this won't be sight (using EM waves from the visible spectrum)

Relativity of the speeds are to the photons in travel.

"made of mc²" means an 'energy being'.
 
  • #12
In the theory of Millenium Relativity it shows that light-or any form of EM radiation, or c, is the only thing that is not relative, and is not affected by the motion of the source. Assuming that these photons are travelling through the same medium, then there would be no difference, but if there was, no matter the speed of the observer, the speed of light remains a constant.

-Harry
 
  • #13
George Jones
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In the theory of Millenium Relativity
Note that the Physics Forums rules,

https://www.physicsforums.com/showthread.php?t=5374,

to which all member agree when they register, state
Overly Speculative Posts: One of the main goals of PF is to help students learn the current status of physics as practiced by the scientific community; accordingly, Physicsforums.com strives to maintain high standards of academic integrity. There are many open questions in physics, and we welcome discussion on those subjects provided the discussion remains intellectually sound. It is against our Posting Guidelines to discuss, in most of the PF forums or in blogs, new or non-mainstream theories or ideas that have not been published in professional peer-reviewed journals or are not part of current professional mainstream scientific discussion.
 
  • #14
DaveC426913
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By "observe" I meant any form or method of detection. Obviously this won't be sight (using EM waves from the visible spectrum)
Photons do not detect in any sense at all. Photons do not experience time. Photons do not have a valid frame of reference.

"made of mc²" means an 'energy being'.
Sorry, this is nonsense either way.
 
  • #15
HallsofIvy
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By "observe" I meant any form or method of detection. Obviously this won't be sight (using EM waves from the visible spectrum)
I did not ask for a definition of "observe"- the point is that photons, having no "point of view" cannot observe anything.

Relativity of the speeds are to the photons in travel.
You cannot measure speed relative to a non-existant frame of reference and photons have NO frame of reference.

"made of mc²" means an 'energy being'.
In other words, it doesn't mean anything!
 
  • #16
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I once heard Feynman say something like: ”c is just the mean value, some photons go slower, some faster”.
 
  • #17
DaveC426913
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I once heard Feynman say something like: ”c is just the mean value, some photons go slower, some faster”.
You met him? What was he like?
 
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  • #19
Fredrik
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It might be a little easier to get a handle on this kind of discussion in the context of Light Coordinates.

http://en.wikipedia.org/wiki/Light_cone_coordinates
Do you have a reason to think of those coordinates as the "reference frame of a photon"? And don't just say that a photon is stationary in it. (There are lots of coordinate systems with that property).
 
  • #20
pervect
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If you stick to linear transformations of (x,t) where (x,t) are Minkowskian coordinates, I think there's only the one set that keeps light in both directions stationary.
 
  • #21
Fredrik
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If you stick to linear transformations of (x,t) where (x,t) are Minkowskian coordinates, I think there's only the one set that keeps light in both directions stationary.
Light going in the "wrong" direction isn't stationary in this coordinate system. It moves at infinite speed. More importantly, the method we use to single out the comoving inertial frame as "the reference frame of" a massive particle, doesn't single out this coordinate system. It doesn't even even work for massless particles.
 
  • #22
pervect
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x-ct is constant for light going in one direction, and x+ct serves as an affine parameter. x+ct is constant for light going in the other direction, and x-ct serves as an affine parameter.

Both x-ct and x+ct are null coordinates - I don't quite get the reference to "infinite speed", I'm guessing that you're interpreting the fact that one null coordinate stays constant and the other varies as an indication of "infinite speed". I would prefer to think of speed as being unrelated to the coordinates used, so that the speed of light is always c.
 
  • #23
Fredrik
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Both x-ct and x+ct are null coordinates - I don't quite get the reference to "infinite speed", I'm guessing that you're interpreting the fact that one null coordinate stays constant and the other varies as an indication of "infinite speed". I would prefer to think of speed as being unrelated to the coordinates used, so that the speed of light is always c.
I consider speed to be the absolute value of the velocity in a coordinate system, which of course is "change of position coordinate"/"change of time coordinate". So to even assign a speed to two massless particles going in opposite directions (in the inertial frame that was used to construct this coordinate system), we have to label one of the axes "time" and the other one "position". (Yes, that's pretty weird, as they're both null lines, but how else can we make sense of "speed"?) Now we have a world line that coincides with the t axis (dx/dt=0) and one that coincides with the x axis (dx/dt=infinite).

I'm familiar with a coordinate-independent definition of acceleration (which is quite useful), but I don't know a coordinate-independent definition of speed.
 
  • #24
tiny-tim
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I once heard Feynman say something like: ”c is just the mean value, some photons go slower, some faster”.
rubbish! :rolleyes:

Feynman never said any such thing!
 
  • #25
HallsofIvy
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Feynman was a notorious joker!
 

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