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Absolute rest is not possible as the concept of motion is relative. But can we assert that photons have absolute motion as the observers in all the frames of reference would agree to the same value of speed of light.

Do we consider frames of references moving at speed of light, if we do than above assertion won't be correct.

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  • $\begingroup$ By tagging this with general-relativity you open a whole 'nother can of worms. There are perfectly good ways of looking at things in GR when the speed of light at places distant from you is not constant. $\endgroup$ – dmckee Nov 22 '16 at 5:52
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    $\begingroup$ We cannot consider frames of references moving at speed of light because Lorentz transofmation formulas become undefined at $v=c$. We can only take $v<c$ and try to obtain some limiting results at $v\rightarrow c$, but they should be interpreted with caution. $\endgroup$ – Alexey Sokolik Nov 22 '16 at 9:25
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    $\begingroup$ Related: physics.stackexchange.com/q/29082/50583 $\endgroup$ – ACuriousMind Nov 22 '16 at 14:01
  • $\begingroup$ Also related: physics.stackexchange.com/q/16018/2451 $\endgroup$ – Qmechanic Nov 23 '16 at 8:14
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In a certain sense you are right: there is no inertial frame of reference in which a photon propagating in vacuum is locally at rest.

When measured locally and in an inertial frame, the speed of light is $c$ independently of the inertial frame we choose: this is one of the postulates of special relativity.

Notice anyway the keyword locally: on larger scales, where spacetime cannot be considered flat, we have to use the formalism of general relativity, and things may be very different: see for example this question & answer and also this one.

Also, it is important that we choose an inertial frame: if the frame of reference is not inertial, the speed of light may be different from c.

The similar behaviors found in the presence of gravitational fields and of non-inertial frames is no coincidence: the equivalence principle actually tells us that the forces experienced in a gravitational field are the same as those expereinced in a non-inertial frame of reference.

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All photons move on individual vectors through inertial space at exactly C. From this undisputed experimental fact, we can conclude that all photons move within the same inertial reference frame. This assumption leads to the conclusion that each photon is a material body with an intrinsic mass (m=p/c) and dimension (=h/mc) within this frame of absolute photon rest. Any motion of a photon’s source, relative to rest, has no effect on the photon’s velocity, but transforms its intrinsic mass and wavelength in proportion to the absolute motion of the source. This same transformation occurs between the photon’s intrinsic momentum at c and its observed momentum of . The Doppler effect allows the accurate measurement of any difference in velocity along a vector between source and observer, but it prevents any measurement of the three intrinsic inertial vectors of source, observer, and photon. To verify the existence of this elusive preferred reference frame, and to measure its position, the observer must look to the results of the Michelson-Morley experiment, the Uhuru pulsar observations and the Pound-Rebka measurements.

Source: http://www.circlon-theory.com/HTML/motionofphotons.html

I would just comment but I don't have enough reputation (sorry ;))

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  • $\begingroup$ Welcome on Physics SE and thank you for the contribution :) You might want to see this help page for typesetting formulas :) $\endgroup$ – Sanya Nov 23 '16 at 10:20
  • $\begingroup$ There are two problems with your first statement: it is difficult, if not impossible, to make any sense of the notion of an inertial frame moving at $c$. Secondly, even if we were talking about a group of objects moving at the same speed $v<c$ relative to some observer, one cannot infer that they share the same inertial frame from their all moving at the same speed $v$. The directions of the velocity vectors can be different, in which case so are the inertial frames. $\endgroup$ – WetSavannaAnimal Nov 23 '16 at 10:56

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