Has the velocity of photons, which are coming from black holes, already been measured? I am not aware of such measurements.

The reason for the question is the following: If space-time near black holes has some extra dimensions, than photons created in this higher dimensional space-time might have non-zero components of the wave-vector in these extra dimensions. On the earth however we see just our 3 or 4 components of the wave-vector, so we might see just the projection of the original wave-vector onto our (3,1)-dimensional space-time. In this case the velocity of the photons might be less than the standard velocity of light.

I have in mind a special model of our universe. I believe, that our universe is the horizon of a (4,2)-dimensional space-time. On this horizon (which is our universe) we do not see the extra dimensions, however, they should be visible near black holes. So, in this model, black holes are the windows into the extra dimensions.

  • $\begingroup$ No, I don't think so. We have detectors that receive a photon which is all the information we have. We use local experiments to deduce the speed of light, and more experiments to deduce that the speed of light in vacuum is constant. Then we work backwards to say, "This photon must have left the vicinity of the black hole X years ago". We only have 1 piece of information, "received photon at time=now"; we do not know "when" the photon was emitted. I think we would have to create a black hole locally and run experiments to verify your model. Simulations with assumptions can do that. $\endgroup$
    – Quantic
    Jul 6, 2016 at 15:35
  • $\begingroup$ I presume you mean "coming from near black holes"? No (Hawking) radiation has been observed to come from a black hole. $\endgroup$
    – ProfRob
    Jul 6, 2016 at 22:18
  • $\begingroup$ @RobJeffries Sorry for being not exact. I meant "coming form near black holes". $\endgroup$ Jul 7, 2016 at 10:14
  • $\begingroup$ @Quantic We measure the speed of light exactly up to at least nine decimal digits. So I do not understand, why this should not be possible with light coming from near black holes. $\endgroup$ Jul 7, 2016 at 10:40

1 Answer 1



  1. Photons can't come from the singularity of a black hole, or from beyond the event horizon.
  2. While some very good candidates for black holes exist, none have been certainly observed.
  3. We have never observed Hawking radiation: it remains theoretical.
  4. We have observed radiation from the accretion disk of likely black holes, however the matter in this disc is orbiting in 'normal' spacetime.
  5. Direct measurement of the speed of individual photons is pretty much out of the question. We can measure the speed of light by creating photons at one point and then timing when those photons arrive at another. You can't measure a photon's passing, as to measure it you would have to interact with it. If you interact with it, it is no longer the same photon. This leads to...
  6. When we speak of a photon travelling from one point to another, remember that it is a quantum process: the photon interacts with matter and with itself. Photons are not classical particles. Everything about them needs to be considered through quantum electrodynamics.
  7. The flashes from pulsars are coming from a region of intense gravity. They do propagate at the speed of light: pulsars measured from different locations flash at slightly different times due to light travel time. There is nothing special about the space-time outside of a black hole as compared to that around a neutron-star, only a difference of degree. Gravity is still pretty intense around a neutron star, $g\approx10^{12}\mathrm{ms^-2}$
  8. Gravitational redshift from white dwarfs is well known. Gravitational redshift is predicted by relativity, which assumes a constant speed of light.
  9. Constant speed of light is pretty fundamental to relativity. If a theory breaks this, a lot of other physics is broken (exceptional claims require exceptional evidence)
  • 1
    $\begingroup$ ad 1-4): I meant "photons coming from NEAR black holes". ad 5): We meassure the Speed of light rather exactly, so why shouldn't this be possible for light from black holes too. ad 6): I do not understand the relevance of this comment. ad 7): This is an important argument for me. The question is, if space-time is really strongly deformed around pulsars. ad 8): White dwarfs do not have such a strongly deformed space-time, as far as I know. ad 9): Yes, that's right, but the question is, if there is something different in heavily deformed space-time near black holes. $\endgroup$ Jul 7, 2016 at 10:56
  • $\begingroup$ discussed in body. If photons don't have constant speed, I'm not even sure what "space-time" means, I certainly don't know if black holes exist. $\endgroup$
    – James K
    Jul 7, 2016 at 12:33
  • $\begingroup$ ad 5): There are not single photons coming from a black hole, but instead a constant flow of photons. I do not really know how the velocity of light is measured. About 120 years ago a rotating disc with a slit was used by measuring when the pinches of light arrived at a detector in some distance. At least such a procedure should be possible. But of course measuring a single photon is quite difficult or impossible. $\endgroup$ Jul 8, 2016 at 10:10
  • $\begingroup$ ad 7): Pulsars emit very regularly (millisecond) flashes of light. So this light cannot has a variable velocity. On the other hand, I do not know where this light is produced (e.g. on the surface or somewhere high in the "sky"). However light from black holes is rather smeared out, therefore I think, that there is a greater chance to measure such a variable velocity effect. $\endgroup$ Jul 8, 2016 at 10:19

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