In practice, given that in the vacuum of the universe is not perfectly empty, does the photon actually reach exactly c?

Is it even possible? If it travels at exactly c, then from the photon perspective, it arrives at the same time it leaves its point of origin, so for him the distance between the 2 points is zero?

Many thanks.

  • 1
    $\begingroup$ There is no such thing as a rest frame associated to a photon, so "from the photon perspective" doesn't make sense in special relativity. $\endgroup$
    – ChocoPouce
    Apr 30 '14 at 7:01
  • $\begingroup$ And so is it traveling at the speed of light exactly even if the medium is not perfectly empty? $\endgroup$
    – benji
    Apr 30 '14 at 7:04
  • $\begingroup$ Even if the vaccum is not perfectly empty, it doesn't mean that the density of matter in space is continuous. There might be some particles at some points, but if the photon doesn't encounter them it travels at the speed of light. If the photon meets a particle in the vacuum then it's going to interact with it in some way. $\endgroup$
    – ChocoPouce
    Apr 30 '14 at 7:13
  • $\begingroup$ I think an equivalent question is "is the index of refraction of space 1.00000[...]?". $\endgroup$ Apr 30 '14 at 7:24
  • 1
    $\begingroup$ You can't have a photon in a perfect vacuum ;-) and you can't measure it's speed either without another two objects for the start and end flag, and then you need a timer who can race down the rabbit hole between the two. However SI already has it defined exactly, so it can't be wrong ;-) $\endgroup$ Apr 30 '14 at 9:17

The short answer is yes, it travels at exactly c in vacuum.

In a non-vacuum where the speed of light is said to be c/n, this is a picture which only applies if you can't see what is happening on the microscopic scale. On the microscopic scale, a non-vacuum is actually a lot of vacuum with some atoms or other particles in it. When a photon scatters off a small lightweight particle, it is slightly delayed by the interaction. In a uniform medium, the interactions with atoms happen the same number per unit length for every unit length, so the wave is delayed the same amount in each unit length so it is "as if" the photon was slowed down to c/n. In fact, the photon is still traveling at c between atoms, and is experiencing time delays in its motion as it scatters off the atoms in the medium.


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