If all photons travel at the speed of light, then why are two photons traveling in opposite directions relative to each other also traveling at the speed of light? Why don’t the speeds add up?
This note is a preparation for the next one, which will pose a more complex question. The answer is actually quite simple. And the short answer is that it is precisely because the photon itself travels at the speed of light.
But it takes quite a bit of mental effort to understand why this is so.
What happens if we choose one of these two photons as one of the reference frames? But it turns out this is practically impossible. At the speed of light, time stands still. And a single photon arrives at its final destination without losing time. Of course, as a reference frame, it is possible to choose a point in space along the trajectory(as far as this is possible in quantum physics) of this photon, or even a set of points representing the most probable trajectory of this photon. But then we immediately fall out of the reference frame associated with the photon into ordinary space. This means that there is a habitual flow of time and a limitation on the maximum speed of information transfer.
The same thing will happen at the final destination point. If, at the moment when this photon reaches its final point, the second photon still exists, then it will also move away from this point at the speed of light.
Similarly, if we choose as a reference frame the point from which both of these photons were emitted, then relative to this point each of the photons will move at the speed of light.
It turns out that it is impossible to choose a frame of reference where the speed of light would double.