# Acceleration of a particle at the speed of light [duplicate]

Let's take a particle with the speed of light and throw it to a reflective surface with a 90 degree angle. We know that the speed of light is constant thus this particles speed is constant. But doesn't it's speed reduces to 0 at the fraction of reflection ? If it accelerates, doesn't this phenomenon contradict with Einstein saying speed of light is a constant ? (I know that light can accelerate due to its form of wave therefore I used I single particle at the speed of light.)

• Light, and all electromagnetic radiation, in fact all massless particles, travel at the speed of light. Hence they do not accelerate. – Mozibur Ullah Jan 17 '18 at 17:47
• But light does have mass. And I know they don't accelerate but it contradicts with the experiment above – Canberk Özçelik Jan 17 '18 at 17:49
• Photons aren't little balls that bounce off a surface of the mirror. Photons are observable manifestations of the quantum electromagnetic field represented by the wave function. For example, if photons were little balls, a single photon would bounce off a particular very small area of the mirror. In reality however every single photon bounces off the entire surface of the mirror. In fact, it takes longer to get to the far corner of the mirror, so a single photon bounces of f the entire mirror not even at the same time. There is no acceleration while a change in direction is not prohibited. – safesphere Jan 17 '18 at 18:21
• Light has mass energy by virtue of its motion. It's rest mass is zero. – Mozibur Ullah Jan 17 '18 at 18:29
• "Mass" is a vague term. The specific term used in modern relativity is "invariant mass". For massive particles, the invariant mass is the same as the rest mass. For massless particles like photons, the invariant mass is zero. – safesphere Jan 17 '18 at 18:36

## 2 Answers

When you say "particle" it implies volume and mass. A photon always travels at the speed of light, there is no acceleration. It can be absorbed and emitted, however.

As far as the speed of light being a constant, is understood to be the speed of light in a vacuum.

• The explanation that each photon is absorbed and re-emitted by the mirror is incorrect. If this were true, the wave function would collapse at the mirror, but it does not. The coherency of light is preserved by the mirror. You can reflect a beam of light and still see interference and even quantum entanglement. Therefore, each photon is reflected off the entire surface of the mirror as an observable of the quantum wave interacting with electrons in the mirror and reflecting of it, but photons are absorbed in the photoelectric effect where the wave function does collapse localizing each photon. – safesphere Jan 17 '18 at 18:33

Maybe this is not the answer but the concept is light has no mass and has a constant speed c in vacuum, if you change the medium you can observe less than that but not more. If you wonder that how does acceleration effects the photon, maybe you can see gravitational shift (red shift), in this case velocity cannot change but the frequency can. That is, no contradiction.