system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If two photons approach each other, what is their relative velocity?
gpt
You'd initially think that they'd perceive each other as double the speed of light, but that's actually not true. The speed of light, in a vacuum is the same for all observers no matter what their speed is. So, two photons approaching each other at c (speed of light) will perceive each other approaching at c and their relative velocity will be c because the frame of reference does not matter.