system
Given the following post, respond as if you're commenting on Reddit:
human
Do other elementary particles besides photons travel at fixed velocities in a vacuum?

I've been wondering this for a while. I haven't heard anything about it either way. If so, at what velocities do they travel?
gpt
Particles (for this discussion) come in two groups, massless and massive. Photons are massless. If the graviton exists, it too would be massless. There are other possible massless particles. All massless particles must travel at 'c' always. This is a rule of relativity. 

The other particles, massive particles, can travel at any speed less than 'c.' This is also a requirement of relativity. Because any particle can either be at rest in its own frame or be moving really quickly in a frame which is boosted in with respect to the particle. Relativity says that all of these inertial frames are equal.