What particles besides photons can travel at the speed of light?

Answer 1

In order for a particle to travel at light speed, it has to be what's called "massless." There was a period of time when it was thought that particles called neutrinos fit this bill as well as photons, but it was recently discovered (say, within the last ten years) that neutrinos actually have a very tiny rest mass. Consequently, while they can travel very close to light speed, they can't reach it.

In so far as I know, there aren't any particles which have been observed that are massless, other than photons. Gravitons would be massless if they exist - and they may - but no one has seen one yet, so they have to fall into the "theorized but not observed" category.

Answer 2

Any particle with zero rest mass will travel at the speed of light. Gravitons have zero rest mass therefore they also travel at the speed of light.

Particles with rest mass (e.g. electrons, protons and neutrons) cannot ever reach the speed of light, although they can get arbitrarily close to it.

Since neutrinos have recently been shown to have a small but nonzero rest mass, they cannot travel at the speed of light.

Answer 3

None, but tacheon particles are supposed to travel faster than the speed of light.

Answer 4

If we're talking about observed particles and not sub-nuclear physics; I don't know of any although I wouldn't be suprised if neutrinos traveled near that speed since they are so so light.

Answer 5

neutrinos. When a supernova outside our galaxy was seen with optical telescopes in 1987, the neutrinos arrived at the same moment. Both had taken 170 000 years to get here.

Answer 6

Neutrinos

Answer 7

Gravitons?