How are massless particles detected ,and how many in the Universe?

Question

Where has there been any experimental proof that light particles are massless?

Answer 1

Are you serious? Photons of light are massless. They are detected by the retinas in your eye when you look at anything. They are detected by your skin when you feel warmth from the sun. They are detected by telescopes, by cameras, and by life.

I don't know how many photons there are in the universe. It's a nice little bunch, that's for sure.

Other massless particles are supremely difficult to detect. The neutrino is a massless particle which interacts very weakly with matter, and could pass through a wall of lead several light-years thick as if the wall weren't there. They pass through your retina the same way, so we cannot see them.

Answer 2

The mass of a particle has nothing to do with its detection.

To be detectable a particle has to interact. There are only 4 known interactions in the universe - electrostatic, weak, strong and gravitational.

Photons are detectable because they interact via the electrostatic force with electrons in atoms (quantum electrodynamics). They also happen to be massless.

Neutrinos are difficult to detect precisely because they do not interact via the electrostatic or strong forces. They may or may not have mass.