In such cases, the mass of single photons is zero.

They could blow us out of the galaxy with a single photon torpedo.

Replies: A single photon can only be seen by one observer.

Actually, the eye is not sensitive enough to "see" a single photon.

It was like staring into a black hole- not a single photon of light escaped the darkness.

The sensors are so sensitive that they need only very little light and can even detect single photons.

How could this explain the fact that a single photon refracts?

He basically answers your exact question about a single photon with "who knows?"

However, they are massless and the energy of a single photon is related only to its frequency.

This will result in the single photon interfering with itself.