In Chapter 10 we found that an infinite amount of energy is required to accelerate a massive particle to the speed of light. What does this imply about the mass of a photon?

Answer 1

must be pretty light :)

Answer 2

....like vanished

Answer 3

maybe we should have called it a wave?

Answer 4

The trick is stopping it to weigh it.

Answer 5

In fact it has ZERO rest mass but does have momentum and energy at speed. Therefore you can assign mass from E = m c^2 for example.
Google "momentum of photon"