Mass and momentum of a photon
The photon has from the mass conservation law deducing a mass, even if it has no rest mass according to Einstein. Accordingly, after the equivalence of space and time is that mass and momentum are equivalent. So a photon has an impulse. This corresponds to the elementary mass multiplied by the speed of light. This applies to the elementary frequency. Otherwise, you can compute the mass of a photon with m = h * f / c². The momentum is corresponding to p = h * f / c. In especially it has no virtual mass, etc. .. or any kind of that. It is also shown by the quantum theory. In the Heisenberg's uncertainty principle, it is assumed that even if a test object is fired by a photon, this makes the measurement uncertain. This is because the photon with its mass and its momentum shifts the test object so that the location can not be accurately determined.
Mass and rest mass of the photon
Some scientists say that the photon would have no mass, because it has no rest mass. The rest mass is only a theoretical concept introduced by Einstein, which is defined as m (0) = m * the relativistic root (= square root of (1 - v ² / c ²)). The rest mass is the concept, that is artifically. It describes in theory, the value assumed by a particle when it would be absolutely not moving forward. As for the theory of relativity, nothing happens at absolute rest, it is only theoretical. But as I said: The rest mass is the artifically concept, the actual mass depends on it after the above given formula. The rest mass of the photon after Einstein is 0. Nevertheless, the photon has a true mass which is greater than 0. Others in Germany do state this, too, but they call it relativistic or kinetic mass. This is a bit reminiscent of Lorentz, one of the founders of the theory of relativity, which speaks of longitudinal and transverse mass of the electron. Today it is safe and speaks only of the mass of the electron.
Mass of the photon is not defined?
In former times it was said, that the photon has an undefined mass, as is so divided by zero as the relationship between mass and rest mass. m = m (0) / relativistic root. Since both the photon tends to zero, or even becomes zero, it was assumed that the mass of the photon is not defined. But that we can define division by zero, you can read here at Division by zero. We might also say, if we disregard this, that the limit tends to zero, but the term is 1, which corresponds to the elementary mass. This is because both tend the rest mass and the relativistic root to zero and this in a uniform qay. Why is excluded in the recent literature, this question is claimed per se, the photon has no mass, is beyond me. So easy to answer this question is not.
Photon and gravitation
According to Einstein, is subject to the photon, ie solar radiation of gravity. He showed the deflection of the rays of the sun during a solar eclipse, when the moon moves in front of the sun. The rays which nevertheless passed, the sun then forms a circle in the sky, was just as distracted as he had calculated it. Also, I take this as an indication that the photon has a mass. Lose weight particles are not subject to gravity, even if Einstein gravity is defined more as a curvature of space-time and viewed in isolation from the rest. Nevertheless to any force, as is the gravitational force, according to F = m * a part is the mass which accelerates or is deflected in this case. Without this mass, the force would be 0 and thus does not exist. Or, if we assume the force would be infinite at a mass of zero, which is obviously not the case. It should be normal with the exciting new Einstein equations that we do not forget that there are well-formed definitions before that you just can not deny.