# Does light have mass? Why? [duplicate]

I've been wondering whether light has mass. Yet given the wave-particle duality of light, the statement seems to be affirmative. With that, how to calculate it?

## 2 Answers

In quantum field theory, a photon's rest mass is proven to be zero. But relativistically, the photon's energy leads to the relativistic mass $$m=\frac{h\nu}{c^2}$$.

I would avoid mass concept of photon at all, because it doesn't have rest mass. Relativistic mass is very slippery thing and is not unambiguously defined. Someone just put Lorentz factor $${\frac {1}{\sqrt {1-{\frac {v^{2}}{c^{2}}}}}}$$ besides rest mass (multiply by $$m_0$$) and has called it "relativistic mass". Now we know for sure that photon has NO rest mass, so you can't substitute something for $$m_0$$ in photon case. Thus photon doesn't have relativistic mass too !

However, it has momentum: $$p = mc$$ using Einstein famous relation between mass and energy $$E=mc^2$$ and photon energy $$E=h\nu$$, we get momentum as: $$p = \frac{h\nu}{c}$$

Still a bit strange that some object without a rest mass can have momentum, but it is so !
There are numerous experiments which proves light pressure. To me photon and fields in general are very strange things.