Question 16: Why must the rest mass of photon be zero?
Photon is energy particle moving with the velocity of light. If it has some rest mass, then according to Einstein’s special theory of relativity, its relativistic mass must be infinite.
Here m is the relativistic mass of photon, m0 is the rest mass of photon and v = c is the relativistic velocity of photon.
Similarly, applying Einstein’s mass-energy equation
So if the photon has some rest mass, its relativistic mass and energy should be infinity (equations 1 and 2).
But photon does not possess infinite mass or energy, therefore, it is concluded that its rest mass must be equal to zero.