Question

Photons are massless particles but even they are energetic particles but according to Einstein equation
energy is directly proportional to mass why can we not say that it has mass.


Give reason to support your answer
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Answer 1

According to theory it has energy and momentum but no mass, and this is confirmed by experiment to within strict limits. ... Energy, they say, is equivalent to mass according to Einstein's famous formula E = mc2. They also say that a photon has momentum, and momentum p is related to mass m by p = mv.

As an explanation of why a large gravitational field (such as a black hole) can bend light, I have heard that light has momentum. This is given as a solution to the problem of only massive objects being affected by gravity. However, momentum is the product of mass and velocity, so, by this definition, massless photons cannot have momentum.

Answer 2

Answer:

According to theory it has energy and momentum but no mass, and this is confirmed by experiment to within strict limits. ... Energy, they say, is equivalent to mass according to Einstein's famous formula E = mc2. They also say that a photon has momentum, and momentum p is related to mass m by p = mv.