How much mass does a photon of light have?

Answer 1

"it is so light i cant be measured"

This is 'sort of' right. The mass of a photon is a difficult thing to talk about because we tend to think of there being one type of mass but, in physics, there are at least two types. One is the invariant mass and the other is the relativisticmass. These two masses are different - even for the same particle - if the particle is being observed from a reference frame whose velocity is not that of the reference frame of the particle itself.

We're used to the equation E = mc², but that is really only a part of the story. The full equation is E² = m²0c⁴ + p²c² ... and this contains a component related to motion: p (for momentum) is included in the second term of the full equation. The subscript 0 in the first term denotes the first type of mass: invariant or rest mass.

We know that momentum is a property that depends on mass as well as velocity: p = mv (using bold letters here to denote vector quantities: in the energy-mass equivalence relation, we use only scalar quantities, since energy is a scalar quantity and has no direction). So if m = 0kg, then p = 0 kgms⁻¹. The same is true if |v| (or p, the absolute value of v) is 0 ms⁻¹.

So now we get to the important bit. When dealing with photons, we have - in the full equation - a term for the invariantmass (also known as the rest mass): this is the m0²c⁴ bit. We also have a term relating to the relativistic mass: this is the p²c² bit. And the total eneery of a photon is given by the sum of these two terms.

When a photon has no speed (this is the absolute value of velocity), it has no rest mass. so the first term (m0²c⁴) is redundant. But when it is moving, this term remains unaffected. When it is stationary, the second term is also redundant. But when the photon is moving, this term now assumes relevance. Photon moves, and momentum is acquired. No photon, incidentally, can exist at rest.

The energy of a photon is given, then, by the square root of the product of the square of its momentum and the speed of light squared. Thus: E = (p²c²)½ which means E = pc. If we know the actual speed of the photon through a given medium, then we can calculate the relativistic mass of the photon by substituting p =mv in this equation, giving E = mvc.

This - when divided through by vc - gives E/vc = m, which is the relativistic mass of the photon. This curve is akin to the y = 1/x curve and so, as speed increases, mass actually decreases. But, because of the c multiplier, this decrease is probably quite insignificant.

Other than this somewhat mathematical treatment given here, I cannot go more specific because of the absence of any specific data on speeds and energies of photons.

To recap, then:

1- The invariant or rest mass of a photon is 0 kg. Always.

2- The relativistic mass if a photon is given by dividing the energy of the photon by the product of the speed of light in a vacuum and its actual velocity in the medium through which it is being propagated. This is a mass that decreases slightly as speed increases, but probably not by any measurableamount.

Answer 2

A photon of light is a massless particle, meaning it has no rest mass. However, it does have momentum and energy due to its wave-like and particle-like properties.