Special Relativity Question?

Question

Why do photons not have infinite mass???

Answer 1

It's a mathematical limit thing. As a particle approaches the speed of light, its relativitic mass (E/c^2) equals its rest mass multiplied by a number that approaches infinity. However, since a photon has zero rest mass, that number (infinity times zero) is indeterminant. That means a photon can have any energy and still go the speed of light. It also means it *must* go the speed of light to have any energy at all (that is, to exist).

Answer 2

The mass m of a particle moving with a speed v is calculated using the formula

m = mo / √ ( 1- β^2) where β = V/C.

When V= C the denominator becomes zero and hence m becomes infinite.

But in the case of photon, m0 = 0 and also V = C.

And hence m = 0/0 is in the indeterminate form.


This does not mean that we cannot calculate m.
We know that the kinetic energy of photon is hn where n is the frequency of the photon.
hn = mC^2 and hence m can be calculated. The m here denotes the equivalent mass of photon moving with the speed of C. But there is no internal energy and m0 = 0.
And since h n and C are constants, m is a constant.

Answer 3

Remember, mass and energy are equivalent. If any of the energy in a photon were converted to mass, it would no longer travel at the speed of light - and would no longer be a photon. The rest mass of a photon is entirely converted to energy - light energy.

Answer 4

Because in order for mass to increase as you approach the speed of light, you must first have some mass to work with - and photons are classically massless particles.