mass of a photon?

Question

How can it be that photons can travel at the speed of light but objects that have mass cannot? Is it E=mc^2 wrong? I know that energy = mass and that mass = energy. But here it says the contrary when it is stated that photons don’t have mass.

Answer 1

Rest mass of a photon is equal to zero. (if not; it would have an infinite mass at the speed of light and would bend the space-time curve infinetely which means that all the universe would just be a point).

Answer 2

This question comes up in the context of wondering whether photons are really "massless," since, after all, they have nonzero energy and energy is equivalent to mass according to Einstein's equation E=mc2. The problem is simply that people are using two different definitions of mass. The overwhelming consensus among physicists today is to say that photons are massless. However, it is possible to assign a "relativistic mass" to a photon which depends upon its wavelength. This is based upon an old usage of the word "mass" which, though not strictly wrong, is not used much today. See also the Faq article Does mass change with velocity?.

Answer 3

Only objects that have a nonzero restmass are limited to speeds below the speed of light.

It is impossible to have a photon at rest, they are light and always travel at the speed of light for a particular medium, because they never are at rest it makes no sense to talk about a photon restmass, so we say it zero. The truth is that equations the deal with rest mass do not work for photons.

Answer 4

A photon at rest is considered to be mass-less. But a photon is never "at rest." Because a photon moves at the speed of light, it has energy which does affect mass (for example, the sun's light heats up the surface of a rock). A gamma ray will punch holes into living tissue or spaceships. Pretty massive if you ask me.

Answer 5

This question comes up in the context of wondering whether photons are really "massless," since, after all, they have nonzero energy and energy is equivalent to mass according to Einstein's equation E=mc2. The problem is simply that people are using two different definitions of mass. The overwhelming consensus among physicists today is to say that photons are massless. However, it is possible to assign a "relativistic mass" to a photon which depends upon its wavelength. This is based upon an old usage of the word "mass" which, though not strictly wrong, is not used much today. See also the Faq article Does mass change with velocity?.

The old definition of mass, called "relativistic mass," assigns a mass to a particle proportional to its total energy E, and involved the speed of light, c, in the proportionality constant:

m = E / c2.

This definition gives every object a velocity-dependent mass.

The modern definition assigns every object just one mass, an invariant quantity that does not depend on velocity. This is given by

m = E0 / c2,

where E0 is the total energy of that object at rest.

Answer 6

Photons have zero "rest" mass, but that isn't the same as having no mass. In Special Relativity, mass is not a fixed quantity, but depends on the motion of an object relative to an observer.