# Light Mass

*Most recent answer: 06/03/2009*

Q:

It is said that mass increases as it approaches the speed of light and that reaching the speed of light means that the mass is infinite, requiring infinite amount of energy. If that is the case, why is light able to travel at the speed of light and why does light not have an infinite amount of mass?

- Billy Posey (age 15)

Kalamazoo, Michaigan, United States

- Billy Posey (age 15)

Kalamazoo, Michaigan, United States

A:

Nice question.

The rule about how mass gets bigger as something speeds up applies to anything which has some mass to begin with, when it's at rest. If it had zero mass to begin with, then multiplying by any finite number would still give zero.

If you had a particle with zero rest mass and it were to travel at the speed of light, trying to calculate the energy this way would give zero*infinity, which is undefined. Instead one can use a more general energy-momentum equation, which says that anything with zero rest mass has energy equal to its momentum * c. So there's another category of things, with zero rest mass, which have non-zero energy (or mass) only when they travel at the speed of light. Light is in that category. So are gluons, which carry the strong nuclear force.

Mike W.

The rule about how mass gets bigger as something speeds up applies to anything which has some mass to begin with, when it's at rest. If it had zero mass to begin with, then multiplying by any finite number would still give zero.

If you had a particle with zero rest mass and it were to travel at the speed of light, trying to calculate the energy this way would give zero*infinity, which is undefined. Instead one can use a more general energy-momentum equation, which says that anything with zero rest mass has energy equal to its momentum * c. So there's another category of things, with zero rest mass, which have non-zero energy (or mass) only when they travel at the speed of light. Light is in that category. So are gluons, which carry the strong nuclear force.

Mike W.

*(published on 06/03/2009)*