Light’s Weight

Most recent answer: 10/22/2007

Does light have weight? Why?
- homi (age 18)
Light does have weight, meaning that it is a source of gravitational fields. A box of light weighs more than an empty box. The reason is that ALL forms of energy have weight. However, under ordinary earthly conditions, the weight of light is insignificant compared to other weights.

Mike W.

(published on 10/22/2007)

Follow-Up #1: Does darkness weigh anything?

Does darkness have weight too?
- Tayven (age 9)
Fort Lauderdale, FL
Dear Tayven,

That is a very interesting question! Darkness actually refers to the absence of light. It doesn't have any weight in the same way that "ideas" don't have weight.

I hope that answers your question!
Sincerely, John

(published on 02/07/2011)

Follow-Up #2: mass, light, gravity

If light has weight, it means it has mass. And if it has mass, light cannot travel at light speed...what the heck?
- Anonymous
This one keeps coming up. Weight (gravitational effects) comes from energy and momentum in General Relativity. Light has energy and momentum and therefore participates in gravitational effects. Not only does it move through the curved space-time produced by its neighbors, but it also contributes a little to curving spacetime.

So your main question is answered, but now we get to the troublesome word "mass". Does light have mass? That depends on your definition of mass. If you mean the inertial mass m, as in momentum p=mv, where v is the velocity, the answer is yes. That's the same m which is given by E/c2. Since light has energy and momentum, it obviously has m in this sense of the term.

If you mean rest mass, also called invariant mass, the answer is sort of no. That's the m in E2=p2c2+m2c4. Since for a single photon E=pc, that m is zero. As soon as you have multiple photons going different directions, their p's partially or completely cancel even though their E's just add up. In case the p's exactly cancel, you end up with E=mc2, i.e. for that batch of light the invariant mass and the inertial mass are the same and not zero. The inertial mass of that batch of light is just the sum of the inertial masses of the parts, but the invariant mass is completely different from the sum of the invariant masses of the parts. Now the gravitational effect of that light can't depend on whether we call it a bunch of separate photons, with total rest mass zero, or call it a big bag of light, with rest mass E/c2. It's easier then to just think of E as the source of gravity. (p is too, but that's a more complicated term.)


Mike W.

(published on 03/07/2011)