Can you weigh sunlight

So that's easy, right? Light is composed of photons, which have no mass, so therefore light has no mass and can't weigh anything. Not so fast. Because photons have energy -- and, as Einstein taught us, energy is equal to the mass of a body, multiplied by the speed of light squared. How can photons have energy if they have no mass? One imagines Einstein thinking about photons re: mass and shrugging, hoping that nobody noticed the discrepancy.

Actually, what Einstein was proving is that energy and mass could be the same thing -- all energy has some form of mass. Light may not have rest or invariant mass -- the weight that describes the heft of an object. But because of Einstein's theory and the fact that lightbehaves like it has mass, in that it's subject to gravity , we can say that mass and energy exist together.

In that case, we'd call it relativistic mass -- mass when an object is in motion, as opposed to at rest [source: Gibbs ]. So our answer is a grab bag of yeses and nos. Does light have a mass that can be weighed on the bathroom scale? Most certainly not.

But it is a source of gravitational fields, so we could say that a box of light weighs more than a box without light -- as long as you're comfortable understanding that the "weight" you're measuring is a form of energy and not, say, pounds or kilograms [source: Ask the Van ]. Understanding the relationship between light and mass and photons and energy is daunting. It's enough to make one wish Einstein could patiently walk us through it himself. Sign up for our Newsletter! Best Answer No best answer has yet been selected by fatgaz.

The short answer is that light does not have any mass and does not weigh anything. I think the confusion here lies in the distinction between mass and weight. You are correct to say that a photon has zero mass. Weight is caused by the pull of gravity on something and light IS affected by gravity. A black hole for example pulls all light back into itself. Light passing a massive object is refracted.

Maybe this is the source of the calculated weight given in the question. Another possibility hinges on the interpretation of the word 'sunshine'. It consists mostly of electrons and protons. Now these DO have a mass and will exert a force on the earth equivalent to one of the choices offered. I would imagine it to be well above the weight of a flea, but I don't have enough data to know if it equates to an ocean liner or not. Tom Braider. Light does not have weight but it does exert pressure.

As early astronomers checked out the universe, they noticed a strange thing. No matter where a comet was or which way it was flying, its tail always pointed away from the sun. What was causing such a thing to happen?

In , James Clerk Maxwell put out a theory that light itself exerts a mechanical force on the objects it touches. The idea was sound, and it explained the tails of comets well enough, but Maxwell worked out that a square mile of sunlight would only put about four pounds of pressure on the ground on Earth.

Scales were obviously out. No one had the resources to make an instrument that was either huge enough to feel the weight of the sunlight, or delicate enough to be a practical size, and so the idea had no experimental proof.

In , a young physicist known as Peter Lebedev finally announced he'd found a way. He made tiny five millimeter panels of different types of metal and suspended them on poles which hung on an ultrafine thread.

Basically, they were tiny weathervanes. He then put them in a bottle from which the air had been pumped.