If there were a simple answer to how much light weighs, we'd all know it. There would probably be some sort of elementary school rhyme to help us remember the exact figure, much like our useful little ditty about 1492 and the ocean blue.
Instead, we're still asking, "Is light matter?" and must wade through complicated half-answers and concepts like light waves and electromagnetic waves and end up at an answer that goes something like, "Um, it kind of weighs a little, but not like how regular things weigh." Which is a terrible jingle to sing on the playground.
ContentsWe know now that light is both a particle and a wave. Back in the 17th century, Sir Isaac Newton proposed the corpuscular theory of light, suggesting that light consisted of tiny particles. However, his theory didn't account for phenomena like diffraction.
Early experiments by scientists like Thomas Young in the 19th century demonstrated that light exhibits wavelike behavior. A single-slit experiment demonstrated the wavelike nature of light as it passed through a narrow slit or aperture.
A light wave can also interfere with another, creating patterns of light and dark bands. This wave nature of light is fundamental to understanding phenomena such as diffraction and interference.
Later, experiments conducted by Albert Einstein and Max Planck showed that light can also behave as discrete particles, now known as photons. Photons are bundles of energy that carry momentum and can even exert pressure when they interact with matter, a phenomenon known as radiation pressure.
Now do photons have mass? Photons are the smallest measure of light, and no, they don't have mass. So that's easy — photons make up light, and they have no mass; therefore, light has no mass and can't weigh anything, right?
Not so fast. Because photons have energy — and, as Albert 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 light behaves 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].
Light is a form of electromagnetic radiation. The electromagnetic spectrum encompasses a vast range of wavelengths, from radio waves to X-rays to gamma rays. Visible light, the portion of the spectrum detectable by the human eye, consists of colors that vary with different wavelengths. Light energy is the energy that an electromagnetic wave carries in the visible spectrum of light.
Electromagnetic radiation can transmit energy through space without the need for a medium. This property distinguishes light and other electromagnetic waves from mechanical waves, such as sound waves, which require a material medium to travel.
This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.
