Life's Great Mysteries - Does light weigh anything?

Caution: Mind-bending concepts ahead...

Perhaps you’ve heard of light sails (or solar sails). These are huge parachute-like sails that can be expanded in space to allow the pressure from light emitted from the sun to propel a spacecraft. The first spacecraft to demonstrate that this actually works was the Japanese craft IKAROS in 2010. Think about how odd this seems—light emitted by the sun can actually push something. This must mean that light must weigh something, right?

Well, first it’s important to understand the difference between weight and mass. MASS is the total amount of matter, or stuff, an object contains. WEIGHT is the force of gravity on an object. I typically weigh 190 pounds (86 kg). On Earth, my weight and my mass are the same—190 pounds. But, if I go to the moon, where there is less gravity, my weight and mass are no longer the same. My mass is still 190 pounds, but my weight is only 32 pounds. On Mars, my mass is still 190 pounds, but I weigh 72 pounds. Remember, mass is how much stuff there is in an object, so mass does not change in situations with different gravity. Weight does change.

Light is made up of photons, and photons do not have mass. So, a simple answer to this question is NO… light does not have mass, and therefore it does not have weight. But things aren’t that simple. Light actually has momentum (if you’re wondering how something without mass can have momentum, that’s beyond my ability to explain… it has to do with the fact that light also has energy… light is kind of weird). Anyway, light does indeed have momentum, and it can exert pressure on a surface. This is why light sails work. This is why, when I stand in direct sunlight, I weigh slightly more than when I stand in the shade. The sunlight from above pushes me downward with a slight amount of force. On a sunny day, the city of Chicago weighs 140 kilograms more than at night, simply because sunlight is pushing down on it.

Here’s a tidbit that blows my mind: If you captured all the sunlight falling on Chicago in any moment and put that sunlight in a box that has perfect mirrors on the inner walls—mirrors that reflect 100% of the light so that the photons are continually reflected back and forth in the box, the box would then weigh 140 kg more than it did before. And in space, where the box has mass but no weight, more force would be required to accelerate the box. So, this seems like light has mass, right? Not exactly. Because it is the energy and momentum of the light that causes this to happen. Instead of saying the light in the box has mass, it is more accurate to say the light in the box contributes to the total mass of the box.

What a weird and wonderful universe we live in!

(this is a conceptual image of a spaceship with a solar sail)

Photo Credits:
- Ship with solar sail - Midjourney v. 6.1