How Much Does a Shadow Weigh?

I’m sure that we all love to have fun with hand shadows, but how much does a shadow weigh? It might sound like a silly question, because it is. I mean, a shadow cannot be put on a scale and weighed, but the material that it falls on top of can be weighed. And we know that light has energy.

In fact, when light encounters an object, it pushes that object just a little bit. On the surface of Earth, when sunlight is hitting it, every square inch is being pushed with a force of about one-billionth of a pound, which is basically nothing. But, over a large enough surface area, the results can be pretty fun. On a sunny day, the city of Chicago weighs 300 pounds more, simply because sunlight is falling on it, pushing it. In outer space, where solar wind isn’t filtered by Earth’s atmosphere or magnetic field, the results are even bigger.

A space craft, traveling from Earth to Mars, would be pushed by light 1,000 km off course. So these things have to be factored into journeys to Mars. We’ve actually already created things that can sail with light: giant reflective solar sails that are pushed by the Sun’s light. So, in a way that is calculable, though difficult to measure, an area covered in shadow technicaly weighs less than surrounding areas being pushed by light.