When you feel the sun on your face, you're feeling weight as well as heat. But it took scientists thirty years of calculations to figure out how to measure the sunlight's weight.

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 1871, 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 1900, 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. By using a high-powered electric lights and a set of mirrors, he could direct beams of light onto the different panel surfaces, and make them move. This was not as easy as it sounded. He had to ascertain through repeated testing with the color and composition of the panels that nothing else was putting pressure on the panels. A similar object, called the Crookes Radiometer, was at first thought to prove a similar thing. It was found, though, that the heat of the panels caused the residual gas inside the glass to move a certain way and make the panels turn. The difference between Lebedev's contraption, called a Nichols Radiometer, and the failed Crookes' set up was Lebedev's panels twisted on their string even in a complete vacuum. When the other contraction was totally evacuated of air, the paddles stopped. In the absence of any other force on the paddles (including heat), Lebedev finally proved that it was light pushing the paddles around.

Lebedev went on to study the effect of light pressure on gases, an even more difficult task. If you want to try a similar experiment, but don't have the fine motor skills, just grab some force-meters and a quarter-mile of parachute silk and some friends. You can all suspend the silk in the shade, and then in the sun. If holding it in the sun makes it a pound heavier, you're golden.


Top Image: Flagstaff Photos

Second Image: NASA

Via P.N. Lebedev and Manhattan Rare Books.