We come from the future
We come from the future

# Poisson's Spot: The Greatest Burn in Physics

Poisson's Spot is proof that even if you're right about things, you can be humiliated forever. Find out why this physics concept has caused the name Siméon Poisson to live in infamy for almost 200 years.

In the early eighteen hundreds, the French Academy decided to hold a friendly competition. The members of the Academy, like scientists around the world, noticed that when light travelled from one medium to another, it bent. Glass, water, gel, anything that allowed light to get through would only give it passage if it changed its course. People could measure the bending of light, but were baffled as to what made it happen. The French Academy attempted to solve the problem with by pitting the minds of various scientists against each other.

Enter Augustin Fresnel. He was an engineer, and had studied the effects of light. Working backwards from there, he came up with a novel idea; light was emitted in waves perpendicular to the direction it travelled in. His idea caused a stir, especially with Poisson and Poisson's peers. They believed that light travelled as a series of particles, all of which had complicated interactions with each other. Poisson, Fresnel, and other scientists got together and had a rousing series of debates on the nature of light and the observable effects of its journey through various media.

Towards the end of the debates, Poisson came up with an argument that seemed to burn Fresnel's theory to the ground and salt the earth beneath it. If someone is being pelted with objects, their best chance to avoid injury is to hide behind a larger object (at least until heat-seeking missiles ruined it for everyone). Hide behind a rock, and a stream of pebbles coming after you will either bash into the rock or fly harmlessly by. If, on the other hand, a person is waist-deep in water and wants to hide from a wave, they won't be safe crouching behind a rock. Waves bend around solid objects.

Poisson reasoned that if light really was a wave, then when a light was turned on a perfectly spherical object, the light waves would bend around the sides of that object. The perfect symmetry of a sphere meant that all the light waves would meet in the exact center of the shadow behind it. There, people would be able to see a bright spot of light.

Clearly, this was preposterous. After everyone finished laughing, they went off to wench, or drink, or whatever famous and successful French scientists did in those days. At least one person didn't join them. Dominique Arago, one of the judges, realized that Poisson had described the perfect experiment. He found a round object, he found a light, and pretty soon, he found a spot. It was right were Poisson said it would be.