History of Science & Tech #7: The 19th Century Experiment that Broke Reality

The "double-slit" experiment may seem like a very simple and ordinary experiment. However, it may be one of the most iconic experiments of all time. It proved that light behaves like a wave, it is an integral part of the exciting field that is quantum mechanics and most of all, it broke reality.

Double-slit experiment with electrons.
Image source: Wikimedia

Challenging Newton

In the 17th century, Isaac Newton developed the corpuscular theory, which basically states that light is composed of a stream of particles known as corpuscles. This theory was heavily flawed but due to the prestige of Isaac Newton, this theory was accepted by the community. That is until a young prodigy by the name: Thomas Young, stepped up. Young was a remarkable medical student, who was fascinated by the ear and eye. During his study of the eye, Young proposed a theory of coloured vision and during his study of the ear, he came up with a 47 letter alphabet that covered all human sounds. It wasn't long before Thomas Young spotted some similarities between sound and light, but with sound known to be a wave and light only known to be a particle, these similarities were odd. So Young dug deeper and studied Newton's corpuscular theory and quickly spotted its flaws. For example, the corpuscular theory failed to explain why different colours of light are refracted to different degrees, and why some light is refracted and some is reflected in water. So with the similarities between light and sound, and the flaws in the corpuscular theory, Young theorised that light was a wave. To prove his theory, Young created the double slit experiment.

The Experiment

The experiment is done by shining a beam of light onto a screen and placing a board, with two slits, in between. This results in a pattern called "the interference pattern", as you can see on the screen in the picture below. This pattern proves that light behaves like a wave. If light did not behave like a wave, the screen would only show two streaks of light in the shape of the two slits, as it would if you used something like spray paint. However, the interference pattern is a clear indication that the waves of light interfere with each other. Where the peak meets the through they cancel each other out, leaving nothing, this is called "destructive interference". Where peak meets peak, they add on top of each other, this is called "constructive interference". This whole procedure is why we have the interference pattern.

The double-slit experiment
Image source: Wikimedia

Reality = Broken

Looking at everything so far, you might be thinking that everything seems to be relatively ordinary, so what's with all the "reality is broken" thing that I'm talking about. Well here comes the part that's going to tickle your brain. Instead of shining a light through the slits, how about we break it up and fire single photons through them, but for the time being let's just block one slit. On the screen, all that would show is a shape roughly the same as the slit, still nothing out of the ordinary. Now let's clear the screen, open the second slit and proceed with firing one photon at a time, somehow this will eventually result in an interference pattern. So we firing one photon at a time, this photon passes through a slit, interferes with no other photon, yet somehow when you fire enough of them, they form an interference pattern! WHAT THE ACTUAL PHYSICS!!?? How the hell is this possible? But wait, it gets even more strange! Maybe the photons somehow split in half and go through each slit and somehow interfere with themselves on the side. So let's put a detector to detect which slit each photon is passing through, and sure enough, we see that 50% of the photons pass through the one slit and the other 50% passes through the other, but when we do this, for some reason, the interference pattern disappears and on the screen all we see is the shape of two slits.This is as if though the photons somehow knew that we were spying on them!

This experiment that was invented over 200 years ago has opened the door to so many possibilities. It suggests to us that particles like electrons somehow have characteristics of both, particles and waves. This suggestion is a massive part of quantum mechanics. At first glance, this experiment may seem ordinary, but what is actually happening is something that, to this day, baffles some of the world's greatest minds.

Thanks for reading.

References:

Thomas Young and the Nature of Light
Physics in a minute: The double slit experiment
The Double-Slit Experiment Cracked Reality Wide Open