The double slit experiment is the most famous experiment in quantum mechanics.
It shows that quantum mechanical sized particles (e.g. particles that are even smaller than the protons and neutrons of an atom) can act with the properties of particles and waves at the same time. It’s pretty weird, because those two things are usually mutually exclusive.
But what the experiment really shows us is that when you look closely enough at the universe, it acts in strange and unfamiliar ways.
The double slit experiment works like this: You set up a light bulb, a metal plate with two thin slits cut into it, and a screen that’s coated with a special chemical that changes colour when exposed to light. Then turn on the light bulb and see what gets through.
If the particles of light (which are called quantum mechanical particles called ‘photons‘) act like particles, i.e. like a miniature version of tennis balls, this is the pattern you would expect.
But that is not what you get.
Instead, you end up with multiple lines that do not line up with the two slits.
That is pretty interesting. It’s called an ‘interference pattern’, and scientists recognised it as a pattern that all waves make. It tells us that between the plate and the screen, light must be moving in waves; something like in the video below.
This is where things get weird.
If you look more closely at the back plate (you’ll need some special sensors to look closely enough), you’ll see that the interference pattern is made of tiny spots. But spots are made by particles, not waves. How can you get a wave-like pattern that is made of particles?
You can run this experiment a million different times in as many different ways, and you’ll always get the same thing. Physicists have concluded that quantum mechanical sized particles have particle and wave-like properties at the same time.
Because all particles below a certain size have this effect, and everything including us is ultimately made from these tiny particles, it is the ‘dual’ nature of reality that the double slit experiment reveals.
This experiment is a proof of concept of the ‘particle-wave duality’. You might like to check out the best theory we have that explains how it happens, called the ‘correspondence principle‘.