The Double Slit Experiment

This experiment, the one you are about to learn about, was absolutely crucial in developing our understanding of quantum physics.

Wait wait wait! Don't run off!

I promise, it is not hard to understand. Just a little unintuitive.

I have introduced quantum physics before in a previous article, and like I said before, it has a reputation of being incredibly hard and impossible to understand.

Now, I won't go on to diminish the hard work of all the scientists in the field, because it is a very hard field of physics. Despite that, it is certainly not impossible to understand.

So on we go.

Quantum physics, in the simplest sense, looks at how stuff works at a really, really small scale, by examining the behaviour of very small pieces of matter.

Photons, or the force carriers of light, are very small.

For the longest time, the exact behaviour of photons was unknown. Is it a particle? Is it a wave? No one really knew.

Now onto the experiment.

Lets imagine a piece of cardboard with a slit, and a blackboard behind it.

Now throw spitballs at it.

What do you expect is going to happen?

Simple. A line of spitballs forms on the blackboard.

Let's add a second slit on the piece of cardboard.

Now that there are two slits, two spitball lines form on the blackboard.

This is all simple enough.

Now let's get rid of the spitballs, and replace them with photons.

With one slit on the cardboard, exactly what you expect happens. There is a single line of photons on the blackboard.

Let's add a second slit. This is where it gets interesting.

You would expect two lines of photons to appear on the blackboard.

But that doesn't happen.

There's this weird pattern, a series of lines, with the intensity of each getting stronger as it reaches the center. You do it again (because you are a good scientist and believe in repeated trials leading to more reliability).

This time, you place an 'observing machine' by the blackboard, and conduct the experiment.

There are two lines formed on the blackboard, just like with the spitballs.

There is only one explanation for this strange behaviour; light can act both as a particle and as a wave.

That weird pattern mentioned is known as a wave interference pattern, (shown in the picture below) and it's the behaviour of a wave. The two lines that were formed when the photon's activity was measured, was the behaviour of a particle in such a situation.

Don't worry if you feel a little light headed, this was a heavy article. I actually mentioned two very important quantum physics concepts, that of wave-particle duality and the uncertainty principle.

So congratulations! You just learnt a little bit of quantum physics.

E X T R A S T U F F

If you would like to learn more about one application of quantum physics, click here to learn a little bit about quantum computers.

This experiment was one of the first physics related things I ever learnt about, and I remember it being one of the main reasons I ever started liking the subject. This experiment never fails to impress me.

This was the video I watched and immediately sent to all my friends. I highly recommend you watch it. You can very easily tell I was highly influenced by it, even without meaning to have been.

https://www.khanacademy.org/science/chemistry/electronic-structure-of-atoms/bohr-model-hydrogen/v/photoelectric-effect

https://en.wikipedia.org/wiki/Wave%E2%80%93particle_duality

http://whatis.techtarget.com/definition/superposition

http://www.dwavesys.com/quantum-computing

https://blogs.scientificamerican.com/guest-blog/what-does-the-new-double-slit-experiment-actually-show/