The Basics of the Double Slit Experiment

The quantum world is a strange place. Many quantum systems behave in manors which even the most experienced teams of scientists cannot explain. One central mystery to quantum mechanics is the double slit experiment. In this demonstration, a source emits quantum particles through a pair of slits (or more) and onto a screen on the other side. When observed, a very clear interference pattern is present which proves the spectacularity of this phenomenon.

When the source emits non quantum material, two outcomes have been found. You can think of the first outcome of as if the source were to shoot out golf balls. The golf balls would either pass through the slits or get stopped by the wall and eventually, would create two distinct areas, roughly the shape of the slits. Now, with the golf balls, you may simply shoot one from your source and you would be able to detect its exact position. This is how particles behave in classical mechanics. The other outcome would be that of waves in classical mechanics. If your source were to create a ripple or wave, it would pass through the two slits and create two new sources which would interfere with each other either constructively or destructively. Our detector would pick up an interference pattern similar to that of our quantum particles. However, since waves are not a physical object and are only the movement of a collection of mass, we cannot send out singular units from our source and record their exact position.

This is where the quantum particles get their unique attributes. They act as both waves and particles. You can shoot a singular quantum particle through the double slits and record its pin-point location like a particle whilst at the same time, the outcome creates an interference pattern like a wave. However, when you close one of the slits, the quantum particles will act just like the golf balls.

Now, say we wanted to know which of the two slits each quantum particle goes through to try to collect data to figure out why this phenomenon is happening. We can set up a detector on one of the two slits (let’s call it slit A) and every time a quantum particle goes through slit A, our detector goes off. What we would see is what would be expected. 50% of the particles from our source pass through slit A and 50% pass through slit B. Nothing is expected to be different and the data you have just collected matches your prediction. Except for one massive problem. The detector on the other side of the slits no longer resembles an interference pattern.

But how could this be? We are in no way interacting with the particles and we have not changed the experiment in any way. In fact, if you were to turn off your detector, you would be back were you started and you would see the original interference pattern. So what is it about detecting the particles that changes the experiment?

If you were looking for an answer, I’m sorry but I don’t have one. The world of quantum mechanics is beyond confusing and there just seems to be no logical conclusion as to why quantum particles behave the way they do. Maybe the electrons are aware of the observer. Maybe the observer does, in some way, interact with the particles. But most likely of all, the quantum world is beyond our understanding.