The Double-slit experiment

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The Double-slit experiment

Perhaps the most famous experiment in modern physics, the double-slit experiment. The results of this experiment have caused many physicists to doubt and think about what reality actually is. It turns out that particles can be in 2 places at the same time, that particles behave as a wave and as a particle and that when we observe it they seem to make a choice by being in 1 place. How about all this?

What is it about?

Let's do a thought experiment. Imagine you have a device that shoots tennis balls on a wall. You immediately see where the tennis balls hit the wall. Then put a wall in between with a hole to the left and right of the middle. You can imagine that the balls hit the firste wall or go trough one of the holes and hit the seconde wall. By drawing a line from the devide, trough the holes of the firste wall, onto the seconde wall you can say that the tennis balls between those lines must hit the wall. Still quite logical, right? But now we are going to do the same again, but with electrons (it is also possible with photons, these are light particles).

Dots: Tennis balls. Lines: The tennis ball must move in between. Dashes: The area where the tennis ball can hit the wall.

First of all, it is important to know what electrons are. Maybe you have heard of atoms? An atom consists of an atomic nucleus (consisting of protons and neutrons) and an electron cloud (if you want more information about what an atom is and how the quantum world works, click here). During the experiment, these electrons are fired towards the walls by a device. You would expect the same thing to happen with the tennis balls, right? Well, no. As soon as no observation is made, it appears that a certain pattern is created on the 2nde wall, called an interference pattern.

Interference pattern

How does this work than?

This also sounds very tricky, let's try to clear that up. Throwing a stone into the water creates a wave that goes in all directions, let's just take the part that moves further into the water from your point of view. The wave keeps moving away from you and nothing crazy happens. Now throw 2 stones next to each other and look at what the 2 waves are. You will see that the 2 waves react with each other, we call this interference.

The interference pattern is a bit special. Of course it does not happen with the tennis balls, as you have seen, but with electrons, for example. You probably wonder how that is possible, since both consist of matter. It turns out that particles in the quantum world can behave as both a particle and a wave. This effect is known as the wave-particle duality. We therefore describe them with a probability wave. In it you can see where you are likely to find the particle, or at least where you are more likely to find it. This is also reflected in the interference pattern. The further away from the, the less likely it is to find the particle. When 2 waves come together, they get bigger where they meet, so the chance of finding them there increases. Where the 2 waves do not touch each other, the probability almost completely disappears. That is why you also see the pattern.

Green: interfaces, here the probability increases. Blue: Waves. Red: Probability.

Even stranger things happen

And that is by no means the strangest thing that has been noticed. As soon as we try to see what is happening and why the particles behave so crazy, the wave behavior suddenly disappears and "chooses" normal particle behavior. So as soon as we start looking, we suddenly see the same pattern as with the tennis balls. Then we stop observing again, and suddenly the interference pattern is there again. As soon as the particle interacts with the environment, the wave behavior is over. This is known as the measurement problem in quantum mechanics. So it is as if the particle, during the wave behavior, is in several places at the same time. We call this superposition. If we start looking at it, it suddenly becomes a particle and we know exactly where it is. It is also known as the collapse of the wave function.

We still have no idea how this is possible. It's as if there is a world where the waves are and our world where the particles are. So the gap in between is called the measurement problem and nobody knows what's going on. For now it is a gap in our knowledge.

Het tweespletenexperiment toont aan dat de wereld om ons heen veel gekker is dan we onszelf kunnen voorstellen. Mocht je je afvragen wat voor ideeën er zijn rond de problemen die deze experimenten ons hebben opgeleverd kun je onderstaande link bekijken. Hier is er eentje die je misschien nog wel gekker vindt dan dit artikel zelf: Veel-Werelden Interpretatie. Daarin komt het erop neer dat elke keer als de golffunctie ineenstort en we dus 1 deeltje op 1 plek zien eigenlijk al die andere mogelijkheden ook echt gebeuren. Dus de realiteit splitst zich als het ware voortdurend in oneindig veel mogelijkheden. Oftewel, er zijn oneindig veel parallelle universums. Maar daar is nog niks van bewezen, alhoewel snaartheorie (een erg ingewikkelde theorie die probeert te verklaren hoe alles ontstaan is) wel aanwijzingen geeft voor het bestaan van een multiversum.

Mocht je je ook afvragen wat voor ideeën er zijn rond het multiversum, klik hier.

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