Easy experiments sometimes lead to amazing ideas and discoveries.

Quantum mechanics is far removed from anything we understand and observe in our daily lives. This happens to the point where even the simplest experiments can have some of the wildest implications we have ever thought of. The double slit experiment is one example of this, it’s so simple that it is a staple in the modern high school physics classroom.

The Double Slit Experiment

The double slit experiment is one of the most famous experiments in quantum mechanics. This is because this experiment proves wave-particle duality. The double slit experiment is simple, and is often done in the physics classroom, but the implications are massive.

The double slit experiment is usually done with photons, but it can be done with any particle. In the average physics classroom visible light is the most commonly used. With a single slit the particles don’t show any major signs of being waves. Imagine cutting a rectangular slit into a piece of paper and shooting photons through it, the photons go straight through the slit, spread out in a bit, but will still be roughly rectangular when it hits the wall. When another slit is added this changes. Instead of forming neat rectangles on the wall, they form a series of lines.

https://upload.wikimedia.org/wikipedia/commons/thumb/8/8b/Two-Slit_Experiment_Light.svg/2000px-Two-Slit_Experiment_Light.svg.png source

The photons form a series of lines because of the inherent probability within quantum mechanics. Each particle only take on a position and velocity when they are interacted with. Because of this, each particle does not really have an exact position or velocity and instead all aspects of the particle appear based on random probability, and this takes on the aspects of a wave. One of the aspects of a wave is interference. Interference takes on two forms, constructive and destructive. In constructive interference the crests of two or more waves line up, in destructive interference the crest and the trough line up. In constructive interference the amplitude of the wave can be greater than each wave would be individually, while in destructive interference the waves can completely cancel each other out. For constructive interference, water will be higher, sound will be louder, and the chance a particle lands in a position higher. Destructive is the opposite, the water may not move at all, there may be no sound, and there may be nearly zero chance for a particle to show up in a position. There is still one question to ask in this situation, what waves are the photons in this experiment interacting with? The answer to the question is the particle is interacting with itself. This is possible because of the two slits and because in quantum mechanics nothing has a set position or velocity. When the slit the wave goes through is not measured, it enters a state where it goes through both slits as a probability wave, and then interacts with the target as a particle. When this happens an interference pattern emerges, which is based on the position of the particles. Measuring the particle at the slit forces it to choose which slit the particle goes through, which causes the interference pattern to disappear. This is great evidence that what is happening is the probibility wave interacting with itself.[[1]](https://en.wikipedia.org/wiki/Double-slit_experiment) [[2]](https://plus.maths.org/content/physics-minute-double-slit-experiment-0)

Infinite Slits

There will be no major change in the outcome of the experiment when a third slit is added. The only change will be the shape of the probability wave, not its characteristics. This holds true as four, five, six, and even seven slits are made, all the way up to infinity. A new question arises from this, what happens when empty space is treated as a set of infinite slits? The answer is that the particle has a chance to take every possible path. This includes looping back on itself, visiting the center of the sun, and disappearing off to somewhere on the other side of the universe. Even though the particle can travel all paths, some are more likely than others. This can be determined through path integral formulation. In this complicated calculation each of the infinite paths is given a number, and after all these numbers are added together, the probability of each path through spacetime is determined. Some paths happen to cancel each other out, while others increase the probability of the particle landing at a location. This cancels out many paths a particle can take, and is what causes the strangeness of the double slit experiment. [[3]](https://tinyurl.com/yc7hcuva) [[4]](http://www.einstein-online.info/spotlights/path_integrals.html)

https://upload.wikimedia.org/wikipedia/commons/5/5d/Three_paths_from_A_to_B.png source

Infinite Possibilities and Many Worlds

The many worlds interpretation of quantum mechanics is mostly based on the information above. In the many worlds interpretation when something about a particle is determined our reality “splits”. All of reality enters a state like the particle going through the slits, and act out all possibilities. In other words, when we interact with a particle and cause it to determine its position, velocity, or anything else about it, we become just one probability within the universe’s probability wave. One example is if we have Schrodinger's cat in a box. Schrodinger's cat is either dead or alive within in the box. When somebody opens the box to determine the state of the cat they themselves enter two states, one for each possible state of the cat. When anybody observes this person they enter the state of observing this person in one of the two states, and so on. That explanation leaves a lot out though. Everything is made out of insane amounts of smaller particles and they are constantly being “measured” by their own interactions with the environment and themselves. So the split does not happen only when we observe it, but that does not really matter too much. [[5]](https://plato.stanford.edu/entries/qm-manyworlds/)

Quantum mechanics is very different from what we normally observe in interactions throughout our daily lives.

[[1]](https://en.wikipedia.org/wiki/Double-slit_experiment) [[2]](https://plus.maths.org/content/physics-minute-double-slit-experiment-0) [[3]](https://www.youtube.com/watch?v=vSFRN-ymfgE) [[4]](http://www.einstein-online.info/spotlights/path_integrals.html) [[5]](https://plato.stanford.edu/entries/qm-manyworlds/)