The Double Slit Experiment

The famous double slit experiment is a classic demonstration of the strangeness of quantum physics. It highlights the fuzzy nature of reality at the quantum level. It's hard to get your mind around it, but in this essay I make an attempt to explain quantum physics. But before I get into the details of quantum physics (as related to the double slit experiment), first watch a cool video below that talks about the experiment. Don't skip this video otherwise everything I say next will sound like gibberish!



Interesting stuff isn't it? In my opinion it's utterly mind-blowing! How do the results of this experiment even begin to make sense? It's so contradictory to our everyday experiences and to what we know (or think we know) about the world.

Naturally you want more explanation on the quantum physics that's taking place in the experiment. So that's what I'm going to do next, but before discussing the results of the experiment I first want to explain some fundamentals of quantum physics, which you will no doubt find very weird!





Single quantum particles, such as photons and electrons, can take a superposition state in which they are located in all their possible positions at the same time. But if, at any one of these possible positions, a detector is placed, the quantum particle suddenly takes one fixed position. And this position can be at a location different from that of the detector.

Furthermore, these single quantum particles, while in their superposition state, behave as a regular wave, which interacts with itself and with objects it comes into contact with, such as going through two slits and producing an interference pattern as a result. This wave acts as an infinite collection of (quantum) particles which interact with each other, and with other objects, which is how real waves behave. Hence, this (quantum) wave behaves as a physical (real) wave until the instant that any part of it contacts a detector, at which point the location of the quantum particle suddenly takes a fixed position somewhere in the wave space. This phenomenon is commonly referred to as the collapse of the wave function.

This wave can also be thought of as a probability wave (in addition to being thought of as a regular wave). The instant the wave encounters a detector, the quantum particle takes on a certain fixed position within the wave space and the probability of taking this particular position is given by the probability density of the wave at this position. The areas of the wave that are larger are the areas that have a larger probability of the quantum particle appearing there.

For example, in the double slit experiment involving a single electron fired repeatedly through a double slit, an interference pattern is seen on the screen (which acts as a detector). In this experiment the single electron takes on a superposition (wave) state and passes through the double slit, just as a regular wave would, and creates an interference pattern on the screen as a result, just as a regular wave would. Once this wave reaches the screen the position of the electron suddenly becomes fixed (wave function collapse) and it appears at some point on the screen. After many firings of the electron, an interference pattern appears on the screen corresponding to the shape (and probability density) of the wave.

If a detector is placed just behind any of the two slits (and well before the screen), the wave passes through the slits as it normally does but then collapses immediately upon contacting the detector. As a result, the electron position suddenly becomes fixed and its position is at one of the two slits (not necessarily the slit where the detector is placed). The particle then travels in a straight line and impacts the screen. And the impact pattern on the screen corresponds to what we would intuitively expect upon firing a bunch of particles at the two slits -- which is, two lines of impact points appearing behind the two slits.

As mentioned, the probability wave contacts (and interacts) with objects just as a regular wave does, but contact with objects does not cause wave function collapse unless the object is a detector. Hence, detectors have a certain property which normal objects don't, which is what's responsible for them causing wave function collapse. What is this property? It's a property related to information acquisition, where you're trying to measure some physical quantity of the quantum particle, such as its position or momentum. But regardless of what's being measured, it's important to keep in mind that only detectors (ie. measuring devices) will collapse the wave function. But normal objects, such as a double slit apparatus, will not.

It has been suggested by numerous well-respected physicists that the mysterious and "fuzzy" nature of quantum particles can be resolved by the Many-Worlds theory. This theory states that there exist parallel universes, and that wave function collapse is only an observer state corresponding to what we observe in our universe. The Many-Worlds theory states that every possible outcome of a quantum event corresponds to different parallel universes being created at the instant of the event. So if, for example, there are only two possible outcomes in a quantum experiment, where an electron is either in location A or location B, the universe actually creates, at that instant, two parallel and equally real universes, which cannot interact with each other. In one universe the experimenters see the electron as showing up in location A, and in the other universe the experimenters see the electron as showing up in location B. It follows that if a quantum event occurring in our brains leads us to decide one thing instead of another thing, then the universe accommodates both of these outcomes. So if you turned down some opportunity to do something in the past, and you wonder what your life would have been like if you had taken that opportunity, there's a whole other universe (with another you in it) in which that happened.



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