A weirdly incorrect but effective intuition for quantum mechanics

Forming an intuition for an area of knowledge helps greatly in gaining an understanding on a deeper level. The intuition doesn't need to be based on accuracy and correctness to be effective. For example, African culture is rich with idioms, metaphors and other figurative devices which aren't necessarily accurate but help with understanding concepts on a very intimate level. Can we form an intuition for quantum mechanics that would make it less weird? We will start with a brief explanation of some concepts of quantum mechanics and build up to an interesting setup for an intuition.

Quantum mechanics is a very interesting and seemingly weird part of physics. The most widely accepted interpretation of quantum physics, the Copenhagen interpretation, seems to go in a very different trajectory from other branches of physics (classical physics) and science in general. For the most part, classical physics seems to be very concerned with determinism, in particular, the main premise was that if we cannot accurately determine the behaviour of a system it is because we do not have enough data, or cannot measure some things accurately enough. That is to say that everything would be deterministic if we could be accurate enough and gather enough information. Quantum mechanics, on the other hand, comes with the claim that things (at least subatomic things) are inherently not deterministic. This was so weird when it came out that even Einstein was famously taken aback by this and said the famous quote: "God doesn't play dice".

As mentioned one of the defining characteristics of quantum mechanics is how the subatomic particles behave in a non-deterministic way. Instead, they behave in a probabilistic nature - that is to say, we don't know where a subatomic particle (e.g. an electron) is in space, we instead rather have an idea of the probability of finding it in certain places. This, however, begs the question of why do we experience determinism if everything is made out of these subatomic particles. The answer to this question is even weirder. As it turns out, the subatomic particles only exhibit probabilistic behaviour when they aren't being observed, once they are observed they suddenly behave in a deterministic behaviour. This is the so-called wave-function collapse.

This has been popularised by another non-accurate and technically incorrect famous thought experiment called Schr?dinger's cat. In this thought experiment, a cat is both dead and alive in a box with a vile of poison which may or may not break (superposition). Upon opening the box and discovering the state of the vile of poison - only at that point the cat is then definitely either dead or alive (wave-function collapse).

While Schr?dinger's cat is a good thought experiment that illustrates how weird the world of quantum physics, there are also actual experiments which are easy to grasp and still convey the main points highlighted above. The two main points being firstly the non-deterministic nature of quantum mechanics and secondly the weird apparent awareness of being observed which suddenly changes how the subatomic particles behave. The best experiment for this is the double-slit experiment. The setup is straightforward: firstly there is a source that is firing the subatomic particles one at a time. This source fires the particles at the double slit. As some of the subatomic particles pass through the double-slit they eventually hit the screen and make a 'mark' on the screen. You can imagine firing a paintball gun at a white wall through some slits.

In the classical world, we would expect the particles to travel in straight lines, whereby the particles will either miss the slits and stop, or go through the slit. We would expect that if enough particles are fired we would eventually see two distinct lines on the screen. Surprisingly, that's not what happens, what instead happens is that an interference pattern forms just like how two ripples in water interact.

This result already is confusing in the sense that it's weird that we do not understand what sort of path the subatomic particles could be taking to end up creating that wave-like pattern. With that in mind, the next best thing would be to observe or measure the path subatomic particles takes before they reach the double-slit. This is where it becomes even weirder, because once we start observing or measuring then the suddenly the subatomic particles behave predictably. This is because the measurement effectively collapsed the wave-function before it hits the slits, the same way observing what is in the box collapses the wave function which describes the state of Schr?dinger's cat (i.e. the cat becomes either dead or alive).

In absence of understanding, this result can be quite unexpected as we normally do not think of the act of measuring or observing something changing how that thing behaves. This is very curious indeed.

With the above information, can we create an intuition for quantum mechanics that makes the above results less weird? Before we do that it is important to briefly discuss what observation or measurement is in the quantum world. Put in simple terms, it is any sort of leaking of information about a system, whether or not there is someone to observe it or not. So if a tree falls in a forest with nobody there, its fall is effectively measured - as the tree fell it interacted with the air, the ground etc. In other words, the universe observed and measured the tree falling (if you could somehow find out later how the air molecules moved you could reproduce the trajectory the tree took as it fell). This effectively means even seeing something is effectively a measurement in quantum physics, because for the photons to reach your eyes they interacted with whatever you are looking at first, and as such changed their trajectory. Even if you do not see the thing yourself and some light hits it, it is effectively observed. Furthermore, even if there is no light hitting the object, but the object has a temperature above absolute zero, it would start radiating energy and thus interacting with the universe and will effectively be observed or measured.

With this understanding, we can now start with our weird thought experiment to build up our intuition. Let's call this thought experiment 'The quantum music playlist' Imagine that you are listening to music on your favourite music app. You then decide to click on the shuffle option on your music app. Let's say the playlist contains 10 tracks. If we think back to the double-slit experiment we can think of the 3 main components as follows:

• subatomic particles source: music library source (maybe the cloud or songs saved on your mobile phone)
• double-slit: this is the music player as it's playing the music (i.e. the act of a song being played is like a subatomic particle going through one of the slits)
• screen: this would be the very act of listening to the songs or simply just the songs being played

If you just click shuffle on your music app and start listening, as expected you wouldn't know what song would play first, or second etc. You aren't totally in the dark though. If you wanted, you could work out that with 10 tracks, you have 10! (3628800) different ways the tracks could be ordered. If you further had insight into the type of randomisation used when shuffling on your app, you could further draw up an accurate probabilistic model of how songs would be played. If you run through the songs enough times while reshuffling each run you will find that you are aligned with your probabilistic model. This is analogous to the case of the double-slit experiment whereby an interference pattern is formed.

In this case, when listening, there are 10 tracks from A to J, each with a specific probability of being some track between track 1 and 10.

What would happen now, if after you click shuffle you look at the order of the songs? Something interesting now happens, because suddenly your probabilistic model is no longer relevant, in-fact technically it becomes incorrect because you aren't dealing with a random system anymore. If you know that track 3 will play first, it doesn't make sense to ask what is its probability of being played second. You are now dealing with a known order - for example, if you had only two tracks you have two possible orders, (1,2) or (2,1) so you could enumerate them and call them sequence 1 and sequence 2. This is analogous to collapsing the wave function in the double-slit experiment by detecting the trajectory before hitting the slit.

This thought experiment is effective because it precisely shows how access to information effectively changes how we experience a phenomenon. A closed system in which we have no information about behaves in a probabilistic manner while a system which is open and we can access its information behaves in a classical deterministic way.

As prompted in the title, technically this thought experiment is entirely incorrect on many levels - let us look at one way in which this example is technically incorrect. As mentioned measurements or observations aren't about conscious people accessing information - it's about information being leaked in any way possible to the universe. The shuffling of the music is already leaking information - computers use what are called pseudo-random number generators to simulate any type of random data. These are called pseudo because they are not truly random - they are at best chaotic and use hidden variables. This effectively means there is some information leaked to the universe about that number generator which if you had access to, you could determine the seemingly random numbers accurately. Ironically, if we wanted a truly random process we would probably need to again go to the quantum world, we would keep a subatomic particle in a superposition and then collapse its wave function (observe it) when we want a random number.

A powerful aspect of this 'Quantum music playlist' is that its blatant inaccuracies also gives a deeper intuition into why we do not observe quantum mechanical phenomena in every-day life. As an example, you cannot reproduce the double-slit experiment results using a paint gun. As discussed, the thought experiment is flawed in the sense that the shuffling result is leaked to the universe either way (even though we cannot practically access the order). Similarly, in the macro world, our universe is so full of things that there is almost always some sort of interaction and thus a leaking of information. If you fire a paintball gun without looking, its information is still leaked as the paint interacts with the air, and even as photons bounce around the room and also hit the paint.

Quantum physics is an intriguing and exciting branch of science. Gaining deeper intuitions into this field, I believe, would propel this field to greater heights and open it up to more people. I hope new ways of gaining intuitions such as this one can be explored so that we can deepen our understanding.