Introduction
In order to truly grasp quantum queerness, we must quickly do a real experiment called Double-Slit Experiment (this is not a thought experiment but actually performed in the physics lab). It is a famous starting point to understanding the intriguing nature of the QM phenomenon. This experiment is based on the original concept devised by Thomas Young (1773-1829), a brilliant English scientist and polymath, who at the age of 14, was acquainted with over 13 languages.
What Is Double Slit Experiment?
It begins by considering a source of electrons such as an electron gun that fires electrons which hit a screen placed a metre away. This is similar to the light shone from a torch on a wall. The only difference is that the unassuming torch throws out light (photons), while the intricate electron gun emits electrons. Each electron that hits the screen produces a single dot, so by counting the number of dots on the screen, you can actually count the number of electrons fired. If you now insert a plate with a single slit or a hole, between the electron gun and the screen, the electrons will pass through the sit and create the dots as earlier. If you now make two slits instead of one, as shown in Figure 2, and repeat the experiment, what you will now see on the screen is the interference (wave-like pattern of black and white strips), instead of dots. It is understandable that the electrons passing through both the slits may have combined together (since electrons also behave like waves as we saw in the previous section), to form the interference on the screen. Just like when you throw a pebble in the pond, you will see water ripples and when you throw two pebbles, the two ripples combine to form a sort of interference in the water.
Assume that you fire the electron gun in such a way that only one electron is emitted. This is possible to achieve practically in the lab. You will obviously expect that this time the single electron will pass through any one of the two slits on the plate and land on the screen, making a dot. But what you actually see on the screen is some kind of interference (black and white bands), as though several electrons have passed through both slits to form these patterns. But we are quite sure that we have fired only one electron, so we don't expect it to make any wavy patterns on the screen. So what went wrong? How can just one electron make an interference pattern on the screen?
After years of experimentation and analysis, the conclusion came as a rude shock. The conclusion was that the single electron that was fired from the electron gun actually passed through both slits simultaneously, combining with itself and resulting in the interference pattern on the screen! How can a single electron pass through both slits at the same time? It is like walking out of your house from both the front and back door at the same time. Isn't it magical?
Out of curiosity, let's actually follow the electron and track it all the way from the gun to the screen to find out how a single electron actually passed through both holes at the same time. So, when you try to observe and follow the electron's path all the way from the gun to the slit and then to the screen, the electron simply behaves as a point particle passing through one of the two slits creating a dot on the screen. But on the other hand, if you don't look at the electron and simply focus on the screen, the single electron passes through both slits at the same time, forming the interference on the screen.
We know for sure, that we fired only one electron from the gun, but we still ended up getting interference on the screen. How could just a single electron produce an interference that needs at least two electrons or two waves? This implies that the electron was actually passing through both slits at the same time and combining with itself to form the interference. But when we tried to follow its path, it suddenly decided to behave itself by going through only one slit. So, if you don't look at it, the electron goes through both slits simultaneously and produces interference. However, on the other hand, if you do look at it, then it suddenly collapses into a single electron particle passing through any one of the two slits to produce a dot. That means the electron was somehow aware of you watching it and therefore behaved in a normal way as was expected of it. The moment we stopped looking at it, it behaved in an entirely different way. Did the electron 'actually go through both the slits at the same time and form interference on the screen?
Yes. At least this is what the experiment seems to suggest. And this is not a thought experiment like that of John and his friends So, the obvious conclusion is that the electron plays a double-role by existing both as a wave and a particle, but more importantly, it also suggests that your very act of looking or inquiry makes the wavy electron collapse to its physical state. It is as though they have a mind of their own and are aware of someone watching and intruding on them. This reconfirms beyond doubt the bizarreness of the microscopic world that feels alien to the classical world we live in. This quantum phenomenon led to a speculative debate about the very nature of reality as it exists in the world. Why do sub-atomic particles behave so strangely? How can electrons or protons be present at two or many places at the same time? Why do they collapse from their wavy nature to matter or particle nature when we try to observe them or make measurements? Since we humans are made up of trillions of these sub-atomic particles, do we also exist in superposition states? Are we all present at many places at the same time? These are some of the questions that have been haunting both scientists and philosophers for the past 80 years.
Conclusion
Greats scientific minds like Neil Bohr, Albert Einstein, Schrodinger, Heisenberg, and many more, who developed Quantum Theory, had differing views on the interpretation of this strange phenomenon called superposition. For some, it meant that, at a very small-scale, electrons or protons, etc. existed as waves (called more appropriately, wave function), which were not real physical entities and that these wave functions were just a mathematical probability of finding them. What it means is that, every particle or object in the universe (and this includes larger objects like a car, human, dog, ocean, cloud, moon, galaxy, etc.), exists in an indeterminate state with no fixed location. It is as if, all particles or objects are smeared out and hazy in their appearance. This haziness is represented in the language of mathematics in the form of a complex equation that represents the probability of finding it in some place and is called a wave function. The equation gives us the probability of finding the actual object within this continuous wave form, which may be spread from here to anywhere. For example, the wave function of your dog may be spread across a kilometre or so, with the probability of finding him next to you being very high. So, it's all a game of probability. You are located everywhere in a superposition state with a high probability of appearing at only one place. When someone observes you in your superposition state, your wave function will collapse at that location where the probability of finding you is the highest. In Einstein's own words, the moon does not exist when you are not looking at it. Before looking at the moon, the moon was everywhere. That is to say, the moon's wave function was spread all over the universe and the moment you looked, it collapsed to a location where there is a high probability of finding it.
Likewise, when you try to measure the wave function of a moving electron, it suddenly collapses to its particle nature at a location where the probability of finding it is quite high. So nature, on a very small scale, is essentially probabilistic. Compare this to a situation where your friend is asking you to guess which fist she is hiding a ring in. Your probability of guessing it to be either in her right or left fist is 50%. This is a subjective probability, because for you, the probability of getting it correct is 50%, but for her it is 100%. This is because each of you has a different knowledge of the same situation. For you, the ring can be either in her right or left hand, but for her it is certainly a known fact, as she is the one hiding the ring. The world that you and your friend exist in is a classical world and hence, probability is based on one's own knowledge of the system which varies from person to person.
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