In reference to the findings of the 2022 physics noble prize winners work. What are entangled photons? What are Bell inequalities? how the findings establish the violation of Bell inequalities?
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First of all, at the risk of sounding like an arrogant ass, it isn’t always possible to ELI5 advanced concepts in physics. When you simplify something to this point, you often end up reducing it to the point of inaccuracy. I’ll do my best, but this sort of question comes up a lot and the answer is sometimes “in order to understand this properly, you probably have to take a few graduate level classes”. In fact, most people’s conception of what quantum mechanics *even is* is fundamentally flawed.
So while I will explain this in generalities, be aware that everything I say is a reference to a highly detailed, specific, technical finding.
John Stewart Bell was a North Irish physicist who did a lot of work on the idea that “distant” things might affect quantum physics (distant in quotations marks because it has some highly technical definitions that may not be the same as your concepts of distance).
So, what is Bell’s Inequality? Or, more generally, Bell’s Inequalities? Let’s talk about something called quantum entanglement (or, to give it its original name, “Spooky action at a distance”. Quantum entanglement is a situation that comes about when one or more particles find themselves in the situation where you cannot completely describe the state of a particular particle without knowing things about others of these particles. This is considered to be interesting because even though particles need to be nearby in order to develop this situation, they can retain it even if the particles are later moved away from each other. There are significant implications to communications: most immediately, it might be possible to use these techniques to send a message that is completely secure and cannot be intercepted in any way. The important thing for us right now is that quantum entanglement was an unproven theory for a long time, and a lot of quantum physicists hated the idea.
Bell’s Inequality *per se* is a fairly simple mathematical statement. It says that if you have three pairs of random variables whose value may be -1 or +1, and each pair is such that the two elements of that pair **must** have different values, then the probability of certain combinations of these elements being different from each other (the first element of the first pair is different from the second element of the second pair, or the first element of the second pair is different from the second element of the third pair, or the first element of the third pair is different from the second element of the first pair) is greater than or equal to 1. Since probabilities must always be less than or equal to 1, this implies that if you have this set up, then it **must** be the case that at least one of these inequalities is true.
This was important because it turned out that one of the common ideas in quantum mechanics at the time (“local hidden variables”) could not mathematically be true. For reasons that are hard to explain, proving that local hidden variables couldn’t exist provided strong evidence that quantum entanglement was probably true.
Aspect, Clauser, and Zeilinger received the Nobel Prize for a series of experiments each of them did in the 60s and 70s, shortly after Bell published his theoretical findings. These experiments tested whether or not Bell’s work could be demonstrated in the real world and, it turned out, it could!
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