By far, the most famous of all of the Nobel prizes is the Nobel Peace Prize, for contributions towards peace and human rights. However, there are also Nobel prizes for Physics, Chemistry, Physiology or Medicine, and Literature (As well as the Sveriges Riksbank prize in Economic Sciences, which is given in Nobel’s memory). This year, the in physics was given to three physicists: John Clauser, Alain Aspect, and Anton Zeilinger, who performed experiments that show that the world we live in is even weirder than we thought. The prize was given—and warning, this is a mouthful–“For Experiments with Entangled Photons, Establishing the Violation of Bell Inequalities and Pioneering Quantum Information Science.” This description is rather opaque, and so it is hard to understand what exactly is significant or interesting about the work that was done. Before we dive into the specific research that was done, what the results and significance were, which is simply all too much to fit into one article, let’s take some time to understand some of the principles behind these physicists’ work.
At the core of this work is investigation of two principles, what physicists have titled ‘Locality’ and ‘Realism.’ Locality is essentially the idea that nearby things influence nearby things. If I want to knock a water bottle off of a shelf, I have two options: either I can directly knock it off by walking over to it and pushing it, in which case I have to be near the bottle to push it, or I can throw a ball at it to knock it off, in which case I first have to be near the ball to throw it, and then the ball moves until it is close enough to the bottle to hit it off. In both cases, every interaction is between things that are close to each other. Locality takes this idea and says that it is fundamental, that everything must only be able to interact with what is immediately in its vicinity. Nothing can magically affect something far away immediately without some kind of information or influence having to travel between them.
Realism can be summarized as the idea that the universe exists in the exact same way when it is being observed as when it is not. So, it is kind of like the idea that after you knocked the poor water bottle off of its ledge, if it fell behind a cloth, it would continue to fall, move, and act in some definite way, just like it did before it fell behind the cloth. You could almost say that realism is like a kind of object permanence for the universe itself. While infants have to learn that objects continue to exist even when you can’t seem them, realism is the idea that everything exists in some definite way even when you are not observing it.
Physicists then take these two principles, locality and realism, and combine them into one concept, ‘Local Realism.’ This is the idea that the universe has some definite state even when you are not observing it, and that two different locations can’t influence each other without some influence or information traveling between them. Both of these ideas are, at least I think, extremely intuitive, and they seem like quite natural features to expect from our universe. However, the problem comes from one specific word from that long, pretentious title for the work, ‘Quantum.’
‘Quantum Mechanics’ is a field of physics which studies how the smallest pieces of matter move and interact. Quantum mechanics makes the claim that, at the smallest scales, around the size of atoms and electrons, things behave very differently than at the massive scales of humans and our everyday life. For example, in quantum mechanics, we find that it is impossible to say with certainty exactly where a particle will be when you try to measure its position, and instead, you can only calculate the probability, or how likely it is, that you will find it at a specific spot when you try to measure it. Bizarrely, before we measure the position of the particle, the most literal interpretations of quantum mechanics actually say that the particle is in many places at the same time! Now, you may note that this idea of not being able to know where a particle is, only being able to calculate where it might be, seems to clash with realism, and you would be right. After all, if you know where the particle was, and what it was doing before, then it should be certain what it will be doing in the future. So, do we have to abandon realism when we are dealing with things at the quantum scale? Actually, no! You see, physicists are clever, and realized that if you imagine that the particles actually are carrying secret information that we can’t access that tells them where to go, then since we don’t know the information, we didn’t know everything about the particle when it started its journey. If we don’t know this information, then of course the best we can do is calculate the chances that the particle will go to a specific spot. This type of theory is called a ‘local hidden variable theory,’ where the variable/information that the particle is carrying is local, because each particle carries its own information with it, and hidden, because that information is unobservable. However, how could we ever possibly prove that particles carry information that is unobservable? After all, if the information is unobservable, then we could never measure it or see that it is there!
This is where the Bell Inequalities come in. John Bell managed to show that quantum mechanics, as it is formulated, is actually fundamentally incompatible with local hidden variable theory, and that there are experiments that can differentiate between a quantum mechanical world and a local hidden variable world. To do this, Bell came up with a set of conditions—what are now called the Bell Inequalities—that could be compared with the results of certain experiments to tell us whether the universe is consistent with local hidden variable theory, or with quantum mechanics. This is amazing—it is actually possible to tell if there is information that we can’t measure! If Bell’s Inequalities hold, we might be in a universe with local hidden variables—but then quantum mechanics, at least as we usually understand it, is wrong. If Bell’s Inequalities are broken, then the quantum mechanics we know and love is accurate, but local hidden variables are impossible, and we can’t have local realism. It is exactly this—experiments and theory related to testing Bell’s Inequalities—which earned the first two physicists, John Clauser and Alain Aspect, their part of the Nobel prize. Further investigation of a related phenomenon, quantum teleportation, earned Anton Zeilinger his part, but that is all a story for another time.
This article is part one of two in a series discussing the 2022 Nobel Prize in Physics. The second article, which will be released in January 老司机传媒 2023, will explore the specific phenomenon used to test Bell’s inequalities by Clauser and Aspect, the experiments that they performed, the related work of Zeilinger, and finally, what these results mean about how we should think about the universe.
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