The Spooky Nature of Our Physical World

This article is the second part in a series discussing the 2022 Nobel Prize in Physics. The first article, released December 2022, explored the major principles and background that was investigated in this research, and should be read first to best understand this article. Part one can be read here.

Last time, we learned about the principle of local realism, the idea that objects in the universe exist in some certain way, and that objects can only influence their immediate surroundings. We also discussed quantum mechanics, and how Bell managed to show that local realism is actually fundamentally incompatible with how physicists have developed quantum mechanics, and that even if we tried to save local realism by supposing that there was information that was fundamentally immeasurable, it would still lead to differences in how matter behaves. But how can you go about measuring if there is information that is immeasurable? At the heart of these experiments is the experimentation with a particular phenomenon that is unique to quantum mechanics, and is really counterintuitive, entanglement.

Entanglement in quantum mechanics is the idea that separate random events can still have correlations. What this means is that even if two events are known to (by themselves) be random, so you can’t predict what the results of each will be independent of the other, there can still be patterns between them. This behavior only happens with tiny particles like electrons and atoms, but as an analogy, it would be like if you and a friend each took two coins, and you took the coins into separate rooms, and you both flipped the coins 100 times, writing down the sequence of heads and tails. When you are flipping the coins, of course each flip is random. But if the coins were quantum coins that were entangled, then when you and your friend would compare your lists of flips, you would find that you actually got exactly opposite patterns of coin flips! Every time you got heads, they got tails, and vice versa. This is extremely weird behavior, and not what you would expect from the universe, at any scale, and when quantum mechanics was being developed, this prediction led many at the time to reject the theory as nonsense. Albert Einstein, by far one of the most famous physicists of all time, really did not like quantum mechanics, in part because of entanglement, referring to it as “spooky action at a distance.”

It is this prediction of quantum mechanics that John Clauser and Alain Aspect used to test local realism in hidden variable theory, and actually, that makes sense. Entanglement predicts that completely random events, that is, non-real, can have long distance instantaneous correlations, that is, non-local. So if entanglement happens exactly as is predicted by quantum mechanics, then the universe can not be locally real, and Bell’s inequalities that we discussed last time would be violated. But if entanglement experiments deviate from quantum mechanics, we may have local hidden variables. John Clauser was the first person to develop and perform a Bell Test, that is, an experiment that tested Bell’s inequalities. Clauser performed his experiments using photons. Photons can be thought of as little packets of light waves, and these photons can wave in different directions, just like if you are holding a tight string, you can make different types of waves in the string by either shaking it side to side, or up and down. In the same way, the photons can “shake” left and right,  or up and down, or anything in between. The direction that the photons wiggle in is called the polarization. Clauser realized that if you took two photons that were produced in a particular way from a hydrogen lamp, then the polarization of the photons would be entangled. So, Clauser took these two entangled photons and sent each of them to two different detectors that would measure whether each photon was horizontally or vertically polarized, so waving left-right or up-down, with this, he could measure how correlated, or interconnected, the two photons were. What Clauser then did was change the relative angle of the two detectors and then measured how the correlation between the entangled photons changed. By repeating the experiment many times for many different angles, he was able to measure the details of the nature of the correlations between the photons. And what did he find? He found that the correlations violated Bell’s inequality!

So, are we done? Is local realism dead? Well, not quite, because physicists are very careful with experiments, and they realized there was one loophole that could still save Bell’s inequalities: what if the choice of how to set up the detectors influenced how the photons are produced, and that led to the correlation, and not the photons being entangled? This seems really far fetched—after all, why should setting up a measuring device influence the light a lamp produces? But physicists realized it was still technically possible, so they had to consider it. So, how could this loophole be resolved? This is what Alain Aspect solved. He designed an experiment where the angle between the detectors would actually be decided randomly while the photons were still traveling after being produced, so that there was no way for that decision to influence the polarizations of the photons. This was really tricky, since he had to make the experiment randomly decide between different directions, and then set up the detectors to measure in that direction while the light was still traveling, which is a really short time, since light travels at over 670 million miles per hour. But he was able to do it, and again his experiment confirmed the violation of Bell’s inequalities. Thus, Clauser and Aspect provided the first experiment, and the first loophole free experiment respectively, of Bell’s inequalities, showing that our universe is not locally real, and earning them their place in the 2022 Nobel Prize.

But what about Anton Zeilinger, the third person to be part of the prize, what did he contribute? Well, glad you asked! Zeilinger’s work is again also based around quantum entanglement, but taking it in a different direction. In quantum mechanics, it has been proven that there is no way to take a particle in a given state, and “copy” that state onto another particle, in other words, it is impossible to clone something, in the physics sense of the word “clone” where it is an absolutely perfect copy, even down to how the individual particles are moving, while still maintaining the original object. This fact is known as the “No Cloning Theorem.” However, while cloning is not possible, teleportation is. Given some particle in some state, it is possible to “teleport” that state onto another particle, but this will erase the state in the original particle. Zeilinger and his group managed to perform the first ever quantum teleportation, teleporting the state of a photon onto another photon, and it is this that earned Zeilinger his part of the Nobel Prize.

But why should we care? Here we have some physicists hiding in their labs doing obscure experiments shooting light all over the place and looking at patterns in how the light wiggles. Why does this matter? Well, you should care because this work done studying the entanglement and interactions of particles at the quantum scale has created an entirely new field that is already revolutionizing computers and technology, and will continue to do so: Quantum Information. As quantum information continues to develop, and the quantum computers that use these quantum phenomena such as entanglement become more powerful, it has the potential to break the primary algorithms that are used to securely exchange data on the internet, threatening the global security of almost everything in our daily lives. But at the same time, it also provides new algorithms unique to quantum systems that the computers we use every day are incapable of, thus providing new means of even more secure exchange of information. Besides this, quantum computers also have the potential to vastly improve simulations of physical systems such as chemical reactions, which could in the future lead to massive improvements in medicine, chemistry, and technology. Quantum artificial intelligence could lead to computers being able to solve more problems more quickly. That is all well and good, but, taking another perspective, for the more philosophically inclined among you, these discoveries about entanglement and the loss of local realism tells us something really profound about the universe we live in. It tells us that we can never know everything about what will happen in the future, and that things can be in multiple places at the same time. It tells us that things may not really be there when we aren’t looking at them, and that things that are like random coin flips can have almost magical connections that you would think are impossible. It tells us that the universe we live in is a strange, wonderful, mysterious place, and maybe even—to take Einstein’s words—a little spooky.