Local Realism, Bell’s Inequality, and T-Shirts: An Entangled Tale

For this past Christmas my wife, Rebecca, gave me a T-shirt that says "Quantum mechanics: The dreams stuff is made of." This is an allusion to the book The Dreams That Stuff Is Made Of: The Most Astounding Papers of Quantum Physics—and How They Shook the Scientific World, edited by the late Stephen Hawking. For some time, I hesitated to wear the T-shirt because I found its message to be problematic. My feelings toward this very thoughtful gift have to do with NIST’s experiments in quantum foundations, the world’s most random numbers, and why I happily wear the T-shirt now.

I am a physicist who does research in quantum information theory. One of the things that other physicists love to do is argue about the interpretation of quantum theory. There are MANY different interpretations. Wikipedia lists only (only!) 13, and you can see 26 of them categorized in "Interpretations of quantum theory: A map of madness." To explain these interpretations in detail is, as we sometimes write in academic papers, "beyond the scope" of this blog, but I will give a few examples to show how they tell wildly different stories about what is happening at the quantum level of our world.

Some interpretations claim that quantum particles can send faster-than-light signals to one another. Some claim that everything from quantum particles to humans to galaxies evolves in an ever-expanding superposition that contains all possible events happening together like images projected on top of one another, but we can only see the one image of the events we experience. Some claim that human consciousness can "collapse the wavefunction." The wavefunction describes all the possible outcomes that may occur when quantum systems are measured. When a human measures a quantum system, they observe one thing out of all the possibilities. This is the "collapse." Some others claim that quantum particles are not "real." That’s a bit of a misnomer: It means that some of their properties are determined only when measured. Before measurement, they exist in a dream-like state of unreality.

The T-shirt Rebecca gave me seems to endorse this last view that stuff is made of dreams, which is a problem for me.

While most physicists working in quantum foundations tend to choose one interpretation and champion it, I like most of the 26, even the dreamy one. When I told Rebecca, "I don't know if I can wear this shirt," my dilemma was that I didn't want to champion just one interpretation. I can’t decide between these many interpretations. How should I choose? We learn in elementary school that when faced with several different hypotheses, we should do an experiment to reject one or more of them.

Quantum theory has been subjected to many experimental tests over the past century, and it keeps passing the tests. It is so reliable that, despite its apparent strangeness, quantum mechanics is now essential for nearly every modern technology. We trust our lives to quantum mechanics every day, in the computers that control our cars, in the lasers that speed our communications around the world, in the atomic clocks that guide the global positioning system (GPS), and in the medical scans that diagnose our diseases.

However, as good scientists, we continue to test quantum theory.

That’s why I was excited to participate in a recent experiment at NIST to prove that quantum particles violate the principle of "local realism." According to local realism, all particles have definite properties for all possible measurements (realism) and communication between particles is no faster than the speed of light. Classical theories of physics, like those that describe how everyday objects, planets and stars move, obey this principle, but quantum theory predicts that entangled quantum systems can violate local realism and that this violation can be observed in an experiment called a "Bell Test," after John S. Bell.

In a typical Bell Test, two particles are entangled with one another, separated and sent to two measurement stations. At each station, a random number generator chooses how each particle will be measured. The experiment is repeated many times, and the relationships between choices and outcomes are analyzed. The relationships between particles that obey local realism are limited by a set of mathematical rules called "Bell's inequalities," but quantum particles can violate Bell’s inequalities. Since Bell discovered the inequalities in 1964, many Bell Test experiments have shown violation of local realism. However, before 2015, all Bell Tests suffered from loopholes—potential mechanisms, however contrived, that would allow systems that obey local realism to violate the inequalities. In 2015, our experiment and two others (at the University of Vienna and the Technical University of Delft) were the first to close all loopholes and show violations of local realism.

Since 2015, we have improved the optical devices we used in our Bell Test experiment and advanced our mathematical analysis tools. Through the heroic effort and skill of my experimental colleagues, violation of local realism has become routine in our laboratories. We participated in The Big Bell Test in which humans provided the random measurement choices. More than 100,000 people entered 0s and 1s into the Big Bell Test website, and those choices were distributed to 13 labs around the world, each of which successfully performed its own version of a Bell Test.

At NIST, we are now using our Bell Test to produce the world’s most random numbers, meaning that our random numbers are the most difficult to predict before they are generated in our laboratory. By proving that the data from a Bell Test violates local realism, we also prove that the data is unpredictable even if a hacker may have secretly compromised the experimental devices. We use new mathematical tools to quantify the amount of secure randomness in the Bell Test data and to extract a string of random bits, each of which is 0 or 1 with 50 percent probability. Eventually, we hope to incorporate a Bell Test randomness source into the NIST Randomness Beacon, which publishes a certified 512-bit random number every minute. These public random numbers can be used for tasks such as choosing which voting machines should be audited, authenticating identity online, and performing secure multiparty computation. The Bell Test is advancing from an exotic test of fundamental physics to a useful technology.

Krister Shalm Explains NIST's Participation in the BIG Bell Test

After decades of work, the predictions of quantum theory have been confirmed, and the hypothesis of local realism has been rejected! So, did that allow us to rule out any of the many interpretations? Unfortunately not. The Bell Test tells us that either locality or realism (or both) should be discarded, but it doesn’t tell us which. In fact, the problem goes much deeper than the Bell Test because most of the interpretations make exactly the same predictions for all possible experiments. In other words, all the interpretations work. Experiments can only test predictions about things we can measure, and predictions are calculated from the theory’s mathematical structure. All the various interpretations are derived from the same mathematical foundation, which is, in my opinion, the essence of quantum mechanics. The different interpretations are our human attempts to describe the mathematical essence of quantum mechanics with human language. Because the interpretations are built on the same mathematical foundation and make identical predictions, there is no possible experiment that anyone can ever do that can support one interpretation over others.

Because experiments are not helping, many physicists argue for their favorite interpretation of quantum mechanics by emphasizing virtues such as simplicity, aesthetic beauty, appeal to intuition, usefulness as clickbait and others. However, I usually find these arguments unconvincing because, for example, I don’t know how to weigh the simplicity of one theory against the elegance of another. In the past, I despaired over my inability to know the truth because I can’t choose a single interpretation. Now, however, I rejoice because we do have some access to the truth: It is encoded in the mathematical structure of the theory. There are many good ways to interpret or translate from the mathematics to natural language. Maybe no single translation perfectly captures the mathematical source, but they can all be useful tools for understanding.

I am happy that we have a wealth of interpretations, so, after careful consideration, I decided to wear the "Quantum mechanics: The dreams stuff is made of" T-shirt. I like its nonrealist interpretation. But to avoid favoritism, as I told Rebecca, I need more T-shirts with slogans supporting the other 12 (or 25, depending on how you look at it) interpretations. If you have a good quantum T-shirt slogan, or know of one, leave it in the comments below