What Is Real?: The Unfinished Quest for the Meaning of Quantum Physics

by Adam Becker

The soundest fact may fail or prevail in the style of its telling. —Ursula K. Le Guin

Picking a good story, and then searching for holes in that story, is how science progresses.

This epistemology-soaked orgy ought to come to an end. —Albert Einstein, letter to Erwin Schrödinger, 1935

Newton’s masterwork, the Principia, opened with the assumption that space and time were absolute entities unto themselves, with real existence out in the world. This “conceptual monstrosity of absolute space” was, in Mach’s view, “purely a thought-thing which cannot be pointed to in experience.” Mach thought that a proper science of mechanics would dispense with these kinds of ontological claims—claims about what things actually exist in the real world—and instead simply lay down descriptive, mathematical laws that accurately predict the observed motion of all objects. Good theories, according to Mach, were about connecting observations, not about positing things that couldn’t be observed at all.

While Mach believed that physics was merely about organizing perceptions of the world, to Einstein, physics was about the world itself. “Science,” he said, “has the sole purpose of determining what is.”

Einstein once said he went through life as a “one-horse cart”; he rarely collaborated with other physicists and almost never took on students of his own. He was eternally suspicious of the status quo, both scientifically and elsewhere; he characterized common sense as the collection of prejudices accumulated by the age of eighteen.

According to Bohr, there cannot be a more complete description of an electron, or of anything—merely incomplete and incompatible analogies that never overlap. This, Bohr said, was the heart of complementarity, and it was inevitable and unavoidable. The new quantum theory had shown it was impossible to give a single consistent account of an electron that would work at all times.

But Bohr was wrong. There was nothing inevitable or necessary about complementarity—other interpretations of quantum physics are possible. Indeed, the claim of inevitability is an awfully strong and strange claim to make about any interpretive issue in science, precisely because it is always possible to reinterpret any theory.

Entanglement, Schrödinger found, is pervasive in quantum physics. When any two subatomic particles collide, they almost always become entangled. When a group of objects forms some larger object, like subatomic particles in an atom or atoms in a molecule, they become entangled. In fact, nearly any interaction between any particles would cause them to become entangled, sharing a single wave function in the same way as the particles in the EPR thought experiment.

Two theories that are identical in their predictions can have wildly different pictures of the world—like putting the Earth at the center of the universe rather than the Sun—and those pictures, in turn, determine a lot about the daily practice of science. If you think that the Sun is at the center of the solar system, rather than the Earth, you’re likely to conclude that there’s nothing special about Earth, or our solar system, and that there could easily be planets around other stars, even though both astronomical theories give the same predictions about how different lights will move across the sky here on Earth. The story that comes along with a scientific theory influences the experiments that scientists choose to perform, the way new evidence is evaluated, and ultimately guides the search for new theories as well.

Everett didn’t add particles—he didn’t think he needed them. Instead, he insisted that a single universal wave function was all there was: a massive mathematical object describing the quantum states of all objects in the entire universe. This universal wave function, according to Everett, obeyed the Schrödinger equation at all times, never collapsing, but splitting instead. Each experiment, each quantum event, spun off new branches of the universal wave function, creating a multitude of universes in which that one event had every possible outcome. Everett’s shocking idea came to be known as the “many-worlds” interpretation of quantum physics.

All someone had to do was actually construct and perform Bell’s modified version of the EPR thought experiment, or another experiment along those lines involving entangled particles. If the results showed that Bell’s inequality was violated, quantum physics was safe but nature was nonlocal; if his inequality held, then quantum physics was wrong but nature could be local. Bell’s impossibility proof had taken the question of nonlocality out of the realm of debate and turned it into an experimental challenge. This proof, now known as Bell’s theorem, has rightly been called “the most profound discovery of science.”

In short, Bell’s theorem really leaves only three unequivocal possibilities: either nature is nonlocal in some way, or we live in branching multiple worlds despite appearances to the contrary, or quantum physics gives incorrect predictions about certain experimental setups.

Science was an instrument for predicting perceptions, nothing more. This view of science came to be known as instrumentalism.

So you can never verify a single statement: you’re always stuck verifying the entirety of your knowledge about the world, or at least a very large fraction of it. As Quine put it, “Our statements about the external world face the tribunal of sense experience not individually but only as a corporate body.”

Kuhn argued that both the observable and unobservable content of scientific worldviews—what he called “paradigms”—play vital roles in the actual practice of science. These scientific paradigms influence what experiments are done, how they’re performed, and how the results are interpreted.

At every step of the way—forming hypotheses, designing and conducting experiments, even simply observing the results of those experiments—the paradigm of atomic theory informed the actions of the nineteenth-century chemists. And they were wildly successful, discovering the periodic table of the elements decades before physicists discovered electrons or learned anything about atomic structure. Yet, according to the best science of the time, atoms were unobservable. So, Kuhn concluded, it’s not just the observable part of a theory that matters—the full content of scientific paradigms influences how science is done. The interpretation of a physical theory, like quantum physics, is important for the day-to-day practice of science itself.

Scientific realism is what it sounds like: the view that there is a real world out there, independent of our observations of it, and that science gives us an approximate description of that world.

The aim remains: to understand the world. To restrict quantum mechanics to be exclusively about piddling laboratory operations is to betray the great enterprise. A serious formulation will not exclude the big world outside the laboratory. —John Bell, 1989

As Quine said, our beliefs about the world can only be tested against the world as a group, not individually, and this holds for falsification just as much as for verification. No theory, in isolation, is falsifiable.

Philosophers of physics, and most other philosophers, are far removed from this picture: they work on well-defined questions with logical rigor and with input from the most recent developments in science and from the immediate experiences of the senses. How the practice and the image of philosophy have diverged so wildly is a subject for an entirely different book, but one part of the answer probably lies in the split between the two major branches of modern Western philosophy, analytic and Continental philosophy.

If these extraneous factors could have such a profound influence on fundamental physics, what part of science could possibly remain untouched? And, indeed, this isn’t limited to quantum foundations: all of science is vulnerable to human biases and to influences from all the other spheres of human endeavor—politics, history, culture, economics, art—that some of those biases spring from. Most scientists, by and large, will agree to this. But agreeing with the abstract existence of these nonscientific biases within science is different from being faced with a concrete example. The idea that something as pervasive and central as the Copenhagen interpretation might be dominant for “accidental” nonscientific reasons can be scary, especially for people who have devoted their entire lives to physics.

There’s a wide middle ground between “science is Pure and Perfectly Rational” and “science is just some bullshit somebody made up.” There’s still plenty of room for humans to interfere in that middle ground, as we’ve seen throughout this book.

But the short answer is that science involves a combination of experiment, mathematical and logical reasoning, unifying explanations, and biases that scientists bring to the table from their own lives and the cultures they live in. We work to reduce those biases; we don’t always succeed, but the explicit attempt to account for and reduce those biases is an important part of the process, properly conducted. The whole edifice of science is geared toward this goal. And, given the phenomenal explanatory power and predictive success of science, it would be foolish in the extreme to give scientific truths no more credence than idle speculation, religious articles of faith, or deeply held cultural values. Science, done right, works hard to respect absolutely no authority at all other than experience and empirical data. It never succeeds entirely, but it comes closer and has a better track record than any other method we apes have found for learning about the world around us, a world we never made.

So many people today—and even professional scientists—seem to me like somebody who has seen thousands of trees but has never seen a forest. A knowledge of the historic and philosophical background gives that kind of independence from prejudices of his generation from which most scientists are suffering. This independence created by philosophical insight is—in my opinion—the mark of distinction between a mere artisan or specialist and a real seeker after truth. —Albert Einstein