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

by Adam Becker

Quantum physics works, but ignoring what it tells us about reality means papering over a hole in our understanding of the world—and ignoring a larger story about science as a human process. Specifically, it ignores a story about failure: a failure to think across disciplines, a failure to insulate scientific pursuits from the corrupting influence of big money and military contracts, and a failure to live up to the ideals of the scientific method.

Einstein had read Mach’s History of Mechanics as a student and was deeply impressed with his criticism of the Newtonian ideas of absolute space and time. “This book exercised a profound influence on me,” he wrote decades later. Taking Mach’s ideas about eliminating extraneous unobservable entities to heart, Einstein had tackled the problem of the aether, finding it to be an unnecessary hypothesis in special relativity. And, better still, special relativity also consigned to oblivion the absolute space and time that Mach had so despised.

And the name “relativity” itself, which suggests a rejection of absolutes, was introduced by the physicist Max Planck, not Einstein—Einstein disliked the name “relativity” precisely because it connoted a kind of relativism. He preferred the name “invariant theory,” which conjures up a very different set of associations.

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.”

Heisenberg started thinking about what would happen if you tried to measure the position of a single particle, like an electron, to very high precision. He realized that you could do this the same way you’d look for a lost wallet in a dark field: shine a flashlight around until you’ve found what you’re looking for. An ordinary flashlight wouldn’t work for an electron, though—the wavelength of visible light is far too large for that. But Heisenberg knew you could find an electron using higher-energy light, with a shorter wavelength: gamma rays. Shine a gamma-ray flashlight around the room, and you’ll find your electron. But gamma rays pack a punch—bounce one gamma-ray photon off an electron, and the electron will go careening off in some random direction. So you’ll know where the electron was, but you won’t know how fast it’s going or where it’s heading now. Heisenberg wondered if this kind of trade-off between measuring an object’s position and its momentum was unavoidable, or if it was just an artifact of his thought experiment. To his delight, he discovered that these limits on measurement were fundamental: buried in the mathematics of his own matrix mechanics, Heisenberg found a precise formulation of how much information you have to give up about an object’s momentum in order to learn more about its position, and vice versa. You could know a lot about where an object was or a lot about how it was moving—but you couldn’t know both at the same time.

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.

A typical atomic nucleus is 100,000 times smaller than the surrounding electron cloud, which is itself a million times smaller than the width of a human hair. At such close quarters, the electrical repulsion between the protons in the nucleus, left unchecked, would send them flying off at nearly the speed of light. Instead, atomic nuclei are held together by an even stronger force, unimaginatively dubbed the “strong nuclear force.” The strong force binds together the protons and neutrons in atomic nuclei. Neutrons are electrically neutral—hence the name—but they feel the strong force just like protons. They play a crucial role in the nuclear tug-of-war between electrical repulsion and strong force attraction, aiding the latter without affecting the former. While the strong force isn’t quite strong enough to keep two protons together by itself, adding a neutron to the mix increases the “stickiness” of the strong force without adding any electrical charge, creating a stable atomic nucleus of two protons and one neutron (helium-3).

Bohm accounted for the strange results of the double-slit experiment by doing exactly what the Copenhagen interpretation said was impossible: he gave a detailed account of what happens in the quantum world whether or not anyone is looking. Photons, according to Bohm, are particles surfing on waves. While a particle can only pass through one slit, its pilot wave passes through both and interferes with itself. That self-interference, in turn, affects the motion of the particle, because it is guided by the wave. The wave pushes the particle onto a path ensuring the appearance of an interference pattern on the photographic plate after enough photons have been sent through the double slit (Figure 5.4). Putting photon detectors on each slit affects each photon’s pilot wave—no matter how ingenious the design, any photon detector must alter a photon’s pilot wave, as ensured by Heisenberg’s uncertainty principle, which in Bohm’s interpretation places limits on how much measuring devices can avoid interfering with the things they attempt to measure. The effect of these measurements on the photons’ pilot waves alters their trajectories, causing them to form a pair of clusters on the photographic plate rather than an interference pattern. In Bohm’s account, although measurement can influence a particle’s motion, all particles have definite positions whether or not anyone is looking at them.

Bryce DeWitt, Wheeler’s colleague, fellow quantum cosmologist, and co-organizer of the Chapel Hill quantum gravity conference, was skeptical of Everett’s thesis at first. “I am afraid that it is precisely at the most crucial point in Everett’s argument where many people, including myself, will be unable to swallow your implication.… What I am not prepared to accept” is the branching of worlds required by Everett’s theory, wrote DeWitt to Wheeler. “I can testify to this from personal introspection, as can you. I simply do not branch.” Wheeler passed DeWitt’s reply on to Everett; in his reply, Everett drew an analogy between DeWitt’s objection and early objections to the Copernican Sun-centered model of the solar system, tinged with his usual irony: One of the basic criticisms leveled against the Copernican theory was that “the mobility of the earth as a real physical fact is incompatible with the common sense interpretation of nature.” In other words, as any fool can plainly see the earth doesn’t really move because we don’t experience any motion. However, a theory which involves the motion of the earth is not difficult to swallow if it is a complete enough theory that one can also deduce that no motion will be felt by the earth’s inhabitants (as was possible with Newtonian physics). Thus, in order to decide whether or not a theory contradicts our experience, it is necessary to see what the theory itself predicts our experience will be. Now in your letter you say, “… I simp...

And because Bohm’s theory is contextual, it evades all of the proofs that supposedly rule it out. “What is proved by impossibility proofs,” concluded Bell, “is lack of imagination.”

The logical positivists were understandably opposed to philosophical castles in the sky and the tortuous prose they were often defended with. But the logical positivists weren’t merely against metaphysics—they believed they could actually dismiss metaphysical claims as meaningless. Meaning, they held, was a matter of verification: knowing what a statement means is equivalent to knowing how to verify it using your senses. According to the positivists, when you say “it’s hotter outside than it is in here,” you really mean “if you go outside, you will feel hotter than you do in here.” The statement’s meaning is the method of verifying it empirically—and if there’s no way of verifying a statement against your senses, then that statement has no meaning.

The philosophers had successfully overthrown positivism and had a good understanding of the mathematical intricacies of quantum physics—but the physicists were still blinkered, walled off from philosophy and the developments there. They had no idea any of this had happened. While Einstein and Bohr’s generation was widely schooled in philosophy, the push toward specialization after World War II had taken its toll on the liberal arts education of the new crop of physicists. Academic departments had become Balkanized as they had grown in the postwar boom, and physicists, busy with enormous grants and hard-nosed calculations, were generally dismissive of philosophy. So physics trudged along, not knowing that a major revolution had happened in an adjacent field.

There were only a handful of journals that would accept papers on quantum foundations, among them Foundations of Physics, where Zeh’s paper had finally ended up. To solve this problem, the quantum underground founded a new ersatz “journal” called Epistemological Letters. Billing itself as a permanent written symposium on “hidden variables and quantum uncertainty,” this samizdat was hand-typed, published by mimeograph, and overseen by an informal collection of editors, including Shimony.

Feynman himself had explained Bell’s theorem during his keynote speech at an MIT conference in 1981 (though strangely he did not actually mention Bell himself in the course of doing so). The conference was on a seemingly unrelated subject—the physics of computation—yet Feynman showed that Bell’s theorem held the answer to a crucial question in this field. “Can physics be simulated by a universal computer?” Feynman asked the conference. “[The] physical world is quantum mechanical, and therefore the proper problem is the simulation of quantum physics—which is what I really want to talk about,” he continued. The answer, for a normal computer operating under ordinary conditions, was no: using simple ones and zeros in the usual fashion, without strange long-range connections within the computer or some other kind of trick, limited the computer to simulating local physics, rendering it impossible to fully simulate quantum effects. But, Feynman speculated, there might be another way to get the job done. “Can you do it with a new kind of computer—a quantum computer?” Feynman wondered. “I’m not sure.… So I leave that open.” Several years later, a young physicist named David Deutsch took up where Feynman had left off. In 1985, Deutsch proved that a quantum computer—a computer taking full advantage of the difference between quantum physics and classical physics—could perform tasks more efficiently than an ordinary classical computer. Deutsch’s proof opened up the possibility of pract...

if subatomic particles can behave so strangely and we and the objects in our everyday lives are composed of such particles, why don’t we see such strange behavior on a regular basis? According to spontaneous-collapse theory, the answer lies in two key facts: entanglement and the vast number of particles that comprise the objects of our everyday experience. Though a single-particle wave function might not collapse on average until a billion years have passed, the solid objects of our everyday lives, like this book, are generally composed of at least 10 million billion billion individual particles. If each one of those particles’ wave functions is compulsively pulling the handle of its own slot machine (Figure 10.3b), then, on average, at least one of them will hit the collapse jackpot every millionth of a second. But because the particles in this book are all continually interacting with each other, they’re all entangled—which means they all share a single wave function. So when one of them hits the jackpot, the wave function for the entire book collapses, meaning this book can’t be in two places at once for much longer than a microsecond or so—a hundred thousand times faster than a blink of an eye.

DeWitt’s article, “Quantum Mechanics and Reality,” did review several interpretations. But DeWitt made his own opinions quite clear. “The Copenhagen view promotes the impression that the collapse of the [wave function], and even the [wave function] itself, is all in the mind,” he wrote. “If this impression is correct, then what becomes of reality? How can one treat so cavalierly the objective world that obviously exists all around us?” When dealing with a system in a quantum superposition, like Schrödinger’s cat, DeWitt said most physicists “conceive the [measurement device] to have entered a kind of schizophrenic state in which it is unable to decide what value it has found for the system,” a living cat or a dead one. This problem, he concluded, was not resolved by the Copenhagen interpretation. And other interpretations, like Bohm’s, added hidden variables to quantum physics, a move that DeWitt thought was unnecessary. “What if we assert that the [Schrödinger equation] is all, that nothing else is needed?” he wrote in Physics Today. “Can we get away with it?The answer is that we can.”

General relativity didn’t merely deal with abstruse situations—it was also written in abstruse mathematics. The theory is very mathematically complex, far more so than quantum mechanics. Einstein famously had to enlist the help of a mathematician friend, Marcel Grossman, just to learn the differential geometry necessary to formulate and understand his own theory. This combination of unfamiliar subject matter and obscure mathematics made it difficult to be sure of what the theory was saying and led many physicists to be suspicious of its conclusions.

Several years later, in his seminal paper on quantum computing, Deutsch claimed that only the many-worlds interpretation could explain the fabulous increase in speed that quantum computers offered. “The Everett interpretation explains well how the [quantum] computer’s behaviour follows from its having delegated subtasks to copies of itself in other universes,” Deutsch wrote. “When the [quantum] computer succeeds in performing two processor-days of computation, how would the conventional interpretations explain the presence of the correct answer?Where was it computed?”

Part of the problem is that there is no single “Copenhagen interpretation” and never really was. “The name ‘Copenhagen interpretation’ has gotten pretty slippery,” said Nina Emery, a philosopher of physics at Mt. Holyoke. “The semantic confusion makes it easy for physicists to avoid dealing with those flaws directly. For instance, when you push them on the idea that measurements cause collapse… they shift and start talking about some kind of Bohrian view or about the [mathematics of the theory]. And if you point out the issues with those views (e.g. who knows what the former is; and the latter isn’t even a complete interpretation), they go back to talking about [measurements causing collapse].” The flexibility afforded by these contradictory positions makes it easy to fend off any attacks on “the” Copenhagen interpretation. Physicists can simply hop from one position to another—sometimes without even realizing that they’ve done it.

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