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

by Adam Becker

But idealist and theological claims aren’t the only kinds of statements that make no contact with the senses. There are also more straightforward claims, like “the couch is in the living room even when nobody’s there,” that can’t be confirmed directly. Statements like these, about the existence and persistence of material objects independent of perception, are realist claims—they’re statements about a real world that exists whether or not there are any humans around. These statements are fundamental to science. Yet some of the positivists, throwing the baby out with the bathwater, dismissed realist claims as meaningless too, because they can’t be verified by experience. All that is meaningful, on the positivists’ account, are statements about perceptions, along with the purely logical statements of mathematics.

Hmm. Wrong turn

This left positivists in a pickle. They thought it was meaningless to talk about a world that existed independently of perception, but they also wanted to be able to say that science worked. They got around this problem by developing a view of scientific practice that meshed well with their verification theory of meaning. Science, on their account, was about organizing perceptions. Scientific theories were merely methods of predicting future perceptions by churning past perceptions through mathematical machinery. Science wasn’t about an objectively real world that existed independently of our perceptions, because anything that existed beyond perception—even a putatively “real” world—was simply metaphysics. Any statements that scientists made about nonobservable yet “real” things on the basis of their scientific theories were dismissed as unnecessary hypotheses, extraneous metaphysical baggage irrelevant to the true task of science.

Hmm

Some biologists of the time believed in vitalism—the idea that living organisms were not subject to the same laws of physics as inanimate matter, that there was something nonphysical in cell division and inheritance that defied thermodynamics. The positivists rejected this claim, and others like it, as meaninglessly vague metaphysics. Even philosophy itself was to be subsumed by the unity of science, according to the Vienna Circle manifesto: “There is no such thing as philosophy as a basic or universal science alongside or above the various fields of the one empirical science.” Philosophy, like the natural sciences, should be grounded in statements about observation and sensations.

Despite its emphasis on empiricism and logic, the Vienna Circle didn’t limit its concerns to science and philosophy—the unity of science extended to all human activity. “We witness the spirit of the scientific world-conception penetrating in growing measure the forms of personal and public life,” the manifesto boldly claimed, “in education, upbringing, architecture, and the shaping of economic and social life according to rational principles.” The Circle’s members forged connections with artistic and social movements that shared a similar ethos, like the Bauhaus school of architecture and design. And the Circle also had politics to match its revolutionary rhetoric.

The members saw themselves in the tradition of the great Enlightenment empiricist philosophers such as Hume and Locke, and promoted Enlightenment values: international cooperation over nationalism, reason over faith, humanism over fascism, and democracy over authoritarianism. They saw industrialization not as an oppressive force, but as a modernizing one. The Vienna Circle believed these political causes were intimately connected to its philosophical work.

True to their humanist and internationalist politics, Schlick and his colleagues reached out to the world. “The Vienna Circle does not confine itself to collective work as a closed group,” their manifesto declared. “It is also trying to make contact with the living movements of the present, so far as they are well disposed toward the scientific world-conception and turn away from metaphysics and theology.” In this, they succeeded for a time.

Philosophers like Hans Reichenbach in Germany (who had his own Berlin Circle of philosophers) and A. J. Ayer in England visited Vienna, then returned home and promoted logical positivism across national borders and language barriers. Rudolf Carnap became the leading exponent of the Vienna Circle’s views; his landmark 1929 book The Logical Structure of the World established him as a towering figure in the positivist movement, and many of his students went on to become important philosophers in their own right.

Ayer

Logical positivism and quantum physics came out of the same time and place: the Vienna Circle and Berlin Circle both formed in the 1920s, the same decade Heisenberg and Schrödinger (who were German and Austrian, respectively) first developed full-blown theories of quantum physics. This is not a coincidence, but it’s not a conspiracy either. There were hazy ideas floating around the intellectual culture in their shared time and place that may have contributed to the ideas of both the early positivists and the first quantum physicists. But there were definitely specific common inspirations for both groups—most importantly, the work of Ernst Mach.

After the war, there were attempts to reorganize the circle, but Neurath’s sudden death in December 1945 largely put an end to those efforts. Positivism continued on as a philosophy under the new name “logical empiricism,” but the Vienna Circle’s grand dream of an organized political, philosophical, and scientific movement was dead.

Any remaining hope of reviving a unified movement around positivism was dashed by the postwar political environment in the United States. Anticommunist hysteria in the United States rose sharply after World War II, and the nascent Cold War had chilling effects on all arenas of intellectual discourse, including philosophy. To some, the Unity of Science movement, with its left-wing politics, its antireligious philosophy, and its internationalist aspirations, sounded suspiciously like a Communist Party front. During the “red scare” that effectively exiled David Bohm, J. Edgar Hoover’s FBI compiled dossiers on Carnap, Frank, and other leading positivist lights. Under enormous pressure to refrain from all political activity, the positivists were forced to focus solely on issues in logic and the philosophy of science—pushed to what their now-distant manifesto had called “the icy slopes of logic

Politics and environment of logical empiricism

Quine’s paper, “Two Dogmas of Empiricism,” took aim at the core of the positivist program, the verification theory of meaning. Quine pointed out that there was no way to verify single statements—all attempts to verify a statement inevitably involve the assumed truth of other statements, which are themselves subject to the same problem. For example, say the remote control for your TV isn’t working, and you can’t turn the TV on. You suspect that the batteries in the remote are dead. You can verify this by replacing the batteries and trying to turn on the TV with the remote again. You do this, and the TV turns on. Does that mean you were right? No. It’s entirely possible that the batteries in the remote weren’t dead at all. Maybe the remote has a short and will only work intermittently, no matter what batteries you use. Or maybe the old batteries were in backward, and you didn’t notice; you just popped them out and put in new ones pointing the right way. Or maybe something more exotic is going on: maybe the remote always worked, but when you tried it before, the TV mysteriously shifted its picture to the infrared and its sound to the ultrasonic, so you couldn’t see or hear it; the TV happened to go back to normal after you changed the batteries, but not because you changed them. This last idea is clearly preposterous—how could that happen?—but the point remains that in testing out the new batteries in the remote, you’ve assumed a wide variety of basic facts about the world, a...

Logical empiricim to everything is wrong

With the verification theory of meaning left gasping for air, Quine dismissed the idea that it’s meaningless to talk about unobservable things. Unverifiable statements must have meaning, because no individual statement is verifiable. Thus, the “metaphysics” so dreaded by the positivists came roaring back: rather than speaking simply of sensations, Quine contended that it was perfectly intelligible to speak of physical objects with existence independent of the speaker.

Quine

while there was much they didn’t agree on, a new consensus began to form among professional philosophers of science, a position in opposition to logical positivism, which they called scientific realism. 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. When a new scientific theory is accepted in place of an old one, this is generally because it gives us a better approximation of the true nature of the world in some important way. This is not to say that the world is entirely insensitive to our probings of it—quantum contextuality assures us that our measurements have some impact on the world—but, by and large, the world proceeds on whether or not we interfere with it. And

The realists also argued that the distinction between what is observable and what is unobservable was neither meaningful nor relevant to science. This, of course, was anathema to the positivists. Some positivists had even gone so far as to say that objects seen in microscopes were not truly real, because they were not “directly” perceived. The scientific realists thought this was preposterous. “If this analysis is strictly adhered to, we cannot observe physical things through opera glasses, or even through ordinary spectacles, and one begins to wonder about the status of what we see through an ordinary windowpane,” wrote Grover Maxwell, one of the most vocal defenders of scientific realism. Maxwell also pointed out that the very idea of something that is “unobservable in principle” is subject to revision with new theories and new technology. Before the development of optics and the microscope, he pointed out, something “too small to be seen” would have been unobservable in principle. “It is theory, and thus science itself, which tells us what is or is not … observable,” Maxwell wrote, echoing Einstein’s words to Heisenberg. “There are no a priori or philosophical criteria for separating the observable from the unobservable.”

The wrong turn/the extreme position

many positivists still clung to instrumentalism, the view that science is simply a tool for organizing and predicting perceptions, and that the metaphysical content of theories was unnecessary. The realists pointed out that this was untenable as well. If the unobservable “metaphysical” content of our best scientific theories—stuff like electrons—really bears no relation at all to the actual stuff in the world, then why do our scientific theories work at all? The theories themselves suggest explanations for the phenomena that we see based on the unobserved stuff. But if the unobserved stuff is just a convenient picture that just happens to come along with the “real” content of the theory (i.e., the predictions about the observable world) and that picture really doesn’t line up with the stuff in the world in any way, then we’re astonishingly lucky that our theories work so well!

“If you make a measurement, you get an entanglement between the system and the apparatus and the observer,” Zeh said. “The observer sees only one component [of the Schrödinger’s cat state] and not the superposition of all the others. So, that solves the measurement problem.” Zeh had unknowingly reinvented Everett’s many-worlds interpretation from scratch—and, along the way, he had also developed a mathematically sophisticated account of the interactions between small quantum systems, like atoms, and the relatively large quantum objects around them, like rocks and trees and measurement devices. This, in turn, explained why the different branches of the universal wave function would not interact, and did so in a much more detailed way than Everett ever had. Zeh’s approach to these interactions was later dubbed “decoherence.”

Decoherence

The Clauser-Horne-Shimony-Holt (CHSH) paper recast Bell’s mathematics in a form more amenable to a laboratory test and laid out a detailed proposal for an experiment that would determine whether Bell’s inequality was violated

Just as each roulette ball at the casino landed on red or black, each photon would either pass through a polarizer or be blocked by it. Comparing the behavior of many pairs of these photons would test Bell’s theorem. If each pair of entangled photons had a prearranged plan for how to behave at each of the two polarizers, then the results would satisfy Bell’s inequality.

A second test of Bell’s inequality, conducted by Holt and Francis Pipkin at Harvard, directly contradicted Clauser’s results—they found that Bell’s inequality held, suggesting that nature was local and quantum physics was wrong. Another experiment was needed to break the tie. At Berkeley, Clauser set up a modified version of Holt and Pipkin’s experiment, hoping again to find that quantum physics was wrong. Meanwhile, Ed Fry and Randall Thompson at Texas A&M University set up a similar experiment but used cutting-edge “tunable” lasers to dramatically cut down on the time needed to collect the data. In 1976, both Clauser and the Texas team announced their results: quantum mechanics was vindicated, and Clauser and Freedman’s original result stood. Quantum nonlocality was real.

Bohm didn’t live to see much of this work. He died of a heart attack in the back of a London taxicab in 1992, at age seventy-four. He had survived the blacklist, he had suffered four decades of exile with dignity and integrity—and he had unequivocally proven that alternatives to Copenhagen were possible. His work put the lie to von Neumann’s proof and directly prompted Bell’s marvelous theorem. If John Bell was the father of the quantum revival, then surely David Bohm had been its grandfather.

Why don’t we ever see dead-and-alive cats in the real world? Why does the Schrödinger equation work so well for small objects yet appears to fail so miserably for the objects of everyday life? Zeh, unsurprisingly, agreed that “environment-induced decoherence by itself does not solve the measurement problem”—he contended that Everett’s many-worlds interpretation was needed to complete the picture.

When two photons with entangled polarization go flying off in opposite directions and we measure the polarization of one of them, we do instantly learn the polarization of the other one—but there’s nothing mysterious or nonlocal about that, any more than there’s something mysterious or nonlocal about being able to instantly infer the time in Buenos Aires by looking at a clock in Beijing. And since there’s nothing nonlocal about this, there’s no longer any puzzle about why entanglement can’t be used for faster-than-light communication.

Interesting

In spontaneous-collapse theory, the quantum wave function is real, but it doesn’t obey the Schrödinger equation perfectly. Instead, sometimes the wave function collapses. But this collapse has nothing to do with observation or measurement—the collapse happens entirely at random, for no reason at all, whether or not anyone is looking.

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. As Bell put it, in spontaneous-collapse theory, Schrödinger’s cat “is not both dead and alive for more than a split second.” This neatly resolves the measurement problem: all objects, large and small, obey the same laws, with no special role played by measurement. Wave function ...

Spontaneous-collapse theory. (a) A single-particle wave function only has one slot machine and is unlikely to hit the collapse jackpot for millions or billions of years. (b) A wave function shared by many entangled particles has many slot machines and is likely to hit a collapse jackpot much sooner.

On July 19, 1982, Everett died of a heart attack at the age of fifty-one. In accordance with his wishes, his family had him cremated, and left his ashes out with the trash.

WHAT

Inflation, first proposed by the physicist Alan Guth in 1981 and refined by Andreas Albrecht and Andrei Linde shortly thereafter, says that the very early universe expanded extraordinarily quickly for a minuscule fraction of a second—increasing in size by a factor of about 100 trillion trillion in about a billionth of a trillionth of a trillionth of a second—then resumed expanding much more slowly. This rapid expansion was driven by hypothetical “inflatons,” high-energy subatomic particles, which decayed into normal matter at the end of inflation.

the Copenhagen interpretation couldn’t explain how the early universe worked—and the mathematics of quantum physics couldn’t handle such situations either. The early universe was fabulously small, suggesting that quantum physics was needed, but also fabulously dense, requiring the forbidding mathematical machinery of general relativity. Unfortunately, a theory unifying general relativity with quantum physics hadn’t been found, despite having been sought for decades by an army of physicists, including Einstein.

Surprisingly, both string theory and inflation, which were developed quite independently, seem to point to a common conclusion: the existence of a multiverse, an enormous number of multiple independent universes. According to inflation, the universe is unable to escape “eternal inflation”: as inflation ends in one part of the universe, it continues in others, and “bubbles” of noninflating universe continually appear in the inflating region. We live in one of these bubbles; other bubbles would be their own universes, cut off from all the others, and each might have its own laws of physics and assortment of fundamental particles. And because inflation is eternal, there would be an infinity of these bubbles—an infinite multiverse of inflation.

Wow

String theory, meanwhile, doesn’t describe a single universe but instead describes a “string landscape,” a phenomenally huge number of possible universes—10500 or more. The similarities to the many-worlds interpretation’s multiverse were not lost on quantum cosmologists. The appearance of multiverses independent of Everett’s interpretation made its strange profusion of worlds downright appealing. Some physicists even proposed that all three of these multiverses—Everettian many-worlds, eternal inflation, and the string landscape—were in fact a single multiverse, and the three theories were simply describing the same reality in different ways. In any case, the many-worlds interpretation was (mostly) no longer laughed out of the room without serious consideration. Indeed, by the start of the twenty-first century, many-worlds had become the most popular rival to the Copenhagen interpretation itself among physicists, with particular popularity among cosmologists. But with wider consideration came the realization of a new problem, one that any theory with an infinite multiverse faced: the problem of probability.

Wow

At its core, the measurement problem asks when wave functions obey the deterministic harmony of the Schrödinger equation and when they undergo the random process of collapse. The many-worlds interpretation gets around the measurement problem by denying that wave function collapse happens at all.

Main

The best we can do is to use the mathematics of quantum physics to say how likely, how probable, it is that we’re currently in a particular branch of the wave function—which means we’re assigning a probability to seeing a particular outcome of the experiment when we do look. So probability is still an essential part of quantum physics in the many-worlds interpretation; it’s just that the probability isn’t, strictly speaking, about outcomes of experiments, but rather about where you find yourself in the universe right now.

Hmm

“Here’s a ‘multiverse’ that basically everyone believes in,” says David Wallace, a philosopher and advocate of the many-worlds interpretation. “Think about the planets of the stars of distant galaxies. Pretty much everyone thinks that there are indeed planets around the stars in distant galaxies, and that there are rocks on the surface of those planets. … That’s not an infinite multiverse, but [ten thousand billion billion] solar systems is quite a lot to be getting on with. And the reason you take that seriously is not really because we can observe it. … It’s more that it’s a completely unavoidable consequence of a theory that we think is just rock-solid.”

Interesting

Popper promoted a scientific worldview based on falsification. Theories that could be proven false, Popper declared, were potentially scientific theories—and theories that could not be proven false were not scientific at all.

Interesting

Popper’s views became unusually popular among practicing scientists, and, by the end of the twentieth century, many physicists believed that falsifiability was a vital acid test that any potential theory must pass. Viewed through this lens, any multiverse theory appears quite suspect. If the other universes are not accessible and can never directly influence our own universe, then what possible experimental data could falsify the theory that we live in a multiverse?

theoretical physics risks becoming a no-man’s-land between mathematics, physics and philosophy that does not truly meet the requirements of any.” Straying from Popper’s dictum, they warned, was a “drastic step” with potentially dire consequences. “This battle for the heart and soul of physics is opening up at a time when scientific results—in topics from climate change to the theory of evolution—are being questioned by some politicians and religious fundamentalists. Potential damage to public confidence in science and to the nature of fundamental physics needs to be contained by deeper dialogue between scientists and philosophers.”

True

“My theory has been falsified!” But it ain’t necessarily so. Despite the fact that the remote still isn’t working, it’s still possible that the old batteries were dead. Maybe the new batteries are dead too. Maybe a rat gnawed through the TV’s power cable while Popper was at the corner store. Maybe the laws of physics actually change depending on where you are, and while Popper was at the store, the Solar System moved through its orbit around the center of the Milky Way and entered a patch of space where the laws of electromagnetism that govern the behavior of batteries in remote controls are different. The problem is that Popper’s “dead battery theory” of his remote control doesn’t actually make any predictions on its own: it only makes predictions in conjunction with a huge number of other basic assumptions that Popper has made about the functioning of the world. So Popper is wrong: his theory was not falsified.

Popper

“There is always a well-known solution to every human problem—neat, plausible, and wrong

Lol

In the wooded foothills of the Austrian Alps, on the outskirts of Vienna, there is a hut in a vineyard with a small mirror in the window. The vineyard has been there for centuries; it was already old when one of the founders of the Vienna Circle, Otto Neurath, met with Einstein and other scientists on the neighboring hill in 1920 to discuss his idea of an International Encyclopedia of Unified Science.

Zeilinger is an experimental master of photon manipulation. Zeilinger’s group has already demonstrated that it can send and receive single photons over distances much longer than the 10 kilometers from the lab to the mirror in the vineyard and back. In 2012, they successfully sent entangled photons over 143 kilometers, between La Palma and Tenerife in the Canary Islands. And Zeilinger has also spent decades conducting improved versions of Aspect’s Bell experiments, verifying the existence of quantum nonlocality with enormous experimental precision.

Interesting

There have been physicists who have tried to bring the idea that quantum physics is fundamental into their calculations. To do so, they had to give up Copenhagen’s solution to the measurement problem and perform conceptual work to solve the problem a different way. In other words, these people, people like David Bohm and Hugh Everett, had to develop new interpretations of quantum physics, because the Copenhagen interpretation doesn’t take quantum physics seriously.

Hmm

The history of quantum foundations is soaked in personalities. If David Bohm had held more palatable political convictions, if Hugh Everett hadn’t hated public speaking, if Einstein had had Bohr’s charisma, the story told in this book likely would have been dramatically different. So many of the key events were driven by political or social or interpersonal interactions, not by scientific considerations. This suggests another reason the Copenhagen interpretation is so popular: not because it is somehow better or more suited to the needs of physicists but simply because it was first.

Main

That being said, the story of quantum foundations does seem to call into question how science works. We’ve seen how it doesn’t work—it’s not about verification or purely empirical statements, as the positivists thought; it’s not about falsifiability, as Popper thought; nor is it about being completely independent from the complex historical forces that have buffeted and buoyed the characters we’ve met over the course of this book. So how does science work? Echoing the end of Chapter 11, that’s a fabulously complex question. The long answer would take another 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.

Main

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.

Main

The debates about evolution, global warming, and homeopathy have been explicitly manufactured and funded by a variety of corporate, religious, and political entities from outside science, who are not in the least interested in the quest to divorce our human biases from our understanding of the world. They are not committed to taking the science seriously and are instead devoted to taking their own aims and giving them a thin patina of scientific respectability, enough to justify their claims to equal or greater validity than the existing and overwhelming scientific consensus. These groups are not interested in examining the data and happily reject it when it does not meet their preordained conclusions, inventing new “data” to suit their purposes.

So true

There is a correct interpretation, though it may not be any of the ones that we have yet. Simply dismissing the quantum world as a convenient mathematical fiction means we aren’t taking our best theories of the world seriously enough, and we are hobbling ourselves in the search for a new theory. Stating that the conclusions of the Copenhagen interpretation are “inevitable” or “forced upon us by the mathematics of the theory” is simply wrong. It is not true that it’s meaningless to talk about reality existing independently of our perceptions, that we must think of the world solely as the subject of our observations. Solipsism and idealism are not the messages of quantum physics.

Main

Hold on to them loosely, not dogmatically, and keep a fresh perspective on the work we do.

Interesting

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