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

by Adam Becker

Your house keys are a temporary alliance of a trillion trillion atoms, each forged in a dying star eons ago, each falling to Earth in its earliest days. They have bathed in the light of a violent young sun. They have witnessed the entire history of life on our planet. Atoms are epic.

Quantum physics—the physics of atoms and other ultratiny objects, like molecules and subatomic particles—is the most successful theory in all of science. It predicts a stunning variety of phenomena to an extraordinary degree of accuracy, and its impact goes well beyond the world of the very small and into our everyday lives. The discovery of quantum physics in the early twentieth century led directly to the silicon transistors buried in your phone and the LEDs in its screen, the nuclear hearts of the most distant space probes and the lasers in the supermarket checkout scanner. Quantum physics explains why the Sun shines and how your eyes can see. It explains the entire discipline of chemistry, periodic table and all. It even explains how things stay solid, like the chair you’re sitting in or your own bones and skin. All of this comes down to very tiny objects behaving in very odd ways.

What is real, at the most fundamental level, and how does it work?

The Einstein-Bohr debates have entered into the lore of physics itself, and the usual conclusion is that Bohr won, that Einstein’s and Schrödinger’s concerns were shown to be baseless, that there is no problem with reality in quantum physics because there is no need to think about reality in the first place. Yet quantum physics is certainly telling us something about what is real, out in the world. Otherwise, why would it work at all?

Physics is about the world around us. It aims to understand the fundamental constituents of the universe and how they behave. Many physicists are driven to enter the field out of a desire to understand the most basic properties of nature, to see how the puzzle fits together. Yet, when it comes to quantum physics, the majority of physicists are perfectly willing to abandon this quest and instead merely “shut up and calculate,” in the words of physicist David Mermin.

Despite the popular view among physicists, Einstein clearly got the better of Bohr in their debates and convincingly showed there were deep problems that needed answering at the heart of quantum physics. Simply dismissing questions about reality as “unscientific,” as some of Schrödinger’s opponents did, is an untenable position based on outdated philosophy.

This is an astonishing state of affairs, and hardly anyone outside of physics knows about it.

The discovery that the Earth was not at the center of the universe, Darwin’s theory of evolution, the Big Bang and an expanding universe nearly 14 billion years old, containing hundreds of billions of galaxies, each containing hundreds of billions of stars—these ideas have radically altered humanity’s conception of itself.

Finding this Pandora’s box of weird questions lying at the heart of fundamental physics is disturbing, to say the least. Yet despite all this weirdness, quantum physics is wildly successful at describing the world—much more so than simple old Newtonian physics (which was already pretty good).

But this makes the measurement problem even more urgent—it means there’s something about the nature of reality that we don’t understand.

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.”

For nearly two decades, almost nobody other than Einstein believed in photons.

This “wave-particle duality” shows up in all quantum phenomena.

De Broglie, who had defended his PhD thesis only three years prior, had been the first to suggest that all of the fundamental constituents of matter had both a particle and a wave aspect.

esse est percipi [to be is to be perceived].

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.

Einstein would have been first among the newly unemployed—but he had seen Germany’s fate coming. He and his wife Elsa had left their home in Berlin to tour the United States months before Hitler came to power. “Take a very good look at it. You will never see it again,” Einstein had told Elsa as they left. Once the Nazis took over, Einstein, the most famous Jew in the world, was a marked man. Einstein’s stepdaughter managed to smuggle his papers safely out of his Berlin apartment before the Nazis could destroy them; Hitler’s goons ransacked the apartment four times in three days, but Einstein’s family and papers all made it out of the country. Einstein met his family and collected his belongings in Belgium, renounced his German citizenship, then returned to the United States, where he took up a post at the newly minted Institute for Advanced Study in Princeton. He remained in America for the rest of his life. Physicists who lacked Einstein’s foresight but shared his Jewish heritage fled Nazi Germany after the Civil Service Act. They mostly landed in the United States and the UK, drastically shifting the center of the physics world (and changing the international language of physics from German to English).

The best-known physicist in Austria, Erwin Schrödinger, wasn’t Jewish, but his wife was. Schrödinger had been at the University of Berlin in 1933 but had quit in protest when Hitler came to power. After Hitler invaded Austria, Schrödinger publicly recanted his anti-Nazi views, but this wasn’t enough for the new regime. Dismissed from his university post for “political unreliability,” Schrödinger fled to Ireland with his wife. Once there, he wrote a letter to Einstein, apologizing profusely for his “great duplicity.”

“The racial campaign… acquired momentum at an amazingly fast pace,” wrote Laura Fermi. “We at once decided to leave Italy as soon as possible.” Her husband, Enrico, was the pride of Italian physics, one of the foremost experts on both theoretical and experimental nuclear physics in the world. But with Italy unsafe for the family of a Catholic man and a Jewish woman, Enrico and Laura quietly made plans to leave. Their plans were complicated by Mussolini’s fascist economic policy, which made it illegal to take more than pocket change out of Italy. Then Niels Bohr intervened. When Fermi came to Copenhagen for a conference that summer, Bohr took him aside and—breaking an unwritten rule of the physics community—told Fermi that his name was in the running for a Nobel Prize that year. Would the prize, which came with about $500,000 (in today’s money) and an excuse to travel abroad, be useful this year, Bohr asked? Or would another time be more convenient, given the political situation? Fermi told Bohr that this year would be a particularly good time for the prize. Returning home, Fermi found that the Italian government had confiscated all Jewish passports, including Laura’s; pulling a few strings, he managed to get her passport back in time to attend the Nobel ceremony in Stockholm. After Stockholm, the Fermis went to Copenhagen to visit Bohr, and, finding the wheels of American immigration greased by the words “Nobel Prize winner,” they set sail for Manhattan just before Christm...

“My heart aches at the thought of the young ones,” Einstein wrote to Born in 1933. Einstein was soon involved with a British-led effort to help the academic victims of the Nazi regime, which met with some success. By the time Hitler invaded Poland and started World War II on September 1, 1939, more than a hundred physicists had emigrated from the European continent to the United States and the UK—some of the younger ones simply fleeing, refugees without the promise of a job in their new country, coming with a single small bag across the Channel or the Atlantic. Some came with nothing. Some never came at all.

U-238 is a different story. Those three extra neutrons give it a little more stability, so hitting it with a neutron won’t split it as easily. This makes it impossible to build a bomb out of U-238. And fortunately, about 99.3 percent of uranium in nature is U-238. To build an atomic bomb, you would need to separate the tiny quantity of U-235 from the enormous bulk of U-238—and since they’re chemically identical, the only way to separate them is to take advantage of the fact that U-238 is 1.3 percent heavier than U-235. This guaranteed that nuclear power would be phenomenally difficult to achieve, requiring enormous quantities of uranium and city-sized industrial diffusion and centrifuge facilities. “It can never be done unless you turn the United States into one huge factory,” concluded Bohr. Yet the risk of not pursuing nuclear power was too high. If Nazi Germany were to build an atomic bomb, the war would be over. The Einsteins and Fermis and Borns of the world would never be able to escape Hitler’s Reich. “A little bomb like that,” said Fermi, cupping his hands as he looked out over Manhattan, “and it would all disappear.”

The plan worked, to a point: the letter did get FDR’s attention, but he appointed Lyman Briggs, the ineffectual leader of the Bureau of Standards, to head a Uranium Committee. Briggs and his committee did little, and the project stalled for over a year while Hitler occupied Denmark, captured Paris, and relentlessly bombed London. When the US government finally started to seriously investigate atomic power in the fall of 1941, Wigner met with Arthur Compton, an American physicist who was preparing a report for FDR’s Top Policy Group on the feasibility of developing atomic bombs. “[Wigner] urged me, almost in tears, to help get the atomic program rolling,” wrote Compton. “His lively fear that the Nazis would make the bomb first was the more impressive because from his life in Europe he knew them so well.”

Finding the Nazi ideology appealing for aesthetic reasons—and in line with his idealist stance on the philosophy of science—Jordan joined not only the Nazi Party in 1933, but also the Brownshirts, Hitler’s paramilitary storm troopers. And other physicists, like Johannes Stark and Philipp Lenard, had been Nazis even before Hitler came to power, and applied Hitler’s racial “philosophy” to physics, declaring relativity and quantum theory to be “Jewish physics.”

Educating physicists was no longer seen as a matter of mere scientific necessity but an essential investment in military infrastructure.

The results convince you of what you had already suspected but hadn’t dared to believe: the photons are deliberately messing with you.

When you aren’t measuring the position of a photon, it is a wave. Thus, photons can interfere with themselves as they pass through the double slit. But measuring the location of a photon forces it to behave as a particle: when the photon hits the screen behind the double slit, it must strike in only one spot.

This was a truly radical idea: taking quantum physics seriously as a way of accounting for the entire world.

The Copenhagen interpretation, in Bohm’s view, was “guided to a considerable extent” by the idea that objects that can’t be seen aren’t real, an idea Bohm ascribed to positivism. Yet, as Bohm pointed out, “the history of scientific research is full of examples in which it was very fruitful indeed to assume that certain objects or elements might be real, long before any procedures were known which would permit them to be observed directly.” Bohm then gave the example of atoms, the existence of which Mach resisted to the end, despite the overwhelming evidence to support them, because they couldn’t be seen.

Bohm’s theory had also appeared during the height of Zhdanovism, an ideological campaign by Stalin’s USSR to stamp out any intellectual work that had even the faintest whiff of a conflict with the ideals of Soviet communism.

This is a hallmark of the many-worlds interpretation: the appearance of a single world, despite the true existence of many.

Somehow, the answer to the question, “Is the ball on a red number?” is actually affected by which other question you ask! This is contextuality: the answer to a question depends on its context of surrounding questions asked at the same time.

As Pascual Jordan said, “We ourselves produce the results of measurement.”

You can’t look at the quantum world without altering it—but that doesn’t mean the quantum world isn’t there before you look. Quite the opposite: if it weren’t there, you wouldn’t be able to alter it by looking!

The single wave function shared by the two entangled photons guarantees that they will always behave in the same manner when encountering two polarizers with matching axes.

If the quantum prediction for Bell’s experiment is correct, and Bell’s inequality is violated, then something is nonlocal, and locality is merely an illusion. That suggests a need for a radical revision of our conception of space and time, far beyond Einstein’s relativity. Any story of the world that could incorporate a violation of Bell’s inequality would have to be truly strange.

And entangled photons really do behave this way, so something very strange must be going on in quantum physics.

In short, the results of real experiments with entangled photons mean that something, some influence, is going faster than light. Entanglement is not just an artifact of the mathematics of quantum physics: it’s a real phenomenon, an actual instantaneous connection between far-distant objects. This is an astonishing result.

Physics and chemistry were at odds for much of the nineteenth century—chemists mostly believed in atoms, whereas physicists were often skeptical that atoms existed—and only in the first decades of the twentieth century did the two fields start to build a consistent picture of chemical interactions.

Suddenly, entanglement and Bell’s theorem weren’t just concerns for a handful of physicists and philosophers in an abstruse and neglected corner of science. Practical questions of computing technology and cryptography were at stake, and naturally, governments and militaries took a fierce interest in the subject. Mastering control of entanglement, decoherence, and other phenomena first described by researchers in quantum foundations was potentially big business—and the race to build a quantum computer was on. The funding floodgates opened. Within a decade of Shor’s breakthrough, the Department of Defense was funding a $20 million initiative in quantum information. By 2016, multiple US government agencies, both military and civilian, were funding quantum information technology; the EU was funding €1 billion of research and development in the subject; and China was testing a quantum communication satellite. Private corporations, like Google and Microsoft, were also doing work in this field. In short, quantum information processing was no longer a part of quantum foundations—it had spun off and become its own billion-dollar industry. But little of this money was going to quantum foundations. The flood of new grants were almost entirely for developing practical things like building quantum computers themselves, not for new approaches to the measurement problem. Quantum foundations had borne this new fruit, it had proven it wasn’t useless, but the advances in quantum information...

Yet this still leaves us with the question from the Introduction: 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 sam...

“I think you’re stuck with the nonlocality,” Bell told the small group who had come to hear him speak that day. “I don’t know any conception of locality which works with quantum mechanics.”

“WMAP’s data supports the notion that galaxies are nothing but quantum mechanics writ large across the sky,” Brian Greene said in 2006. “This is one of the marvels of the modern scientific age.”

The early universe was fabulously small, suggesting that quantum physics was needed, but also fabulously dense, requiring the forbidding mathematical machinery of general relativity.

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.

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.

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.

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.

Part of the problem is that physicists generally don’t know much about philosophy. There is a massive asymmetry between the two fields: while philosophers usually take physics very seriously indeed—philosophers of physics are mathematically conversant in physics and often have advanced degrees in both fields—physicists are rarely trained in philosophy at all. Despite their ignorance of philosophy (or more likely because of their ignorance), some physicists are openly contemptuous of the subject.

Just a few generations ago, at the birth of quantum physics, all physicists received some schooling in philosophy. Einstein read Mach, Bohr read Kant. But the shifts in research funding and physics classrooms after World War II also led to broader shifts in university curricula. For Einstein and Bohr’s generation, philosophy was part of the core educational curriculum in central Europe. But in postwar America, it was (and is) relatively easy for an intelligent student to go all the way from kindergarten to a PhD in physics at a top-tier university without ever darkening the door of a philosophy classroom.

Quantum physics works. The calculations enabled by the theory are astonishing in their range of applicability and the accuracy of their results. Quantum physics tells us how long it will take to heat up your frying pan to cook your eggs and how large a dying white dwarf star can be without collapsing. It reveals the exact shape of the double helix at the core of life, it tells us the age of the immortal cattle on the rock walls at Lascaux, it speaks of atoms split beneath the stone heart of Africa eons before Oppenheimer and the blinding light of Trinity. It predicts with uncanny accuracy the precise darkness of the blackest night. It shows us the history of the universe in a handful of dust.