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

by Adam Becker

The radiation emitted by the lump of metal is composed of subatomic particles, breaking away from the atoms in the metal and flying off at high speeds. Like all sufficiently tiny things, those particles obey the laws of quantum physics. But, instead of reading Shakespeare, the subatomic particles in the metal have been listening to the Clash—at any particular moment, they don’t know whether they should stay or they should go. So they do both: during the time the box is closed, the indecisive lump of radioactive metal will and won’t emit radiation. Thanks to these punk-rock particles, the Geiger counter will and won’t register radiation, which means the hammer will and won’t smash the vial of cyanide—so the cat will be both dead and alive. And this, Schrödinger pointed out, is a serious problem. Maybe an atom can travel down two paths at once, but a cat certainly can’t be both dead and alive. When we open the box, the cat will be either dead or alive, and it stands to reason that the cat must have been one or the other the moment before we opened the box.

Explanation

Yet many of Schrödinger’s contemporaries piled on, denying exactly that point. Some claimed that the cat was in a state of dead-and-alive until the moment the box was opened, when the cat was somehow forced into “aliveness” or “deadness” through the action of looking inside the box. Others believed that talking about what was going on inside the box before it was opened was meaningless, because the interior of the unopened box was unobservable by definition, and only observable, measurable things have meaning. To them, worrying about unobservable things was pointless, like asking whether a tree that falls in the forest makes a sound when nobody’s around to hear it.

the two directions of understanding

figuring out what quantum physics is saying about the world has been hard. This is, in part, due to the sheer weirdness of the theory. Whatever is in the world of the quantum, it is nothing familiar at all. The seemingly contradictory nature of quantum objects—atoms that are here and there at the same time, radiation that has both been emitted and remains latent in its source—isn’t the only alien aspect of the theory.

There are also instantaneous long-distance connections between objects: subtle, useless for direct communication, but surprisingly useful for computation and encryption. And there does not appear to be any limit to the size of object that is subject to quantum physics. Ingenious devices built by experimental physicists coax larger and larger objects to display strange quantum phenomena almost monthly—deepening the gravity of the problem that no such quantum phenomena are seen in our everyday lives.

Bohr was maddeningly unclear about the location of the boundary between the worlds. And Werner Heisenberg, the first person to discover the full mathematical form of quantum physics, was no better. Bohr and Heisenberg’s approach to quantum physics—known as the “Copenhagen interpretation,” named after the home of Bohr’s famous institute—was pervaded by the same vagueness that Bell had found in his quantum physics courses.

According to the Copenhagen interpretation, this question has a very simple answer: quantum physics tells us nothing whatsoever about the world. Rather than telling us a story about the quantum world that atoms and subatomic particles inhabit, the Copenhagen interpretation states that quantum physics is merely a tool for calculating the probabilities of various outcomes of experiments. According to Bohr, there isn’t a story about the quantum world because “there is no quantum world. There is only an abstract quantum physical description.” That description doesn’t allow us to do more than predict probabilities for quantum events, because quantum objects don’t exist in the same way as the everyday world around us.

They dont exist the same way

As Heisenberg put it, “The idea of an objective real world whose smallest parts exist objectively in the same sense as stones or trees exist, independently of whether or not we observe them, is impossible.”

Einstein couldn’t stand the Copenhagen interpretation. He called it a “tranquilizing philosophy—or religion” that provides a “soft pillow to the true believer … [but it] has so damned little effect on me.” Einstein demanded an interpretation of quantum physics that told a coherent story about the world, one that allowed answers to questions even when no measurement was taking place. He was exasperated with the Copenhagen interpretation’s refusal to answer such questions, calling it an “epistemology-soaked orgy.”

Einstein

But quantum physics is significantly stranger than Newtonian physics, and its math is stranger too. If you want to know where an electron is, you need more than three numbers—you need an infinity of them. Quantum physics uses infinite collections of numbers called wave functions to describe the world. These numbers are assigned to different locations: a number for every point in space. If you had an app on your phone that measured a single electron’s wave function, the screen would just display a single number, the number assigned to the spot where your phone is. Where you’re sitting right now, the Wave-Function-O-Meter™ might display the number 5. Half a block down the street, it’d display 0.02. That’s what a wave function is, at its simplest: a set of numbers, fixed at different places.

The Schrödinger equation ensures that wave functions always change smoothly—the number that a wave function assigns to a particular location never hops instantly from 5 to 500. Instead, the numbers flow perfectly predictably: 5.1, 5.2, 5.3, and so on. A wave function’s numbers can go up and down again, like a wave—hence the name—but they’ll always undulate smoothly like waves too, never jerking around too crazily.

Wave

Newton could give you the location of any object using just three numbers. Apparently, quantum physics needs an infinity of numbers, scattered across the universe, just to describe the location of a single electron.

The wave function doesn’t tell you how much of the electron is in one place—it tells you the probability that the electron is in that place. The predictions of quantum physics are generally in terms of probabilities, not certainties. And that’s strange, because the Schrödinger equation is totally deterministic—probability doesn’t enter into it at all. You can use the Schrödinger equation to predict with perfect accuracy how any wave function will behave, forever.

Probability

the Schrödinger equation holds all the time, except when you make a measurement, at which point the Schrödinger equation is temporarily suspended and the wave function collapses everywhere except a random point. This is so weird that it gets a special name: the measurement problem (Figure 1.1).

Why does the Schrödinger equation only apply when measurements aren’t happening? That doesn’t seem to be how laws of nature work—we think of laws of nature as applying all the time, no matter what we’re doing. If a leaf detaches from a maple tree, it will fall whether or not anyone is there to see it happen. Gravity doesn’t care whether anyone is around to watch.

The observations effect

now we have a new challenge: what is a “measurement,” anyhow? Does a measurement require a measurer? Does the quantum world depend on whether it has an audience? Can anyone at all collapse a wave function? Do you need to be awake and conscious for it, or can a comatose person do it? What about a newborn baby? Is it limited to humans, or can chimps do it too? “When a mouse observes, does that change the [quantum] state of the universe?” Einstein once asked.

Lol

The wavelength of visible light is thousands of times larger than the size of an individual atom.

Bohr proposed a “planetary” model of the structure of an atom, with a tiny yet massive nucleus surrounded by orbiting electrons. In Bohr’s model, the electrons were restricted to a particular set of allowed orbits. Electrons could never be between Bohr’s allowed orbits, but they could “jump” from one orbit to another. Each orbit corresponded to a different energy, and, as the electrons jumped, they would emit or absorb light equal to the change in their energy, producing the spectrum seen in the lab. These discontinuous jumps of certain energies were known as quanta, from the Latin for “how much”—hence the new science of the atomic world came to be known as “quantum physics

The light or spectrum emitted from the jump between orbits, quanta

Put an atom in a magnetic field, and its spectrum changed. Put it in an electric field, and its spectrum changed in a different way. Colors shifted, blurred, and split, dimmed and brightened, with no larger pattern in sight—until Heisenberg.

Heisenberg focused on what he could actually see: the spectrum of light emitted from the jumps between energy levels themselves.

Heisenberg’s matrix mechanics was technically forbidding and impossible to visualize—but it offered the prospect of a theory not just for atomic spectra but for the entire quantum world.

Good theories, according to Mach, were about connecting observations, not about positing things that couldn’t be observed at all.

Interesting

Einstein was accustomed to isolation. He had changed the world in 1905 working alone in a Swiss patent office and continued that habit for the rest of his life. 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

Einstein

“On principle, it is quite wrong to try founding a theory on observable magnitudes alone. In reality the very opposite happens. It is the theory which decides what we can observe.” Einstein then went on to explain that the information about the world around us that we receive from scientific instruments—or even from our own senses—would be totally incomprehensible without some kind of theory about the way the world works.

Interesting

The Danish Academy of Arts and Sciences chose Bohr to be the resident of the Carlsberg House of Honor, built and funded by the Carlsberg corporation, the great Danish beer-brewing company.

Bohr was a peculiar kind of sage—brilliant and insightful, yet plodding and obscure, sometimes infuriatingly so. “It is practically impossible to describe Niels Bohr to a person who has never worked with him,” said George Gamow, a Russian physicist and former student of Bohr (who had a famously large personality himself). “Probably his most characteristic property was the slowness of his thinking and comprehension.”

Bohr

Max Born had discovered a piece of the puzzle that summer. He found that a particle’s wave function in a location yields the probability of measuring the particle in that location—and that the wave function collapses once measurement happens. Born’s insight ultimately won him a Nobel Prize, and rightly so.

measurement again

Heisenberg wasn’t particularly concerned with solving the measurement problem. He was more concerned with getting another offer for a tenured professorship. He was worried that Schrödinger’s accomplishments had eclipsed his own and that he had made a mistake in returning to Copenhagen rather than accepting the permanent professional safety that had been offered by Leipzig.

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.

gamma ray for electron

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 Schrödinger’s wave 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.

the action of measurement (gamma ray) will compromise the object's momentum to pinpoint location.

Heisenberg used the term “uncertainty principle” to describe this insight. Heisenberg’s uncertainty paper paid off as he had hoped: the University of Leipzig again offered him a tenured professorship. He accepted, and in June 1927, Heisenberg, at twenty-five, became the youngest tenured professor in all of Germany.

In other words, one could not ask what was really happening inside of an atom when nobody looked

And the behavior of the objects in that world, as indicated by such an apparatus, would be best described as either particles or waves, but never both simultaneously. These descriptions are contradictory—a particle has a definite location, which waves don’t; waves have frequencies and wavelengths, which particles don’t—yet Bohr claimed that this “inevitable dilemma” was not a problem for quantum physics. “We are not dealing with contradictory but with complementary pictures of the phenomena,” claimed Bohr, which are “indispensable for a description of experience.”

interesting. complementary pictures of the phenomena

So sometimes the electron behaves like a wave, and sometimes it behaves like a particle, but never both. 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.

interesting

It’s certainly true that the thought experiment illustrates a world in which there are limits on our knowledge, but it’s also a world where particles have well-defined positions and momenta at all times. Hitting an electron with a gamma ray can’t alter the electron’s momentum unless it has a momentum in the first place. We don’t know what that momentum is—but that’s certainly not the same thing as saying it doesn’t exist.

hmm

Paul Dirac. (Dirac wasn’t merely sniping—he had in fact discovered a new equation himself. He had skillfully fused quantum physics with special relativity, leading to a new theory of particle physics that came to be known as quantum field theory. Dirac’s theory correctly predicted the existence of antimatter, a feat that would win him a Nobel Prize in 1933.)

Paul Dirac

Rather than particles and waves being incomplete, contradictory, “complementary” pictures of the quantum, de Broglie offered a quantum world where particles and waves lived in a peaceful coexistence, with particles surfing along “pilot waves” that govern their motion—anticipating Bohm’s interpretation of quantum physics a quarter century later. De Broglie’s particles moved in an entirely deterministic way, despite Born’s statistical rule identifying the wave function as a tool for calculating probabilities. Yet the particles satisfied Heisenberg’s uncertainty principle, because their paths were hidden from view—no experiment could reveal a particle’s full trajectory, just as Heisenberg had said. De Broglie had found a way to restore determinism and causality to the quantum world without sacrificing the extraordinary match between theory and observation of the new quantum physics.

De Brogile. Brought deterministic back to quantum physics

And Heisenberg and others were definitely influenced by Ernst Mach and his successors, the “Vienna Circle” of philosophers, who developed a school of thought called “logical positivism.” Logical positivism picked up where Mach left off—according to them, any statement that made reference to something unobservable was not only bad science, it was literally meaningless.

Yet, despite their differences, Bohr, Heisenberg, and the rest of the Göttingen-Copenhagen group had a few things in common. They all agreed that it was pointless to talk about what was “really” happening in the quantum world. Making accurate predictions about the outcomes of measurements was, for them, enough.

As Bohr put it years after Solvay, “It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature.” Quantum physics, then, didn’t have to present a coherent or consistent picture of how the world operated—indeed, according to Bohr’s complementarity, such a picture was necessarily impossible.

Hmm

Einstein reminded the group, Heisenberg and Born had claimed that quantum physics was closed, complete, perfect as it was. In that case, there cannot be anything that determines the particular location at which the electron hits the film. But this is a problem—and not because it introduces randomness into nature. Instead, the problem is one of locality: the principle that something that happens in one location can’t instantly influence an event that happens somewhere else.

When they gave up. Einstein took the initiative

In quantum physics, the situation is a little trickier. According to the Copenhagen interpretation, particles don’t have properties like position or momentum (or anything else) until those properties are measured. But, EPR argued, measurements made on one particle couldn’t instantly affect another particles far away. So, to get around the uncertainty principle, just wait until particles A and B are very far apart, then find the momentum of A. Measuring A’s momentum lets you infer B’s momentum without disturbing B at all. Then simply measure the position of B. Now you know B’s position and momentum, to arbitrary precision, at the same time. Therefore, argued EPR, a particle can have a definite position and momentum at the same time.

Problem solved

because quantum physics doesn’t let you simultaneously predict the position and momentum of a single particle, EPR argued that it must be incomplete—there must be features of the world that quantum physics doesn’t account for.

EPR Paper

For Einstein, the crucial bit of the EPR thought experiment once again had to do with locality. If you measure A’s momentum, you know B’s momentum too. But because B is far away from A, then, assuming locality, there’s no way that making a measurement on A could have affected B immediately. B’s momentum must have been set when A and B collided, just like billiard balls.

But quantum physics doesn’t let you calculate the momenta of A and B once they collide. Instead, the quantum wave function connects A and B in a strange way. Because of their collision, A and B share a single wave function, rather than having their own individual wave functions. But that shared wave function doesn’t say what the particles’ momenta are before a measurement is made. It simply ensures that, once A’s momentum is measured, B’s momentum will always be equal and opposite.

According to the Copenhagen interpretation, particles don’t have definite properties until those properties are measured. So if A and B have definite momenta before they’re measured, then the Copenhagen interpretation is wrong and quantum physics is an incomplete description of nature. But if A and B don’t have definite momenta before they’re measured, then the very act of measuring A’s momentum must affect B instantly, in order to ensure that its momentum is equal and opposite to A’s—even if A is in New York City and B is on the Moon. And that violates locality. In short, quantum physics is either incomplete or nonlocal. This forced choice was what had been “smothered” in the EPR paper, according to Einstein.

Main

Schrödinger dubbed this connection “entanglement.” 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.

Entanglement

For any entangled system, Einstein’s choice applied: either the system is nonlocal, or quantum physics can’t fully describe all the features of that system. And Schrödinger had just shown that nearly any quantum interaction would result in an entangled system. Thus, the challenge the EPR paper posed wasn’t limited to some tiny corner of quantum physics—it was deeply embedded in the fundamental structure of the theory.

Connection

But this is a weird view. Stating that consciousness collapses wave functions does arguably solve the measurement problem but only at the price of introducing new problems. How could consciousness cause wave function collapse? Since wave function collapse violates the Schrödinger equation, does that mean that consciousness has the ability to temporarily suspend or alter the laws of nature? How could this be true? And what is consciousness anyhow? Who has it? Can a chimp collapse a wave function? How about a dog? A flea? “Solving” the measurement problem by opening the Pandora’s box of paradoxes associated with consciousness is a desperate move, albeit one that seemed reasonable at the time, in the absence of other fully developed solutions to the measurement problem.

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).

Atomic nucleus structure

Uranium is far past that point. With 92 protons, it doesn’t matter how many neutrons you add to uranium—it will eventually decay. But there are two forms of uranium nuclei that will stick around for billions of years before they do: uranium-235 and uranium-238. The numbers refer to the total number of protons and neutrons in the nuclei: U-235 has 143 neutrons and 92 protons, for a total of 235. U-238 has 3 more neutrons, which makes it slightly heavier. But they’re both uranium: the chemical identity of an atomic nucleus is determined solely by the number of protons that it has.

Uranium 235 238