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

by Adam Becker

Chemistry is all about electromagnetic interactions between atoms. The chemical properties of an atom are entirely determined by the number of electrons it has—and the number of electrons that surround a particular atomic nucleus is determined in turn by the number of protons in that nucleus. Nuclei with the same number of protons but different numbers of neutrons are different isotopes of the same element—they differ in weight but not in their chemical properties.

Chemistry

Specifically, hitting a U-235 nucleus with a neutron leads the nucleus to fission: it splits into two smaller nuclei, releasing a fabulous quantity of energy, along with a few free-floating neutrons. With enough U-235—a critical mass—the neutrons left over from fission will hit more U-235 nuclei, which will split in turn, releasing even more neutrons and starting a chain reaction. Left uncontrolled in 120 pounds of pure U-235—a small sphere of the dense metal, less than twenty centimeters across—a nuclear chain reaction would explode with the power of 15,000 tons of TNT, enough to instantly level a small city. Controlling the reaction by absorbing some of the excess neutrons would allow you to power a small city instead, for days on end, with the same 120 pounds of U-235.

Fission.u235.

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.

On the evening of August 6, 1945, just before dinner, Major Rittner, the British military intelligence officer in charge of Farm Hall, quietly took Otto Hahn aside and told him that the Americans had dropped an atomic bomb on Hiroshima. “Hahn was completely shattered by the news,” wrote Rittner: He felt personally responsible for the deaths of hundreds of thousands of people, as it was his original discovery which had made the bomb possible. He told me that he had originally contemplated suicide when he realized the terrible potentialities of his discovery. … With the help of considerable alcoholic stimulant he was calmed down and we went down to dinner where he announced the news to the assembled guests. As was to be expected, the announcement was greeted with incredulity.

Damn

And the bickering of the other scientists at Farm Hall confirmed what documents captured by Alsos had already suggested: the Nazi bomb program, unlike the Manhattan Project, was a disorganized mess, with vital information compartmentalized and no clear vision of how to proceed. Yet, in those same few days, the Farm Hall transcripts make it clear that Heisenberg and his student, Carl von Weizsäcker, purposefully constructed a revisionist narrative of their wartime activities. According to them, while the Americans had built a weapon of death and destruction on unprecedented scales, they, the Germans, had deliberately pursued only a nuclear reactor, being unwilling to build a massive new weapon for Hitler’s Reich—thereby placing the responsibility for their failure on their supposed moral clarity, rather than their sheer incompetence.

There was no trying to elbow one’s way to power, for the simple reason that there wasn’t any place to exercise power.

Pascual Jordan maintained that he had never truly supported the Nazi cause, despite his publications extolling the virtues of a National Socialist approach to science. He even had the audacity to send a letter to Max Born, his mentor who had been forced out by Hitler’s racist policies, explaining that he hadn’t really been a Nazi and asking for a character reference for his “de-Nazification.” Born replied with a list of his friends and family members murdered by the Nazis.)

Damn

Most physicists were perfectly happy with the jumble of ideas that purportedly constituted the Copenhagen interpretation itself, since questions about the meaning of quantum physics had little bearing on their work. The mathematical formalism of the theory continued to work remarkably well in a wide variety of postwar applications of physics to the military-industrial complex, which turned most physicists to work in nuclear physics or solid-state physics (the branch of physics that, shortly after the war, led to the development of the silicon transistor, as well as many of the other materials later responsible for the shrinking size and ballooning importance of computers).

Dresden finally looked at Bohm’s paper, and he was surprised by what he found. Bohm had discovered a totally new way to interpret quantum physics. Rather than refusing to answer questions about the quantum world, as the Copenhagen interpretation did, Bohm’s interpretation depicted a world of subatomic particles that existed whether or not anyone was looking at them, particles with definite positions at all times. These particles, in turn, each had “pilot waves” that determined their motion, which also behaved in an orderly and predictable fashion.

Bohm

Bohm’s brilliance was obvious to his friends and professors alike, as were his personal quirks. Bohm had a “talent for getting people to want to take care of him,” according to his friend Melba Phillips—and “a talent for being unhappy.”

Bohm thought the research being done at Caltech was incremental rather than fundamental, and he found the environment too competitive for his liking. “I wasn’t really happy there at Caltech,” he recalled later. “They’re not interested in science. They were more interested in competition and getting ahead and mastering techniques and so on.”

Bohm joined the Berkeley campus chapter of the Communist Party in November 1942. But he found the reality of the party less appealing than the idea. “I began to feel that they did nothing but talk about things of no significance, about trying to organize protests of affairs on the campus, and so on. … The meetings were interminable.” Bohm left the party after several months, but he remained a Marxist in his political convictions for many years afterward.

“We discussed it and he felt that one needs a theory in which one could discuss some reality which was existing and would stand by itself and did not always have to be referred to an observer,” Bohm recalled. “He really felt quite definite that the quantum theory was not doing this. Therefore, though he accepted that it was giving the right results … he felt that it was incomplete.”

Einstein's take

On May 31, he appeared in federal district court in Washington, DC, where he was cleared of all charges. But the next month, under tremendous pressure from President Dodds, the Princeton physics department announced that they would not be renewing Bohm’s contract, leaving him out of a job. Einstein wrote several letters of recommendation for Bohm but to no avail. Despite his legal innocence, Bohm remained on the blacklist.

politics

In Bohm’s interpretation of quantum physics, much of the mystery of the quantum world simply falls away. Objects have definite positions at all times, whether or not anyone is looking at them. Particles have a wave nature, but there’s nothing “complementary” about it—particles are just particles, and their motions are guided by pilot waves. Particles surf along these waves, guided by the waves’ motion (hence the name). Heisenberg’s uncertainty principle still holds—the more we know about a particle’s position, the less we know about its momentum, and vice versa—but according to Bohm, this is simply a limitation on the information that the quantum world is willing to yield to us. We may not know where a given electron is, but in Bohm’s universe, it’s always somewhere.

main

The Copenhagen interpretation doesn’t let you ask what’s happening to Schrödinger’s cat before you look in the box, insisting only that it’s meaningless to talk about the unobservable. But, in Bohm’s pilot-wave interpretation, not only can you ask but there’s an answer: before you look in the box, the cat is either dead or alive, and opening the box merely reveals which is true. The act of observation has nothing to do with the condition of the cat.

main. Bohm.

Waves in the double-slit experiment interfere with each other.

interesting experiment

There’s nothing particularly quantum about this—waves create interference patterns all the time, whether they’re overlapping waves from two stones thrown in a pond or sound waves coming from two stereo speakers. Wave interference isn’t mysterious: in spots where the peaks of one wave line up with the valleys of another, they cancel out and the waves vanish; when the peaks of both waves line up with each other, they’re amplified. This creates the patterns of dark and light bands in Figure 5.2.

just wow

But photons aren’t really tennis balls of light, and they do something extraordinary instead: though each one hits the plate in a single location, their impacts collectively form an interference pattern on the plate (Figure 5.3b). Even though each photon went through the double slit individually, they still somehow “knew” where to arrive on the photographic plate in order to form an interference pattern. Something was interfering with each photon as it went through,

wow

despite the fact that particles don’t interfere with each other, and there was only one particle in the double slit at a time anyhow.

Figure 5.3. (a) We wouldn’t expect individual photons passing one at a time through the double slit to produce an interference pattern. (b) Somehow, individual photons passing through the double slit do manage to interfere with themselves.

just wow!!

The idea of particles, Copenhagen claims, is complementary to the idea of waves. The ideas are contradictory—photons cannot be both particles and waves—but both are necessary, in alternation, for describing this experiment. 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.

Copenhagen interpretation

putting photon detectors on each slit causes the photon to behave as a particle as it passes through the double slit: the detectors force each photon to pass through only one slit, and thus not interfere with themselves, when before they were free to behave as waves and pass through both slits. But asking where the photon was before the measurement is meaningless: waves have no singular location. The property measured was created by the measurement itself, and to ask about its location beforehand is mere sophistry.

main

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).

Photon

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.

Photon

This was a truly radical idea: taking quantum physics seriously as a way of accounting for the entire world. In Bohm’s pilot-wave interpretation, strange quantum behaviors are minimized for larger objects, which is why we don’t see them in the everyday world. But every object, big and small, is ultimately governed by the same set of quantum equations.

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.”

True

A single particle, wandering the universe on its own without bumping into anything, is guided in its path by its own pilot wave and is perfectly local. But introduce a second particle that interacts in any way with the first, and suddenly they are linked—entangled—and the pilot wave of one particle will change depending on the precise location of the other particle, no matter how distant it may be. This kind of “spooky action at a distance” also appeared in the Copenhagen interpretation—it was exactly what Einstein had argued against in the EPR paper. But many physicists were still unaware of the EPR argument, and most that were aware of it profoundly misunderstood it.

Bohm

Everett’s Princeton friends were also impressed with his brilliance. “It surprised me after I got to know him that he was as brilliant as he was,” recalled Arnold. “It didn’t come across until you got close to him. And then you would recognize that this guy would be on top of the world. He was smart in a very broad way. I mean, to go from chemical engineering to mathematics to physics and spending most of the time buried in a science fiction book. I mean, this is talent.”

Be really good at something.

And even worse, according to Everett, von Neumann’s approach doesn’t even tell you what measurements are. If a measurement only happens when someone looks at a system, who, in particular, has to look? Everett argued that this line of reasoning leads inevitably to solipsism—the idea that you are the only being in the universe, and everyone else is somehow illusory or secondary, existing in states of indeterminate reality until you, the High Arbiter of Wave Function Collapse, deign to observe them.

interesting

Rejecting both von Neumann and Bohr, Everett came up with his own solution to the measurement problem. Rather than explaining wave function collapse, Everett stated that wave functions never collapse at all. This in itself was not new; Bohm said the same thing. But Bohm had also added particles with definite positions into the theory, which accounted for the outcomes of measurements. 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.

Everett. The many worlds interpretation of quantum physics.

But what does the mathematics say? What does Schrödinger’s equation say about Schrödinger’s cat? Well, the wave function of the lump of metal is half “radiation was emitted” and half “no radiation was emitted.” That interacts with the wave function of the detector, which means they get entangled. So now instead of two wave functions, one for the lump of metal and one for the detector, you have one for the both of them, and now that’s in a weird state: half “radiation was emitted and the detector detected it” and half “no radiation was emitted and the detector didn’t see anything.”

If I ask the “you” that sees the living cat how many cats you see, you’ll answer “just one.” And if I ask the same question of the “you” in the other branch of the wave function, the one with the dead cat, the answer will be the same (though your tone of voice will probably be quite different). The same thing happens, Everett pointed out, if I ask each copy of you how many selves you see. There is only one copy of you in each branch of the wave function, and, even if you repeat the experiment, this will still be true—there will be more branches, but each branch will still only have one copy of you. And the Schrödinger equation dictates that each branch will carry on independently of the others, with hardly any interaction between branches.

As you interact with the things in your environment, they get entangled with you, and then other things get entangled with them, and so on. Eventually, we have a single complicated and messy wave function for the entire universe—the universal wave function. And as more events happen, that universal wave function splits into more and more noninteracting parts, each merrily marching along to the deterministic beat of the Schrödinger equation. These are the many worlds of Everett’s interpretation.

Many world

it won’t do much good unless you go and fight with the greatest fighter.

Hmm

A hidden-variables interpretation assigns definite locations or other properties to quantum objects before they are observed, even if those properties can’t be calculated from the theory itself. These properties go unseen in the mathematics of quantum physics, hence “hidden” variables. Bohm’s pilot-wave interpretation is a prime example of such a theory: in Bohm’s world, particles always have positions, even though those positions are largely hidden from view and can’t be calculated from Schrödinger’s equation. The proofs of von Neumann, Jauch, and Gleason all suggested that this kind of scheme must be impossible—yet Bohm’s pilot-wave interpretation clearly worked, as Bell knew quite well. Something must be wrong, and Bell thought he knew what it was.

Bell showed that the purported “no-hidden-variables” proofs collectively suggested something else entirely, something the original creators of these proofs did not intend or fully understand. Specifically, Bell found that a hidden-variables theory could avoid the traps laid by these proofs if it had a rather peculiar property, later dubbed contextuality.

contextuality

Contextuality means that the outcome of a measurement on a quantum system depends on the other things you measure about that system at the same time. In other words, if you measure a property of a thing, the outcome of your measurement can depend on what other stuff you measure about that thing at the same time. In a contextual world, if you measure the energy of a neutron along with its momentum, you’ll get an answer about the neutron’s energy—but if you had measured the energy along with the location, you could have gotten a completely different answer about the neutron’s energy, simply by virtue of the context in which you made the energy measurement.

contextuality

The hidden variables in Bohm’s pilot-wave interpretation behave in exactly this way. Particles, according to Bohm, always have positions—but those positions can be dramatically altered by small disturbances and changes in experimental setups. Ask a slightly different set of questions to an electron, in Bohm’s world, and you can get an enormously different set of answers—but the electron has a definite position all the while. 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.”

interesting

Despite his definitive demonstration that Bohm’s theory was not impossible, Bell was still concerned about the strangest feature of pilot-wave theory: it was “hideously nonlocal.” “Terrible things happened in the Bohm theory,” said Bell. “For example, the [paths of] particles were instantaneously changed when anyone moved a magnet anywhere in the universe.”

hmm

Bohm’s version of the EPR experiment made the whole thing easier for Bell to play around with in his head. Instead of two particles colliding and flying away from each other with entangled momentum, Bohm’s version of EPR involved photons with entangled polarization. Polarization is a property of light—light is an electromagnetic wave, and the polarization is the direction that the wave is doing its waving in.

polarization is sort of like a little arrow that each photon carries with it that can point in different directions. But it’s not quite that simple. For one thing, we can’t actually tell what direction a photon’s polarization arrow is pointing. All we can do is measure a photon’s polarization along one particular axis at a time in a somewhat indirect manner, by shooting it at a polarizer (like a lens in a pair of polarized sunglasses). When a photon hits a polarizer, it either passes through or gets blocked; the closer the photon’s polarization is to the polarizer’s axis, the more likely it is to pass through.

polarization

Bell’s stroke of brilliance was to consider imperfection, rather than perfection. After all, the perfect correlations in the EPR-Bohm setup are easily compatible with locality—the photons could be sharing hidden instruction sets at their common origin. But if you rotate the axis of one of the polarizers, quantum physics predicts that pairs of entangled photons arriving at the polarizers will no longer behave in exactly the same way every time. And Bell showed that the imperfect correlations predicted by quantum physics were too strong for any local theory of nature to be able to account for them. So either the predictions of quantum physics are wrong and nature can be local, or quantum physics is right and “spooky action at a distance” is real.

Interesting

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.

Hmm

Bohm’s pilot-wave interpretation of quantum physics has no trouble at all with Bell’s theorem, because Bohm’s theory is explicitly nonlocal.

Bell’s theorem strongly suggests that quantum physics must be nonlocal; pilot-wave theory merely makes this strange quantum behavior so obvious that we can’t ignore it.

Now we have two assumptions: locality and living in a single universe. One of them must be wrong, since Bell’s inequality is violated in real experiments.

Bell’s theorem assumes the idea that quantum objects have well-defined properties before they’re measured, and that this is what is meant by “realism.” But this is simply not true, as previously stated—there’s no assumption of preexisting properties (i.e., hidden variables) in Bell’s theorem at all.

Others claim that the form of realism assumed by Bell’s theorem is the very idea that anything at all exists independently of observation. Denying this, they claim, is the true insight of the Copenhagen interpretation, and this is what allows the Copenhagen interpretation to remain local despite Bell’s ingenious proof. Ignoring the problem of solipsism that this introduces into physics—whose observations make things real?—another problem arises. Without the assumption that reality exists independently of observation in some form, the idea of locality itself is meaningless. How can it mean anything to talk about effects moving faster than light from one place to another when neither the objects nor their locations exist at all? Denying realism to break Bell’s proof invariably breaks the concept of locality as well—a Pyrrhic victory for the antirealists determined to keep physics local at all costs. As Bell himself said, “I don’t know any conception of locality which works with quantum mechanics. So I think we’re stuck with nonlocality.”

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. So abstruse statements like Hegel’s pronouncements about substance and form, and other metaphysical claims like “there is a God,” are meaningless, since they make no contact with the observable world.