This book is about the controversies involved in the foundational knowledge of quantum physics. Quantum physics has been integral in developing such practical science as nuclear warheads and silicon transistors. Despite this, the very basic foundations of how and why quantum calculations actually work are still in dispute. In explaining the different theories still posited, Becker explains some of the basic science, the history behind various theories, the methods of various experimental setups, and why a clear conclusion has yet to emerge.
Most physicists subscribe to the Copenhagen Theory. “The Copenhagen interpretation states that quantum physics is merely a tool for calculating the probabilities of various outcomes of experiments…. Quantum physics proves that small objects simply do not exist in the same objectively real way as the objects in our everyday lives do.” It does not exist in the real world. Niels Bohr stated, “there is no quantum world. There is only an abstract quantum physical description.” Pascal Jordan continued, “the electron is forced to a decision. We compel it to assume a definite position; previously, it was, in general, neither here nor there…. We ourselves produce the results of measurement.” The measurement itself affects the outcome. The electron before was in a superposition, but the measurement interacted with it, it moved to a definite position. “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…. The Schrodinger 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…. The wave function doesn’t tell you how much of the electron is in one place—it tells you the probability that the electron in in that place…. Once you find that electron, a funny thing happens to its wave function. Rather than following the Schrodinger equation like a good wave function, it collapses—it instantly becomes zero everywhere except in the place where you found the electron…. The Schrodinger equation holds all the time, except when you make a measurement.”
Heisenberg’s uncertainty principle stated that either you could know the location of an electron or how fast and in what direction it was going, but not both at once. “Heisenberg found a precise formulation of how much information you have to give up about an object’s momentum in order to learn more about its position, and vice versa. You could know a lot about where an object was or a lot about how it was moving—but you couldn’t know both at the same time.” Bohr added to the uncertainty principle the issue of complementarity. He stated, “any observation of atomic phenomena will involve an interaction with the agency of observation not to be neglected…. an independent reality in the ordinary physical sense can neither be ascribed to the phenomena nor to the agencies of observation.” Becker explains, “the quantum world could only be considered real in conjunction with some kind of measurement apparatus to study that world. 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.” Bohr did not view this as a problem, however. He stated, “we are not dealing with contradictory but with complementary pictures of the phenomena.” According to Copenhagen, this wave/particle duality is inherent in all quantum phenomena.
Within Copenhagen there was not complete consensus, but all the physicists agreed that it was pointless to talk about what was really happening in the macro world. Quantum physics was not a theory of the world as it actually is. It is a tool for making predictions. “Making accurate predictions about the outcomes of measurements was, for them, enough.” Bohr stated, “there is no quantum world. Isolated material particles are abstractions, their properties on the quantum theory being definable and observable only through their interaction with other systems.” Heisenberg stated, “the atoms or the elementary particles are not as real [as phenomena in daily life]; they form a world of potentialities rather than one of things or facts…. 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…. The transition from the ‘possible’ to the ‘actual’ takes place during the act of observation…. The transition from the ‘possible’ to the ‘actual’ takes place as soon as the interaction of the object with the measuring device, and thereby with the rest of the world, has come into play; it is not connected with the act of registration of the result by the mind of the observer.” John von Neumann threw his considerable weight behind the Copenhagen interpretation with a proof, which stated that wave functions normally obey the Schrodinger equation, but collapse upon measurement. Von Neumann stated, “we therefore have two fundamentally different types of interventions which can occur in a system. [When an object remains undisturbed, the equation] describes how the system changes continuously and causally in the course of time. [But once measurement is made] the arbitrary changes by measurement [are] discontinuous, non-causal, and instantaneously acting…. We must always divide the world into two parts, the one being the observed system, the other the observer. Quantum mechanics describes the events which occur in the observed portion of the world, so long as they do not interact with the observing portion, with the aid of the [Schrodinger equation], but as soon as such interaction occurs, i.e. a measurement, it requires the [collapse of the wave function].”
Einstein disagreed with the Copenhagen consensus. He felt, “the programmatic aim of all physics [was] the complete description of any (individual) real situation (as it supposedly exists irrespective of any act of observation or substantiation.)” His greatest problem with the Copenhagen interpretation was the issue of locality: “the principle that something that happens in one location can’t instantly influence an event that happens somewhere else.” He viewed the instantaneous wave function collapse as violating locality. Schrodinger brought to light the issue of entanglement. “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…. For any entangled system, Einstein’s choice applied: either the system is nonlocal, or quantum physics can’t fully describe all the features of the system.”
David Bohm was the first man to come up with a counter interpretation that could explain the foundations of quantum physics. This was the Pilot Wave Theory. Becker relates, “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 us. We may not know where a given electron is, but in Bohm’s universe, it’s always somewhere…. Bohm’s theory is mathematically equivalent to the Schrodinger equation, the central equation of quantum physics, and therefore it must make the same predictions as any other interpretation.”
The famous double-slit experiment showed the differences between Copenhagen and Bohm. Richard Feynman quipped that the double-slit experiment “has in it the heart of quantum mechanics…. In reality, it contains the only mystery.” Becker states, “the idea of particles, Copenhagen claims, is complementary to the idea of waves. The ideas are contradictory—photons cannot be both particles and waves…. 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.” Becker contrasts that explanation with Bohm’s theory, “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…. Putting photon detectors on each slit affects each photon’s pilot wave—no matter how ingenious the design, any photon detector must alter a photon’s pilot wave, as ensured by Heisenberg’s uncertainty principle, which in Bohm’s interpretation places limits on how much measuring devices can avoid interfering with the things they attempt to measure…. 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…. 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.” Bohm’s interpretation did away with the superpositions of electrons and was able to incorporate measurement devices into the quantum descriptions. However, it did not solve Einstein’s problem of nonlocality, “allowing particles to influence each other instantaneously at long distances. 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…. Because Bohm’s theory involved faster-than-light connections between particles, it appeared difficult to extend Bohm’s ideas to incorporate special relativity.”
Hugh Everett III was the next physicist to find the Copenhagen interpretation lacking. He took the measurement problem, that wave functions collapsed upon measurement, seriously. He understood that it placed any observer in a solipsistic position. Instead, Everett proposed a single universal wave function, “a massive mathematical object describing the quantum states of all objects in the entire universe. This universal wave function, according to Everett, obeyed the Schrodinger 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…. 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 Schrodinger equation dictates that each branch will carry on independently of the others, with hardly any interaction between branches…. To each person in each branch on the universal wave function, their world appears to the the only world.” This was Everett’s Many-Worlds Theory.
John Bell was another Copenhagen heretic. He believed Von Neumann’s proof was incorrect. It purported to rule out any explanation of quantum physics that used hidden variables. “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.” Bohm’s pilot wave interpretation is one such theory. Bell thought Einstein’s EPR paper, which questioned the locality of quantum physics, was another way around hidden variables. Bell tried to resolve the differences between Copenhagen, Bohm, and EPR. “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…. Therefore, if nature is local, the wave function is not everything—there must be hidden variables. So either quantum physics is incomplete, or nature is nonlocal. We cannot have both locality and completeness in quantum physics…. 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.” Bell explained, “certain particular correlations, realizable according to quantum mechanics, are locally inexplicable, They cannot be explained, that is to say, without action at a distance.” This was Bell’s impossibility proof. “Bell’s theorem really leaves only three unequivocal possibilities: either nature is nonlocal in some way, or we live in branching multiple worlds despite appearances to the contrary, or quantum physics gives incorrect predictions about certain experimental setups.”
Dieter Zeh was the next quantum physicist to poke holes in the Copenhagen consensus. He independently came up with a Many-Worlds Theory, which in many ways resembled Everett’s. (Everett left academia for the Pentagon after barely receiving his PhD at Princeton after squabbles with Bohr, his thesis advisor, John Wheeler, and others at Bohr’s institute in Copenhagen and so his theory was promptly forgotten.) Zeh started by positing, “let’s assume that the universe is a closed system, like a nucleus.” Becker continues that Zeh’s “general idea, a system in a superposition, with its components strongly entangled—could explain how measurement works in quantum physics, without resorting to any of the tricks the Copenhagen interpretation used…. Quantum physics says that the measurement device will become strongly entangled with the thing it’s measuring…. The measuring device interacts with the experimenter, and everything else in the room, and eventually the entire universe—so when a small quantum system interacts strongly with a large object, ultimately, the entire universe ends up like Schrodinger’s cat, splitting into dead-cat and alive-cat “branches.” And the inhabitants of each branch of the universe only see one outcome…. The different branches of the universe are extraordinarily unlikely to interact.” Zeh explained, “the observer sees only one component [of the Schrodinger’s cat state] and not the superposition of all the others. So, that solves the measurement problem.” Bryce DeWitt added, “the universe is constantly splitting into a stupendous number of branches, all resulting from measurementlike interactions between its myriads of components. Moreover, every quantum transition taking place on every star, in every galaxy, in every remote corner of the universe is splitting our local world on earth into myriads of copies itself.” This interpretation, therefore, never requires any wave functions to collapse. It does require an almost infinite number of universes, however. We are actually living in a multiverse.
This Multi-Worlds interpretation was bolstered by the discoveries of String Theory and of the concept of inflation, which “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 more slowly…. 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 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. String theory, meanwhile, doesn’t describe a single universe but instead describes a “string landscape,” a phenomenally huge number of possible universes—10^500 or more.”
Another possible interpretation is Information Theory. “If the wave function is information of some sort, rather than being a physical object, then many of the puzzles at the heart of quantum physics seem to melt away. In particular, the measurement problem seems much easier to explain if the wave function is information—your information changes when you make a measurement, so it’s no surprise that wave functions change dramatically when measurements occur…. [However] information-based interpretations of quantum physics ran the risk of collapsing into solipsism as well. If the information that the wave function represented was your information, what makes you so special?”
A final quantum interpretation is Spontaneous-Collapse Theory, which “manages to leave most of the predictions of standard quantum physics intact, while altering them enough to solve the measurement problem…. In Spontaneous-Collapse Theory, the quantum wave function is real, but it doesn’t obey the Schrodinger 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…. Though a single-particle wave function might not collapse on average until a billion years have passed, the solid objects of our everyday lives…. 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, then, on average, at least one of them will hit the collapse jackpot every millionth of a second. But because the particles…. are all continually interacting with each other, they’re all entangled—which means they all share a single wave function…. As Bell put it, in Spontaneous-Collapse Theory, Schrodinger’s cat “is not both dead and alive for more than a split second.””
Quantum physics has enormous predictive power in the real world. The equations have yielded amazing advancements in science and technology. Therefore, it somewhat amazing that its basic foundations are still in dispute. Most physicists today believe in the Copenhagen interpretation. In graduate programs it is taught that Bohr was right and Einstein wrong. However, dissent has also grown. Today, the multi-worlds interpretation has garnered a few famous adherents, especially among those who still work specifically on the foundations of quantum theory. Perhaps one day a true consensus will truly develop.
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