The Mind and the Brain (35 page)

There is, to put it mildly, something deeply puzzling about the collapse of the wave function. The Schrödinger equation itself contains no explanation of how observation causes it; as far as that equation is concerned, the wave function goes on evolving forever with no collapse at all. And yet that does not seem to be what happens. All that we know from experiment and hard-nosed mathe
matical calculations is that the Schrödinger wave equation, describing a microworld of superposed wave functions, somehow becomes a macroworld of definite states. In sharp contrast to the unqualified success of quantum physics in predicting the outcome of experiments stands the mess of diverse opinion that lies under the umbrella “interpretations of quantum mechanics”: what happens to turn the Schrödinger wave equation into a single observed state, and what does that process tell us about the nature of reality?

There are at least three ways to account for the shift from a microworld of probabilities defined by the Schrödinger wave equations to a macroworld of definite states that we measure. Each interpretation implies a different view of the essential nature of the world. One view, preferred by Einstein, holds that the world is governed by what are called
hidden variables
. Although so-far undiscovered and perhaps even undiscoverable (hence the
hidden
part), they are supposed to be the certainties of which the wave function of quantum physics describes the probabilities. This view can be thought of as the way a tank of goldfish might think about the arrival every day of food flakes drifting through their water. It seems to be random, and without cause. There is (say) a fifty-fifty chance that food will arrive before the shadow of a little plastic skin diver reaches the little plastic mermaid, and a fifty-fifty chance that the food will land later. If only our little finned friends knew more about the world, they would understand that the arrival of the food is completely causal (the human walks over and sprinkles flakes on the water’s surface). The hidden variables view, in other words, says that things look probabilistic only because we are too stupid to identify the forces that produce determinism. If we were more clever, we would see that determinism rules. Einstein’s beliefs tended in this direction, leading him to his famous pronouncement “God does not play dice” with the universe. Einstein notwithstanding, however, hidden variables have been out of favor since the 1960s, for reasons too technical to get into. Suffice it to say that the physicist John Bell showed that hidden variables require instanta
neous action at a distance—that is, causal influences that break the Einsteinian speed limit and travel faster than the speed of light.

A second interpretation of quantum physics holds that superposed waves exist for quantum phenomena, all right, but never really collapse. This
many-worlds view
is the brainchild of the late physicist Hugh Everett III, a student of John Wheeler, who suggested it at Wheeler’s urging in a 1957 paper. Instead of attempting to answer how the act of observation induces the wave function to collapse into a single possibility, the many-worlds view holds that no single possibility is ever selected. Rather, the wave function continues evolving, never collapsing at all. How, then, do we manage to see not superpositions—electrons that are a little bit here and a little bit there—but discrete states? Every one of the experiential possibilities inherent in the wave function is realized in some superrealm, Everett proposed. If the wave function gives a fifty-fifty probability that a radioactive atom will decay after thirty minutes, then in one world the atom has decayed and in another it has not. Correspondingly, the mind of the observer has two different branches, or states: one perceiving an intact atom and the other perceiving a decayed one. The result is two coexisting parallel mental realities, the
many-minds view
. Every time you make an observation or a choice your conscious mind splits so that, over time, countless different copies of your mind are created. This, needless to say, is the ultimate in having your cake and eating it, too: sure, you may have uttered that career-ending epithet in this branch of reality, but in some other branch you kept your mouth shut.

From its inception, this theory caused discomfort. “For at least thirteen years after Everett’s paper was published, there was a deafening silence from the physics community,” recalls Bryce DeWitt, a physicist who championed the many-worlds view even into his eighties. “Only John Wheeler, Everett, and I supported it. I thought Everett was getting a raw deal.”

As a psychiatrist I was certainly familiar with the idea of a split personality, but here was an interpretation of quantum mechanics
that took the concept absolutely literally. Many scholars investigating the ontological implications of quantum mechanics squirm at the notion that all possible experiences occur, and that a new world and a new version of each observer’s mind are born every time a quantum brain reaches a choice point. But many others prefer this bizarre scenario to the idea of a sudden collapse of the wave function. They like, too, that many-worlds allows quantum mechanics to operate without requiring an observer to put questions to nature—that is, without human consciousness and free choice rearing their unwelcome heads, and without the possibility that the human mind can affect the physical world. In fact, at a 1999 quantum conference in England, of ninety physicists polled about which interpretation of quantum mechanics they leaned toward, thirty chose many-worlds or another interpretation that includes no collapse. Only eight said they believed that the wave function collapses. But another fifty chose none of the above. (And of course, if they were honest, the ones who chose many-worlds would have to believe they had simultaneously made a near-infinity of different choices on their other branches.)

A third view of the change from superpositions to a single definite state is the one advanced by Niels Bohr. In this case, the abrupt change from superpositions to single state arises from the act of observation. This is the interpretation that emerged in the field’s earliest days. During a period of feverishly intense creativity in the 1920s, the greatest minds in physics, from Paul Dirac and Niels Bohr to Albert Einstein and Werner Heisenberg, struggled to explain the results of quantum experiments. Finally, at the fifth Solvay Congress of physics in Brussels 1927, one group—Bohr, Max Born, Paul Dirac, Werner Heisenberg, and Wolfgang Pauli—described an accord that would become known as the Copenhagen Interpretation of quantum mechanics, after the city where Bohr, its chief exponent, worked. Bohr insisted that quantum theory is about our knowledge of a system and about predictions based on that knowledge; it is not about reality “out there.” That is, it does
not address what had, since before Aristotle, been the primary subject of physicists’ curiosity—namely, the “real” world. The physicists threw in their lot with this view, agreeing that the quantum state represents our knowledge of a physical system.

Before the act of observation, it is impossible to know which of the many probabilities inherent in the Schrödinger wave function will become actualized. Who, or what, chooses which of the probabilities to make real? Who, or what, chooses how the wave function “collapses”? Is the choice made by nature, or by the observer? According to the Copenhagen Interpretation, it is the observer who both decides which aspect of nature is to be probed and reads the answer nature gives. The mind of the observer helps choose which of an uncountable number of possible realities comes into being in the form of observations. A specific question (Is the electron here or there?) has been asked, and an observation has been performed (Aha! the electron is there!), corralling an unruly wave of probability into a well-behaved quantum of certainty. Bohr was silent on how observation performs this magic. It seems, though, as if registering the observation in the mind of the observer somehow turns the trick: the mental event collapses the wave function. Bohr, squirming under the implications of his own work, resisted the idea that an observer, through observation, is actually influencing the course of physical events outside his body. Others had no such qualms. As the late physicist Heinz Pagels wrote in his wonderful 1982 book
The Cosmic Code
, “There is no meaning to the objective existence of an electron at some point in space…independent of any actual observation. The electron seems to spring into existence as a real object only when we observe it!”

Physical theory thus underwent a tectonic shift, from a theory about physical reality to a theory about our knowledge. Science is what we know, and what we know is only what our observations tell us. It is unscientific to ask what is “really” out there, what lies behind the observations. Physical laws as embodied in the equations of quantum physics, then, ceased describing the physical
world itself. They described, instead, our knowledge of that world. Physics shifted from an ontological goal—learning what is—to an epistemological one: determining what is known, or knowable. As John Archibald Wheeler cracked, “No phenomenon is a phenomenon until it is an observed phenomenon.” The notion that the wave function collapses when the mind of an observer registers a new bit of knowledge was developed by the physicist Eugene Wigner, who proposed a model of how consciousness might collapse the wave function—something we will return to. But why human consciousness should be thus privileged has remained an enigma and a source of deep division in physics right down to today.

It is impossible to exaggerate what a violent break this represented. Quantum physics had abandoned the millennia-old quest to understand what exists in favor of our knowledge of what exists. As Jacob Bronowski wrote in
The Ascent of Man
, “One aim of the physical sciences has been to give an exact picture of the material world. One achievement of physics in the twentieth century has been to prove that that aim is unattainable.” The Copenhagen Interpretation drew the experiences of human observers into the basic theory of the physical world—and, even more, made
them
the basic realities. As Bohr explained, “In our description of nature the purpose is not to disclose the real essence of phenomena but only to track down as far as possible relations between the multifold aspects of our experience.” With this shift, Heisenberg said, the concept of objective reality “has thus evaporated.” Writing in 1958, he admitted that “the laws of nature which we formulate mathematically in quantum theory deal no longer with the particles themselves but with our knowledge of the elementary particles.” “It is wrong,” Bohr once said, “to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature.”

To many, this surrender was nothing short of heresy. Einstein, perhaps the most passionate opponent of the abandonment of efforts to understand nature, objected that this shift from what exists to what we know about what exists violated what he considered to
be “the programmatic aim of all physics,” namely, a complete description of a situation independent of any act of observation. But Einstein lost. With the triumph of quantum, physics stopped being about nature itself and instead became about our knowledge of nature.

Right about here every discussion of quantum epistemology invokes Schrödinger’s cat, a thought experiment that Schrödinger proposed in 1935 to illustrate the bewilderments of quantum superpositions. Put a pellet inside a box, he said, along with a radioactive atom. Arrange things so that the pellet releases poison gas if and only if the atom decays. Radioactive decay is a quantum phenomenon, and hence probabilistic: a radioactive atom has a finite probability of decaying in a certain window of time. In thirty minutes, an atom may have a 50 percent chance of decaying—not 70 percent, not 20 percent, but precisely 50 percent. Now put a cat in the box, and seal it in what Schrödinger called a “diabolical device.” Wait a while. Wait, in fact, a length of time equal to when the atom has a fifty-fifty chance of decaying. Is the cat alive or dead?

Quantum mechanics says that the creature is both alive and dead, since the probability of radioactive decay and hence release of poison gas is 50 percent, and the possibility of no decay and a safe atmosphere is also 50 percent. Yet it seems absurd to say that the cat is part alive and part dead. Surely a physical entity must have a real physical property (such as life or death) ? If we peek inside the box, we find that the cat is alive or dead, not some crazy superposition of the two states. Yet surely the act of peeking should not be enough to turn probability into actuality? According to Bohr’s Copenhagen Interpretation, however, this is precisely the case. The wave function of the whole system, consisting of kitty and all the rest, collapses when an observer looks inside. Until then, we have a superposition of states, a mixture of atomic decay and atomic intactness, death and life.

Observations, to put it mildly, seem to have a special status in quantum physics. So long as the cat remains unobserved, its wave
function encodes equal probabilities of life and death. But then an observation comes along, and
bam
—the cat’s wave function jumps from a superposition of states to a single observed state. Observation lops off part of the wave function. The part corresponding to living or deceased, but not the other, survives.

If the power of the observer to coax a certain value out of a probability wave sounds like the wrong answer to the question of whether a tree falls in an empty wood even if no one hears it, take heart: physicists are just as puzzled about how the world can work in such a bizarre way. As Eugene Wigner stated in 1964, “This last step [the entering of an observation into consciousness] is…shrouded in mystery and no explanation has been given for it so far.” The collapse of the wave function, which gives rise to the centrality of consciousness in physical theory, “enters quantum mechanical theory as a deus ex machina, without any relation to the other laws of this theory.”

One of the things I admired about Stapp’s
Mind, Matter and Quantum Mechanics
was his willingness to address the ethical implications of the change from Newtonian physics to quantum physics. In particular, Stapp made the point that there is no stronger influence on human values than man’s belief about his relationship to the power that shapes the universe. When medieval science connected man directly to his Creator, man saw himself as a child of the divine imbued with a will free to choose between good and evil. When the scientific revolution converted human beings from the sparks of divine creation into not particularly special cogs in a giant impersonal machine, it eroded any rational basis for the notion of responsibility for one’s actions. We became a mechanical extension of what preceded us, over which we have no control; if everything we do emerges preordained by the conditions that prevail, then we can have no responsibility for our own actions. “Given this conception of man,” Stapp argued, “the collapse of moral philosophy is inevitable.” But just as Newtonian physics undermines moral philosophy, Stapp thought, so quantum physics might rescue
it. For quantum physics describes a world in which human consciousness is intimately tied into the causal structure of nature, a world purged of determinism.