The Mind and the Brain (34 page)

Yet gifts from gods, no less than gifts from crafty Greeks, can conceal unwelcome surprises. For quantum physics, in addition to predicting and explaining phenomena that range over fifteen orders of magnitude in energy, has done something else: it has triggered a radical upheaval in our understanding of the world. In place of the tidy cause-and-effect universe of classical physics, quantum physics describes a world of uncertainties, or indetermin
ism: of limits to our knowledge. It describes a world that often seems to have parted company with common sense, a world at odds with some of our strongest intuitive notions about how things work. In the quantum world, subatomic particles have no definite position until they are measured: the electron orbiting the nucleus of an atom is not the pointlike particle we usually imagine but instead a cloud swathing the nucleus. In the quantum world, a beam of light can behave as a wave or a barrage of particles, depending on how you observe it. Quantities such as the location, momentum, and other characteristics of particles can be described only by probabilities; nothing is certain. “It is often stated that of all the theories proposed in this century, the silliest is quantum theory,” the physicist Michio Kaku wrote in his 1995 book
Hyperspace
. “In fact, some say that the only thing that quantum theory has going for it is that it is unquestionably correct.”

Correct it may be, but at its core quantum physics departs from classical physics in a very discomfiting way. Integral to quantum physics is the fundamental role played by the observer in choosing which of a plenitude of possible realities will leave the realm of the possible and become actual. For at its core, quantum physics challenges the ontology that permeated the scientific enterprise for centuries, the premise that a real world—independent of human choice and interference—is out there, uninfluenced by our observation of it. Quantum physics makes the seemingly preposterous claim (actually, more than claim, since it has been upheld in countless experiments) that there is no “is” until an observer makes an observation. Quantum phenomena seem to be called into existence by the very questions we ask nature, existing until then in an undefined fuzzy state. This feature of the quantum world led the American physicist John Archibald Wheeler to say that the world comes into being through our knowledge of it—or, as Wheeler put it, we get “its from bits” (bits of knowledge). The Danish physicist Niels Bohr captured the counterintuitive world that physicists were now playing in when he told a colleague, “We all agreed that your theory is crazy.
The question which divides us is whether it is crazy enough to have a chance of being correct.”

The role of observation in quantum physics cannot be emphasized too strongly. In classical physics, observed systems have an existence independent of the mind that observes and probes them. In quantum physics, however, only through an act of observation does a physical quantity come to have an actual value. Many are the experiments that physicists have dreamed up to reveal the depths of quantum weirdness. I’ll follow the lead of the late physicist Richard Feynman, who contributed as much as any scientist after the founders to the development of quantum mechanics. Feynman thought that “the only mystery” of quantum physics resides in a contemporary version of an experiment first performed in 1801. It is called the
double-slit experiment
, and Feynman found it so revealing of the quantum enigmas that he described it as having in it “the heart of quantum mechanics.” (I am using the terms
quantum physics, quantum mechanics
, and
quantum theory
interchangeably.) In his book
The Character of Physical Law
, Feynman declared, “Any other situation in quantum mechanics, it turns out, can always be explained by saying, ‘You remember the case of the experiment with the two holes? It’s the same thing.’”

In 1801, the English polymath Thomas Young rigged up the test that has been known forever after as the two-slit experiment. At the time, physicists were locked in debate over whether light consisted of particles (minuscule corpuscles of energy) or waves (regular undulations of a medium, like water waves in a pond). In an attempt to settle the question, Young made two closely spaced vertical slits in a black curtain. He allowed monochromatic light to strike the curtain, passing through the slits and hitting a screen on the opposite wall. Now if we were to do a comparable experiment with something we know to be corpuscular rather than wavelike—marbles, say—there is no doubt about the outcome. Most marbles we fired at, for instance, a fence missing two slats would hit the fence and drop on this side. But a few marbles would pass through
the gaps and, if we had coated the marbles with fresh white paint, leave two bright blotches on the wall beyond, corresponding to the positions of the two openings.

Figure 7
: The Double-Slit Experiment. Monochromatic light passes through a two-slit grating. In Experiment A, only one narrow slit is open.The narrowness of the slit, coupled with the quantum Uncertainty Principle, causes the beam that passes through the slit to spread out and cover a wide area of the photographic plate. But each photon is observed to land in a tiny spot. The bell curve shows the distribution of spots, or photons. In B, only the right slit is open, and again the beam is spread out over a wide area. In C, both slits are open, but the result is not the sum of the single-slit results (dotted curve). Instead, the photons are observed in narrow bands that resemble the interference pattern formed when water waves pass through two openings in a sea wall: as semicircular waves from the two openings ripple outward, they combine where crest meets crest (in the photon experiment, the bands where many photons are found) and cancel when crest meets trough (where photons are scarce). Opening the second slit makes it clear that light behaves like a wave, since interference is a wave phenomenon.Yet each photon is found to land, whole and undivided, in a tiny region, like a particle would. Even when photons are emitted one at a time, they still form the double-slit interference pattern. Does a single photon interfere with itself?

This is not what Young observed.

Instead, the light created, on a screen beyond the slitted curtain, a pattern of zebra stripes, alternating dark and light vertical bands. It’s called an
interference pattern
. Its genesis was clear: where crests of light waves from one slit met crests of waves from the other, the waves reinforced each other, producing the bright bands.
Where the crest of a wave from one slit met the trough of a wave from the other, they canceled each other, producing the dark bands. Since the crests and troughs of a light wave are not visible to the naked eye, this is easier to visualize with water waves. Place a barrier with two openings in a pool of water. Drop a heavy object into the pool—watermelons work—and observe the waves on the other side of the barrier. As they radiate out from the watermelon splash, the ripples form nice concentric circles. When any ripple reaches the barrier, it passes through both openings and, on the other side, resumes radiating, now as concentric half-circles. Where a crest of ripples from the left opening meets a crest of ripples from the right, you get a double-height wave. But where crest meet trough, you get a zone of calm. Hence Young’s interpretation of his double-slit experiment: if light produces the same interference patterns as water waves, which we know to be waves, then light must be a wave, too. For if light were particulate, it would produce not the zebra stripes he saw but, rather, the sum of the patterns emerging from the two slits when they are opened separately—two splotches of light, perhaps, like the marbles thrown at our broken fence.

So far, so understandable. But, for the next trick, turn the light source way, way down so that it emits but a single photon, or light particle, at a time. (Today’s photodetectors can literally count photons.) Put a photographic plate on the other side, beyond the slits. Now we have a situation more analogous, it would seem, to the marbles going through the fence: zip goes one photon, perhaps making it through a slit. Zip goes the next, doing the same. Surely the pattern produced would be the sum of the patterns produced by opening each slit separately—again, perhaps two intermingled splotches of light, one centered behind the left slit and the other centered behind the right.

But no.

As hundreds and then thousands of photons make the journey (this experiment was conducted by physicists in Paris in the mid-
1980s), the pattern they create is a wonder to behold. Instead of the two broad patches of light, after enough photons have made the trip you see the zebra stripes. The interference pattern has struck again. But what interfered with what? This time the photons were clearly particles—the scientists counted each one as it left the gate—and our apparatus allowed only a single photon to make the journey at a time. Even if you run out for coffee between photons, the result is eventually the same interference pattern. Is it possible that the photon departed the light source as a particle and arrived on the photographic plate as a particle (for we can see each arrive, making a white dot on the plate as it lands)—but in between became a wave, able to go through both slits at once and interfere with itself just as a water wave from our watermelon goes through the two openings in the barrier? Even weirder, each photon—and remember, we can release them at any interval—manages to land at precisely the right spot on the plate to contribute its part to the interference pattern.

So, to recap, we seem to have light vacillating between a parti-clelike existence and a wavelike one. As a particle, the light is emitted and detected. As a wave, it goes through both slits at once. Lest you discount this as just some weird property of light and not of matter, consider this: the identical experiment can be done with electrons. They, too, depart the source (an electron microscope, in work by a team at Hitachi research labs and Gakushuin University in Tokyo) as particles. They land on the detector—a scintillation plate, like the front of a television screen, which records each electron arrival as a minuscule dot—as particles. But in between they act as waves, producing an interference pattern almost identical to that drawn by the photons. Dark stripes alternate with bright ones. Again, the only way single electrons can produce an interference pattern is by acting as waves, passing through both slits at once just as the photons apparently did. Electrons—a form of matter—can behave as waves. A single electron can take two different paths from source to detector and interfere with itself: during its travels it
can be in two places at once. The same experiments have been performed with larger particles, such as ions, with the identical results. And ions, as we saw back in Chapter 3, are the currency of the brain, the particles whose movements are the basis for the action potential by which neurons communicate. They are also, in the case of calcium ions, the key to triggering neurotransmitter release. This is a crucial point: ions are subject to all of the counterintuitive rules of quantum physics.

The behavior of material particles means that these particles have associated waves. Like a wave in water, the electron wave goes through both slits and interferes with itself on the other side. The interference pattern at the detector defines the zebra pattern. Particles travel through the hole as waves but arrive as particles. How can we reconcile these disparate properties of such bits of matter as electrons? A key to understanding the whole bizarre situation is that we actually measure the photon or electron at only two points in the experiment: when we release it (in which case a photodetec-tor counts it) and when we note its arrival at the end. The conventional explanation is that the act of measurement makes a spread-out, fuzzy wave (at the slits) collapse into a discrete, definite particle (on the scintillation plate or other detector). According to quantum theory, what in fact passes through the slits is a wave of probability. In fact, quantum physics describes the behavior of a particle by something called the Schrödinger wave equation (after Erwin Schrödinger, who conceived it in 1926). Just as Newton’s second law describes the behavior of particles, so
Schrödinger’s wave equation
specifies the continuous and smooth evolution of the wave function at all times when it is not being observed. The wave function encodes the entire range of possibilities for that particle’s behavior—where the particle is, when. It contains all the information needed to compute the probabilities of finding the particle in any particular place, any time. These many possibilities are called superpositions. The element of chance is key, for rather than speci
fying the location, or the energy, or any other trait of a particle, the equation modestly settles for describing the probability that those traits will have particular values. (In precise terms, the square of the amplitude of the wave function at any given position gives the probability that the particle will be found in some region near that position.) In this sense the Schrödinger wave can be considered a probability wave.

When a quantum particle or collection of particles is left alone to go its merry way unobserved, its properties evolve in time and space according to the deterministic wave equation. At this point (that is, before the electron or photon is observed), the quantum particle has no definite location. It exists instead as a fog of probabilities: there are certain odds (pretty good ones) that, in the appropriate experiment, it will be in the bright bands on the plate, other odds (lower) that it will land in the dark bands, and other odds (lower still, but nonzero) that it will be in the Starbucks across the street. But as soon as an observer performs a measurement—detecting an electron landing on a plate, say—the wave function seems to undergo an abrupt change: the location of the particle it describes is now almost definite. The particle is no longer the old amalgam of probabilities spread over a large region. Instead, if the observer sees the electron in
this
tiny region, then only that part of the wave function representing the small region where observation has found it survives. Every other probability for the electron’s position has vanished. Before the observation, the system had a range of possibilities; afterward, it has a single actuality. This is the infamous
collapse of the wave function
.