The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos

by Brian Greene

  Let me set the stage.

  The upheaval in understanding that took place between roughly 1900 and 1930 resulted in a ferocious assault on intuition, common sense, and the well-accepted laws that the new vanguard soon began calling “classical physics”—a term that carries the weight and respect given to a picture of reality that is at once venerable, immediate, satisfying, and predictive. Tell me how things are now, and I’ll use the laws of classical physics to predict how things will be at any moment in the future, or how they were at any moment in the past. Subtleties such as chaos (in the technical sense: slight changes in how things are now can result in huge errors in the predictions) and the complexity of the equations challenge the practicality of this program in all but the simplest situations, but the laws themselves are unwavering in their viselike grip on a definitive past and future.

  The quantum revolution required that we give up the classical perspective because new results established that it was demonstrably wrong. For the motion of big objects like the earth and the moon, or of everyday objects like rocks and balls, the classical laws do a fine job of prediction and description. But pass into the microworld of molecules, atoms, and subatomic particles and the classical laws fail. In contradiction of the very heart of classical reasoning, if you run identical experiments on identical particles that have been set up identically, you will generally not get identical results.

  Imagine, for example, that you have 100 identical boxes, each containing one electron, set up according to an identical laboratory procedure. After exactly 10 minutes, you and 99 cohorts measure the positions of each of the 100 electrons. Despite what Newton, Maxwell, or even a young Einstein would have anticipated—would likely have been willing to bet their lives on—the 100 measurements won’t yield the same result. In fact, at first blush the results will look random, with some electrons found near their box’s front lower left corner, some near the back upper right, some around the middle, and so on.

  The regularities and patterns that make physics a rigorous and predictive discipline become apparent only if you run this same experiment, with 100 boxed electrons, over and over again. Were you to do so, here’s what you’d find. If your first batch of 100 measurements found 27 percent of the electrons near the lower left corner, 48 percent near the upper right corner, and 25 percent near the middle, then the second batch will yield a very similar distribution. So will the third batch, the fourth, and those that follow. The regularity, therefore, isn’t evident in any single measurement; you can’t predict where any given electron will be. Instead, the regularity is found in the statistical distribution of many measurements. The regularity, that is, speaks to the likelihood, or probability, of finding an electron at any particular location.

  The breathtaking achievement of quantum mechanics’ founders was to develop a mathematical formalism that dispensed with the absolute predictions intrinsic to classical physics and instead predicted such probabilities. Working from an equation Schrödinger published in 1926 (and an equivalent though somewhat more awkward equation Heisenberg wrote down in 1925), physicists can input the details of how things are now, and then calculate the probability that they will be one way, or another, or another still, at any moment in the future.

  But don’t be misled by the simplicity of my little electron example. Quantum mechanics applies not just to electrons but to all types of particles, and it tells us not only about their positions but about also their velocities, their angular momenta, their energies, and how they behave in a wide range of situations, from the barrage of neutrinos now wafting through your body, to the frenzied atomic fusions taking place in the cores of distant stars. Across such a broad sweep, the probabilistic predictions of quantum mechanics match experimental data. Always. In the more than eighty years since these ideas were developed, there has not been a single verifiable experiment or astrophysical observation whose results conflict with quantum mechanical predictions.

  For a generation of physicists to have confronted such a radical departure from the intuitions formed out of thousands of years of collective experience, and in response to have recast reality within a wholly new framework based on probabilities, is a virtually unmatched intellectual achievement. Yet one uncomfortable detail has been hovering over quantum mechanics since its inception—a detail that eventually opened a pathway to parallel universes. To understand it, we need to look a little more closely at the quantum formalism.

  The Puzzle of Alternatives

  In April 1925, during an experiment at Bell Labs undertaken by two American physicists, Clinton Davisson and Lester Germer, a glass tube containing a hot chunk of nickel suddenly exploded. Davisson and Germer had been spending their days firing beams of electrons at specimens of nickel to investigate various aspects of the metal’s atomic properties; the equipment failure was a nuisance, albeit one all too familiar in experimental work. On cleaning up the glass shards, Davisson and Germer noticed that the nickel had been tarnished during the explosion. Not a big deal, of course. All they had to do was heat the sample, vaporize the contaminant, and start again. And so they did. But that choice, to clean the sample instead of opting for a new one, proved fortuitous. When they directed the electron beam at the newly cleaned nickel, the results were completely different from any they or anyone else had ever encountered. By 1927, it was clear that Davisson and Germer had established a vital feature of the rapidly developing quantum theory. And within a decade, their serendipitous discovery would be honored with the Nobel Prize.

  Although Davisson and Germer’s demonstration predates talking movies and the Great Depression, it’s still the most widely used method for introducing quantum theory’s essential ideas. Here’s how to think about it. When Davisson and Germer heated the tarnished sample, they caused numerous small nickel crystals to meld into fewer larger ones. In turn, their electron beam no longer reflected off a highly uniform surface of nickel but instead bounced back from a few concentrated locations where the larger nickel crystals were centered. A simplified version of their experiment, the setup of Figure 8.1, in which electrons are fired at a barrier containing two narrow slits, highlights the essential physics. Electrons emanating from one slit or the other are like electrons bouncing back from one nickel crystal or its neighbor. Modeled in this way, Davisson and Germer were carrying out the first version of what’s now called the double-slit experiment.

  To grasp Davisson and Germer’s startling result, imagine closing off either the left or the right slit and capturing the electrons that pass through, one by one, on a detector screen. After many such electrons are fired, the detector screens will look like those in Figure 8.2a and Figure 8.2b. A rational, nonquantum trained mind would therefore expect that when both slits are open, the data would be an amalgam of these two results. But the astounding fact is that this is not what happens. Instead, Davisson and Germer found data, much like those illustrated in Figure 8.2c, consisting of light and dark bands indicating a series of positions where electrons do and do not land.

  Figure 8.1 The essence of the Davisson and Germer experiment is captured by the “double-slit” setup in which electrons are fired at a barrier that has two narrow slits. In the Davisson and Germer experiment, two streams of electrons are produced when incident electrons bounce off neighboring nickel crystals; in the double-slit experiment, two similar streams are produced by electrons that pass through the neighboring slits.

  These results deviate from expectations in a way that’s particularly peculiar. The dark bands are locations where electrons are copiously detected if only the left slit or only the right slit is open (the corresponding regions in Figures 8.2a and 8.2b are bright), but which are apparently unreachable when both slits are available. The presence of the left slit thus changes the possible landing locations of electrons passing through the right slit, and vice versa. Which is thoroughly perplexing. On the scale of a tiny particle like an electron, the distance between the slits is huge. So when the electron passes through one slit, how
could the presence or absence of the other have any effect, let alone the dramatic influence evident in the data? It’s as if for many years you happily enter an office building using one door, but when the management finally adds a second door on the building’s other side, you can no longer reach your office.

  What are we to make of this? The double-slit experiment leads us inescapably to a conclusion hard to fathom. Regardless of which slit it passes through, each individual electron somehow “knows” about both. There’s something associated with, or connected to, or part of each individual electron that is affected by both slits.

  But what could that something be?

  Figure 8.2 (a) The data obtained when electrons are fired and only the left slit is open. (b) The data obtained when electrons are fired and only the right slit is open. (c) The data obtained when electrons are fired and both slits are open.

  Quantum Waves

  For a clue as to how an electron traveling through one slit “knows” about the other, look more closely at the data in Figure 8.2c. The light-dark-light-dark pattern is as recognizable to a physicist as a mother’s face is to her baby. The pattern says—no, it screams—waves. If you’ve ever dropped two pebbles into a pond and watched as the resulting ripples spread and overlap, you know what I mean. Where the peak of one wave crosses the peak of another, the combined wave height is big; where the trough of one crosses the trough of another, the combined wave depression is deep; and most important of all, where the peak of one crosses the trough of the other, the waves cancel and the water remains level. This is illustrated in Figure 8.3. If you were to insert a detector screen across the top of the figure that recorded the water’s agitation at each location—the larger the agitation, the brighter the reading—the result would be a series of alternating bright and dark regions on the screen. Bright regions would be where the waves reinforce each other, yielding much agitation; dark regions would be where the waves cancel, yielding no agitation. Physicists say the overlapping waves interfere with one another, and call the bright-dark-bright data they produce an interference pattern.

  The similarity to Figure 8.2c is unmistakable, so in trying to explain the electron data we’re led to think about waves. Good. That’s a start. But the details are still murky. What kind of waves? Where are they? And what have they to do with particles such as electrons?

  The next clue comes from the experimental fact I emphasized at the outset. Reams of data on the motion of particles show that regularities emerge only statistically. The same measurements performed on identically prepared particles will generally reveal them to be in different places; yet many such measurements establish that, on average, the particles have the same probability of being found at any given location. In 1926, the German physicist Max Born joined these two clues together and with them made a leap that nearly three decades later would earn him a Nobel Prize. You’ve got experimental evidence that waves play a role. You’ve got experimental evidence that probabilities play a role. Perhaps, Born suggested, the wave associated with a particle is a probability wave.

  Figure 8.3 When two water waves overlap, they “interfere,” creating alternating regions of more or less agitation called an interference pattern.

  It was an unprecedented and spectacularly original contribution. The idea is that in analyzing the motion of a particle we shouldn’t think of it as a rock hurtling from here to there. Instead, we should think of it as a wave undulating from here to there. Locations where the wave’s values are large, near its peaks and troughs, are locations where the particle is likely to be found. Locations where the probability wave’s values are small are locations where the particle is unlikely to be found. Locations where the wave’s values vanish are places where the particle won’t be found. As the wave rolls onward, the values evolve, going up in some locations, down in others. And since we’re interpreting the undulating values as undulating probabilities, the wave is justly called a probability wave.

  To flesh out the picture, consider how it explains the double-slit data. As an electron travels toward the barrier in Figure 8.2c, quantum mechanics tells us to think of it as an undulating wave, as in Figure 8.4. When the wave encounters the barrier, two wave fragments make it through the slits and undulate onward toward the detector screen. What happens next is key. Much like overlapping water waves, the probability waves emerging from the two slits overlap and interfere, yielding a combined form that looks much like that in Figure 8.3: a pattern of high and low values that, according to quantum mechanics, corresponds with a pattern of high and low probabilities for where the electron will land. When electron after electron is fired, the cumulative landing positions conform to this probability profile. Many electrons land where the probability is high, few where it’s low, and none where the probability vanishes. The result is the bright and dark bands of Figure 8.2c.3

  And that’s how quantum theory explains the data. The description makes manifest that each electron does “know” about both slits, since each electron’s probability wave passes through both. It’s the union of these two partial waves that dictates the probabilities for where the electrons land. That’s why the mere presence of a second slit affects the results.

  Figure 8.4 When we describe the motion of an electron in terms of an undulating probability wave, the puzzling interference data are explained.

  Not So Fast

  Although I’ve focused on electrons, similar experiments have established the same probability-wave picture for all of nature’s basic constituents. Photons, neutrinos, muons, quarks—every fundamental particle—all are described by waves of probability. But before we declare victory, three questions immediately present themselves. Two are straightforward. One is a bear. It’s the latter that Everett sought to answer back in the 1950s, and it led him to a quantum version of parallel worlds.

  First, if quantum theory is right and the world unfolds probabilistically, why is Newton’s nonprobabilistic framework so good at predicting the motion of things from baseballs to planets to stars? The answer is that probability waves for big things usually (but not always, as we will shortly see) have a very particular shape. They’re extraordinarily narrow, as in Figure 8.5a, meaning there’s a huge probability, just shy of 100 percent, that the object is located where the wave is peaked and a minuscule probability, just a shade above 0 percent, that it’s located anywhere else.4 Moreover, the quantum laws show that the peaks of such narrow waves move along the very same trajectories that emerge from Newton’s equations. And so, while Newton’s laws predict precisely the trajectory of a baseball, quantum theory offers only the most minimal refinement, saying there’s a nearly 100 percent probability that the ball will land where Newton says it should, and a nearly 0 percent probability that it won’t.

  Figure 8.5 (a) The probability wave for a macroscopic object is generally narrowly peaked. (b) The probability wave for a microscopic object, say, a single particle, is typically widely spread.

  In fact, the words “just shy” and “nearly” don’t do the physics justice. The chance of a macroscopic body deviating from Newton’s predictions is so fantastically tiny that if you’d been keeping tabs on the cosmos for the last few billion years, the odds are overwhelming that you’d have never seen it happen. But according to quantum theory, the smaller an object, the more spread-out its probability wave typically is. For example, a typical electron’s wave might look like that in Figure 8.5b, with substantial probabilities of being at a variety of locations, a totally foreign concept in a Newtonian world. And that’s why it’s the microrealm where the probabilistic nature of reality comes to the fore.

  Second, can we see the probability waves on which quantum mechanics relies? Is there any way to directly access the unfamiliar probabilistic haze, such as that illustrated schematically in Figure 8.5b, in which a single particle has a chance of being found in a variety of locations? No. The standard approach to quantum mechanics, developed by Bohr and his group, and called the Copenhagen interpretation in their
honor, envisions that whenever you try to see a probability wave, the very act of observation thwarts your attempt. When you look at an electron’s probability wave, where “look” means “measure its position,” the electron responds by snapping to attention and coalescing at one definite location. Correspondingly, the probability wave surges to 100 percent at that spot, while collapsing to 0 percent everywhere else, as in Figure 8.6. Look away, and the needle-thin probability wave rapidly spreads, indicating that once again there’s a reasonable chance of finding the electron at a variety of locations. Look back, and the electron’s wave collapses anew, eliminating the range of possible places the electron might be found in favor of its occupying a single definite spot. In short, every time you attempt to see the probabilistic haze it disappears—it collapses—and is supplanted by familiar reality. The detector screen in Figure 8.2c provides a case in point: it measures the impinging probability wave of an electron and thus immediately causes it to collapse. The detector forces the electron to relinquish the many available options for where it could hit and settle upon a definite landing location, which is then evidenced by a tiny dot on the screen.