by Brian Greene
Nevertheless, these results, coming from both theoretical and experimental considerations, strongly support the conclusion that the universe admits interconnections that are not local. 4 Something that happens over here can be entwined with something that happens over there even if nothing travels from here to there—and even if there isn't enough time for anything, even light, to travel between the events. This means that space cannot be thought of as it once was: intervening space, regardless of how much there is, does not ensure that two objects are separate, since quantum mechanics allows an entanglement, a kind of connection, to exist between them. A particle, like one of the countless number that make up you or me, can run but it can't hide. According to quantum theory and the many experiments that bear out its predictions, the quantum connection between two particles can persist even if they are on opposite sides of the universe. From the standpoint of their entanglement, notwithstanding the many trillions of miles of space between them, it's as if they are right on top of each other.
Numerous assaults on our conception of reality are emerging from modern physics; we will encounter many in the following chapters. But of those that have been experimentally verified, I find none more mind-boggling than the recent realization that our universe is not local.
The Red and the Blue
To get a feel for the kind of nonlocality emerging from quantum mechanics, imagine that Agent Scully, long overdue for a vacation, retreats to her family's estate in Provence. Before she's had time to unpack, the phone rings. It's Agent Mulder calling from America.
"Did you get the box—the one wrapped in red and blue paper?"
Scully, who has dumped all her mail in a pile by the door, looks over and sees the package. "Mulder, please, I didn't come all the way to Aix just to deal with another stack of files."
"No, no, the package is not from me. I got one too, and inside there are these little lightproof titanium boxes, numbered from 1 to 1,000, and a letter saying that you would be receiving an identical package."
"Yes, so?" Scully slowly responds, beginning to fear that the titanium boxes may somehow wind up cutting her vacation short.
"Well," Mulder continues, "the letter says that each titanium box contains an alien sphere that will flash red or blue the moment the little door on its side is opened."
"Mulder, am I supposed to be impressed?"
"Well, not yet, but listen. The letter says that before any given box is opened, the sphere has the capacity to flash either red or blue, and it randomly decides between the two colors at the moment the door is opened. But here's the strange part. The letter says that although your boxes work exactly the same way as mine—even though the spheres inside each one of our boxes randomly choose between flashing red or blue—our boxes somehow work in tandem. The letter claims that there is a mysterious connection, so that if there is a blue flash when I open my box 1, you will also find a blue flash when you open your box 1; if I see a red flash when I open box 2, you will also see a red flash in your box 2, and so on."
"Mulder, I'm really exhausted; let's let the parlor tricks wait till I get back."
"Scully, please. I know you're on vacation, but we can't just let this go. We'll only need a few minutes to see if it's true."
Reluctantly, Scully realizes that resistance is futile, so she goes along and opens her little boxes. And on comparing the colors that flash inside each box, Scully and Mulder do indeed find the agreement predicted in the letter. Sometimes the sphere in a box flashes red, sometimes blue, but on opening boxes with the same number, Scully and Mulder always see the same color flash. Mulder grows increasingly excited and agitated by the alien spheres but Scully is thoroughly unimpressed.
"Mulder," Scully sternly says into the phone, " you really need a vacation. This is silly. Obviously, the sphere inside each of our boxes has been programmed to flash red or it has been programmed to flash blue when the door to its box is opened. And whoever sent us this nonsense programmed our boxes identically so that you and I find the same color flash in boxes with the same number."
"But no, Scully, the letter says each alien sphere randomly chooses between flashing blue and red when the door is opened, not that the sphere has been preprogrammed to choose one color or the other."
"Mulder," Scully sighs, "my explanation makes perfect sense and it fits all the data. What more do you want? And look here, at the bottom of the letter. Here's the biggest laugh of all. The 'alien' small print informs us that not only will opening the door to a box cause the sphere inside to flash, but any other tampering with the box to figure out how it works— for example, if we try to examine the sphere's color composition or chemical makeup before the door is opened—will also cause it to flash. In other words, we can't analyze the supposed random selection of red or blue because any such attempt will contaminate the very experiment we are trying to carry out. It's as if I told you I'm really a blonde, but I become a redhead whenever you or anyone or anything looks at my hair or analyzes it in any way. How could you ever prove me wrong? Your tiny green men are pretty clever—they've set things up so their ruse can't be unmasked. Now, go and play with your little boxes while I enjoy a little peace and quiet."
It would seem that Scully has this one soundly wrapped up on the side of science. Yet, here's the thing. Quantum mechanicians—scientists, not aliens—have for nearly eighty years been making claims about how the universe works that closely parallel those described in the letter. And the rub is that there is now strong scientific evidence that a viewpoint along the lines of Mulder's—not Scully's—is supported by the data. For instance, according to quantum mechanics, a particle can hang in a state of limbo between having one or another particular property—like an "alien" sphere hovering between flashing red and flashing blue before the door to its box is opened—and only when the particle is looked at (measured) does it randomly commit to one definite property or another. As if this weren't strange enough, quantum mechanics also predicts that there can be connections between particles, similar to those claimed to exist between the alien spheres. Two particles can be so entwined by quantum effects that their random selection of one property or another is correlated: just as each of the alien spheres chooses randomly between red and blue and yet, somehow, the colors chosen by spheres in boxes with the same number are correlated (both flashing red or both flashing blue), the properties chosen randomly by two particles, even if they are far apart in space, can similarly be aligned perfectly. Roughly speaking, even though the two particles are widely separated, quantum mechanics shows that whatever one particle does, the other will do too.
As a concrete example, if you are wearing a pair of sunglasses, quantum mechanics shows that there is a 50-50 chance that a particular photon—like one that is reflected toward you from the surface of a lake or from an asphalt roadway—will make it through your glare-reducing polarized lenses: when the photon hits the glass, it randomly "chooses" between reflecting back and passing through. The astounding thing is that such a photon can have a partner photon that has sped miles away in the opposite direction and yet, when confronted with the same 50-50 probability of passing through another polarized sunglass lens, will somehow do whatever the initial photon does. Even though each outcome is determined randomly and even though the photons are far apart in space, if one photon passes through, so will the other. This is the kind of nonlocality predicted by quantum mechanics.
Einstein, who was never a great fan of quantum mechanics, was loath to accept that the universe operated according to such bizarre rules. He championed more conventional explanations that did away with the notion that particles randomly select attributes and outcomes when measured. Instead, Einstein argued that if two widely separated particles are observed to share certain attributes, this is not evidence of some mysterious quantum connection instantaneously correlating their properties. Rather, just as Scully argued that the spheres do not randomly choose between red and blue, but instead are programmed to flash one particular color when o
bserved, Einstein claimed that particles do not randomly choose between having one feature or another but, instead, are similarly "programmed" to have one particular, definite feature when suitably measured. The correlation between the behavior of widely separated photons is evidence, Einstein claimed, that the photons were endowed with identical properties when emitted, not that they are subject to some bizarre long-distance quantum entanglement.
For close to five decades, the issue of who was right—Einstein or the supporters of quantum mechanics—was left unresolved because, as we shall see, the debate became much like that between Scully and Mulder: any attempt to disprove the proposed strange quantum mechanical connections and leave intact Einstein's more conventional view ran afoul of the claim that the experiments themselves would necessarily contaminate the very features they were trying to study. All this changed in the 1960s. With a stunning insight, the Irish physicist John Bell showed that the issue could be settled experimentally, and by the 1980s it was. The most straightforward reading of the data is that Einstein was wrong and there can be strange, weird, and "spooky" quantum connections between things over here and things over there. 5
The reasoning behind this conclusion is so subtle that it took physicists more than three decades to appreciate fully. But after covering the essential features of quantum mechanics we will see that the core of the argument reduces to nothing more complex than a Click and Clack puzzler.
Casting a Wave
If you shine a laser pointer on a little piece of black, overexposed 35mm film from which you have scratched away the emulsion in two extremely close and narrow lines, you will see direct evidence that light is a wave. If you've never done this, it's worth a try (you can use many things in place of the film, such as the wire mesh in a fancy coffee plunger). The image you will see when the laser light passes through the slits on the film and hits a screen consists of light and dark bands, as in Figure 4.1, and the explanation for this pattern relies on a basic feature of waves. Water waves are easiest to visualize, so let's first explain the essential point with waves on a large, placid lake, and then apply our understanding to light.
A water wave disturbs the flat surface of a lake by creating regions where the water level is higher than usual and regions where it is lower than usual. The highest part of a wave is called its peak and the lowest part is called its trough. A typical wave involves a periodic succession: peak followed by trough followed by peak, and so forth. If two waves head toward each other—if, for example, you and I each drop a pebble into the lake at nearby locations, producing outward-moving waves that run into each other—when they cross there results an important effect known as interference, illustrated in Figure 4.2a. When a peak of one wave and a peak of the other cross, the height of the water is even greater, being the sum of the two peak heights. Similarly, when a trough of one wave and a trough of the other cross, the depression in the water is even deeper, being the sum of the two depressions. And here is the most important combination: when a peak of one wave crosses the trough of another, they tend to cancel each other out, as the peak tries to make the water go up while the trough tries to drag it down. If the height of one wave's peak equals the depth of the other's trough, there will be perfect cancellation when they cross, so the water at that location will not move at all.
Figure 4.1 Laser light passing through two slits etched on a piece of black film yields an interference pattern on a detector screen, showing that light is a wave.
The same principle explains the pattern that light forms when it passes through the two slits in Figure 4.1. Light is an electromagnetic wave; when it passes through the two slits, it splits into two waves that head toward the screen. Like the two water waves just discussed, the two light waves interfere with each other. When they hit various points on the screen, sometimes both waves are at their peaks, making the screen bright; sometimes both waves are at their troughs, also making it bright; but sometimes one wave is at its peak and the other is at its trough and they cancel, making that point on the screen dark. We illustrate this in Figure 4.2b.
When the wave motion is analyzed in mathematical detail, including the cases of partial cancellations between waves at various stages between peaks and troughs, one can show that the bright and dark spots fill out the bands seen in Figure 4.1. The bright and dark bands are therefore a telltale sign that light is a wave, an issue that had been hotly debated ever since Newton claimed that light is not a wave but instead is made up of a stream of particles (more on this in a moment). Moreover, this analysis applies equally well to any kind of wave (light wave, water wave, sound wave, you name it) and thus, interference patterns provide the metaphorical smoking gun: you know you are dealing with a wave if, when it is forced to pass through two slits of the right size (determined by the distance between the wave's peaks and troughs), the resulting intensity pattern looks like that in Figure 4.1 (with bright regions representing high intensity and dark regions being low intensity).
Figure 4.2 ( a ) Overlapping water waves produce an interference pattern. ( b ) Overlapping light waves produce an interference pattern.
In 1927, Clinton Davisson and Lester Germer fired a beam of electrons—particulate entities without any apparent connection to waves—at a piece of nickel crystal; the details need not concern us, but what does matter is that this experiment is equivalent to firing a beam of electrons at a barrier with two slits. When the experimenters allowed the electrons that passed through the slits to travel onward to a phosphor screen where their impact location was recorded by a tiny flash (the same kind of flashes responsible for the picture on your television screen), the results were astonishing. Thinking of the electrons as little pellets or bullets, you'd naturally expect their impact positions to line up with the two slits, as in Figure 4.3a. But that's not what Davisson and Germer found. Their experiment produced data schematically illustrated in Figure 4.3b: the electron impact positions filled out an interference pattern characteristic of waves. Davisson and Germer had found the smoking gun. They had shown that the beam of particulate electrons must, unexpectedly, be some kind of wave.
Figure 4.3 ( a ) Classical physics predicts that electrons fired at a barrier with two slits will produce two bright stripes on a detector. ( b ) Quantum physics predicts, and experiments confirm, that electrons will produce an interference pattern, showing that they embody wavelike features.
Now, you might not think this is particularly surprising. Water is made of H 2 O molecules, and a water wave arises when many molecules move in a coordinated pattern. One group of H 2 O molecules goes up in one location, while another group goes down in a nearby location. Perhaps the data illustrated in Figure 4.3 show that electrons, like H 2 O molecules, sometimes move in concert, creating a wavelike pattern in their overall, macroscopic motion. While at first blush this might seem to be a reasonable suggestion, the actual story is far more unexpected.
We initially imagined that a flood of electrons was fired continuously from the electron gun in Figure 4.3. But we can tune the gun so that it fires fewer and fewer electrons every second; in fact, we can tune it all the way down so that it fires, say, only one electron every ten seconds. With enough patience, we can run this experiment over a long period of time and record the impact position of each individual electron that passes through the slits. Figures 4.4a-4.4c show the resulting cumulative data after an hour, half a day, and a full day. In the 1920s, images like these rocked the foundations of physics. We see that even individual, particulate electrons, moving to the screen independently, separately, one by one, build up the interference pattern characteristic of waves.
This is as if an individual H 2 O molecule could still embody something akin to a water wave. But how in the world could that be? Wave motion seems to be a collective property that has no meaning when applied to separate, particulate ingredients. If every few minutes individual spectators in the bleachers get up and sit down separately, independently, they are not doing the wave. More than that, wave inte
rference seems to require a wave from here to cross a wave from there. So how can interference be at all relevant to single, individual, particulate ingredients? But somehow, as attested by the interference data in Figure 4.4, even though individual electrons are tiny particles of matter, each and every one also embodies a wavelike character.
Figure 4.4 Electrons fired one by one toward slits build up an interference pattern dot by dot. In ( a )-( c ) we illustrate the pattern forming over time.
Probability and the Laws of Physics
If an individual electron is also a wave, what is it that is waving? Erwin Schrödinger weighed in with the first guess: maybe the stuff of which electrons are made can be smeared out in space and it's this smeared electron essence that does the waving. An electron particle, from this point of view, would be a sharp spike in an electron mist. It was quickly realized, though, that this suggestion couldn't be correct because even a sharply spiked wave shape—such as a giant tidal wave—ultimately spreads out. And if the spiked electron wave were to spread we would expect to find part of a single electron's electric charge over here or part of its mass over there. But we never do. When we locate an electron, we always find all of its mass and all of its charge concentrated in one tiny, pointlike region. In 1927, Max Born put forward a different suggestion, one that turned out to be the decisive step that forced physics to enter a radically new realm. The wave, he claimed, is not a smeared-out electron, nor is it anything ever previously encountered in science. The wave, Born proposed, is a proba bility wave.
