A Brilliant Darkness

Meanwhile, Ettore pressed on with his work, against the grain of Via Panisperna. It was not only with regard to the neutron that he was in conflict with Fermi. Pauli had just brought its sissy brother—the neutrino—into the world, and it was in his research on the neutrino that Ettore would see the furthest “beyond.” It wasn’t until several decades later that anyone realized just how odd a particle the neutrino was. To Ettore, however, this would hardly have been news.

Saying that we live in an odd world is often an understatement. I once had a random conversation on a Toronto street that derailed into the most sublime insanity. After a few minutes of pleasant platitudes, my casual acquaintance, out of the blue, revealed that “they” had implanted radioactive isotopes in his testicles. Being high-minded, he refrained from ejaculating, lest he might contaminate the entire universe. He’d done so for years and was sexually desperate—he couldn’t take it any longer.

Later I discovered that there was a psychiatric hospital just up the road, but we do live in an odd world. Ettore was odd; the neutrino is Ettore’s soul mate; ergo, the neutrino is odd. A cheap syllogism, but true: The neutrino is odd. Except that this statement has a precise, concrete meaning—it’s not as if the neutrino believes its balls have been irradiated.

The “oddity” of the neutrino remains one of the major unexplained features of our world. The issue was first raised as a pure mathematical problem by the lonely hearts of physics—Ettore and Dirac—circa 1933. But it wasn’t until 1957 that the full brunt of the news befell the world. Ettore’s science was so visionary that once again we require the services of a time machine, shifting us from the 1930s to the 1950s. The world is literally odd.

Imagine Alice gone through the looking glass, in a world “inverted” by a mirror. “Perhaps looking-glass milk isn’t good to drink,” Alice remarks to her cat, after they’ve walked through to the other side. Such is the issue of “parity” (in scientific jargon): Would the mirror-inverted world be any different from ours? Alice and Ettore were the pioneers of this new method of tripping.

At first it might seem that not much would change. Most people look the same in the mirror-world with the possible exception of their hair. Some haircuts don’t essentially change in the mirror (see Figure 9.1a)—these are an example of what physicists call parity symmetric. Other haircuts manifestly do (see Figure 9.1b): These are an example of what physicists call chiral, or parity violating. If you look in more detail, however, even leaving moles and scars aside, everyone looks quite different to their mirror-world twin. The left hand appears in the mirror like the right, but they’re “palpably” not the same. Try shaking someone’s right hand with your left (or shake hands with your mirror-world self): It just wouldn’t work.

To drive the point home, the term chiral, coined to describe parity asymmetry, comes from the Greek word for “hand” and is sometimes replaced by “handedness.” Our hands have handedness: That is, they are parity violating or chiral, each distinct from its mirror image. The same goes for our feet—scientists in particular are well acquainted with the experience of putting on the wrong shoes. Chiral wearing of nonmatching socks is another ubiquitous feature of the absentminded genius.

The more we think about it, the more we realize that our world is abundantly chiral. Alice sees a distinctly different universe on the other side of the mirror. The majority of people are left-handed, unlike in our world; human hearts turn to the right, not the left; seashells and snails look different (see Figure 9.2); writing is gibberish; Alice finds corkscrews and taps awkward to operate. But among these “oddities” she identifies things that are “maximally chiral”: where the mirror world is actually the opposite of the real world. On which side of the road do you drive?

There’s a difference of opinion between the British and the Continental: They make opposite choices. But seen through a mirror, Britain would have adopted Continental driving, and vice versa. Except for parity-symmetric Indian roads, driving is maximally chiral: In such extreme cases, the two opposites are called right- and left-handed states, or even and odd. Left-handed driving is odd.

Chemists and biologists have long been familiar with chirality. Amino acids—the building blocks of the proteins we’re made of—are chiral molecules (see Figure 9.3). Strangely enough, only left-handed amino acids exist in the organic components of food and living beings. When traces of left-handed organic molecules were discovered in meteorites, it sparked controversy. Is the observed handedness of “living” amino acids a peculiarity of life on Earth, or a feature of life everywhere in the universe? Are these meteorites proof of alien life? Mr. Spock, incidentally, was in one of his incarnations made of organic components with the wrong handedness.

It’s not surprising that drugs can also be made of chiral molecules, given the handedness of our inner chemistry. Dramatically different effects may then result from right- or left-handed drugs. A medicine used to cure tuberculosis, for example, has a mirror image that causes blindness. Infamously, thalidomide is chiral; its right-handed version treats morning sickness, whereas its mirror image causes birth defects. The tragedy of thalidomide is that once ingested, the right- and left-handed chemicals naturally interconvert.

Alice was right to voice grave concerns over the quality of looking-glass milk. Like thalidomide, it might be poisonous. I doubt her cat—clever beasts that they are—would have touched it. He’d have disdainfully sniffed it, sneered, then haughtily stalked off.

Yet until 1957, physics was ambidextrous, making no distinction between the right and left hand. The contents of the world, for sure, were chiral, but not the laws governing the world, an entirely different proposition. Of course the solar system is chiral, but if the planets all orbited the sun in the opposite direction, nothing else would change (e.g., the duration of the terrestrial year would be the same) because the laws of gravity don’t distinguish between the world and its mirror image. So much was parity trusted that it was long regarded as an essential tenet when proposing new theories. Imagine the surprise of physicists Chen Ning Yang and Tsung Dao Lee when they examined the beta decay of certain particles found in cosmic rays, only to find that their disintegration laws were chiral.

Cosmic rays are very energetic particles resulting from big explosions inside our galaxy, sometimes even beyond.³⁰ When they hit Earth, they provide physicists with a natural, inexpensive look at high-energy phenomena. In 1956, Yang and Lee were studying new particles found in cosmic-ray showers, deemed to be so weird that they were baptized “strange particles” (a terminology that persists to this day, although they’re now run of the mill in particle accelerators). To their consternation, the beta decay of these particles appeared distinctly different from its mirror image. This was serious stuff! But it was it a general rule? Or was it yet another peculiarity of “strange particles,” which nobody understood anyway?

What followed is even more remarkable sociologically than scientifically. When Yang and Lee scrutinized vanilla-flavored beta decays—involving non-strange, well-known particles and nuclei—they found that parity violation was highly prominent. But these decays had been studied for over fifty years: How could such a conspicuous feature have passed unnoticed? The answer is a monument to prejudice: The parity violations had not been noted because no one had bothered to ask the question. Experiments implicitly assumed parity symmetry, averaging between what they actually saw and a presumed identical mirror version, in a case of tautology derived from carelessness.

But no one was quite prepared for the news that followed. First it emerged that virtually all beta decays were chiral, so that the startling feature couldn’t be blamed on a particular mother or daughter nucleus but on the decay process itself. Second, the decays were maximally chiral; i.e., one of those instances where there are two opposites, and our world picks one but the mirror world plumps for the other.

Physicists were astounded. But the bombshell was so dramatic that at least the logical implications were completely clear. It didn’t take long to discover the culprit; indeed, it’s quite predictable. Elementary, my dear Watson, elementary.

This is how things stood: All beta decays were known to be maximally chiral regardless of the mother or daughter nucleus. Their common trait was the emission of a very energetic electron (the beta ray) and a neutrino. The electron had impeccable credentials regarding parity: Its ambidexterity had been tested over and over again by all manner of experiments.

This left the neutrino. What else?

But although the culprit was found, there remained one important question: What made the neutrino maximally chiral? The answer was not unexpected by 1957, thanks to the work of Ettore, Dirac, and Wigner. In fact, issues of chirality had been central to Ettore’s mathematical constructions since the early 1930s. Recall that quantum particles have a habit of spinning like tops—tops with rather strange behavior. Given a fixed direction, they can only rotate around it with a fixed “speed,” the sense of rotation being their only choice: spin up or spin down. This means all they can do is stand up or perform a headstand, or at least nothing else can be observed. But quantum mechanics, having limited the options for what can be seen, then allows for ghost superpositions when no one is looking. You can’t sit down if you’re a quantum particle, but you can be 80 percent standing up and 20 percent doing a headstand, in a mixed state pregnant with possibility: Ask Schrödinger’s cat.

Spin turns out to be the clue to the neutrino’s chirality. Suppose that we choose its direction of motion as the preferred axis with which to play the quantum-spin game.³¹ The two possible spin states, up and down, translate into “along” and “against” the direction of motion, depending on how the neutrino corkscrews along (see Figure 9.4). One is the mirror image of the other, so we call them right-and left-handed neutrinos, or even and odd. Is this the origin of their chirality?

Figure 9.4: Two particles (here protons) moving from the bottom to the top of the page (as indicated by the arrows). Playing the spin game with respect to the direction of motion, the particle on the left has spin up (it rotates clockwise with respect to its translational motion), whereas the one on the right is spin down. One is the mirror image of the other. (Notice that the translational motion doesn’t change in a mirror.)

Not quite: This classification can be applied to all spinning particles, such as protons, photons, and electrons, which have no chirality. What makes these particles ambidextrous is that they exist in both right- and left-handed states, each behaving in exactly the same way. The probability of emitting a right- or left-handed photon in a particle collision is the same. If a right-handed photon excites an atom, the effect is the same as for a left-handed twin. The world of light is ambidextrous because there are right- and left-handed photons, and they shake hands without any problems: They are interchangeable.

Not so with the neutrino, evidently. Beta decays pointed to the neutrino’s maximal chirality. This implied that in beta decay processes, only one type of neutrino—either right- or left-handed—was present. Which one was it? Was the neutrino right- or left-handed? Curiously, after the remarkable discoveries of Yang and Lee, it took some complex experimentation to settle the issue. It was finally decided—by an experiment conducted by Goldhaber, Grodzins, and Sunyar—that the neutrino is odd. Only the left-handed neutrino exists, or for those with a weird frame of mind, the neutrino has two left hands. This is a very extreme form of chirality: We’re in Transylvania. Look at a (left-handed) neutrino in the mirror and you’ll see nothing, as with Dracula. Because its mirror image—the right-handed neutrino—simply doesn’t exist.

The surprising discovery of the neutrino’s oddity was a moment of glory for Yang and Lee. But hiding behind it lies a nasty tale illustrating much about human nature. The success of one person is often the misfortune of another. A mere hesitation can be the undoing of much genius.

Before Yang and Lee made their discovery, there was a PhD student at Cambridge University who had long been struggling with the mathematical constructions of Ettore and Dirac. Something wasn’t quite right when the neutrino was entered into their equations. His confusion didn’t stop him from getting his PhD (it never does), but he felt uncomfortable. His name was Abdus Salam, and he would later become one of the greatest exponents of twentieth-century physics, codiscovering the “standard model,” for which he was awarded the Nobel Prize in 1979.

In September 1956, while Yang and Lee were struggling with the decay of strange particles, Salam went to a conference in the United States where he heard news of their observations. On the trip back to England, on a crowded military plane (his visit was funded by the US Air Force), he couldn’t sleep and revisited the problem. And that’s when he had a revelation: His mathematical problems miraculously evaporated if the neutrino were maximally chiral, if only left-handed particles, say, existed. Despite the screaming children of servicemen he got off the plane feeling “naturally very elated,” as he later reported in his Nobel Prize lecture.

He rushed to Cambridge to work out the details of his idea and bestow his wisdom upon the world. But Peierls—who’d examined his PhD—was dismissive, telling him, “I do not believe left-right symmetry is violated in weak nuclear forces at all. I wouldn’t touch such ideas.” Unabashed, Salam contacted the world authority on neutrinos, no less than their “inventor,” Wolfgang Pauli. Pauli warned Salam that he’d be committing career suicide by suggesting such a stupid idea. “Give my regards to my friend Salam and tell him to think of something better,” wrote Pauli soon afterwards.

Salam was crestfallen and perhaps unsurprisingly never published his idea. A year later Yang and Lee did. They were awarded the Nobel Prize in Physics for their discovery, and Abdus Salam was never the same.