A Brilliant Darkness

While Ettore was struggling with his identity, so was his elusive soul mate, the neutrino. For years no one believed the neutrino could be detected, assuming it even existed. But then something wild and menacing happened—in the shape of a mushroom cloud!

Estimates of the neutrino’s inability to interact with ordinary matter had been made early on, starting with Pauli himself. He even considered the possibility that perhaps the neutrino didn’t interact at all, scolding himself for proposing a “metaphysical” hypothesis. “I have done a terrible thing. I have postulated a particle that cannot be detected,” Pauli said shortly after proposing the neutrino. Top-brass nuclear physicists Hans Bethe and Rudolph Peierls added, after some calculations, that “one obviously would never be able to see a neutrino.”

But there are some stubborn and optimistic people in science called experimentalists—who usually disregard theoretical pronouncements—and in 1934 James Chadwick (the discoverer of the neutron) made a first attempt at detecting the neutrino. He met with no success, but his experiment established the first landmark in the field, what physicists call a bound to the neutrino’s reactivity (or lack thereof). His failure proved that the neutrino had to be able to travel more than 150 kilometers (about ninety-three miles) through air before interacting with a single atom. This was no theorist’s guess: It was an observational result in view of the known sensitivity of his experiment, plus his negative result. His experiment was powerful enough that if the neutrino reactivity were even slightly superior to that figure, he would have seen one.

Maurice Nahmias then performed an even more sensitive experiment, eliminating unwanted background by going thirty meters (about a hundred feet) underground into the Holborn tube station in London, near the British Museum.¹⁶ His failure to detect the neutrino raised Chadwick’s figure to 31,000 kilometers (about 19,000 miles): The neutrino could travel at least this distance through air without making itself known. Of course it might be able to travel much, much further unimpeded. Nahmias’s number was merely a “lower bound,” in the parlance of experimentalists.

These were the observational results, but in 1933, Fermi (Ettore’s boss and equal) discovered a masterful way to compute any quantity related to beta decay and neutrino emission. Remarkably, his theory predicted a new reaction that could potentially be used to detect the neutrino: inverse beta decay (see Figure 4.1). Schematically, beta decay happens when a neutron decays into a proton, an electron, and a neutrino. Fermi’s theory predicted that a neutrino could hit a proton to produce a neutron and a positron or antielectron. What is an antielectron, you might ask? Is there, then, an antineutrino? Ettore made his mark by answering these very questions. So bear with me—these particulars will later form an important part of his story, but they are irrelevant for the moment.

Details aside, Fermi had just proposed a concrete reaction that could be used to detect the neutrino, and had found a way to compute its predicted intensity. But when Bethe and Peierls actually performed the calculation in 1934, they concluded that the neutrino would require a water target 1,000 light-years across to have any chance of colliding with a proton, inducing inverse beta decay. That’s over 60 million times the distance between Earth and the sun. A very deep ocean indeed would be needed before the neutrino would let its corporeal nature be known. No wonder Pauli was as depressed by his theory as he was by his sex life.

It would be normal to give up at this point, and scientists did in fact give up on the neutrino for a few decades. Something dramatic had to happen to change the landscape of neutrino detection: a couple of atomic bombs being dropped, for example. Indeed, the bomb was essential in bringing the neutrino to light. Ironically, the specter of nuclear weapons was the price paid for exorcising the poltergeist.

Figure 4.1: Inverse beta decay takes place when a neutrino hits a proton (either on its own or inside a nucleus), converting it into a neutron and emitting a positron or positively charged electron (mouth changed for consistency).

As it turned out, the calculation done by Bethe and Peierls was correct if one wanted to detect a given neutrino: In order to detect it, one would need a target 1,000 light-years thick. But the problem in practice was quite different: Scientists wanted to detect any neutrino—it didn’t matter which one, and they didn’t even care if most of the neutrinos passed through the target unnoticed. Since they didn’t have access to oceans light-years deep but instead had to contend with more realistic targets, the probability of a single neutrino interacting was minuscule—as computed by Bethe and Peierls. But what if they bombarded their target with a huge flux of neutrinos? One of them was bound to interact eventually, even if most of them didn’t. The trick was to recognize the ethereality of the target and go for numbers: Shoot an absolutely ridiculous swarm of bullets at the target, hoping that eventually one of them would get stuck. Of course for this to work, they still needed a very powerful neutrino gun. Like a nuclear bomb.

And indeed, the man who eventually detected the first neutrino had a very strange perversion. He was obsessed with the beauty of nuclear explosions.

Fred Reines was a Los Alamos physicist who’d spent his formative years testing nuclear weapons, wondering if “this man-made star could be used to advance our knowledge of physics,” as he’d later say in his Nobel Prize speech. Some sources claim he belonged to a group of scientists violently in love with the bomb, calling it their baby, reassured by the beauty of that mushroom cloud, so much like a star in the sky, but so close to us that its brilliance amounted to darkness. These Dr. Strangeloves did love the bomb: even more so aesthetically than scientifically or politically. Ettore was emphatically not one of them, as will later become clear.

But by 1951, Reines must have had a crisis of doubt: He applied for leave to work on theoretical issues rather than bombs. He was thirty-three years old and had devoted his life to the construction of the death threats upon which world peace so precariously rested. Now he found himself in what he described as a “stark empty office, staring at a blank pad for several months, searching for a meaningful question worthy of a life’s work.” I note a tinge of bitterness in this statement. It occurred to him that detecting a neutrino might cleanse his possibly dirty soul.

Reines soon found a collaborator in Clyde Cowan, after a casual encounter while the two were stranded at the Kansas City airport (there should be a monument erected to plane delays there). What follows sounds like complete lunacy: They had the idea to detect inverse beta decay at the core of a nuclear explosion.

It does appear suicidal: You place a nuclear weapon at the top of a tower thirty meters (about a hundred feet) high. Some forty meters (130 feet)away you dig a well. You cover the entrance and suspend from the top a detector designed to sense the end products of inverse beta decay, the telltale signature of the neutrino. Air would then be pumped out of the deep shaft, to create a near vacuum down the well. As soon as you trigger the bomb, you release the detector down the shaft (by remote control, needless to say). So the detector is freefalling through the vacuum when the massive shock wave passes through. In a vacuum, the shock wave has no support—no medium in which to propagate—so the detector miraculously avoids destruction, falling onto a bed of feathers at the bottom of the well, ready to detect the huge swarm of neutrinos being produced by the concoction of isotopes created all around. A few moments later, a fireball ascends in the sky, and all hell breaks loose, but by then you should already have radioed in your results (see Figure 4.2).

“The idea that such a sensitive detector could be operated in close proximity [to] one of the most violent explosions produced by man was somewhat bizarre,” Reines admitted. “But we had worked with bombs and felt we could design an appropriate system.” I wonder how much of what was released of this initial plan was a joke or a red herring. It certainly has the flavor of classified information.

Eventually, the two men did something somewhat less extreme—they used a nuclear reactor as their neutrino source. In response to this news, Fermi remarked, not without sarcasm, “I was very interested. . . . Certainly your new method should be much easier to carry out and has the great advantage that the measurement can be repeated any number of times.” As Reines himself put it, “Reflecting on the trail that took us from bomb to reactor, it is evident that it was our persistence which led us from a virtually impossible experiment to one that showed considerable promise.”

Their first attempt was made with the Hanford reactor, built during World War II to produce plutonium for the atomic bomb. They arrived in the spring of 1953 and set to work building and installing the appropriate detectors. The results at Hanford were inconclusive, but the party used the opportunity to perfect their detectors and develop an intricate series of unambiguous signatures of inverse beta decay, thereby eliminating the possibility of mistakes. Some of their techniques are still in use today.

No one believed the Hanford results, but Reines and Cowan felt encouraged nonetheless. In the fall of 1955, at the suggestion of John A. Wheeler, the group gained access to the new, much more powerful Savannah River reactor in South Carolina. Suddenly all their hard work paid off. They found that their techniques were already adequately sharp: All they needed was a more potent neutrino beacon—just like the one they now had. After many checks and repeats, on June 14, 1956, Reines and Cowan were fully convinced that they were detecting neutrinos—at the incredible rate of two neutrinos per hour!

At once they felt compelled to send Pauli a telegram: “We are happy to inform you that we have definitely detected neutrinos. . . . Observed cross sections agree well. . . .” To which Pauli replied, “Thanks for the message. Everything comes to him who knows how to wait.”

Dry, you might think. It was later discovered that Pauli and friends drank a case of champagne in celebration.

Many years later, Reines would confront Bethe, the man who’d pronounced the neutrino undetectable, over his statement. Bethe replied with his characteristic humor:

“Well, you shouldn’t believe everything you read in the papers.”

And this is where Ettore’s mystery really begins. He never did believe what he read in scientific papers, and therefore noticed at once—possibly as early as 1932—something about the neutrino that nobody in the scientific community took seriously until very recently. He published his observations, and that’s a knotty story in itself. But what’s worse is what Ettore didn’t publish, and which was subsequently lost.

Frustratingly, all the work from Ettore’s dark years (1934 to 1937) vanished with him. Luciano shared a room with him and saw him at his desk every day, working away. More important, Ettore’s later work refers to notes written at this time. None of these notes have survived.

“In 1966, after grandma died, my father decided to take all of Ettore’s writings to the scientific archive in Pisa,” Fabio tells me. “Why have all this stuff at home, he reasoned, when it could be of use to other scientists? All his notes were at the Rome house, so he asked Maria to collect them. She gave him a box containing about 10,000 pages, and that’s what you can now find in the Domus Galilaeana.

“Now it could be that there were two boxes of scientific writings, the other one containing whatever he produced 1934 to 1937.”

“Then the second box should still be housed in Rome.”

“No. Precisely. It got lost,” continues Fabio. “There are all these crazy theories saying that spies stole his work, that he was trying to build an atom bomb . . .”

“ . . . here in Catania!” says the signora with amusement.

“But the unfortunate truth is that this important work got lost.”

Did he destroy it? Why? Did he lose it? Or did someone else lose it for him? Strange events certainly took place on the morning Ettore vanished from Naples.

“And what of Ettore’s personal things?”

“Those stayed with the family. They were kept in a safe.”

A safe? How cinematographic.

“The family safe,” says the signora. “It was next to Dorina’s bed. Only she had the key, kept in her vast bosom. It had all the family cash, important documents . . . and Ettore’s personal papers.”

“Then she can have destroyed any letters that not nice!” I scream in my atrocious Italian.

“I don’t think so. . . .” They cringe in horror.

I blush. I’ve obviously made a faux pas. Well, I know that I would have destroyed anything a dead relative wouldn’t have wanted exposed, and Dorina was probably a far more determined person than me.

“And who carries these documents now?”

“Ettore Majorana.”

But I’d heard right. Ettore Majorana literally kept these documents.

I needed to go to Rome, to the institute where Ettore did his postgraduate work, where his mind first blossomed. The institute is in a bizarre location, at Via Panisperna. The street name sounds horribly like “pane e sperma,” “bread and sperm”—undoubtedly an odd type of sandwich. Particularly because whatever happened inside that building, it was certainly a hearty way to earn your bread.