The 50-year quest to find the particle that almost broke physics

When physicists first discovered a form of a radioactivity called beta decay it seemed to violate the laws of physics. It took 50 years to work out what was going on

 

Wolfgang Pauli postulated a particle that he believed couldn’t be detected. Shutterstock/Unwind

Physicists have an impressive track record of world-changing discoveries: from the serendipitous discovery of X-rays in 1895, which transformed medicine, to experiments in the 1920s that verified quantum mechanics and enabled modern computing, to the indirect spin-offs from enormous particle collider experiments, including the World Wide Web. Of course, physics breakthroughs aren’t always useful in the real world. And if there is one discovery that epitomises the idea of curiosity-driven research with no eye on practical applications at all, it is the 50-year-long quest to find the neutrino.

 

This story began with a mystery involving a type of radioactivity known as beta decay. In the early 1900s, physicists using rudimentary detectors and dangerous vials of radioactive substances found that beta decay appeared to violate momentum conservation. This was extremely concerning. Momentum conservation is one of the most tightly held laws of physics, which states that the total amount of momentum in a system is constant. In an atom undergoing beta decay there is at first one object, the atom. Afterwards, there are two objects, the atom and the “beta particle” (i.e., an electron). The law of conservation of momentum dictates that the kinetic energy carried away by the projectiles in a simple two-body system like this should take a predictable, unique value. The two other types of radiation known at the time, alpha and gamma radiation, obeyed this law nicely, but in beta radiation the energies seemed random and unpredictable. Try as they might, anyone who did such an experiment couldn’t get the data to come out any other way.

 

Every physicist had a different opinion on what was going on. Some, like Niels Bohr, contemplated throwing out the idea of momentum conservation, or at least sneaking around it by proposing that on the tiny scales inside atoms, energy might only be conserved on average, not in every single decay. One theorist in particular, Wolfgang Pauli, was unable to set the mystery aside. Pauli was well known for his critical and rational approach, which led to his nickname “the scourge of God”. He wasn’t happy with the suggestion of physicist Peter Debye, who told him at a meeting in Brussels, Belgium, to simply not think about beta decay at all. Pauli was determined to save momentum conservation and managed to come up with a theoretical solution, but to his horror it made the situation even worse. “I have done a terrible thing,” he said. “I have postulated a particle which cannot be detected.”

 

That particle was the neutrino, which Pauli first presented to other physicists in a letter in 1930. Perhaps, he suggested, a tiny electrically neutral particle was carrying away the energy? He felt it was so preposterous that he told his addressees he “dare not publish anything” about it. The problem was that Pauli predicted these particles have no mass and no electric charge, making it virtually impossible for them to show up in an experiment.

 

By 1933 Enrico Fermi had dubbed the new particle the neutrino or “little neutral one” and submitted a fully-fledged theory to the journal Nature. It was rejected on the basis that it “contained speculations too remote from reality to be of interest to the reader”. A year later in Manchester, UK, Rudolf Peierls and Hans Bethe calculated that the neutrinos created in beta decay could pass through the entire earth without any interactions with matter. In fact, they could do the same through quantities of lead so thick it would be measured in light years. The neutrino might have solved the beta decay problem in theory, but what use is a particle if it is impossible to detect so it can’t be verified? For years, it was more or less ignored by experimentalists.

 

The problem sat that way for two decades. Finally, in the 1950s, Fred Reines at Los Alamos Laboratory in New Mexico decided to go after the elusive neutrino. He found a willing collaborator in colleague Clyde Cowan, a chemical engineer and former captain in the US Air Force. Where Reines was a sparkling extrovert, Cowan was more measured, less outgoing, but a brilliant experimentalist. They launched their project in 1951, the core team of five gathering in a stairwell around a cardboard sign with a hand-drawn logo of a staring eye and the words “Project Poltergeist”. Behind the sign, one of them was inexplicably holding a large broom in the air. They look in good spirits, as they’d need to be: their proposed experiment involved building an enormous tank, filling it with extremely well-filtered and prepared liquids, surrounding it in delicate electronics and hoping that they’d be able to catch a particle that was nigh-on invisible.

 

After initial shoestring budget experiments gave tantalising but inconclusive results, they realised they would have to move their experiment underground to avoid the effects of cosmic rays, preferably underneath a nuclear reactor – which would produce the neutrinos for the experiment. They found a basement area over at the Savannah River Site in South Carolina, and the owner let the physicists set up their experiment 12 metres beneath it. By late 1955, Project Poltergeist was formally known as the Savannah River Neutrino Experiment. The set-up had grown to a three-layered sandwich of scintillating liquid and detectors, its rectangular tanks weighing in at a whopping 10 tonnes. The detector sat beneath the reactor, shrouded in layers of wax and concrete shielding, while electronic cables carried signals to a trailer outside.

 

The Savannah River experiment lasted for about five months. Once all the chemistry and electronics were worked out, it all came down simply to the careful collection of data, flash by flash. The researchers were filled with hope each time they saw, just once or twice each hour, the characteristic signal of two flashes 5 microseconds apart, which whispered neutrino. Their eureka moment came not as a rush, but in a gradual accumulation of data until there was no doubt left. When all was added up, there were five times as many neutrino signals when the reactor was on compared with when it was off. From the 100 trillion (1014) neutrinos that the reactor emitted each second, they had managed, against the odds, to design a system that could catch a few each hour and measure their interactions.

 

Twenty-five years after Pauli predicted a particle that could not be detected, Reines and Cowan and their team had achieved the impossible. “We are happy to inform you that we have definitely detected neutrinos”, they wrote in a telegram to Pauli, who interrupted the meeting he was attending at the CERN particle physics laboratory in Switzerland to read it out loud and deliver an impromptu mini lecture. Legend has it Pauli later polished off an entire case of champagne with his friends, which might explain why his reply telegram never made it to Reines and Cowan. It read “Everything comes to him who knows how to wait”.

 

In comparison to a zippy electron that interacts with matter via the electromagnetic force, or a neutron that interacts with atomic nuclei via the strong nuclear force, the chargeless and almost massless neutrino is like a barely perceptible puff of a particle that interacts with almost nothing. Unlike many other physics breakthroughs, we have no direct use for neutrinos in our daily lives. Yet many discoveries in physics were premature compared with the technologies of their day: the electron didn’t seem useful at first and its discovery wasn’t aimed at telecommunications and computing. Particle accelerators weren’t invented to produce medical isotopes or to treat cancer. No one was eagerly awaiting these developments except the physicists who made them, and even then the discoveries weren’t always intentional. While it’s likely that neutrinos will never be as directly useful as electrons, the knowledge we have gleaned from them is important and – incredibly – there are a few possible applications in the pipeline.

 

The first uses for neutrinos were for physics researchers. Later experiments confirmed that there are many sources of neutrinos out there in the universe, including our sun. In 1987, neutrino bursts from a supernova were detected by multiple experiments, giving rise to a new field of neutrino astronomy. Confirming our understanding of how neutrinos form in the sun also helped solidify our knowledge of nuclear physics, required for fusion reactors, which may provide abundant electrical energy on Earth in future. They may also one day help us in designing particle accelerators: beyond our galaxy, extremely high-energy particles are created out in space and it is highly likely that neutrinos will one day be the messengers that teach us how those cosmic particle accelerators work, perhaps giving us a mechanism to copy in our laboratories here on Earth.

 

In the Boulby mine in the north of England, a UK-US collaboration is currently building a new experiment called WATCHMAN (Water Cherenkov Monitor for Antineutrinos). This project will use a neutrino detector to monitor nuclear fission reactors remotely. The project could provide a unique contribution to global security by creating a reliable way of checking whether reactors are compliant with non-proliferation treaties. Because neutrinos are so hard to stop, there is simply no way of hiding an operating nuclear reactor from a detector like this.

 

Further in the future, there may be direct applications of neutrinos and the knowledge we have about them. Because of their ability to cover vast cosmic distances at almost the speed of light without hindrance, neutrinos could even one day become a kind of cosmic messaging

 

system. If there are any advanced civilisations out there living on one of the thousands of exoplanets that we have discovered, neutrinos might well be the way they communicate with each other. In 2012, a neutrino experiment called MINERvA (Main Injector Neutrino ExpeRiment to study v-A interactions) at the Fermi National Accelerator Laboratory in Illinois tried this out. The researchers encoded a beam of neutrinos with a message, sent it through half a mile of rock to a detector and successfully decoded it again. This could also be useful on Earth, for submarines trying to communicate through water, for instance, where radio waves get distorted by obstacles. With neutrinos they could communicate not just through water but also straight through the centre of the earth in a direct line.

 

It’s fair to say that neutrinos are not quite ready to use yet, and perhaps they never will be. We cannot predict the future, but what we can say about neutrinos is that the outcome of our quest to understand them has contributed to our lives in indirect, but profound, ways. One of the key neutrino experiments, the Sudbury Neutrino Observatory (SNO) is located in a deep underground laboratory in Canada, which has now been expanded and renamed SNOLAB. When they say deep underground they really mean it: at 2100 metres below ground, the laboratory is located twenty times deeper than the Large Hadron Collider in Switzerland. The air pressure increases by 20 per cent as you take the 6-minute journey down in the lift, which feels a little like descending in an aeroplane while surrounded by rock.

 

The underground lab is not just host to particle physicists. Its creation opened up possibilities in many other areas of science. Being so deep in the earth, it is a unique environment because the laboratory has an incredibly low level of background radiation from cosmic rays. The existence of a stable, clean underground facility with such low radiation levels has enabled a broad research programme looking at the impact of low radiation levels on cells and organisms. No land-dwelling animals have ever lived – or for that matter evolved – without exposure to background radiation from cosmic rays, so these experiments are helping biologists understand what the impact is when you remove this radiation.

 

This is important because it may answer the question of whether radiation is always bad for cells and organisms, whether it always causes damage, or if there is some threshold level of radiation which is harmless or possibly even beneficial to life. It could tell us more about whether evolution is influenced by the random mutations caused by radiation. So far, the results seem to indicate that life actually needs a low level of radiation. If further experiments validate this, it has enormous implications not just for humans and our interactions with radiation, but also for our understanding of the existence of life elsewhere in the cosmos. Without deep underground labs, we simply couldn’t do this research.

 

SNOLAB also happens to be one of the best places on (or in?) Earth to run experiments on quantum computers. There is emerging evidence that the decoherence time – that is, the time for which a quantum bit can store information before it loses it – may be limited by natural background radiation on the surface of the earth. In the future, it may be necessary to run quantum computers underground. For now, at least, these laboratories provide a rare space for this development work.

 

The neutrino has been called a ghost, a messenger, a spaceship, a wisp of nothing. It started life as an apology to save a basic law of physics and over time it led to enormous payoffs in astronomy, cosmology, geology and our most fundamental understanding of matter. What’s more, neutrinos have raised countless questions as we’ve learned more about them: we still don’t know why neutrinos have a tiny mass, instead of none.

 

The neutrino, small as it is, turns out to be a billion times more abundant in the universe than the matter that makes up stars, galaxies and us. It has driven experimenters and theorists alike to ever greater heights, or technically depths, to unravel its secrets. Ironically, in saving one basic law of physics, the neutrino is now one of the richest sources of knowledge gaps in physics. It affirms that there is so much about our universe that we are yet to discover.