The 50-year quest to seek out the particle that nearly broke physics

By Suzie Sheehy

The next is an extract from our Misplaced in House-Time publication. Every month, we hand over the keyboard to a physicist or two to inform you about fascinating concepts from their nook of the universe. You possibly can join Misplaced in House-Time at no cost right here.

Physicists have a powerful monitor file of world-changing discoveries: from the serendipitous discovery of X-rays in 1895, which remodeled drugs, to experiments within the Nineteen Twenties that verified quantum mechanics and enabled trendy computing, to the oblique spin-offs from huge particle collider experiments, together with the World Broad Net. After all, physics breakthroughs aren’t at all times helpful in the true world. And if there may be one discovery that epitomises the thought of curiosity-driven analysis with no eye on sensible purposes in any respect, it’s the 50-year-long quest to seek out the neutrino.

This story started with a thriller involving a kind of radioactivity often called beta decay. Within the early 1900s, physicists utilizing rudimentary detectors and harmful vials of radioactive substances discovered that beta decay appeared to violate momentum conservation. This was extraordinarily regarding. Momentum conservation is without doubt one of the most tightly held legal guidelines of physics, which states that the overall quantity of momentum in a system is fixed. In an atom present process beta decay there may be at first one object, the atom. Afterwards, there are two objects, the atom and the “beta particle” (i.e., an electron). The legislation of conservation of momentum dictates that the kinetic power carried away by the projectiles in a easy two-body system like this could take a predictable, distinctive worth. The 2 different kinds of radiation identified on the time, alpha and gamma radiation, obeyed this legislation properly, however in beta radiation the energies appeared random and unpredictable. Strive as they may, anybody who did such an experiment couldn’t get the info to return out another manner.

Each physicist had a unique opinion on what was occurring. Some, like Niels Bohr, contemplated throwing out the thought of momentum conservation, or at the least sneaking round it by proposing that on the tiny scales inside atoms, power may solely be conserved on common, not in each single decay. One theorist specifically, Wolfgang Pauli, was unable to set the thriller apart. Pauli was well-known for his vital and rational method, which led to his nickname “the scourge of God”. He wasn’t pleased with the suggestion of physicist Peter Debye, who instructed him at a gathering in Brussels, Belgium, to easily not take into consideration beta decay in any respect. Pauli was decided to save lots of momentum conservation and managed to provide you with a theoretical resolution, however to his horror it made the state of affairs even worse. “I have done a terrible thing,” he mentioned. “I have postulated a particle which cannot be detected.”

That particle was the neutrino, which Pauli first introduced to different physicists in a letter in 1930. Maybe, he urged, a tiny electrically impartial particle was carrying away the power? He felt it was so preposterous that he instructed his addressees he “dare not publish anything” about it. The issue was that Pauli predicted these particles haven’t any mass and no electrical cost, making it nearly unattainable for them to indicate up in an experiment.

By 1933 Enrico Fermi had dubbed the brand new particle the neutrino or “little neutral one” and submitted a fully-fledged idea to the journal Nature. It was rejected on the idea that it “contained speculations too remote from reality to be of interest to the reader”. A yr later in Manchester, UK, Rudolf Peierls and Hans Bethe calculated that the neutrinos created in beta decay might go by way of your complete earth with none interactions with matter. In truth, they may do the identical by way of portions of lead so thick it might be measured in gentle years. The neutrino may need solved the beta decay drawback in idea, however what use is a particle whether it is unattainable to detect so it could possibly’t be verified? For years, it was roughly ignored by experimentalists.

The issue sat that manner for 20 years. Lastly, within the Nineteen Fifties, Fred Reines at Los Alamos Laboratory in New Mexico determined to go after the elusive neutrino. He discovered a keen collaborator in colleague Clyde Cowan, a chemical engineer and former captain within the US Air Drive. The place Reines was a glowing extrovert, Cowan was extra measured, much less outgoing, however a superb experimentalist. They launched their venture in 1951, the core crew of 5 gathering in a stairwell round a cardboard signal with a hand-drawn brand of a staring eye and the phrases “Project Poltergeist”. Behind the signal, one in all them was inexplicably holding a big broom within the air. They appear in good spirits, as they’d have to be: their proposed experiment concerned constructing an unlimited tank, filling it with extraordinarily well-filtered and ready liquids, surrounding it in delicate electronics and hoping that they’d be capable to catch a particle that was nigh-on invisible.

After preliminary shoestring price range experiments gave tantalising however inconclusive outcomes, they realised they must transfer their experiment underground to keep away from the consequences of cosmic rays, ideally beneath a nuclear reactor – which might produce the neutrinos for the experiment. They discovered a basement space over on the Savannah River Web site in South Carolina, and the proprietor let the physicists arrange their experiment 12 metres beneath it. By late 1955, Undertaking Poltergeist was formally often called 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, whereas digital cables carried alerts to a trailer exterior.

The Savannah river experiment lasted for about 5 months. As soon as all of the chemistry and electronics had been labored out, all of it got here down merely to the cautious assortment of knowledge, flash by flash. The researchers had been full of hope every time they noticed, simply a couple of times every hour, the attribute sign of two flashes 5 microseconds aside, which whispered neutrino. Their eureka second got here not as a rush, however in a gradual accumulation of knowledge till there was little question left. When all was added up, there have been 5 instances as many neutrino alerts when the reactor was on in contrast with when it was off. From the 100 trillion (10¹⁴) neutrinos that the reactor emitted every second, that they had managed, towards the percentages, to design a system that might catch a couple of every hour and measure their interactions.

Twenty-five years after Pauli predicted a particle that might not be detected, Reines and Cowan and their crew had achieved the unattainable. “We are happy to inform you that we have definitely detected neutrinos”, they wrote in a telegram to Pauli, who interrupted the assembly he was attending on the CERN particle physics laboratory in Switzerland to learn it out loud and ship an impromptu mini lecture. Legend has it Pauli later polished off a complete case of champagne along with his buddies, which could clarify why his reply telegram by no means made it to Reines and Cowan. It learn “Everything comes to him who knows how to wait”.

Compared to a zippy electron that interacts with matter by way of the electromagnetic drive, or a neutron that interacts with atomic nuclei by way of the robust nuclear drive, the chargeless and nearly massless neutrino is sort of a barely perceptible puff of a particle that interacts with nearly nothing. Not like many different physics breakthroughs, we’ve no direct use for neutrinos in our each day lives. But many discoveries in physics had been untimely in contrast with the applied sciences of their day: the electron didn’t appear helpful at first and its discovery wasn’t geared toward telecommunications and computing. Particle accelerators weren’t invented to supply medical isotopes or to deal with most cancers. Nobody was eagerly awaiting these developments besides the physicists who made them, and even then the discoveries weren’t at all times intentional. Whereas it’s probably that neutrinos won’t ever be as straight helpful as electrons, the data we’ve gleaned from them is essential and – extremely – there are a couple of doable purposes within the pipeline.

The primary makes use of for neutrinos had been for physics researchers. Later experiments confirmed that there are various sources of neutrinos on the market within the universe, together with our solar. In 1987, neutrino bursts from a supernova had been detected by a number of experiments, giving rise to a brand new subject of neutrino astronomy. Confirming our understanding of how neutrinos kind within the solar additionally helped solidify our data of nuclear physics, required for fusion reactors, which can present considerable electrical power on Earth in future. They could additionally sooner or later assist us in designing particle accelerators: past our galaxy, extraordinarily high-energy particles are created out in area and it’s extremely probably that neutrinos will sooner or later be the messengers that train us how these cosmic particle accelerators work, maybe giving us a mechanism to repeat in our laboratories right here on Earth.

Within the Boulby mine within the north of England, a UK-US collaboration is at present constructing a brand new experiment known as WATCHMAN (Water Cherenkov Monitor for Antineutrinos). This venture will use a neutrino detector to observe nuclear fission reactors remotely. The venture might present a singular contribution to international safety by making a dependable manner of checking whether or not reactors are compliant with non-proliferation treaties. As a result of neutrinos are so arduous to cease, there may be merely no manner of hiding an working nuclear reactor from a detector like this.

Additional sooner or later, there could also be direct purposes of neutrinos and the data we’ve about them. Due to their capability to cowl huge cosmic distances at nearly the pace of sunshine with out hindrance, neutrinos might even sooner or later turn out to be a type of cosmic messaging

system. If there are any superior civilisations on the market residing on one of many hundreds of exoplanets that we’ve found, neutrinos may nicely be the best way they convey with one another. In 2012, a neutrino experiment known as MINERvA (Major Injector Neutrino ExpeRiment to check v-A interactions) on the Fermi Nationwide Accelerator Laboratory in Illinois tried this out. The researchers encoded a beam of neutrinos with a message, despatched it by way of half a mile of rock to a detector and efficiently decoded it once more. This is also helpful on Earth, for submarines attempting to speak by way of water, as an illustration, the place radio waves get distorted by obstacles. With neutrinos they may talk not simply by way of water but in addition straight by way of the centre of the earth in a direct line.

It’s truthful to say that neutrinos usually are not fairly prepared to make use of but, and maybe they by no means can be. We can not predict the longer term, however what we will say about neutrinos is that the end result of our quest to grasp them has contributed to our lives in oblique, however profound, methods. One of many key neutrino experiments, the Sudbury Neutrino Observatory (SNO) is situated in a deep underground laboratory in Canada, which has now been expanded and renamed SNOLAB. Once they say deep underground they actually imply it: at 2100 metres under floor, the laboratory is situated twenty instances deeper than the Giant Hadron Collider in Switzerland. The air strain will increase by 20 per cent as you’re taking the 6-minute journey down within the raise, which feels just a little like descending in an aeroplane whereas surrounded by rock.

The underground lab is not only host to particle physicists. Its creation opened up potentialities in lots of different areas of science. Being so deep within the earth, it’s a distinctive setting as a result of the laboratory has an extremely low degree of background radiation from cosmic rays. The existence of a secure, clear underground facility with such low radiation ranges has enabled a broad analysis programme wanting on the influence of low radiation ranges on cells and organisms. No land-dwelling animals have ever lived – or for that matter advanced – with out publicity to background radiation from cosmic rays, so these experiments are serving to biologists perceive what the influence is whenever you take away this radiation.

That is essential as a result of it could reply the query of whether or not radiation is at all times dangerous for cells and organisms, whether or not it at all times causes harm, or if there may be some threshold degree of radiation which is innocent or probably even helpful to life. It might inform us extra about whether or not evolution is influenced by the random mutations attributable to radiation. Up to now, the outcomes appear to point that life truly wants a low degree of radiation. If additional experiments validate this, it has huge implications not only for people and our interactions with radiation, but in addition for our understanding of the existence of life elsewhere within the cosmos. With out deep underground labs, we merely couldn’t do that analysis.

SNOLAB additionally occurs to be the most effective locations on (or in?) Earth to run experiments on quantum computer systems. There may be rising proof that the decoherence time – that’s, the time for which a quantum bit can retailer info earlier than it loses it – could also be restricted by pure background radiation on the floor of the earth. Sooner or later, it could be essential to run quantum computer systems underground. For now, at the least, these laboratories present a uncommon area for this improvement work.

The neutrino has been known as a ghost, a messenger, a spaceship, a wisp of nothing. It began life as an apology to save lots of a primary legislation of physics and over time it led to huge payoffs in astronomy, cosmology, geology and our most elementary understanding of matter. What’s extra, neutrinos have raised numerous questions as we’ve discovered extra about them: we nonetheless don’t know why neutrinos have a tiny mass, as a substitute of none.

The neutrino, small as it’s, seems to be a billion instances extra considerable within the universe than the matter that makes up stars, galaxies and us. It has pushed experimenters and theorists alike to ever higher heights, or technically depths, to unravel its secrets and techniques. Paradoxically, in saving one primary legislation of physics, the neutrino is now one of many richest sources of information gaps in physics. It affirms that there’s a lot about our universe that we’re but to find.

Suzie Sheehy’s guide, The Matter of All the pieces: Twelve experiments that modified our world, is obtainable now within the UK and Australia, and within the US and Canada from 10 January 2023.