In September of this year, the UK Government announced a partnership with the US to invest £65 (88$) million into the Deep Underground Neutrino Experiment (DUNE). Currently under construction in South Dakota, DUNE aims to study the unknown nature of the most prolific matter particle in the universe: the neutrino. The study of neutrinos will shed light on the magnitude of stellar explosions, how the forces of nature unify, the imbalance between anti-matter and matter and why galaxies, planets and humans came to exist in the first place.
This article contains a brief introduction to the idea of neutrinos and their history. If you’re already familiar with the basics or you’re only interested in the return you’re getting on your tax money, you can skip straight to Part II.
In the time it takes you to read this sentence, about five trillion particles from the sun will have passed straight through your body at the speed of light (regardless of whether it’s day or night). Almost none of them will take the courtesy of noticing you. This inconsiderate barrage of cosmic particles comes in the harmless form of neutrinos. Neutrinos are in today’s scientific spotlight because they’re still pretty mysterious to modern physics, partially because they’re so nefariously difficult to study. Israeli physicist Haim Harari once said “Neutrino physics is largely an art of learning a great deal by observing nothing.”
‘A terrible thing’
For supposed “nothing”, neutrinos caused a lot of grief (and excitement) for twentieth century physicists. The idea of a particle that does virtually nothing except fly off unnoticed, was conceived of because of the seemingly impossible radioactive decay in Lithium. In rare circumstances, particles called electrons are emitted from the very heart of an atom, its nucleus, in a process called “beta decay”. The energy of these electrons, provided that energy and momentum are conserved (as we were all taught in school), should have been a single, easily determined value. Instead, what physicists observed in the late 1920’s was that electrons came out with all sorts of energies, implying some of the energy was going missing. This suggested that either all knowledge of physics was hopelessly wrong (troubling for my job prospects to say the least) or that there was something else going on.
Enter Wolfgang Pauli: German, perfectionist, and Nobel prize-winning physicist (a term you’ll see a lot of in this article). In a letter addressed to the 1930 meeting of physicists in Tübingen (charmingly addressed ‘Dear Radioactive Ladies and Gentlemen’), he proposed that the missing energy was being carried by an electrically neutral, extremely light particle that he dubbed the Neutron. The disaster for physicists, was that being electrically neutral (having zero charge), this particle would be damned near impossible to find. Other than gravity, all experimental apparatus utilised the force of electromagnetism, which only affects charged particles. Thus, the history of Neutrino Physics begins with the holy verse from Pauli: ‘I have done a terrible thing, I have postulated a particle that cannot be detected.’
As an erroneous aside, the name Neutron would instead be given to the particle discovered by James Chadwick (Nobel prize winner) two years later, a situation remedied when (Nobel prize winner) Enrico Fermi changed the name of Pauli’s proposed particle to Neutrino (meaning “little neutral one” in Italian). Back to the action.
Thankfully, as our understanding of nuclear physics accumulated in the 1950s, Pauli did not turn out to be entirely correct. Although electromagnetism couldn’t detect neutrinos, by studying the nuclear processes in a reactor, the neutrino’s hallmark signature of an atomic collision (two high energy particles of light, called gamma rays) was measured by Frederick Reines and Clyde Cowan (also Nobel prize-winners, henceforth marked with a * ; the lesson here is if you want a Nobel Prize in Physics, predict or discover a particle). Careful observation revealed that an excess of these sorts of events was best explained by a neutrino colliding with the atoms in the reactor.
An important thing to note here is that nuclear processes are much weaker (in the physics lingo) than electromagnetic processes. This means that neutrinos interact extremely rarely with ordinary matter, making them tricky but not impossible to find. The calculated energies of neutrinos were consistent with them travelling at the speed of light, implying they had no mass whatsoever, under Einstein’s theory of Relativity.
The history of Physics is (as labelled by historians) a confounding stream of seemingly impossible chaos, and just as physicists finally had a handle on their existence, by the 1960’s, neutrinos (and their lack thereof) were already causing more trouble for Earthly scientists.
The schizophrenic particle
In the thirty odd years since the neutrino had been theoretically proposed, physicists had also been busying themselves modelling the internal processes of the Sun (the astronomical body, not the British newspaper, whose internal machinations are still unknown to modern physics). From the predictions of nuclear physics, it was clear that the majority of the atomic elements were formed in the sun by fusing different kinds of atoms together (add two Hydrogen particles for a Deuteron, add a Deuteron and Hydrogen to make Helium and so on). This elemental cascade also releases a plethora of neutrinos in the process.
The number of neutrinos being emitted from the sun from this palaver (remember nuclear processes can emit neutrinos) was thought to be well understood, and so Ray Davis* and friends* set out to measure the expected neutrino flux: the total number streaming in from the sun per second per unit area. The basic principle is that since neutrinos only interact in extremely rare nuclear processes with ordinary matter, you need to put a lot of stuff in their way to observe them. Experimenters filled the abandoned Homestake goldmine in South Dakota with a gigantic tank of chlorine and observed tiny flashes of light emitted by electrons, which were kicked out of atoms in neutrino interactions
Curiously though, they only seemed to find about a third as many neutrinos as they expected. Some people suggested that perhaps solar processes weren’t as well understood as we thought, but another possibility emerged: the neutrinos weren’t behaving as predicted.
The resolution, verified through thirty more years of concentrated effort analysing the properties of neutrinos from the atmosphere, deep space, the sun and nuclear reactors was this: neutrinos have a case of multiple personality disorder.
There are three different flavours of neutrino, each one associated with a partner particle, such as our old friend the electron and it’s heavier cousins the muon and the tau. Only the first kind, electron neutrinos, were the sort that Davis and company could measure in their detectors, but something completely unexpected seemed to be happening.
It turns out that neutrinos, being fundamental particles whose behaviour is best described with Quantum Mechanics, are prone to oscillating (wiggling) in such a way that when they travel, they can shift from one type into the other. This changes their fundamental character and how they interact with particles on Earth. Depending on how far they’ve travelled and what energy they started out with, a neutrino will have changed flavours, with a certain probability, into another. So Davis’ missing neutrinos were the electron neutrinos that had turned into muon and tau neutrinos, incapable of being measured at the Homestake mine.
That might all sound very abstract and… frankly kind of dull but the significance is that no other particle in the universe, as far as we know, is capable of this self-motivated, identity-changing behaviour. Particles are known to convert into one another in interactions with other particles, but the neutrino doesn’t need any such interference to undergo metamorphosis: it can spontaneously become something else. The experimental effort to verify this behaviour was recognised in 2015 when Arthur Mcdonald and Takaaki Kajita were awarded, you guessed it, the Nobel Prize in Physics.
This sort of behaviour is only possible if neutrinos have mass. Each of the three neutrino flavours lives an eclectic lifestyle where it has three different masses in varying proportions, and as the neutrino travels through space the contribution of these masses to the neutrino changes and alters its flavour, a state of affairs known as superposition.
Since most previous calculations with neutrinos presumed they had no mass whatsoever but still made accurate predictions, the mass of neutrinos must be extremely tiny. An atom only carries the mass of a billionth, of a billionth, of a billionth of a one gram. An electron is one thousandth the mass of an atom. And a neutrino? It has a mass less than one millionth that of an electron. That is, in technical terms, stupidly, mindbogglingly small.
So small, in fact, it raises questions about the nature of particle masses and why they vary so drastically in the laws of physics. What’s more, neutrinos, play a role in all sorts of other fundamental questions about the universe, and their story isn’t over yet. Click here for Part II.