Before there were gravitational waves, multi-messenger astronomy got its start with the neutrino.

Sometimes, the best-designed experiments fail. The effect you’re looking for might not even occur, meaning that a null result should always be a possible outcome you’re prepared for. When that happens, the experiment is often dismissed as a failure, even though you never would have known the results without performing it.

Yet, every once in a while, the apparatus that you build might be sensitive to something else entirely. When you do science in a new way, at a new sensitivity, or under new, unique conditions, that’s often where the most surprising, serendipitous discoveries are made. In 1987, a failed experiment for detecting proton decay detected neutrinos, for the first time, from beyond not only our Solar System, but from outside of the Milky Way. This is how neutrino astronomy was born.

The conversion of a neutron to a proton, an electron, and an anti-electron neutrino is how Pauli hypothesized resolving the energy non-conservation problem in beta decay. (JOEL HOLDSWORTH)

The neutrino is one of the greatest success stories in theoretical physics history. There were three forms of radioactive decay recognized in the early twentieth century:

  • A bigger atom emits a helium nucleus, which jumps two elements down the periodic table.
  • When an atomic nucleus emits a high-energy electron, a new element is added to the periodic table.
  • Gamma decay occurs when an atomic nucleus produces a high-energy photon while staying stationary on the periodic table.

Whatever the total energy and momentum of the original reactants are, the energy and momentum of the end products must equal under the principles of physics. They always did for alpha and gamma decays. But what about beta decays? Never. There was constantly a loss of energy.

The V-shaped track in the center of the image is likely a muon decaying to an electron and two neutrinos. The high-energy track with a kink in it is evidence of a mid-air particle decay. This decay, if the (undetected) neutrino is not included, would violate energy conservation. (THE SCOTTISH SCIENCE & TECHNOLOGY ROADSHOW)

Wolfgang Pauli suggested a new particle, the neutrino, to address the issue in 1930. This tiny, neutral particle may carry both energy and motion, but it would be impossible to detect. It wouldn’t absorb or emit light, and only very seldom interact with atomic nuclei.

Pauli felt humiliated after receiving the proposal, rather than confident and thrilled. “I have done a horrible thing,” he claimed, “I have proposed a particle that cannot be discovered.” Despite his doubts, experiment proved the hypothesis correct.

Reactor nuclear experimental RA-6 (Republica Argentina 6), en marcha, showing the characteristic Cherenkov radiation from the faster-than-light-in-water particles emitted. The neutrinos (or more accurately, antineutrinos) first hypothesized by Pauli in 1930 were detected from a similar nuclear reactor in 1956. (CENTRO ATOMICO BARILOCHE, VIA PIECK DARÍO)

Neutrinos (or, more precisely, antineutrinos) were first directly discovered as part of nuclear reactor output in 1956. When neutrinos collide with an atomic nucleus, one of two things happens:

  • They either scatter and produce a recoil, like a billiard ball colliding with other billiard balls, or they scatter and cause a recoil, like a billiard ball colliding with other billiard balls.
  • Alternatively, they may produce the emission of new particles with their own energies and momenta.
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In any case, you may construct specialized particle detectors and search for neutrinos in the areas where you anticipate them to interact. The first neutrinos were discovered in this way: around the margins of nuclear reactors, particle detectors sensitive to neutrino signals were built. After all, energy is preserved if you reassembled the total energy of the products, including neutrinos.

Schematic illustration of nuclear beta decay in a massive atomic nucleus. Only if the (missing) neutrino energy and momentum is included can these quantities be conserved. (WIKIMEDIA COMMONS USER INDUCTIVELOAD)

Neutrinos should be created anywhere nuclear processes occur, such as in the Sun, stars, and supernovae, and anytime a high-energy cosmic ray collides with a particle in Earth’s atmosphere, according to theory. Physicists began creating neutrino detectors in the 1960s to search for both solar (from the Sun) and atmospheric (from cosmic ray) neutrinos.

This neutrino detecting technique would be encircled by a significant volume of material with mass intended to interact with the neutrinos inside it. The neutrino detectors were installed far below, in mines, to protect them from other particles. Only neutrinos should be able to enter the mines; the rest should be absorbed by the Earth. Both solar and atmospheric neutrinos had been discovered by the end of the 1960s.

The Homestake Gold Mine sits wedged in the mountains in Lead, South Dakota. It began operation over 123 years ago, producing 40 million ounces of gold from the 8,000 foot deep underground mine and mill. In 1968, the first Solar neutrinos were detected at an experiment here, devised by John Bahcall and Ray Davis. (Jean-Marc Giboux/Liaison)

The particle detection technique developed for neutrino investigations and high-energy accelerators was discovered to be relevant to another phenomenon: proton decay detection. While the proton is predicted to be totally stable in the Standard Model of particle physics, it may decay into lighter particles in various extensions, such as Grand Unification Theories.

When a proton decays, it should generate lower-mass particles at very high speeds, according to theory. You can rebuild the whole energy and check whether it originated from a proton if you can detect the energies and momenta of those fast-moving particles.

High-energy particles can collide with others, producing showers of new particles that can be seen in a detector. By reconstructing the energy, momentum, and other properties of each one, we can determine what initially collided and what was produced in this event.(FERMILAB)

Protons must have a very long lifespan if they disintegrate. The Universe is 1010 years old, yet the lifespan of a proton must be far longer. How much longer do we have? The idea is to look at a large number of protons rather than just one. Whether a proton has a lifespan of 1030 years, you may either take a single proton and wait that long (a horrible idea), or you can take 1030 protons and wait one year to check if any decay has occurred.

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In a liter of water, there are around 1025 molecules, each of which includes two hydrogen atoms: a proton orbited by an electron. If the proton is unstable, a big enough tank of water surrounded by a large number of detectors should enable you to quantify or restrict its stability/instability.

A schematic layout of the KamiokaNDE apparatus from the 1980s. For scale, the tank is approximately 15 meters (50 feet) tall. (©JNN / WIKIMEDIA COMMONS)

In the Kamioka mines in Japan, work on a massive underground detector started in 1982. The detector was given the moniker KamiokaNDE, which stands for Kamioka Nucleon Decay Experiment. It could store over 3,000 tons of water and had over a thousand detectors designed to detect the radiation emitted by fast-moving particles.

The detector had been working for years by 1987, with no evidence of proton decay. This null conclusion entirely excluded the most prevalent model among Grand Unified Theories, with roughly 1033 protons in that tank. As far as we can determine, the proton does not decay. The basic goal of KamiokaNDE was to fail.

A supernova explosion enriches the surrounding interstellar medium with heavy elements. The outer rings are caused by previous ejecta, long before the final explosion. This explosion also emitted a huge variety of neutrinos, some of which made it all the way to Earth. (ESO / L. CALÇADA)

However, something unexpected occurred. A huge star in a Milky Way satellite galaxy had reached the end of its life and erupted in a supernova 165,000 years before. The first time that light reached Earth was on February 23, 1987.

But something amazing occurred at KamiokaNDE a few hours before the light came: a total of 12 neutrinos arrived in a timeframe of around 13 seconds. The nuclear reactions that produce neutrinos are abundant in supernovae, as shown by two bursts, the first of which had 9 neutrinos and the second of which contained 3.

Three different detectors observed the neutrinos from SN 1987A, with KamiokaNDE the most robust and successful. The transformation from a nucleon decay experiment to a neutrino detector experiment would pave the way for the developing science of neutrino astronomy. (INSTITUTE FOR NUCLEAR THEORY / UNIVERSITY OF WASHINGTON)

We have discovered neutrinos from outside our Solar System for the first time. Neutrino astronomy was a new field of study. A number of ground-based and space-based observatories saw the light from that supernova, now known as SN 1987A, over the following several days in a wide range of wavelengths. Based on the minute gap between the neutrinos’ time-of-flight and the light’s arrival time, we discovered that neutrinos:

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That 165,000 light years were moved at a pace that was indistinguishable from the speed of light.

that they might have a mass of little more than 1/30,000th that of an electron,

and that neutrinos, unlike light, are not slowed as they travel from the core of the collapsing star to its photospher.

We may still investigate this supernova remnant and observe how it has developed after more than 30 years.

The outward-moving shockwave of material from the 1987 explosion continues to collide with previous ejecta from the formerly massive star, heating and illuminating the material when collisions occur. A wide variety of observatories continue to image the supernova remnant today. (NASA, ESA, AND R. KIRSHNER (HARVARD-SMITHSONIAN CENTER FOR ASTROPHYSICS AND GORDON AND BETTY MOORE FOUNDATION) AND P. CHALLIS (HARVARD-SMITHSONIAN CENTER FOR ASTROPHYSICS))

This result’s scientific significance cannot be emphasized. It was the first direct discovery of gravitational waves from merging black holes, just as it was the first direct detection of gravitational waves from merging black holes. It was the beginning of multi-messenger astronomy, with the first observation of the same object in both electromagnetic radiation (light) and another way (neutrinos).

It demonstrated the use of big underground tanks for detecting cosmic occurrences. And it makes us hopeful that, one day, we will see the ultimate observation: an event in which light, neutrinos, and gravitational waves all collide to tell us about the workings of the things in our Universe.

The ultimate event for multi-messenger astronomy would be a merger of either two white dwarfs or two neutrons stars that was close enough. If such an event occurred in near-enough proximity to Earth, neutrinos, light, and gravitational waves could all be detected. (NASA, ESA, AND A. FEILD (STSCI))

Most cleverly, it resulted in KamiokaNDE being renamed. Because the Kamioka Nucleon Decay Experiment was a complete failure, KamiokaNDE was no longer an option. However, the spectacular observation of neutrinos from SN 1987A inspired the creation of a new observatory: the Kamioka Neutrino Detector Experiment (KamiokaNDE). This has been renovated many times over the last 30 years, and comparable facilities have sprung up all around the globe.

We would be treated to upwards of 10,000 neutrinos arriving in our detector if a supernova exploded today in our own galaxy. All of them, taken together, have reduced the lifespan of the proton to less than 1035 years, but that is not why we create them. Neutrinos race across the Universe whenever a high-energy catastrophe happens. Neutrino astronomy is alive, healthy, and ready for anything the universe throws at us now that our detectors are online.

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