The Hunt for the Elusive Neutrino

About 100 trillion neutrinos just passed through your body a second ago. Did you feel them? Neutrinos are one of the most abundant particles in the universe, but they’re also the most elusive. They can pass through just about anything, including your body, without being noticed. Now, imagine if we could harness this power. Imagine the possibilities if you could control a particle that can pass through anything undetected.

While an intriguing idea, it’s this exact characteristic that kept neutrinos from being detected for nearly half a century after physicist James Chadwick first theorized their existence back in 1914.

Dr. Chadwick was studying nuclear radiation, a chemical reaction that causes atomic nuclei (the stuff that makes elements like carbon and gold what they are) to spontaneously break apart and release the energy contained within them. He was studying a specific kind of radiation called beta decay, which is when a neutron in an atomic nucleus breaks down into a positively charged proton and a negatively charged electron. But during his experiments, Chadwick observed that when a neutron broke down through beta decay, the resulting products did not equal the sum of its parts: during the decay, the system lost energy. Some of the energy stored in the neutron seemed to have disappeared!

At first glance, Chadwick’s experiment appeared to violate one of the most fundamental concepts in physics, the law of conservation of energy. This law states that energy is never created or destroyed; it only changes from one form to another. But when Chadwick’s neutrons lost energy, it appeared that energy had been destroyed.

This is a big deal. Breaking a physical law of the universe is not like getting a parking ticket; it simply isn’t possible to break them. If you park illegally and you get a ticket, the law is still there and the universe goes on the way it was. If a physical law is broken, the law itself changes, and our understanding of how the universe works changes along with it. It’s like if you sued over your parking ticket and the suit reached the supreme court, where they overturned precedent and make it illegal for states to limit where people could park. Imagine the waves that would make.

So, Chadwick’s discovery – that the products of beta decay (the proton and electron) together carry a smaller amount of energy than the original neutron – caused great controversy. In the following years, other physicists replicated Chadwick’s experiments several times, only to confirm that, indeed, neutrons lose energy when they break down during beta decay. This confusing discovery seemed to be real.

Meanwhile, physicists didn’t just assume that Chadwick disproved the law of conservation, which was first described by Émilie du Châtelet in 1756. Instead, they figured that Chadwick must have been missing something. Something else must have taken up the remaining energy.

To explain the missing energy, Wolfgang Pauli proposed in 1930 that there must be another particle, with barely any mass and no electrical charge, that is emitted during beta decay along with the electron and proton. Two years later, Enrico Fermi named this particle the “neutrino” (Italian for “little neutron”) and included it in his newly developed theory about beta decay.

Pauli and Fermi solved one problem only to introduce another: because they theorized that the neutrino doesn’t have any electrical charge and very little, if any, mass, everyone thought it couldn’t be detected. Place a magnet next to it and nothing would happen. Place it on a scale and it wouldn’t register. Shine a light on it, whether it be ultraviolet, infrared, or visible light, and it wouldn’t reflect anything back. In theory, it was almost invisible.

In fact, using Fermi’s theory, the physicists Hans Bethe and Rudolf Peierls calculated that because it’s so small, virtually massless, and uncharged, a single neutrino could easily go through Earth without interacting with anything. Because it doesn’t interact with anything or emit any sort of detectable radiation, Bethe and Peierls stated in 1934 that “there is no practically possible way of observing the neutrino.” With this assertion, these gentlemen threatened o put an end to 20 years of research dedicated to finding this elusive particle.

It’s not often that scientists give up on something so substantial. Most spend decades hunting down obscure answers to questions that only give rise to several additional questions.  This mysterious subatomic particle just boggled so many minds that everyone was ready to give up.

Fortunately though, the hunt wasn’t abandoned forever. A few years later, American physicists Frederick Reines and Clyde Cowan took up the challenge of trying to figure out how to detect the neutrino. To observe the neutrino in action, they decided to take a completely new approach: Instead of trying to detect the presence of a few neutrinos during beta decay, they decided to look at the opposite reaction where neutrinos and protons combine to produce neutrons. Using this method, all they needed was a known source of neutrinos, such as a nuclear reaction.

They first thought they’d try to capture neutrinos released during a hydrogen bomb blast – an explosion that releases a bunch of neutrinos…and enough energy to pulverize anything within miles. (It was the early 50’s. The Cold War was happening. These bombs were being tested often back then.)

With a little persuasion from a colleague who was concerned that their work would kill them, Reines and Cowan changed plans and decided to use a safer kind of neutrino-generating apparatus: a nuclear reactor where the neutrinos would pass through two 50-gallon tanks of water.

The nuclear reactor released ten trillion neutrinos per second per square centimeter. If each neutrino were a drop of rain, that would equate to the worst rainstorm of your life – 132 million gallons of water falling on each centimeter of your head every second. That’s 175 Niagara Falls’ pouring over an area the size of your fingertip every second. Talk about flooding.

Anyway, in the water tanks, they expected the neutrinos from their reactor to interact with protons in the water to produce positrons and neutrons. Positrons are another funny little subatomic particle — they’re basically electrons, except instead of having a negative charge, they have a positive charge. They predicted that the chance of this reaction happening would be extremely rare, even with so many neutrinos in their giant water tanks. But they also knew that this reaction produced unique products that, if they were to be detected, they would be an obvious sign that neutrinos were there.

Once a neutrino entered the water, it would meet up with the hydrogen atoms in there to produce the positron and neutron. Next, the positron (which I mentioned is basically a positively-charged electron), would join its negatively-charged cousin, with whom it is in a family spat. The fighting is so intense, in fact, that they annihilate each other, a reaction that releases a pair of high-energy gamma rays.

Meanwhile, the poor little neutron is sitting all by his lonesome. But Cowan and Reines, the devilish matchmakers that they were, put some cadmium in the water, an element that really likes neutrons. They surmised that the neutrons would jump on cadmium’s back and ride off into the sunset, leaving behind another gamma ray in the process, just a fraction of a second later.

The physicists knew that if they could detect these three gamma rays, two at first, followed by a third, that would mean they detected a sign that neutrinos were there.

Cowan and Reines turned on their reactor and they saw two gamma rays show up. Then another. Then they saw two more gamma rays followed by another. This kept happening! Finally, 42 years after Chadwick first theorized the existence of the neutrino, Reines and Cowan did it. They were seeing evidence of neutrinos!

Why Should We Care About Neutrinos?

Neutrinos’ ghostly nature made them hard to find, but it might make them valuable in future technologies. Since neutrinos can travel through the Earth’s core without getting the least bit stuck or slowed down, physicists are trying to develop methods to use neutrino beams as a way to send messages directly from one point on Earth to another. Imagine that: instead of running electrical wires or optic cables under the ocean, where they can easily get damaged by a passing ship, we might be able to send information through the center of the Earth to the other side simply using a neutrino beam. We are still far away from seeing anything like this in real life, as we do not yet have the technology to reliably control and detect neutrinos.

When stars blow up (that is, when they turn into supernovas, like the one pictured at the top of this article), they release tons of neutrinos. The Big Bang released a bunch, too (and scientists just found some of these original ones). There is still a lot to learn about what neutrinos are and how they work, and just over 100 years into the search, we’re only at the beginning!