They’re small, nearly imperceptible, and there are 500 trillion of them passing through you right now. Neutrinos are among nature’s most plentiful yet mysterious creations. Science writer James Riordon recently set out to list what was known versus unknown about neutrinos, and he found the second column was longer. “To me, the most interesting thing is how we know surprisingly little about them,” he says. “These are definitely here and definitely mysterious. The exciting science lies in answering these questions.”
In the new book Ghost Particle: In Search of the Elusive and Mysterious Neutrino (MIT Press, 2023), Riordon and his co-author, physicist Alan Chodos, document how the surprising particles were first proposed and discovered and what scientists have figured out so far—plus everything they hope to eventually understand. Because of their many oddities, neutrinos seem like promising conduits for answering some of our biggest questions: Why is the universe made of matter and not antimatter? What is dark matter? And can anything travel faster than light?
Scientific American spoke to Riordon about why these bizarre bits of nature are so cool and how his own family history fits into the story of neutrinos.
[An edited transcript of the interview follows.]
So you actually have a personal connection to neutrinos. What is it?
I am the grandson of one of the co-discoverers of neutrinos, Clyde Cowan, Jr. But he passed away when I was nine. There was always a mythology in my family about him, but it wasn’t really clear what he had done. It wasn’t something I understood until I went to study physics in college. My interest developed more as I became a science writer and started seeing these interesting neutrino results coming out.
I talked to MIT Press about doing a book, and they were interested, but they wanted to make sure there was an expert in the field writing with me. I thought of Alan Chodos, a theoretician who thinks outside the box. He has written some interesting speculations about neutrinos that are a little bit on the edge.
Which of the myriad of questions neutrinos pose intrigues you the most?
My favorite mystery is the determination of whether or not it’s its own antiparticle. To me, I think that’s the biggest and most dramatic question about neutrinos. That one touches on the really big question of the origin of the universe.
If a neutrino does turn out to be its own antiparticle, it could allow us to understand why the universe is made of mostly matter and not antimatter. We know that when the universe first began, it had to be a perfect balance of matter and antimatter. There’d be no matter left if all the matter and the antimatter in the universe had just annihilated. So an imbalance had to arrive somewhere, and neutrinos could be a clue as to the source of that imbalance.
You write, “The very idea of neutrinos was a terrible thing, in the words of the first person who imagined it.” Wolfgang Pauli proposed neutrinos in 1930 to explain why there seemed to be missing energy and momentum in a certain type of particle decay. Why was the neutrino solution so terrible?
It almost seemed like a sleight of hand. They had a problem with beta decay, this nuclear reaction that seemed to have something missing. So to sit and say, “What’s missing? Let’s just scrape all those things that are missing and put them together into a new particle” to answer the question, it feels like a “just-so story”—like “How did Leopard get its spots? Well, some ancient god threw mud at him.” Sure, it’s an answer. But you can’t check it. It solves your problem, but it’s unsatisfying.
Pauli assumed he couldn’t check the answer because he and other physicists thought neutrinos would be completely undetectable. Yet we’ve now seen three different types of them. And is there a chance there are even more?
At Los Alamos [National Laboratory in New Mexico], they were finding there were too many neutrinos turning up in one of their experiments. One explanation would be that there is yet another type of neutrino that only interacts with other neutrinos and perhaps some sort of dark matter. Those are called sterile neutrinos. There’s reason to believe that there may be many types of neutrinos, but that’s just a possibility.
People expected this Los Alamos anomaly to go away. They were testing it at other machines at [Fermi National Accelerator Laboratory in Illinois]. I remember talking to the people at Los Alamos who had first found what appeared to be the suggestive sterile neutrinos, and they all expected there to be absolutely no sign of sterile neutrinos [from the follow-up test]. Instead it confirmed their initial experiment that suggested there were sterile neutrinos. It was a stunning confirmation of something that almost everybody assumed was just a measurement error. The question is still clearly out there, and there are reasons to believe both sides: that there’s some kind of systematic error that both are subject to or that the question’s still out. That hopefully will be answered soon.
Another big mystery is what neutrinos actually weigh. At first, they were predicted to be massless, but now scientists know they must have nonzero mass. Where do we stand on figuring out what that mass is?
One of the things that’s really fun to talk to Alan about is that KATRIN [a German experiment aiming to measure neutrinos’ mass] doesn’t say the neutrinos have a small positive mass. It says they have a small mass that could be either positive or negative mass squared. That means they could have, based on the way they do the experiment, an imaginary mass, which would make them “tachyonic neutrinos.” This would make them potentially travel faster than the speed of light or potentially backward in time, depending on how you think about it.
Of course, the people at KATRIN don’t believe that that’s a possibility, so they just throw that away. But there’s still this slight hope in the minds of people such as Alan that maybe the answer will actually be negative, even though they’re just including it to make sure their statistics don’t get screwed up.
If neutrinos could travel faster than light, wouldn’t we know that already?
It’s true there would be all kinds of problems. I spoke to [physicist] Sheldon Glashow and asked him about that. He pointed out that if neutrinos could travel faster than light, it would lead to a huge burst of radiation, and they would rapidly slow down. So even if they could momentarily travel faster than the speed of light, they would instantly not travel faster than light. I tend to believe in Sheldon Glashow’s answer. Alan holds out hope because he’s a theoretician, and they like to believe weird stuff. It’s not something that anyone, even Alan, seriously expects to see.
After doing all this research and writing this book, did it change how you feel about your grandfather?
It did. I found that there’s a huge amount of humor in what he and Fred Reines [his collaborator on the neutrino discovery experiments] did. They had the audacity to encode a little joke into the design of a tremendous scientific experiment.
Their first idea to look for neutrinos was to take advantage of the nuclear weapons testing being done during the Manhattan Project at Los Alamos, right?
If you look at the initial proposal, which was in itself audacious, they were going to put a detector in a shaft and drop it at the same time that a nuclear weapon went off about 40 meters away. It was an incredibly complicated system to develop. They had to decide where to dig the shaft. And they chose to put it 137 feet away from the tower where the weapon was going to go off. They chose that because the fine structure constant [a fundamental constant related to the strength of the electromagnetic force] is 1 ⁄ 137. But they knew that that was a little too frivolous to put in the description of the experiment for approval by Los Alamos, so they found the metric equivalent, which was roughly 40 meters. They turned it into an inside joke.
Then when they had the experiment that actually discovered the neutrino, in South Carolina, they put up all this shielding to see if they could modulate the signal and, along with the shielding, one pound of hominy grits. I think I romanticize my grandfather as this funny guy, and it became fleshed out. You can see this sense of humor and this sense of fun that went through this very serious activity they were doing.