By Don Lincoln, Fermilab
What would you say if I told you that physicists spend a lot of time trying to detect and understand ghosts? I mean a particle that interacts so little with other matter that it is almost impossible to detect. To give you a sense of what I mean, something like a quadrillion neutrinos pass through you every second of every day of your life and you’ve never felt one of them and have had no negative consequences at all.
What We Know about Neutrinos
Neutrinos can pass through the entire Earth, with just a tiny chance of interacting. Even weirder, neutrinos can actually change their identity, not so different as if you could spontaneously change into your cousin, then your best friend, before turning back into yourself.
And yet researchers have figured out how to detect and study the neutrino. We’ve learned a great deal about this quantum ghost, this cosmic chameleon. It’s a crazy business. So, let’s get to it.
The neutrino is one of those particles that was proposed before it was observed. The story begins in the early years of the 20th century, shortly after radioactivity was discovered. It turns out that there are lots of different types of radioactivity, called gamma radiation, alpha radiation, beta radiation, and several others. But it is beta radiation that is important for our tale.
This article comes directly from content in the video series The Evidence for Modern Physics: How We Know What We Know. Watch it now, on Wondrium.
Beta Radiation and Beta Decay
Beta radiation is a form of nuclear transformation, where a neutron inside the center of an atom spontaneously converts into a proton and emits an electron. Beta radiation was discovered in 1899, just three years after the first discovery of radiation in general occurred. The person who discovered beta radiation was Ernest Rutherford, lea prolific guy.
So, beta radiation was a thing, but it had a very curious property. If you’ve taken any physics class at all, you’ve heard of energy and momentum conservation and these are principles that apply to every phenomenon we’ve ever discovered, and they should certainly apply to beta radiation. If they did, this is what should happen.
To all intents and purposes, atoms don’t move all that much when they are in a solid. So, a neutron inside that atom basically can be considered stationary. If it decays into two other particles, say a proton and an electron, those two particles have to shoot out in opposite directions.
The Problem of Beta Decay
Because the neutron wasn’t moving, it had no momentum and therefore the proton and electron should have opposite momentum to balance each other out. Furthermore, using Einstein’s theory of relativity, it should be possible to predict exactly the energy and momentum the electron should have.
Now, the specific number for energy and momentum depends on what atom is being studied, so I’m going to make up a number for the energy expected that an electron would carry in beta decay. Say it is 10 in some units. And, I want to emphasize, I just made that number up for illustration. Don’t take it seriously.
So, say that the electron should have 10 energy units. When scientists measured the energy of the electron, they saw that it never carried 10 units. In fact, it very rarely carried more than nine. In fact, the electron more often carried like three units of energy, which is way lower than expectation. This was a huge puzzle and it led some scientists to propose that maybe energy conservation didn’t apply in beta decay.
How Wolfgang Pauli Solved the Problem
It took a long time, but in 1930, a chap by the name of Wolfgang Pauli had an idea, which was presented at a conference in Tübingen, Germany. Pauli didn’t present the idea in person, because he had a party to attend in Zurich. I like to mention that part, because it shows that we physicists have been party animals for at least a century. Take that, chemists and biologists.
The letter is fun to read. It starts out, “Dear Radioactive Ladies and Gentlemen,” and it then goes on to propose perhaps in beta decay there was a third particle emitted, which we now call a neutrino. Actually, in the letter, Pauli calls his new particle the neutron, but that was before what we currently call the neutron was discovered. The names were messed up for a while, but eventually the word neutrino came to describe Pauli’s particle.
In any event, in his letter, he noted that the particle should have been discovered if he was right, but only those who dare can win. So, he predicted it anyway and told the listeners, “Thus, dear radioactive people, look and judge.” I think I would have liked Pauli.
The neutrino solved the beta radiation problem by inventing a new particle that would carry off some of the energy. So, some of the energy that should have been carried by the electron was carried by the neutrino instead. And that was that.
Common Questions about Neutrinos, Beta Radiation, and Beta Decay
Neutrinos are particles that rarely interact with other matter rarely, and that’s why they are almost impossible to detect. There are about a quadrillion neutrinos passing through every person’s body every second. Yet, no one ever feels them.
These include gamma radiation, alpha radiation, and beta radiation, among several others. Beta radiation was what proved to be connected with neutrinos.
As Wolfgang Pauli noted in his letter that perhaps there was a third particle emitted in beta decay. He called this new particle neutron since neutrons had not been discovered yet. Later, Pauli’s particles were called neutrinos.