(This is Part 2 of a series on neutrinos, Majorana fermions, and one of the strangest open questions in physics. Read Part 1 first.)
What I'm about to say may be some small comfort for those of you who are left-handed and feel like the world isn't constructed with you in mind. When it comes to fundamental particles, the universe doesn't care if you're left- or right-handed. It doesn't make any difference. Think of all the forces of nature and the ways they interact. Gravity and mass. Electromagnetism and charge. Strong force and color charge. Left-handed? Nobody cares, nobody notices, everything is the same. Right-handed? Same deal.
An electron buzzing around is constantly flitting back and forth between left- and right-handed identities — the massless twins that when combined give us an electron. When it hits you, you don't care if it's left- or right-handed mode. You feel its mass and charge. That's it.
Oh, right, except for the weak nuclear force.
Ahem, right, so, weak nuclear force. The quirky cousin in the family of the forces. The eccentric one. The one living in its own little world, to the beat of a different drum. Or accordion. Or vuvuzela.
The weak force REALLY cares about handedness. It cares SO MUCH that it ONLY, and I mean ONLY, talks to left-handed particles. It's BLIND to right-handed particles. It's like some sort of germaphobe that will ONLY shake hands with left-handed people.
Famed physicist and all-around curmudgeon Wolfgang Pauli once quipped, "I cannot believe God is a weak left-hander."
He said that because even though the weak force is the odd one out, it's absolutely critical for most of the universe as we know it. One of its superpowers is beta decay, which allows it to reach inside a neutron, grab one of its quarks, and change it, transforming the neutron into a proton. That transformation makes nuclear fusion and fission possible, which is responsible for, among other things, making stars shine.
Pauli spent years disparaging, in typical Pauli fashion, the experiments conducted by Chinese-American physicist Chien-Shiung Wu — known as Madame Wu, the first lady of physics — who conclusively demonstrated that the weak force only works with left-handed particles. Her experiments showed that radioactive decay of cobalt-60 tended to prefer one direction over another. When she first made the announcement, nobody liked it because it broke up the nice and tidy picture of the universe we had been so meticulously cultivating for decades. But evidence is evidence, and Madame Wu was exceptionally good at getting it, so even the old curmudgeon relented.
Okay, that's weird. Fine, universe, whatever. In most cases this doesn't matter at all, ever. That's because every particle, like an electron, is constantly flitting between left- and right-handed states. If the weak nuclear force wants to talk to the electron, it just — in a sense — "waits" for the left-handed version to show up (which isn't very long at all), and grabs it. It doesn't need the right-handed version.
To torture this analogy even further: if our germaphobe only shakes hands with left-handed people, that would normally be incredibly restrictive and largely unfriendly...except this germaphobe happens to live in a world filled with ambidextrous people.
So it doesn't matter.
Oh, right, except for the neutrinos.
Neutrinos are exceptionally selective when it comes to the forces. They have no electric charge, so they don't interact via electromagnetism. They have no color charge, so the strong force is out for them. They feel gravity because if you exist in this universe you don't get a choice about that.
But the weak force? Oh, they LOVE the weak force. They're the one kind of particle our left-hand-only germophobic force will actually SEEK OUT at an event. They speak the same language. They eat the same kinds of foods — with their left hands, of course.
And this is where the story comes screeching to a halt. Because there are ONLY left-handed neutrinos.
Seriously. Every single neutrino we have ever observed, ever, in every physical reaction, is only left-handed. They don't flit back and forth. They don't switch identities. They just...are left-handed.
Now the antineutrinos? Those exist, but those are ONLY right-handed.
This is completely unlike any other particle. Any other massive particle has both left- and right-handed identities that constantly swap back and forth. But the neutrino? Only left-handers for neutrinos, only right-handers for antineutrinos. That's it. Trillions of neutrinos passing through your thumb every single second — not a one is right-handed.
For a long time, this was classified under "weird but fine." We thought neutrinos were massless. And massless particles, like the photon, are locked in either left- or right-handed mode forever. There was weirdness — we see both left- and right-handed photons in equal measure, but ONLY left-handed neutrinos and right-handed antineutrinos — but that was tagged as "problem for another day."
And then we discovered that neutrinos have mass.
Massless particles are locked into one hand. Massive particles swap back and forth. The neutrino is both massive AND locked.
I don't know about you, but this sounds like a problem.
In Part 3, we look at the most straightforward fix — and what it quietly hides.
