Naughty, Naughty Neutrinos

Neutrinos are the voodoo child of quantum particles. And a subatomic particle has to be really, really weird to standout in the very strange world of quantum physics. The neutrino was first proposed back in 1931 when physicists determined that there was missing energy when a large particle nucleus experienced something called beta decay.¹ Either there was an undetected particle involved or the law of conservation of energy had been repealed. There was another problem involving nuclear spin, where, again, the numbers wouldn’t add up.² Physicist Enrico Fermi suggested that the unexplained energy imbalance and nuclear spin issues could be resolved if there was a very light, neutral particle emitted that, at the time, they simply couldn’t detect. Neutrinos were proposed as a solution to a couple of fairly obscure problems in nuclear decay.

They turned out to be real buggers to detect, too. Sure, there is indirect evidence like the missing energy and spin issues, but the explanation for the missing energy in beta decay and the issue of nuclear spin imposed constraints on the physical characteristics of the neutrino. The particles had to be electrically neutral. They had to have a spin of 1/2. And they had to have a tiny, tiny mass.³ And neutrinos could not participate in either the electromagnetic interaction or the strong interaction. That leaves only the weak force as a means of interaction and detection.

It took until 1965 to detect a neutrino in the wild. That happened in a test chamber 3 kilometers underground in an abandoned gold mine. Then it turned out that there were three “flavors” of neutrino. Until 1995, neutrinos were believed to be massless, like a photon. But now quantum physicists now think they do have mass, but the mass of inidivudal neutrinos is constrained by the mathematics of the Big Bang: the Big Bang model predicts that there is a fixed ratio between the number of neutrinos and the number of photons in the cosmic microwave background. The Big Bank model predicts that if the total mass of all three types of neutrinos exceeded an average of 50 eV/c² per neutrino, there would be so much mass in the universe that it would collapse.

But in the attempts to determine the actual mass – the numbers in the table are estimates based on constraints – it turns out that as they travel, neutrinos transform across the three flavors. Again, that’s an inference, but it explains the so-called “solar neutrino problem,” in which the solar flux of neutrinos was only about a third of what theory predicted. There’s some nearly evidence that neutrinos oscillate between different flavors in flight. For example, an electron neutrino produced in a beta decay reaction may interact in a distant detector as a muon or tau neutrino. Even in the very weird world of quantum mechanics, it’s hard to imagine a mechanism that would allow the mass (and consequential speed) of a neutrino of any flavor could change mass over two orders of magnitude in flight.

So physicists have taken refuge in quantum superposition: a specific flavor is a specific mixture of all three mass states. That is, whenever the system is definitely in one state you also have to consider it as being partly in each of two or more other states. The original state is regarded as the result of a kind of superposition of the two or more new states, in a way that cannot be conceived based on classical ideas. It also cannot be conceived of in WC’s ancient (and insufficiently flexible) brain.

The neutrino naughtiness doesn’t stop there. Apparently, neutrinos are both particle and antiparticles, at the same time. WC was raised to believe in Newton’s Three Laws and that antimatter always exploded when it came in contact with ordinary matter. Unless, apparently, you are neutrino.⁴

And there are jillions and jillions of these things. If you hold a quarter up to the sun at arm’s length, there are 6 billion – that’s billion with a “b” – streaming through the quarter every second. Essentially none of them have any interaction whatsoever with the quarter.

Almost no mass. No electric charge. Not affected by other force, except gravity, where we just don’t know. The best – and very expensive – neutrino detectors might find 100 neutrinos a year. All of which is why they are called “the ghost particle,” although that’s probably unfair to ghosts.

WC remembers now why it has been almost seven years since he wrote about quantum physics. It will probably be another seven years before tries it again.

Bad, naughty neutrino.

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¹ Nope. Not going to try to explain beta decay in this post.

² Nope. Not going to try to explain the law of conservation of angular momentum, either.

³ The mass of subatomic particles is expressed in electron volts, abbreviated eV. The electron, a very tiny particle itself, has an energy of .511 MeV; that’s 511,000 eV. The electron neutrino has an energy of less than 2.2 eV. Mass is eV/c² where c is the velocity of light in a vacuum. When you pencil it out, an electron neutrino has about 1/5,000,000 the mass of an electron.

⁴ And then there is chirality, the handedness” of subatomic particles, where neutrinos are also very weird, but chirality is another thing WC isn’t going to try and explain.