As a science fiction writer, I feel ashamed to admit that my understanding of quantum physics is murky at best. I realize that even quantum physicists don’t pretend to know all the answers, but still…. So I when I ran across Dolly Setton’s article “Ghosts of the Universe” in Discover (September 2014), I focused in on her sidebar “Neutrino Mysteries: A Guided Tour of Subatomic Weirdness,” with the hope that the author could increase my limited understanding.
Setton summarizes four basic neutrino properties that quantum physicists still struggle with: flavor, mass, antineutrinos (spin), and mirroring.
The first property we don’t fully understand is flavor: electron, muon, and tau. Somehow, neutrinos can change flavor as they travel. Setton explains, “Because neutrinos are quantum particles, and by definition weird, they are not one single flavor at a time, but rather always a mixture of flavors.” She says we can only discern which flavor is dominant in a neutrino’s final moments. When a neutrino collides with another particle, if the collision produces a muon, we can deduce the neutrino was muon-flavored immediately before the collision. If an electron results, the neutrino must have been electron-flavored, and so on.
The second property we don’t understand is neutrino mass. Some neutrinos mass more than others; perhaps mass depends on their mix of flavors at that specific time. Perhaps. The Heisenberg uncertainty principle also creates difficulty: the more precisely we know one property of a subatomic particle, the less precisely we can know another. Flavor and mass are so linked. The more we know a neutrino’s flavor, the less we can know mass. And vice versa.
The third point of weirdness involves neutrinos’ antimatter counterparts. Normally, the antimatter version of a particle (like an electron) is identical to the normal matter version except that it has the opposite charge. My brain struggles a bit with the concept of a positively charged electron (positron), but I can follow the logic. However, because neutrinos are neutral, their antimatter particles can’t have opposite charges. Instead, their spin is reversed. I’m still following. Then Setton explains that neutrinos don’t physically spin like a top or a planet. She says the term “refers to a property that is in some ways equivalent to spin.” She loses me here. I have no idea what that means. Then Setton adds one theorist’s idea that neutrinos may be their own antiparticles, which apparently satisfies one condition for the existence of the universe. Well. That’s as clear as mud to me.
The final point of weirdness involves the mirror effect: a magnetic field will push on an electron and a positron with exactly the same force but in different directions. Physicists hope neutrinos don’t follow this rule, so they are running experiments in Japan and the US to look for asymmetrical behavior to test certain quantum theories. The results may change our ideas about the dawn of time and the big bang.
I think this sidebar summarizes nicely the key issues surrounding our understanding (or lack thereof) of neutrinos and their properties. At least, I understand better what the difficulties are, and I look forward to reading the results of those ongoing particle experiments.
vlsperry 
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