Daring Do and the Mysteries of Particle Physics · 7:14pm Feb 8th, 2020
GMBlackjack has now published a pony particle physics story I commissioned. Check out Daring Do and the Inexplicable Artifact. An A.K. Yearling adventure in which our pegasus hero discovers a mysterious object that appears to be shooting some sort of beam, but while it is evidently drawing a lot of power, it appears to have no effect whatsoever on anything in its path. Daring enlists the help of Twilight Sparkle, and together they travel to the other side of the planet on a quest to solve the mystery. Go read it now.
Now it’s my turn to talk about the particle physics.
The particles in this story are neutrinos. Neutrinos are fundamental particles that have no electric charge, hardly any mass, and hardly interact at all with other particles. Although the universe is full of them, they just zip through space, and most just go straight through any stars or planets that get in their way. As it is so rare for one to hit an atom, they can only be detected with huge super-sensitive detectors. Given this, you might well ask how we learned that they exist at all?
The idea of neutrinos was first suggested to solve a physics mystery in the 1930s. A close examination of the beta decays of radioactive atoms showed this process appeared to not conserve energy. In beta decay, a neutron inside an atom changes into a proton and emits an electron (the beta particle). The energy change inside the atom could be calculated. It was expected the beta particle would take away this energy, yet when this was measured, it was found they were emitted with a spread of energies up to this value. Where was the missing energy going? This was a big mystery even before you looked at the momentum and other conserved quantities.
With some desperation, the Austrian physicist Wolfgang Pauli proposed that the energy was taken away by an invisible particle, emitted at the same time as the electron. This theory was developed by Italian-American Enrico Fermi, who named it the neutrino. For years neutrinos were just a theoretical hypothesis, but in the post-war enthusiasm for experimental nuclear physics, a US team set out to detect them. It was known that as the interaction rate was very low, this would require something able to produce a lot of them. Early ideas involving exploding lots of atom bombs were rejected and instead the first neutrinos detected were created in a nuclear reactor.
The first neutrinos from natural sources were detected shortly after. This required huge detectors in deep underground laboratories, to get away from background radiation. These experiments soon revealed further mysteries. The number of neutrinos produced by the sun was lower than expected. It was generally assumed the nuclear-astrophysicists must have messed up their calculations. Then the Super-Kamiokande experiment in Japan measured neutrinos, created when high-energy cosmic rays wham into the top of Earth’s atmosphere. They could see the neutrinos coming from both directly above Japan, and those created on the other side of the planet. The numbers showed neutrinos seemed to be disappearing while passing through the Earth.
The solution to this mystery was a new scientific phenomenon: neutrino oscillations. Neutrinos come in three flavours: electron neutrinos, muon neutrinos, and tau neutrinos. The mix of flavours would ‘oscillate’ as they travelled, as the different types had slightly different masses and moved at slightly different speeds. Early detectors could only see electron neutrinos and missed other flavours.
Pinning down the details of this behaviour has been an exciting area of particle physics for the last twenty years and has required experiments operating over long distances. A real world version of Daring Do's "temple" is the Fermilab laboratory near Chicago, from where a beam of neutrinos is sent 735km through the Earth’s crust to a detector in northern Minnesota.
This research is being conducted by other experiments including the T2K experiment, which sends a beam across Japan, and the CNGS project sending neutrinos from CERN to Gran Sasso, Italy. There remain unsolved mysteries about neutrinos. Are there just the three flavours, or could there be other sterile neutrinos? Are neutrinos the same as their antiparticle version, antineutrinos? Could they provide a clue about the difference between matter and antimatter and thus explain how our matter-dominated universe came to be?
Back in Equestria, Daring Do has uncovered evidence of an ancient civilization that were tackling these questions. Did they find the answers they sought? Quite likely as they clearly had access to some pretty clever magical particle physics technology, such as a portable super-efficient neutrino gun (wish I had one of those), and small crystal neutrino detectors, sensitive to the different flavours. How could that work? I think that will need to wait for another blog post.
Want to know more? See this manga: Neutrino! (from Kyoto University), or: What’s a neutrino? (by Fermilab)
Past blog posts about neutrinos:
- Pony Adventures in Particle Physics: Twilight, Pinkie Pie and Rainbow Dash visit Sudbury, Ontario and shed light on neutrinos
- T2K Neutrino Oscillations
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I keep thinking, given the style of the maths and the problem, that neutrinos are like the solutuions in quaternion space, where theres three possible complex axis of rotation if scalar variations are always 0, as any change in the scalar value would give a charge? Or something similar.
Then again, that would mean time would be scalar, and space would be complex. x,y,z,w all imaginary. Which physisists hate due to obviously living in realitY?
5199539
I'm not familiar with quaternions. It seems quite possible they could be used as an alternative way to express the PNMS matrix, although I really don't know if there is a significant benefit to doing it that way.
5199539
It'd just be another thing named by a detractor that ended up being critical to their models. You know, like the Big Bang. (What, you thought a name that dumb didn't start out as an attempt to make fun of it?)
5199606
The worst one Ive heard of that is the guy who is recoding all of mathematics to get rid of imaginary numbers, by renaming the square root of minus 1.
I shall read the story. Speaking about stories, I can walk now and got the 2nd Book of Dust.