The sun shines because of fusion. It's in every middle school science book! So scientists must've been 100% sure of this fact for a long time, right?

What if I told you that only a few decades ago, the Solar Neutrino Problem almost made us abandon this theory forever?

Solar Energy Production
The Neutrino
Neutrinos in Matter
The Davis Experiment
Trouble Brewing
Physicists Get Creative (and Momentarily Forget the Past Century of Stellar Astrophysics)
A Solution

When we say the sun is powered by fusion, we are specifically referring to how lighter stars, like our sun, fuse hydrogen nuclei into Helium-4 nuclei in their core through a process theorized in the 1930s called the p-p chain (also known as the proton-proton chain).

Protons fuse to create deuterium (Hydrogen-2), which in turn creates helium-3 nuclei. These nuclei are fused into lithium and beryllium, which eventually result in Helium-4 nuclei. These processes release a number of other particles as well, such as photons and positrons.

What we REALLY care about for this thread are those tiny blue particles- neutrinos! Originally proposed by Wolfgang Pauli (and theoretically fleshed out by Enrico Fermi) to explain the energies of beta decays, the neutrino is a small, neutrally-charged lepton (like electrons).

Neutrinos come in three flavors corresponding to the three flavors of charged leptons- electron neutrinos, muon neutrinos, and tau neutrinos. The standard model tells us that neutrinos are massless, like photons, meaning that they propagate through space at the speed of light.

So through its fusion processes, the sun is generating massive amounts of neutrinos each second; on Earth, this neutrino flux is about 6.36 × 10¹⁰ ν / (cm² • s). When these neutrinos reach a neutrino detector, they can interact with targets (atoms) in the detector.

The neutrinos bump into the nuclei in the detector volume. The matter-neutrino scattering cross section is extreeeemely tiny, so this happens rarely! As a result, the geometry of the detector volume doesn't even matter- a neutrino is equally likely to hit the front or back.

Now here's where things heat up. Now that neutrinos have been theorized (and a few even detected!), a physicist named Raymond Davis wanted to measure the flux of electron neutrinos generated by the sun through the p-p chain.

He built the first dedicated neutrino detector deep underground in the Homestake Mine of South Dakota as a means of mitigating false detections from cosmic ray showers. He pumped a tank full of 100,000 gallons of chlorine and flushed the system every month to check for results.

Davis would count the number of argon atoms in the detector during each flush, as argon is the byproduct of this specific chlorine-neutrino interaction. His results, which were swiftly duplicated by other types of detectors, suggested a baffling conclusion...

The Sun wasn't releasing enough neutrinos! What?? The p-p chain is supposed to be the mechanism generating all of the energy in the sun- we know exactly how much energy is generated in a p-p event, and we know the luminosity (power) of the sun- so where are all the neutrinos?

Davis's and many other experiments suggested that the sun's actual neutrino output was only a third of the theoretically expected rate. This may not seem like a big deal, but the mathematics of the p-p chain were failing us... could we have been wrong about fusion in the sun?

Scientists began positing a number of creative theories as to why this could be the case. Stephen Hawking notably suggested that the energy from the sun could be from matter falling into a small black hole in its core, a callback to Lord Kelvin's famous theory of gravitational

contraction producing solar energy- though Lord Kelvin's calculations of the solar age were orders of magnitude too low and even precluded Darwin's theory of evolution from being possible!

(Darwin was actually extremely nervous about Kelvin's paper on the subject and, as a result, struck his estimate for the timescales of evolution from future reproductions of his works)

A more popular theory was that WIMPs (weakly interacting massive particles) in the solar interior actually suppressed neutrino emissions, sloppily saving the p-p mechanism, but leaving a bad taste in many particle physicists' mouths.

Things changed in 2001 when the Super-Kamiokande experiment in Japan and the SNO experiment in Canada, both pure water neutrino detectors sensitive to not only electron neutrinos, but muon and tau neutrinos as well, published their results.

They found that while only a third of the expected electron neutrinos from the sun were detected... two thirds of the expected electron neutrino signal was detected- but as muon and tau neutrinos instead! A theory by Bruno Pontecorvo from 1968 provided an explanation.

In Pontecorvo's theory, neutrinos are NOT massless afterall. Instead, they have three mass states which are actually offset from their flavor states- meaning, electron neutrons don't have a mass, persay, but are instead a linear combination of the three mass states.

This applies to the other flavors of neutrinos as well! This offset between the mass and flavor states causes the neutrinos to undergo a phenomenon called neutrino oscillations whereby one neutrino will cycle through flavor states as it propagates through space.

The standard model is incomplete and neutrinos undergo oscillations due to their non-zero mass states. Today, neutrinos remain a heavily studied facet of particle physics, and there is much to learn about them- but, at last, we know for sure why the sun shines :)

Normally this is where'd I'd give reading materials- but instead I'll just tell you this is a preview thread for a more detailed thread on neutrino oscillations I'm dropping on Saturday- so keep your eyes peeled! :)

Pic sources in order:
1) Me :)
2) https://en.wikipedia.org/wiki/Standard_Model
3) https://www.insidescience.org/news/planned-neutrino-detector-may-also-reveal-details-earths-interior
4) https://www.pbs.org/wgbh/nova/neutrino/dete-01.html
5) https://www.cora.nwra.com/~werne/eos/text/neutrino.html
6) https://ned.ipac.caltech.edu/level5/Sept05/Gondolo/Gondolo3.html
7) https://www.businessinsider.com/super-kamiokande-neutrino-detector-is-unbelievably-beautiful-2018-6
8) https://www.italyonthisday.com/2019/08/bruno-pontecorvo-Italian-nuclear-physicist-soviet-spy-claim.html
9) https://www.slideserve.com/rhona/1-neutrino-oscillation

Relevant companion thread! https://twitter.com/AstroYuki/status/1294252089112244224?s=20