John Carlos BaezSep 23, 2025

Gold is mainly made when neutron stars spiral in, collide, and explode in a supernova. So if you have a gold ring, it's only thanks to gravitational radiation.

But it may also be thanks to neutrino oscillations! A supernova creates a huge pulse of electron neutrinos - since as the star's core collapses, it uses

p + e → n + νₑ

to make heavier, more neutron-rich nuclei. A new paper shows that when the electron neutrinos νₑ turn into muon neutrinos, it helps the neutron star shoot out more heavy elements like gold and platinum. A lot more: up to 10 times as much!

Can you figure out why? I'll tell you in my next post.

The paper says:

"We report on the first numerical relativity simulations including the dynamical effects of lepton number-preserving neutrino flavor equilibration that could be driven by quantum many-body effects, beyond standard-model physics, or flavor instabilities. Our study demonstrates that such transformations can impact the structure and composition of the merger remnant and of its accretion disk. At high densities, the conversion of electron-type neutrinos and anti-neutrinos into heavy-lepton neutrinos accelerates the contraction of the outer layers of the remnant. [...] At lower densities, the depletion of electron-type neutrinos due to flavor conversion results in more neutron rich ejecta and boosts the production of heavy r-process elements with A≳120 by up to one order of magnitude."

https://www.sciencedaily.com/releases/2025/09/250920214447.htm

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Neutrinos may be the hidden force behind gold and platinum

When two neutron stars collide, they unleash some of the most powerful forces in the universe, creating ripples in spacetime, showers of radiation, and even the building blocks of gold and platinum. Now, new simulations from Penn State and the University of Tennessee Knoxville reveal that elusive particles called neutrinos—able to shift between different “flavors”—play a crucial role in shaping what emerges from these cataclysmic events.

ScienceDaily
Show threadJohn Carlos Baez

Okay, I looked at the paper and learned *how* neutrino oscillations help make more gold. It's ridiculous that the pop-sci articles on this topic don't explain this. But it's completely typical.

Here's the deal, explained quickly. If you have any questions please ask and I'll try to figure out the answers!

When electron neutrinos turn into muon neutrinos, it leaves fewer electron neutrinos in the exploding star. So, the reverse process

n + νₑ → p + e

happens less in the fragments that shoot out of the star. That means more neutron-rich elements like gold and platinum!

Here's the paper:

• Yi Qiu, David Radice, Sherwood Richers and Maitraya Bhattacharyya, Neutrino flavor transformation in neutron star mergers, https://arxiv.org/abs/2503.11758.

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Neutrino Flavor Transformation in Neutron Star Mergers

We present the first numerical relativity simulations including neutrino flavor transformations that could result from flavor instabilities, quantum many-body effects, or potential beyond standard model physics in neutron star mergers. We find that neutrino flavor transformations impact the composition and structure of the remnant, potentially leaving an imprint on the post-merger gravitational-wave signal. They also have a significant impact on the composition and nucleosynthesis yields of the ejecta.

arXiv.org
Sep 23, 2025 at 9:11amWeb
Show threadSimonSep 23, 2025

@johncarlosbaez a little while ago I read an article saying that neutron star collisions produced heavy elements via iron + the r-process, and I get why more neutrons would make this easier.

What I couldn't find out was why this has to be the case. There's not enough iron on the neutron star so it must come from the breakup of the bulk... Is it neutron capture all the way up from single protons, or from larger nuclei, in which case why just iron and not other nuclei?

Show threadaapSep 23, 2025
@johncarlosbaez Do you happen to know anything about how being bathed in neutrinos, like we are on earth, may affect β-decay processes like these?
Show threadJohn Carlos BaezSep 23, 2025

@aap - neutrinos interact with matter on Earth very rarely: you may have heard that a neutrino of this energy has roughly 50-50 chance of going through a light year of lead without interacting with it. So the amount of neutrinos we're bathed has little effect on nuclear reactions here, even though, for example, the flux of solar neutrinos at the earth's surface is on the order of 100 billion per square centimeter per second.

The fact that neutrinos are significant in nuclear reactions in a supernova is a testament to just how intense supernovae are, and how dense the core of a collapsing star becomes.

Show threadaskSep 23, 2025

@johncarlosbaez why can't the reverse muon capture process happen with the muon neutrinos and neutrons then? It's surprising to me that neutrinos are allowed to change flavor but somehow electron neutrinos are special.

https://en.wikipedia.org/wiki/Muon_capture

Muon capture - Wikipedia