Trillions and Trillions

At the instant that you read this, billions of neutrinos are passing through your body. They are not electromagnetic waves. They are real particles: just vanishingly tiny. The smallest particles we know of. Yet these particles hold significant sway in helping us determine the properties of the universe. 

Neutrinos were first theorized by Wolfgang Pauli in 1934 to account for missing mass in nuclear reactions. Enrico Fermi gave it an Italian flair by calling it a neutrino (little neutral one) since they carry no charge. In 1956, it was experimentally confirmed (earning a Nobel prize) by Frederick Reines and Clyde Cowan who generated neutrinos in a nuclear reaction.

These astonishing minuscule particles, a product of fusion, and radioactivity, occur in nuclear weapons, power plants, and interestingly enough, bananas (the potassium-40 in the banana undergoes natural radioactive decay-producing about a million neutrinos a day) 

How small are neutrinos? They are about 4.1 million times less than the mass of an electron: 0.120 eV/c² to be exact. Subatomic masses are expressed in electron-volts/light speed²: from Einstein’s E = mc². To learn how a neutrino’s mass is determined, visit this article titled: Direct Neutrino-Mass Measurement Achieves New, Sub-Electronvolt Sensitivity.

What the neutrino lacks in mass, it more than makes up in its enormous speed. Its kinetic energy is about 1 TeV ( trillion electron volts), equivalent to the energy of a flying mosquito. In particle physics this is enormous. 

Neutrinos are part of the Standard Model of elementary particles: a table that describes how the basic building blocks of matter interact. These interactions are governed by the four fundamental forces. Gravity, Nuclear Strong, Nuclear Weak, and Electromagnetism. The last two are now combined into the Electroweak Force. Neutrinos fall under the Electroweak Force: radioactivity. 

^{See The Standard Model from CERN for a fuller explanation.}

While it would seem that neutrinos should be the focus of particle physics, their role in cosmology is fundamental. 

Neutrinos travel at 99.999% of the speed of light. Unlike EM waves (that can refract and diffract) they can travel in straight lines like lasers. Remarkably, they can travel through everything: undeflected by interstellar gas, or immense gravitational and magnetic fields. Best of all, they can travel like this for billions of years. If we could capture and analyze them, which is devilishly hard, they could inform us of events, a la George Lucas, from “a long time ago in a galaxy far far away” (unless you have been living under a rock, you should recognize the phrase).

To complicate matters even more, neutrinos come in three flavors: They can transform from one flavor to another. Thus we are uncertain about the “history” of any particular neutrino. An April 2024 comprehensive, but technically dense, paper about neutrinos enumerates their cosmological significance:

1. During post-Big Bang cooling, neutrinos, by the trillions, decoupled from the W and Z bosons (all, part of the Weak Force), spewing out into the universe. This happened in an infinitesimally short time. Analysis of this process challenges assumptions made about the Standard Model.
2. The asymmetric distribution of neutrino densities also raises questions about how matter was organized after the Big Bang. See Neutrinos in Cosmology

Neutrinos also play a large role in supernovae. Neutrinos account for 99% of the energy released during a supernova collapse. All stars, under gravitational pressure, undergo the fusion of hydrogen into helium. Gravity compels fusion into heavier elements until the process reaches iron. Because iron is a stable dense element, more energy would be needed to fuse the iron than the energy released by the fusion of iron. Comparable to the cooling period of the Big Bang, a star also undergoes a practically instantaneous emission of enormous numbers of neutrinos. This accompanies the complex reaction leading to a core collapse, initiating the supernova. In addition to the vast thermonuclear energy released, this process also spews untold numbers of neutrinos of which we are recipients. 

^{A sequence of Hubble Space Telescope images from 1994 to 2009, shows the collision of the expanding remnant with a ring of material ejected by the progenitor 20,000 years before the supernova. Image credit: NASA}

Detailed analyses of this process can be found in Neutrinos from Core-Collapse Supernova Explosions and Toward Powerful Probes of Neutrino Self-Interactions in Supernovae. To date, SN 1987A is the only supernova observed since 1604. Yet the study of just this one supernova neutrino emission has provided a great deal of information about stellar collapse. For those intrepid souls who want to get to the “core” of this process, watch this informative video created by David Vartaniayan, Robin Jeffries & Adam Burrows at the Argonne Labs detailing the core collapse of a supernova.

In essence, the enormous energy released takes place in about a 10-second interval with a release of something like 10⁵⁵ neutrinos. We here on earth will get to meet some of them 170,000 years later. Given their elusive nature, the number of neutrinos we can detect can be counted on our fingers and toes (plus some ears and a nose). How this will come to pass is the subject of next month’s column.