About 100 trillion neutrinos pass through your body every second, night and day. Yet in a lifetime you will not stop one of them. You are transparent to neutrinos – more transparent than a windowpane is to light. Even the whole Earth is transparent to neutrinos. In fact, neutrinos are so hard to stop that if a beam of them were passed through a light year (10 trillion km!) of lead, half would get to the other side.
Neutrinos are very, very hard to stop. They have almost no mass and travel at almost the speed of light. They are ghost particles.
Why do we even think they exist? And why are they important?
In the early decades of the 1900s there was a mystery in observations of radioactive decay of elements where there was a small amount of energy missing. In 1930 the physicist Wolfgang Pauli suggested that another invisible particle must be involved, one that came to be named the neutrino (little neutral one in Italian – originally a joke by another physicist, Enrico Fermi).
In 1930 Pauli wrote a letter to his colleagues suggesting a new particle, the neutrino, to account for the missing energy, playfully addressing them as:
“Dear Radioactive Ladies and Gentlemen,”
And going on:
… I agree that my remedy could seem incredible because one should have seen those [neutrinos] very earlier if they really exist. But only the one who dares can win …
It was a brave, but somewhat desperate suggestion. Then in 1956 the confirmation of the detection of neutrinos was published, leading to the award of the Nobel Prize in Physics to Frederick Reines in 1995 – 39 years later!
The first detection of a neutrino in nature was made in February 1965 in a special chamber 3km underground in a gold mine near Boksburg, right here in South Africa. Finally, neutrinos in nature could be detected.
First-year university astronomy students learn to calculate that hydrogen fusion in the Sun creates nearly 200 trillion-trillion-trillion neutrinos every second in the process of generating the energy that shines from the Sun. It is a small fraction of those – 100 trillion – that pass through your body each second. They come straight from the nuclear reactions in the centre of the Sun to you in a little over 8 minutes.
A energy is generated in the core of the Sun by hydrogen fusion, the light from that takes 100,000 years to work its way to the surface through the opaque gasses that make up the Sun. So, the light that falls on Earth tells us how much energy the Sun was making in its core 100,000 years ago, whereas the neutrinos tell us how much it was making 8 minutes ago.
From the 1960s we developed ways to stop a few of the astronomical numbers of neutrinos that pass through the Earth. We do that with tanks of ultra-pure water in mines deep underground, in sea water deep in the Mediterranean, and in ice deep in Antarctica. These detectors are deep underground, under water, or under ice to shield them from cosmic rays that would blind them from seeing the rare neutrino events.
When a neutrino has a rare interaction with a proton in water, there is a cascade of sub-atomic particles travelling at speeds greater than the speed of light in water or ice, 225,000 km per second. Those particles rapidly decelerate, generating blue light, known as Cherenkov radiation, that detectors can see. It is then possible to work backwards and figure out the energy and direction of the neutrino.
Those neutrinos tell us what is happening in the core of the Sun right now; others can come from distant supernovae, explosions that annihilate massive stars. And on February 12, 2025 an incredible neutrino from far out in the Universe was seen in the Mediterranean ocean.
This neutrino was detected by a European collaboration called KM3NeT, which when completed will have 115 detectors spread out in a cubic km of deep ocean in the Mediterranean, hence the KM3 (km3 = cubic km) in the name and NeT for Neutrino Telescope. (Scientists do like their acronym names for projects.)
The February 12 neutrino packed 1000 times more energy than anything we can produce on Earth in our giant particle accelerators, such as the US$5 billion, 27-km long CERN Large Hadron Collider, which is buried 100m underground in Switzerland and France. The neutrino came from an empty part of the sky where we see no star or galaxy, hence it must have come from very far away.
What incredible event could have happened to generate such a neutrino from the far depths of space? We do not know. What fun to begin to figure this out, and to keep watch for more of them with the new neutrino telescopes.
https://www.talkofthetown.co.za/?p=79934&preview=true
- Donald Kurtz is Extraordinary Professor at North-West University in Mahikeng. He has an A-1 rating from the South African National Research Foundation, its highest rating.
- This article was first published in Talk of the Town, February 20 , 2025. The newspaper serving the communities of Ndlambe and the Sunshine Coast, with a weekly wrap of Makhanda news, is available at stores from early on Thursdays.
