The ghosts of reality 🌬️ | Neutrinos

Michelle Ncube
Aug 22, 2025
3 min read

Updated: Oct 3, 2025

When I watch YouTube videos or browse the Internet looking for interesting physics topics, I always stumbled across the word neutrinos but never knew what they actually were. Now that I have a basic understanding of these particles, I’ll exploring them in this blog!

Other than dark matter, neutrinos are the ghosts of reality. We can’t detect a neutrino’s presence directly, only its effects it has on other particles.

So what are neutrinos and how are we able to detect them if they are practically invisible?

What are neutrinos?

Neutrinos are the most abundant subatomic particles that have mass in our universe. They are extremely tiny, have no electric charge and interact so weakly with matter making them difficult to detect. They can pass through almost everything and anything, just like a ghost!

Relic neutrinos

These neutrinos have existed ever since the Big Bang and were produced in huge numbers. About one second after the Big Bang, these neutrinos decoupled from other matter and began to travel freely through space. As the universe expands at an accelerating rate, their wavelengths continue to stretch due to cosmological redshift, reducing their energy.

This behaviour is possible as neutrinos exhibit wave particle duality, so act like particles and waves.

Even before cosmological redshift started to take place, relic neutrinos had relatively lower energy compared to solar neutrinos, so to this date, they are the hardest to detect as their energies are so low.

Solar neutrinos

Unlike relic neutrinos, solar neutrinos are much higher in energy. They born during the nuclear fusion reactions that occur in our sun. Physicists call these solar neutrinos. Millions of neutrinos are emitted from the sun every second, travelling close to the speed of light and aimlessly sailing throughout our universe.

How can we detect them?

About 1 in 10 billion solar neutrinos interact with matter so detection is very difficult. In the late 1960s, physicist Raymond Davis Jr and John Bahcall built a 380 cubic metre tank filled with perchloroethylene in a gold mine deep underground.

They believed when the electron neutrino (a type of solar neutrino) interacted with the chlorine atoms in the fluid, it would convert it into argon. So, by detecting how much argon was produced, they would be able to calculate the amount of neutrinos that passed through as only the effects neutrinos have on other particles/elements can be measured.

However, only a third of the expected neutrinos were detected which led to the Solar Neutrino Problem. For about 4 decades, scientists debated the source of the error. Was it the experiment ? Was the physics all wrong? Many other physicists repeated the experiment and got the same results.

It wasn’t until 1998 when two teams led by Takaaki Kajita and Arthur B. McDonald discovered neutrino oscillations.

Neutrino oscillations

It was found that electron neutrinos which were produced in the sun could change into muon and tau neutrinos on their way to earth. This is formally known as changing flavours, hence why only a third of expected neutrinos were detected as the original experiment was not sensitive to the other types of neutrinos.

This discovery also had a new implication that neutrinos must have mass as the observations that physicists Kajita and McDonald carried out implied that the masses of the neutrinos involved cannot be equal to one another. (They won a Nobel prize for this in 2015!) Furthermore, the fact that they have mass implies they have gravitational effects, despite its magnitude being small and therefore hard to detect.

Applications?

The discovery of neutrinos has led to talks about its applications in the real world. Here’s a paper written by Natti detailing the industrial applications of this [click here]

So to round of, even though neutrinos are invisible and we may never be able to see them directly. We are still able to detect their presence through the subtle effects they have on matter.

Really fitting for their ghost like behaviour ;)

Further reading / Fun topics

I came across some really good mysteries around neutrinos as well as topics that go into