Closing in on the Ghost Particle

Can we weigh the lightest particle in the universe?

Alastair Williams
Predict
Published in
3 min readFeb 25, 2022

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Photo by Paul Volkmer on Unsplash

At first physicists thought the neutrino, like the photon, was a massless particle. That, as Einstein told us, allows it to move through the cosmos at the speed of light. Observations seemingly confirmed this fact. Neutrinos do indeed move very fast, close to the speed of light (and not beyond, as one experiment mistakenly found in 2011).

That should give neutrinos some rather unusual properties. According to relativity, time appears to slow the closer one travels to the speed of light. In the limit — at c itself — time ceases to flow altogether. Photons, when they travel across the universe, do not experience the passing of time — and, therefore, cannot change or decay as they do so.

Photons follow this rule quite happily. But neutrinos, as physicists discovered with some shock in 1998, do not. Indeed, neutrinos do appear to change as they move, changing from one type of neutrino into another. That means, unless something is wrong with relativity, that they must experience time — and therefore must have a mass.

Since neutrinos are literally everywhere — countless trillions are passing through you right now — this mass could add up to something quite substantial. Indeed, some even speculate that neutrinos make up almost…

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Alastair Williams
Predict

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