What are neutrinos?
And what the ghost particles could tell us about the universe
Neutrinos — they’re everywhere, in every second of time. They’re passing through your body and you’re not even noticing. They’re ubiquitous, yet they’re full of mysteries. Investigating these particles could help us unveil how the universe really works.
We’re in the amusement park. You’re having fun going up and down on the slides. I’m watching you from the ground. You whizz down one of the slides.
‘Wow, keep on like that and you’ll reach the speed of light.’
‘Pah, I’m too heavy for that! That’s what you explained to me yesterday. Heavy particles can’t go that fast. And I’m made of lots of particles, so I’m very heavy.’
‘Correct. And what did I tell you, what are the lightest particles?’
Why neutrinos matter — Sílvia Bravo Gallart
Elementary particles are the smallest known building blocks in the universe-and the neutrino is one of the smallest of them.
‘Indeed. And the second-lightest?’
‘Ummm. I think we didn’t talk about that.’
‘I mentioned it yesterday. The second lightest particles are… Neutrinos!’
‘Oh, yeah, now I remember. But I didn’t understand what they are…’
‘They’re tricky to understand. Even scientists are puzzled about them. Let’s take a break and I’ll explain it all to you.’
‘But only if I get some cotton candy!’
‘All right then.’
We sit down on one of the benches with a big portion of candy.
‘What do neutrinos do?’
‘Nothing. Almost nothing. There are trillions of neutrinos whizzing through your body every second. But you’ll never notice a thing.’
‘But if they do nothing, how do we even know they exist?’
‘Well, in very rare cases they will do something. They’ll leave a trace in a particle detector. But while we know that they exist, there are still a lot of mysteries that remain unsolved.’
‘What kind of mysteries?’
‘First of all, we have no idea where they come from. I mean, we know where neutrinos with low energies come from — from radioactive decays and from the sun, for example. But we’re also seeing neutrinos with very high energies. And we have no idea what objects in space they’re coming from.’
‘But if they’re flying around everywhere, can’t we just trace back their trajectories?’
‘That’s exactly what we’re doing. Since neutrinos hardly interact with any particles, they can fly in a straight line for billions of years. This is why we’re assuming that high-energy neutrinos come from very, very far away in space. We call these strange neutrinos cosmic rays.
‘The problem is, so far we’ve found nothing special in the locations in the sky where the high-energy neutrinos seem to be coming from. We would have expected some big quasars, galaxies, or supernovas in these points in space. But so far, the locations don’t seem special at all.’
Neutrinos can fly in a straight line for billions of years
‘Oh. That’s weird.’
‘It is indeed. It will certainly take another few years to determine why these locations are the origins of cosmic rays.
‘But there’s another property of neutrinos that scientists are excited about.’
‘Neutrinos may be their own antiparticle!’
‘Every particle has an antiparticle — a particle which is identical in all its properties except for having the opposite electrical charge. When a particle meets an antiparticle, they both vanish and leave a big explosion behind. Physicists call that annihilation.
‘Shortly after the Big Bang, there were as many particles as antiparticles. Then these annihilated. What’s mysterious is that while almost all antiparticles vanished, some particles were left over. These leftover particles are what we are made of.
‘If neutrinos are their own antiparticles, this could explain why we exist.’
‘Wow, so those funny particles could actually explain some really fundamental things!’
‘I think so, too! That’s just one of the reasons why the next few years will remain exciting when it comes to particle physics.’
‘I’m still more excited about the slides…’
‘You’re a child, it’s normal to be excited about slides! Go ahead and have fun.’
You dash off to the slides. But your head is still spinning from all the facts about neutrinos.