If you have ever eaten a banana, then you have consumed an active radiation source. They are rich in potassium, a small fraction of which is the naturally occurring radioactive isotope 40K (potassium-40). While this might sound scary, the levels are safe for human consumption, and they have a fun side effect: neutrinos. Beta decay, one of the ways unstable atoms can undergo radioactive decay, converts the potassium into calcium with a side helping of elementary particles: an electron and a neutrino.
Sometimes referred to as “ghost particles”, neutrinos are actually cousins of the electron. Just like electrons, they are elementary particles that can’t be broken into smaller parts. Their intrinsic, quantum sense of rotation has the same value as the electron’s. There are fundamental differences, though. While electrons have an electrical charge and hence interact strongly with matter, neutrinos don’t and are said to be ghost-like because they interact with almost nothing. While you have been reading this, about 6000 trillion neutrinos passed through your body. None stopped to hang out, though, because we are essentially invisible to them.
Although every electron in the universe is identical to every other electron, neutrinos come in three flavours: the electron neutrino, the muon neutrino and the tau neutrino. Neutrinos also differ from electrons in another way: we understand a lot less about them. Explaining how they get mass remains a major mystery in particle physics. We aren’t even sure what their masses are, except that they are very small – so small that, for a while, we wondered if they were completely massless, which would mean they were capable of travelling at the speed of light. We know now that while they can get close to this maximum speed, they are slowed by a wee bit of mass.
Advertisement
But this isn’t the most amazing feature of neutrino behaviour as they move through space. Neutrinos are deeply unusual because their identities may change as they mature. An electron neutrino doesn’t necessarily remain an electron neutrino; it may at some point become a muon or tau-type instead. In other words, neutrinos are non-trinary, oscillating between and mixing three possible identities. We particle physicists think that this non-trinaryness is somehow related to how neutrinos get their mass, but we still aren’t sure.
The neutrino's birthplace was probably near an extremely massive black hole actively accreting material
Neutrinos may be a mystery in their own right, but they can give us an insight into other cosmic questions. In fact, neutrinos have become an important alternative to photons for observing the cosmos because phenomena like stars, black hole accretion discs and supernovae emit them.
The search for these cosmic neutrinos occasionally yields very exciting results. Last month, scientists at the European Cubic Kilometre Neutrino Telescope (KM3NeT) announced they had seen evidence of a 220 peta-electronvolt (PeV) neutrino, the most energetic one anyone has detected. An energy this high is comparable with the energetic environment around a neutron star, which is the second most compact type of object in the universe, after a black hole. By contrast, past measurements of neutrinos “only” came in at around 10 PeV. This new one is 20 times more energetic.
KM3NeT didn’t capture the neutrino directly, since, as I mentioned, these particles are hard to get our hands on. Instead, the telescope’s detector captured a very energetic muon, another elementary particle. Using our understanding of the theory behind neutrino and muon interactions, researchers were able to reconstruct the muon’s path. The most likely scenario is that the muon was created by a 220 PeV neutrino colliding with some other elementary particle.
The discovery raises new questions. Because the neutrino was unusually energetic, it may have a different origin story than other cosmic neutrinos we have seen. There are very few cosmic environments that create the conditions for such energy levels, which suggests that the neutrino’s birthplace was in the vicinity of a particularly massive black hole with a very active accretion disc.
One of the delightful aspects of KM3NeT’s discovery is that it creates the possibility neutrinos will help us get information about these extremely massive black holes, while also validating a 30-year-old proposal that such black holes could be used to better understand the non-trinary nature of neutrinos. This is a reminder that science can be a slow burn, requiring patience and persistence, while neutrinos are a reminder that identity transitions are a natural phenomenon. And the mystery around how and why these transitions occur helps us appreciate that the universe remains full of opportunity to better understand reality, when we are open to all the possibilities.
Chanda’s week
What I’m reading
I have been enjoying Never Say You Can’t Survive: How to get through hard times by making up stories by Charlie Jane Anders.
What I’m watching
For the first time, I am viewing every episode of The Golden Girls.
What I’m working on
During these hard times, I am trying to stay in touch with my sense of wonder and possibility.
Prepare to have your mind blown by CERN, Europe's particle physics centre, where researchers operate the famous Large Hadron Collider, nestled near the charming Swiss lakeside city of Geneva.
CERN and Mont Blanc, dark and frozen matter: Switzerland and France
