A Side of Science: Ghostly Neutrinos

In my latest cosmic game connection, I discuss a recent discovery of a neutrino that came from near a black hole in a galaxy more than 4 billion light years away. These ghostly neutrinos may not be familiar particles, however, so what are they all about?

Credit: CERN
  • Neutrinos are elementary particles like electrons and quarks. That means that they can not be broken down into smaller particles.
  • Neutrinos shouldn’t be confused with neutrons, even though the name is very similar. Neutrons are the particles that make up the neutral part of an atom’s nucleus, and they can be broken down into quarks.
  • The existence of neutrinos was first proposed in the 1930s, a quarter century before they were directly observed. A particle was needed to conserve energy in a particular kind of decay, and neutrinos would fit the bill.
  • Neutrinos come in three different types, or “flavors” as physicists call it – electron neutrino, muon neutrino, and tau neutrino.
From nobelprize.org
  • The reason they are so “ghostly” is because they only interact with other matter through gravity and the weak nuclear force. Gravity you’ve probably heard of, but you may have only encountered the weak nuclear force in high school physics or chemistry. The important thing here is that while the weak force is actually stronger than gravity, it is only effective at very tiny distances. How tiny? It only works at a distance of a fraction of a proton. Compare that to gravity, which works at all distances. That’s why you remember gravity, but maybe not the weak force.
  • Because the weak force only works on very tiny distances, the neutrino needs to get very, very close to another particle before interacting with it.
  • When I first started studying physics (many moons ago), it wasn’t clear if neutrinos had mass or not. They appeared to travel at close to the speed of light, but it was hard to tell if they traveled *at* the speed of light. If they could go that fast, they would *have* to be massless, like photons (particles of light).However, there was another problem. Theory predicted a certain flux of neutrinos from the sun, but experiments were only seeing about a half of those predicted neutrinos. So, were the theories wrong? Or were we missing some of the neutrinos? One proposed solution was that the neutrinos could change flavor. The detectors of the time were only detecting electron neutrinos. Flavor oscillations isn’t unheard of in physics – quarks also do it. However, this could only work if the neutrinos had mass.This question has since been settled. Neutrinos have mass, and do change flavor. Solar neutrino problem solved. However, the fact that neutrinos had mass required modifications to the Standard Model of physics.
  • Even though we now know that neutrinos have mass, researchers can only place limits on that mass – they can’t pinpoint it. At this point that limit implies that the sum of the three neutrino masses is less than a millionth the mass of an electron. That’s tiny!

And, in case you missed my blazar neutrino video, be sure to check it out!


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