Did you know Neutrinos have Mass? I didn’t even know they where Catholic!

Terrible jokes aside, this was actually a HUGE discovery in the world of physics, so it’s no surprise that two of the scientists responsible, Takaaki Kajita and Arthur B. McDonald, where awarded this year’s Nobel Prize. Their research led to the discovery of the phenomenon now called “Neutrino Oscillations”, proving that these elementary particles do in fact have mass. Now, at this point that will likely not mean anything to you (it meant f**k all to me at first!), and before we dive into the explanation, we’re going to need a brief history of these elusive particles.

Neutrinos were first proposed by physicist Wolfgang Pauli when he attempted to explain conservation of energy in beta-decay; a type of radioactive decay in atomic nuclei. Noticing that some energy was missing upon this decay, he suggested that some of it was carried away by an electrically neutral, weakly interacting, and extremely light particle. This concept was such a mind-f**k that Pauli himself had a hard time accepting it’s existence – “I have done a terrible thing, I have postulated a particle that cannot be detected.” But this all changed in June 1956 when physicists Frederick Reines and Clyde Cowan noticed that these particles had left traces in their detector. This was big news, and as a result many experiments began to both detect and identify them.

So! Where do these particles come from? Well some have been around since the very beginning of the Universe, created during the Big Bang, and others are constantly being created in a number of processes both in Space and on Earth. These processes include exploding supernovas, reactions in nuclear power plants, and naturally occurring radioactive decay. This can even occur inside our bodies, with an average of 5000 per second being produced when an isotope of potassium decays. Don’t worry! These things are harmless (remember – weakly interacting) so there’s no need to go on a neutrino freak-out. In fact most of the neutrinos that reach Earth originate in nuclear reactions inside the Sun, a fact we’ll need to remember for later. There are also three types (or “flavors”) of neutrino according to the Standard Model of Physics (electron-neutrinos, muon-neutrinos, and tau-neutrinos) and the exact flavor is determined by which charged particle is also produced during the decay process (electron / muon / tau-lepton). The Standard Model also requires these particles to be massless, which will also be important later on.

Now that we know all this, we can let the experimentation begin! Both of the Nobel Prize winning scientists were working with research groups attempting to detect, quantify, and identify neutrinos arriving on Earth, albeit on different parts of the globe. It is also worth noting that both detectors were built deep underground in order to reduce interference from neutrinos produced in the surrounding environment. Takaaki Kajita was working at the Super-Kamiokande detector, which became operational in 1996 in a mine north-west of Tokyo. This was able to detect both muon and electron-neutrinos produced when cosmic radiation particles interact with molecules in Earth’s atmosphere, and could take readings from both neutrinos arriving from the atmosphere above the detector, and from those that had arrived on the other side of the globe and moved through the mass of the whole planet. Given that the amount of cosmic radiation doesn’t vary depending on position, the number of neutrinos detected from both directions should have been equal, but more were observed arriving from above the detector. Neutrinos were the cause of yet another mind-f**k… and it was suggested that if they had changed flavor, from muon / electron to tau-neutrinos, then this discrepancy would make sense.

Fast forward a few years to 1999 and the Sudbury Neutrino Observatory had become active in a mine in Ontario, Canada. This is where Arthur B. McDonald and his research group began measuring neutrinos arriving on Earth from the Sun using two methods; one could only detect electron-neutrinos, the other could detect all three flavors but not distinguish between them. Remember that most of the neutrinos arriving on Earth come from the Sun? Well it was also known that reactions within the sun only produce electron-neutrinos. This meant that both detection methods should have yielded the same results, as only electron-neutrinos would be detected. However, measurements of all three flavors were greater than the readings for electron-neutrinos only. This could really only mean one thing, the neutrinos must be able to change flavors.

This is where things get REALLY confusing, as neutrinos need to have mass to be able to change flavors. Why? The answer lies in Quantum Mechanics, and a phrase i’ve frequently heard is: if you claim to understand Quantum Mechanics, that only confirms how much you don’t. Now, I’m gonna need you to bear with me here, as I’m going to attempt to explain this while confusing you as little as possible, a task that gave me a BAD headache while planning and researching. We’ll start this endeavor by stating that neutrinos can be classified in one of two ways, by their flavor (three types) or by their mass (also three types). We’ll also need to point out that, thanks to the “Uncertainty Principle”, if you know the flavor of a Neutrino, you cannot know it’s mass, and vice versa. This means that you cannot know the mass of a muon-neutrino / electron-neutrino etc. At all. It’s simply not possible. This ALSO means a neutrino of a precise and identified flavor exists as a precise superposition (or mix) of all three mass types. It’s also worth noting that each flavor is a different mix of all mass types, but it is exactly this property that allows a neutrino to change identity. Welcome to the f**ked up world of Quantum Mechanics!

Einstein’s theory of special relativity states that a particle’s velocity is dependant on its mass and its energy. So, if we have an electron-neutrino moving through space, each of the three mass types it consists of move at slightly different velocities. It is this small difference that causes the mix of mass types to change as the particle moves, and by changing the mix, you change the flavor of the neutrino. Congratulations! You are now somewhat closer to understanding (or not understanding I guess…) the phenomenon of “Neutrino Oscillations”!

While all of this is excellent at causing brain pain, it also opens the gateway to completely new physics as, like I mentioned before, the Standard Model REQUIRES neutrinos to be massless, which is clearly not the case. This discovery marked the first successful experimental challenge to this model in over 20 years, and it is now obvious that it cannot be a complete theory of how the fundamental constituents of the Universe function. Physics now has many new questions.

Did you make it this far? Well done! Go lie down and let your brain rest. It won’t make any more sense tomorrow.


  • Neutrino Types and Neutrino Oscillations. Of Particular Significance: Conversations about Science with Theoretical Physicist Matt Strassler. Link: http://profmattstrassler.com/articles-and-posts/particle-physics-basics/neutrinos/neutrino-types-and-neutrino-oscillations/
  • How Are Neutrino Flavors Different? Maybe There Is Only One Vanilla. Cosmology Science by David Dilworth. Link: http://cosmologyscience.com/cosblog/how-neutrino-flavors-are-different/
  • Neutrino Physics. SLAC Summer Institute on Particle Physics (SS104), Aug. 2-13, 2004. Author: Boris Kayser. Link: http://www.slac.stanford.edu/econf/C040802/papers/L004.PDF
  • The chameleons of space. The Nobel Prize in Physics 2015 – Popular Science Background. The Royal Swedish Academy of Sciences. Link: http://www.nobelprize.org/nobel_prizes/physics/laureates/2015/popular-physicsprize2015.pdf
  • Velocity Differences of Neutrinos. Of Particular Significance: Conversations about Science with Theoretical Physicist Matt Strassler. Link: http://profmattstrassler.com/articles-and-posts/particle-physics-basics/neutrinos/neutrino-types-and-neutrino-oscillations/velocity-differences-of-neutrinos/

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