document: My favourite particle : the neutrino I started writing about neutrinos because I love them . They are quite magical really ; the Universe is completely swarming with them -LRB- they are the second most abundant particle after photons -RRB- but we know practically nothing about them . And what we do know is really weird . I wrote down '' `` My favourite particle : the neutrino '' '' to fit in with the other particle articles . But it is n't my favourite particle . I toyed with '' `` Not my favourite particle : the neutrino '' '' but that just felt disrespectful . Then I decided it does n't matter what I write at the top of the page . Why do I need this post to fall into line with the others ? I suppose it 's because I want symmetry . I want that warm , comfortable-yet-exciting feeling you get when starting to read the next book in a trilogy . The neutrino was postulated -LRB- imagined-up -RRB- for a similar reason : the desire for symmetry . A hundred years -LRB- ish -RRB- ago there was lots of great experimental science going on . There was no clear idea what atoms were , but there was an understanding that they were n't solid balls . Ernest Rutherford and others had established , through experiments , that there was a nucleus in the center of an atom and had postulated that electrons were '' `` in orbit '' '' around this nucleus . He came up with this description based purely on his experimental results : there was no theory at the time that predicted this . I feel quite nostalgic for this sort of experiment-driven theory . In the era of the LHC it is not the done thing to come up with a new theory to describe what we see experimentally . In general , our results have to fit some theory that has already been proposed . When they do n't -LRB- they do n't -RRB- we tune the theoretical predictions to match our data , like twiddling a load of knobs -LRB- most of these theory predictors , which we call Monte Carlo , have twenty or thirty knobs -RRB- until we get some agreement . I am moaning about this , but really we do n't have a choice . Anyway , back to the good old days . Something called Beta decay had been observed . They had a name for it before they knew it for what it was . It was a nice bit of real-world , useful , observational science . It was noticed that some atoms would emit radiation without any provocation to do so . This is what we call radioactivity . Active radiation . Nothing to do with the radio , other than that radio also works via radiation . Radio is carried by electromagnetic radiation -LRB- my first love - the photon -RRB- . What radiation is is the loss of energy by something . That energy invariably travels somewhere else in some form or other . But the thing with radioactivity is it is not deliberate . It just happens '' `` naturally '' '' . When first discovered it triggered a whole host of money-making schemes , similar to modern-day homeopathy I suppose , except homeopaths make their millions out of plain water and packaging , whereas back in the day the quacks were raking it in by selling people radioactive face-cream . The scientists rebelled , lots of people got cancer and the business end closed . I laughed at these adverts and showed my mates , and then I felt really sad because I thought about people in the future -LRB- okay , the present -RRB- laughing at my friends for buying 0 % proof hogswart . Anyway -LRB- again -RRB- , the science continued , and we realized that what was happening was that part of the atom , in fact part of the nucleus , was '' `` decaying '' '' . It was established that the radiation being emitted was electrons , because the measurement of the emitted particles ' charge and mass was exactly the same as for electrons . The electrons were n't being thrown '' `` out of orbit '' '' , they were being emitted from the nucleus . This is interesting , because the atomic nucleus only contains protons -LRB- with a charge of +1 -RRB- and neutrons -LRB- with a charge of zero -RRB- . To get a negatively charged particle out of this we thought that the neutron must decay into a proton and an electron : zero charge = +1 and -1 . Great . This fitted nicely with the experimental measurement of the atom before and after it went through Beta decay . But this is where things stopped being comfortable . This picture predicts precisely the energy that the emitted radiation -LRB- the electron -RRB- must have . Depending on what kind of atom is doing the decay , we should be able to predict exactly how much energy the electron has.When they measured the energy carried by these emitted electrons , they found that the electrons could have any energy . This was a huge problem because conservation of energy was -LRB- and is -RRB- the closest we get to feeling comfortable with any knowledge in particle physics . It always works . We do n't have to appeal to the fact that the theory is beautiful and elegant , we just do cold , hard experiments . And warm , soft ones . Any kind of one . We always get the same answer : energy is conserved . Always . But not in radioactivity . Wolfgang Pauli was n't having any of it . There must be something else emitted , something that the instruments were not detecting . Pauli described the properties that this invisible particle must have ; It is neutral -LRB- no charge -RRB- , it has a small -LRB- or zero -RRB- mass and it has spin half like the electron . If the new particle was emitted along with the electron , then the electron could have any energy , because the total energy emitted could still add up to be the exact value predicted by the theory . The new particle would have to be a ghost , traveling through matter without interacting at all . Then we could explain why it was not detected . Of course this was not a comfortable place to be , but it was slightly less uncomfortable than violating the law of conservation of energy . Pauli was aware of his responsibility , famously saying '' `` I have done a terrible thing . I have postulated a particle that can not be detected '' '' . Wolfgang Pauli : he knew his stuff . Is it okay to make a prediction that we can not test ? Science is about trying to describe the way the world works ; our understanding should be based on what we observe and our theories should be testable . This is why some experimentalists laugh at string theory . I for one am happy to ignore it until someone comes up with a way for us to test its validity . Okay , some of the reason I ignore it is because it is hard , but I would be much more inclined to make the effort if I thought it was any use . Thankfully , the little neutral ones , neutrinos , can be detected . It is just very , very difficult to do . This is because they simply do not interact with the particles that make up matter . Neutrinos can fly past an atom as if it were not there . They are neutral , so they are not affected by electromagnetic fields . The only thing in nature they have anything to do with is the weak nuclear force , so they tend to just whizz through the universe , through planets and through us , leaving no trace . We do detect the odd one . The first was 25 years after Pauli 's prediction , by Cowan and Reines . Back in 1987 we saw a whopping 24 of them in just a few seconds , thanks to a supernova going off in the vicinity . But generally they are a bugger to catch . So what do we know about neutrinos now ? Almost nothing , but the little bit we do know is very interesting indeed . The standard model of particle physics -LRB- the theory that we are most happy with when putting together all the little bits of knowledge we have gathered from thousands of experiments over a hundred years -RRB- tells us that neutrinos have no mass , like photons . We now know that this is not the case . Neutrinos do have mass . We know that the three kinds of neutrinos must all have different masses , so at least two of them can not be zero . I feel like nobody really talks about this . Perhaps I hang out with the wrong kind of physicists . Neutrino experiments have shown the standard model to be deeply flawed , yet we still persist in calling it '' `` the standard model '' '' . I have finished writing this now without actually saying a single thing about why I love the neutrino . I have n't mentioned that they are able to change flavour -LRB- neutrino oscillations -RRB- or that they could be their own antiparticle -LRB- are they Majorana or Dirac particles ? -RRB- or that they can help us understand dark matter , or that the fate of the universe is in their hands . Each one of these things is completely deserving of its own post , written by someone who knows their onions and is able to write something that is not 90 % digression .
doc_bio_tags: O B I O O B O O B O B O O O O O O O O O O O O B O B I O O O O O O O O O B O B O O O O O O O O O O O O O O O O B O O O O O O O B I O O B O O O O O O O O B B O O O O O O B I O O O O O O O O B I O O B O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O B O O O O O O O O O O O O B O O B O B O B O O O O O O O O O B O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O B O O O O O B O O O O O O O O B O O O O O B I O O O O B O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O B O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O B I O O O O B I O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O B I O O B O O O O O O O O O O O O O O O O O O O O O O B O O O B O O O O O O O O O O B O O O O O O O O O O O O B O O B O O O O O O O O O O O O O O O B O O O O O B I O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O B O B O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O B O O O O B O O O O O O O O B O B O O O O O B O O B O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O B O O B O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O B I B O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O B O O O O O O O O O B I O O O O O O B O O O O O B O O O O O B B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O B I O B O O O O O O O O O O O O O O O O B O B B O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O B O O O O O B O O O O O O O O O O O O O B O O O O O O O O O O O O B O B O O O O O O O O O O O O O O O O O B O O O O O O O O O B O O O O O O B O O O O O O O O O B O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O B O O O B I I I I O O O O O O B O O O O O O O O O O O O O O B O B O O O O B O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O B I O O O O O O O O O O B O O O O O O O O O O O O O B O O O O O O O O B O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O B O O B O O O B O O O O O O O O O O O O O O O O O O B O O O O O O B O B O O O O O O O O O O O O O O B O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O B B O O O O O O O O O O O O O O O B O B O O B O O O B O O O O O O O O B O O B O O B O O O O O O O O O O O B O O B O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O B B O O O O O O O O B O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O B O O O O B O O O O O O O O B O O O O O O O O O B O O O B O B I O O O O O O O O O O B O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O