Why Do Neutrinos Have Mass? A Small Question with Huge Consequences

[Music] if I were to ask you hey what do you think are some of the biggest unsolved problems in physics you might think of things like what happened during the Big Bang and is there a theory of everything because that's what you tend to hear about in documentaries or on random science news sites but actually those questions and many others are all connected to a kind of obscure unsolved mystery why do neutrinos have mass neutrinos are weird little particles that are notorious for being super hard to detect they rarely interact with other particles so they have a reputation for being kind of ghostly and this ghostliness brings with it a lot of mysteries and every time we solve one another one seems to pop up but these mysteries are really worth solving because it turns out they reveal a lot about the fundamental nature of reality now neutrinos aren't rare particles they're actually more common than every kind of particle that we know of except photons the particles of light in fact billions of them are passing through you every second but you'd be lucky to see even one hit any atom in your body they just really don't like interacting with stuff so like you might expect doing experiments with them is really hard but we have managed to work out a few things like we know that there are three kinds or flavors of neutrino and each is related to one of three other particles electrons muons and Tau's goals so there's an electron neutrino a muon neutrino and a Tau neutrino we also know that neutrinos have mass and that might seem obvious because most things do but we didn't know this for a long time in fact it took us until the late 1990s to figure it out around then two teams ran a clever experiment that observed neutrinos coming from the Sun they expected to see a certain number of electron neutrinos coming from our star just based on what they knew about it but instead the number of electron neutrinos that arrived was only a third of what they had predicted after some follow-up tests the researchers found that the other two-thirds had spontaneously turned into the other two flavors into muon and tau neutrinos now it might not sound like a huge deal but this process now called neutrino oscillations was super surprising like it the team a nobel-prize surprising see according to the laws of physics changes like that can only happen when particles have different masses but why is besides the point here but the key thing is for something like an electron neutrino to have a different mass than a towel or muon neutrino that means some of those flavours need to have mass in the first place measuring exactly what those masses are though is a whole other story for one there's some messy quantum probability stuff involved and that's because any flavor of neutrino can come in one of three different masses in other words when you detect say an electron neutrino there's a certain chance you'll see it with mass number one but also a small chance that it'll have mass number two or number three and the odds of seeing each of the three possible masses is different for each of the three flavors of neutrino and as if that weren't enough we don't know what exactly those masses are because they're so ridiculously tiny that we haven't been able to measure them directly like after neutrinos electrons are the next lightest particle and electrons are tens of thousands of times lighter than typical atoms but from what we can tell the heaviest neutrino is at least a half million times lighter than the electron and the fact that they are so much lighter than all the other particles is something that actually bothers a lot of particle physicists see we think we have a good understanding of how particle mass happens at a fundamental level it's called the Higgs mechanism and it says that the mass of a particle is determined by how strongly it interacts with something called the Higgs field so the question is what makes neutrinos interact so much less strongly with this field than the other particles and here's where the problem shows up the current best guess is that neutrinos don't interact with the Higgs field at all because they just behave too differently from other particles in too many ways for that to make sense normally that would mean they don't have any mass but well they clearly do and so now maybe you can see where the big question comes in after decades of research and countless experiments we've finally figured out that neutrinos have mass but we don't know why after all this searching we've basically run into a brick wall now it is possible that the answer will become clear with more research for instance some theorists predict that we'll eventually find that two of those mysterious neutrino masses are very close to each other that would be a very strong hint that neutrinos have some kind of sub structure like how protons and neutrons are actually made of smaller quarks and that would imply that we need to look even deeper inside neutrinos to see what makes them tick but that's not the most popular idea out there instead the most popular explanation has something to do with a property called the handedness of neutrinos to work out the handedness of a particle you look at the direction it's traveling and the axis it's rotating along I mean it's not really rotating because it's particle physics and everything is weird but it's a close enough approximation if the rotation axis points the same way as the direction of travel we say a particle is right-handed and if the access points the opposite way then it's left-handed now normally we see right-handed and left-handed particles but for neutrinos the rules seem to be a little different we only ever see left-handed neutrinos so where are the right-handed ones well the proposed idea is whimsically called the seesaw mechanism it predicts that the missing right-handed neutrinos do exist but are way too big for us to detect in other words the extremely light left-handed neutrinos also have extremely heavy right-handed counterparts and the idea is the two groups are connected essentially the lighter neutrinos get their mass from coupling to the heavier one somehow and as the mass of the left-handed group goes down the mass of the right-handed group goes up hence the seesaw so just as the left-handed groups mass is tiny the right-handed groups mass should skyrocket past the mass of every other particle like many estimates say there are tens of millions of times the mass of the heaviest particle we know of and unfortunately that would make them way too heavy to find with our current technology you need an accelerator that can reach outrageously high energies to find super-heavy particles so it's unlikely that anyone will confirm this idea anytime soon still there are big reasons to keep looking even beyond the fact that well we're just super curious and want to know why neutrinos have mass for one understanding neutrinos could help us find new theories for all of physics right now the gold standard in particle physics is called the standard model and like we've said before on this channel it's great but we also know that it can't be the end of the story I mean it's meant to model all particle interactions but it doesn't include gravity and well I'm not drifting off into space right now so that definitely exists for decades we've been looking for experiments to guide us to theories beyond the standard model and neutrino masses are one of those few windows that we have to that new physics after all to make progress and come up with new hypotheses physicists need experimental evidence for things that don't fit the standard model picture and the seesaw mechanism is exactly that a new idea that goes beyond the standard model based on new evidence but that's not the only reason people are so interested in this either and as is so often the case in physics a discussion about the smallest parts of the universe naturally leads to a discussion about the biggest our understanding of the very early universe is almost entirely based on mathematical models where everything is mixed together in a hot dense soup of particles we rely on our knowledge of physics to make those models and neutrinos play a big part in them that's because even if each one on its own has a miniscule amount of mass the sheer number of them means that their collective mass affects the way galaxies evolve and if there were any of these super heavy right-handed counterparts in those early days that would have added even more mass to things so that's another reason to keep looking into all of this it could tell us how galaxies changed and as a result how today's galaxies will change to but even beyond that we may also need to understand neutrino mass to understand why the universe still exists at all right after the universe formed physics predicts there should have been equal amounts of matter and antimatter antimatter being matter with properties like the opposite electric charge for instance an electron has a negative charge and an anti electron has a positive one we know that matter and antimatter behave identically in virtually all situations and when they touch they annihilate each other so why didn't everything blow itself up in the very beginning nobody knows but neutrinos could help explain it because some people think it's possible that neutrinos and antineutrinos do behave differently specifically we think that spontaneous shape-shifting we mentioned earlier the neutrino oscillations may happen differently for neutrinos and antineutrinos if that's true that could influence why matter one out over antimatter but to understand exactly how and what that means for the big picture we need to know more about the neutrinos masses which a the rates of those oscillations and we all know how much of a mess that is so in the end it seems like every time we learn something about neutrinos more questions come up like we know that neutrinos are the lightest particles that have mass except they might be made of even smaller particles and also they might be the heaviest particles by a factor of a hundred million or more and we don't know exactly how light they are and we don't know why they're left-handed and we don't know why they have mass at all neutrinos are weird but we are making progress in 2020 the t2k experiment in japan revealed tentative evidence that neutrinos can behave differently to anti neutrinos which would be huge if confirmed to be true and we'll have another episode about that soon so as time goes on these ghostly little particles are coming a little more into view and one day soon we'll hopefully bust this mystery wide open thank you for watching this episode of scishow article physics as you might have noticed is kind of a hard topic and we wouldn't have been able to spend so much time figuring out the science and putting together the graphics without the support of our patrons on patreon so thank you to everyone who makes this all possible if you want to support the work we do here and help bring more free educational content to the Internet you can go to patreon.com/scishow you [Music]

Channel: SciShow

Views: 406,088

Keywords: SciShow, science, Hank, Green, education, learn, Stefan Chin, Physics, Neutrino, Mass, particle, light, power, electron, muon, tau, Sun, universe, galaxy, space, astronomy, neutrino oscillations, Higgs mechanism, Higgs field, handedness, left-hand, right-hand, seesaw mechanism, Standard Mode, gravity, T2K experiment, Kamiokande

Id: pIq654AMHEw

Length: 10min 13sec (613 seconds)

Published: Wed Jun 10 2020