The Map of Particle Physics | The Standard Model Explained

you know how sometimes kids ask the most difficult questions if a four-year-old asks you why do things exist you'd probably struggle to give them a decent answer because we don't really know if we peel away the layers of complexity from humans to cells to molecules to atoms to subatomic particles we reach the standard model of particle physics which is our best description so far of the fundamental machinery of the universe but it doesn't really answer why anything exists but it does describe what exists and how it behaves and that's what we'll be discovering in this video although it's probably not actually suitable for four-year-olds uh but for the rest of us this is a crash course in the fundamental rules of particle physics and it will help set the context for future news when people claim to have evidence beyond the standard model of particle physics you know it's good to first of all understand what the standard model is so that's what this video is for we know the standard model isn't complete there's lots of unresolved mysteries which i'll be taking a look at at the end of the video particle physics is one of the main branches of fundamental physics i've briefly covered it before in the map of physics and in the map of quantum physics but here we're going to dig deeper and i'm going to tell you all the basics of the fundamental particles of the standard model which i've summarized in this map which like all my others is available for free digitally and for money as a physical poster links in the description so here are all the fundamental particles we've discovered so far this set is what everything in the universe is made of there are a few distinctions to point out these are called the fermions and these are the bosons fermions make up the physical matter in the universe and the bosons mediate how those matter particles interact with each other and so also known as force carriers or exchange particles but technically the thing that sets them apart is a specific quantity called spin the fermions all over a spin of a half and the bosons have got a spin of one or zero for the higgs boson you can see these values in the chart here spin in quantum mechanics is a form of angular momentum which in general is some kind of rotating motion but spin specifically exists within elementary particles it's also known as intrinsic angular momentum but spin is a bit of a confusing name because it's not the same as classical spin like spinning a basketball there's no classical analog for spin so i've actually got no idea how to draw a picture of quantum spin so this is the best i can do but this is just an analogy in quantum mechanics every particle has a wave function and the spin of a particle tells us how the wave function behaves under rotation in 3d space so when we rotate the spin one particle we see face one then two then face 1 again so for a spin one particle the wave function looks the same after one full rotation but for a spin half particle you need two full rotations so as you rotate it you see face one then face two then face three then face two then back to one again so the wave function looks the same only after two full rotations hopefully that gives you a decent picture of spin but um it's a complex topic which i won't really be able to do justice in the time frame of this video but how it works isn't the important thing for us for this video the important things are the consequences of these different kinds of spin and there's two things you need to know about the first is that spin has to be conserved is one of the conservation laws of particle physics basically there are a bunch of rules about how particles can interact with each other these are known as the conservation laws of particle physics and they say that there are certain quantities like spin that you can't create or destroy in a particle interaction the same amount of spin going into a reaction needs to come out at the other end one example is an electron and an anti-electron known as a positron which come together and annihilate into a photon this is allowed because the particles going in have got a spin of a half each and they add together to give the photon spin of one so the same amount of spin came out as went in and so spin is conserved there's a number of other conservation laws which we'll meet throughout this video i'll just mention the two most fundamental conservation laws energy and linear momentum which along with angular momentum apply to everything in physics not just particles actually that's not quite true these aren't conserved in general relativity if space-time is changing but we don't have to worry about that for this video so this is the first consequence of spin the second is even more dramatic when you get a bunch of fermions together collectively they behave very differently to when you get bosons together in a group when you get fermions together they obey a rule called the parolee exclusion principle which says that fermions can't share the same quantum state and this is a very good thing the paraly exclusion principle is what makes the electrons in atoms populate all of the energy shells if it didn't exist they'd all collapse into the lowest energy state and atoms would behave very differently because it's the outer electrons in each of the elements that sets the kind of chemical bonds those atoms can make and chemistry creates all of the complexity in the universe so without the parallel exclusion principle every atom in the periodic table would behave almost identically to hydrogen and so none of the complexity that makes up chemistry and biology including us would exist because nothing would be solid when we sit on a chair it's essentially the power exclusion principle that's stopping us from just slothing through bosons are different they're allowed to share the same quantum state and this results in some really interesting quantum phenomena like super fluidity superconductivity lasers and many more and a more fundamental level all of the forces we know of come from a host of virtual bosons interacting with real particles interestingly it isn't just the fundamental particles that these rules apply to the bose statistics for bosons the parallely exclusion principle for fermions they apply to collections of particles as well so for example helium is known as a composite boson as it has two electrons which have spin half each if you add these spins together you get a whole number a boson this means that helium atoms behave like bosons and if you collect a load of helium atoms together and cool them down into a liquid they will all inhabit the same quantum state and become a superfluid with zero viscosity which means it can do weird things like flow up walls also all the amazing properties of superconductors like having zero resistance comes from electrons coming together to form a boson called a cooper pair and all these cooper pairs having the same quantum state so fermions and bosons are more general than i previously stated a fermion is anything with a total spin of a half integer and a boson is anything with a total spin being a whole integer okay that covers the consequences of spin now let's take a closer look at the spin half particles the fermions now you'll notice that the fermions are also split into the quarks and the leptons we'll look at the quarks first a single quark can't ever exist on its own quarks are always found bundled together the up and down quarks make protons and neutrons which make up all of the elements but quarks can also be joined together in other configurations like a pion which is a particle made of a quark and an antiquark pair like the electrons these quarks all carry electric charge but they have different amounts these ones have a charge of two-thirds and these have a charge of minus one-third these all add together to give the overall charge for example a proton is made up of up up down quarks and so has an overall charge of a plus one but a neutron is made up of up down down quarks where the charges all cancel out for an overall charge of zero charge is another quantity that's conserved in particle interaction so we should add that to the list quarks interact with all of the fundamental forces let me just explain what i'm showing here each of these show which particles interact with each of the fundamental forces we've got the electromagnetic force the strong force and the weak force so you can see here that the quarks interact with all of these forces also of note the quarks are the only particles that feel the strong force along with the carriers of the strong force the gluons you'll notice i've used the word field here instead of force and this comes from quantum field theory which are the fundamental rules that dictate how particles interact quantum field theory states that there is a field associated with each of the fundamental particles and particles are really excited states also known as quanta of their field so these forces are carried by specific fields which is what we mean when we say that these bosons are force carriers we also need to add the higgs field which doesn't cause a force but these particles highlighted here get their mass from interacting with the higgs field the other quarks the charm strange top and bottom quarks they're just like the up and down quarks but they've got larger masses also they're unstable in the first fraction of a second after the big bang all of these decays down into the up and down quarks by the weak force but that doesn't mean they don't exist in the universe today you can still find these heavier quarks in high energy cosmic environments things like supernovae or neutron stars and we also create them in our particle accelerators now from the quarks we've got a couple more conservation rules to add to the list barion number and color charge a barian is another technical term which i'll explain the proton and neutron are both baryons but more generally a baryon is any particle made up of an odd number of quarks but at least three quarks because quarks can't exist on their own barrier number is plus a third for each of the quarks and minus a third for the antiquarks so these add together to give a plus one barion number for the proton and the neutron and is minus one for antiprotons and antineutrons i haven't talked about antiparticles yet so let's do that now here's a version of the standard model but i've drawn all of the antiparticles as well as the particles as you can see all of the fermions have got antiparticle partners which have got the same mass as the regular particles but have got an opposite everything else like charge or barrier number and other kinds of quantum numbers we'll be meeting some more quantum numbers along the way the next one is color charge which we'll look at now color charge is a property of quarks and gluons and is an important part of the strong force interaction conceptually color charge is similar to the electric charge but instead of two kinds of charge there are actually three kinds of color charge which physicists have labeled red green and blue there's a restriction on the kinds of quarks that can join together to make a proton or neutron the rule is that you need to have the composite particle be color neutral which means for a proton or a neutron you need one quark of each colour the analogy physicists are using here is that the primary colours red green and blue add together to make white which we're calling colour neutral in fact you can see an example of this color edition here if you look really closely at a pixel on your screen right now from a distance it looks white but if you zoom in they're actually red blue and green added together but we shouldn't take this analogy too literally quark color has got nothing to do with color as we experience it through photons of light physicists discovered the rules of quark colour conservation and the mixing of primary colours of light just happened to be a useful analogy that fit quite well antiquarks have got anti-color so a green and an anti-green quark can come together and make a pion particle which has got neutral colour charge because the colours cancel out now there's actually an alternative analogy to the colour charge thing which i quite like instead of colors we've got arrows that always point in specific directions as shown here to have red color charge means to point in this direction specifically and no other these arrows can't be rotated now you can use these arrows to create valid particles a valid combination of quarks is one where the quark arrows make a closed path so a proton or a neutron would look like this just an equilateral triangle an antiproton or antineutron would look like this and we can extend this out to other shapes like a pion or a particle made of two quarks and two antiquarks called a tetra quark and what's cool is any other shape you can come up with which makes a closed path is also possible this is nice because i find it a more intuitive analogy than the color wheel so i thought i'd tell you about it now quarks can interact with each other through the strong force and this is where the gluons come in there are actually eight kinds of gluons each with a different colour anti-colour pair and through these gluons the different quarks interact with each other so now if i add the colour charges to the standard model diagram along with all the anti-particles we get this this is actually all of the fundamental particles that make up the universe which i think is important to show because there are a lot more than in the normal standard model diagrams okay so that completes my description of the quarks so now let's add some extra conservation rules to the chart conservation of barrier number and conservation of color charge now let's move on to the other half of the fermions the leptons here they all are and here's the most famous one the electron which is the hardest working particle responsible for chemical bonds electricity atoms emitting and receiving photons of light amongst others modern society is entirely built on our ability to get electrons to do what we want them to do so they're pretty cool the muon and tau particles are identical to the electron just with higher masses and they're unstable just like the heavier quarks all the muon and tau particles that were created in the big bang decayed into electrons in the first fraction of a second via the weak force which released lots of neutrinos and photons that are still flying around the universe today new muons are also created in natural processes one example is cosmic rays hitting the atmosphere but any other natural process with high enough energy will produce muons and towers as well here are the field interaction charts again and this time let's look at the leptons the electron muon and tau interact with everything except the strong force and get their masses from the higgs field below them are the neutrinos which have got very small masses and don't carry electric charge and so these only interact with the weak force because of this they're very difficult to detect as they interact with mata very very rarely there are three kinds of neutrino an electron neutrino a muon neutrino and a tau neutrino and these different kinds of lepton each have their own quantum number which you can think of as electroneus muon-nus and taunus tauwishness i don't know i'm making these words up in physics we actually call them different lepton flavors these lepton flavors are another conserved quantum number we can see this in this vitamin diagram of beta decay where the w boson decays into an electron and an electron anti-neutrino the electron has an electron number of plus one and the electron anti-neutrino has got an electron number of minus one because it's an anti-particle so these two numbers cancel out to zero which is what we started with and so the electron lepton number is conserved it's important to note that the different flavors of lepton are independent so for example an anti-muon wouldn't cancel out an electron this means we've got three more conservation rules to add to the conservation laws chart i want to say a little bit more about the neutrinos because we still don't fully understand them it's difficult to study neutrinos because they don't interact with matter very often the sun is constantly spewing out neutrinos from the nuclear processes inside and in any second we've got 400 billion neutrinos traveling through our body like it's not even there for a long time neutrinos were thought to be massless but we now know that each neutrino has its own very small mass we don't know what this mass is and it's still being investigated but we know that it's greater than zero and the sum of all three neutrino masses must be less than 0.3 electron volts because they have mass neutrinos can oscillate between the different lepton flavors for example electro neutrinos produced by beta decay have been observed to have a different flavor when measured in a distant detector another mysterious property of neutrinos is to do with their chirality which here i'm going to be simplifying as their handedness or helicity in order to draw you a picture which basically all means whether they're left-handed or right-handed we've observed left-handed or right-handed versions of every other particle but with neutrinos we've only ever observed them to be left-handed never right-handed and for anti-neutrinos we've only ever seen right-handed anti-neutrinos and not left-handed ones here i've actually drawn the helicity which is how the spin aligns with linear momentum because i don't actually know how to draw chirality i'm only mentioning this because there's a subtle difference between chirality and helicity in any particle with mass in massless particles chirality and helicity end up being the same thing but i'm i'm really getting into the weeds here we don't really need to worry about that for this video the main point is we don't know why we only get left-handed neutrinos and it breaks a couple of symmetries in physics called parity symmetry and charge symmetry okay before we get into the next bit i'm just going to give us a bit of a break because the next bit's quite a lot to get your head around so this is a bit of a breather basically neutrinos are kind of weird and there's still a very active area of research going on to try and study them but they're hard to study because they don't really interact with anything but it's interesting there's more neutrinos in the universe than photons so clearly they're very important okay let's get back to the symmetries of physics asymmetry in physics is an important concept to understand if we change some global property like in an instant we turn the universe into a mirror image copy of itself the question is would the laws of physics be the same after that transformation if so the universe would be symmetrical with respect to mirror images which is technically known as parity symmetry so is the universe actually parity symmetric the answer is yes except for the neutrinos this is because a left-handed neutrino in the original universe gets turned into a right-handed neutrino in the mirror universe but right-handed neutrinos don't exist so the two universes aren't symmetric for a similar reason neutrinos also break another symmetry called charge conjugation symmetry which is if you instantaneously turned all of the particles in the universe into their antiparticle counterparts would the universe look and continue the same mostly it would but again not for the neutrinos if you have a left-handed neutrino and you flipped it into an anti-particle it would become a left-handed anti-neutrino which doesn't exist so if we go over our conservation laws again in most cases in physics the mirror image or parity symmetry does hold it's conserved but not by the weak force interaction that produces neutrinos and the same goes for charge symmetry but interestingly if you apply both transformations parity and charge this combination is conserved by these weak force interactions because both transformations get you back to a valid neutrino so this combination called charge parity conservation or cp conservation is a more general conservation law which actually applies to everything except it doesn't in the 1960s it was found that the chaos a specific kind of particle made up of a strange quark and an up or down antiquark they don't obey cp conservation and subsequently many other examples of cp violation have been found in particle interactions but if we also add time reversal symmetry into the mix which is does the universe look the same if we run time backwards compared to forwards so we take particle interaction then we flip parity and then charge and then time this combination of transformations is conserved by all particle interactions and this is known as cpt symmetry did that make sense uh maybe not it's a lot to take in but historically physicists have been more and more frustrated that they need to relax these conservation laws as it makes the overall standard model less and less elegant the main takeaway point from all of this is that we've got different conservation laws which are conserved by the different forces anything within this box is conserved by the weak force anything in this box is conserved by the electromagnetic force and anything in this box is conserved by the strong force and as we're here i'll just fill in the rest of these conservation laws strangeness is a quantum number attached to strange quarks which is violated by the weak force but conserved elsewhere and this is also true for the other flavour quantum numbers charm top and bottom i mentioned time reversal earlier but it basically says the laws of physics look exactly the same if we flip the arrow of time and run it backwards interestingly this is actually equivalent to cp symmetry which i just talked about and this isn't conserved by the weak force but it is by the others and finally we have isospin z component and isospin magnitude which are quantum numbers to do with the up and down quarks but i'll be honest i don't understand these well enough to do a concise summary here but i've included them for completeness and so now at least you know that they exist okay so let's do a quick review of what we've covered so far i've described the fermions which contain the quarks and the leptons we've also seen all the conservation laws as well as all the field interactions for all of the particles these standard model interactions at the bottom are a summary of all the possible particle interactions that are allowed by the standard model which i looked at in my previous video on feynman diagrams so go watch that if you'd like more details on them i won't cover them in this video this brings us to the last part of the standard model we need to look at the bosons also known as exchange particles also known as force carriers the glue ones we've already covered when i talked about color charge and the strong force the photons are how you are seeing right now they carry the electromagnetic force and interact with anything with electric charge which is all of these particles however the photons don't carry electric charge themselves so they don't interact with each other which is why i haven't included them on this chart even though they're the carriers of the electromagnetic force it's a funny thought that because light is how we see things we can't actually see light as in we can't see it traveling past us because we can't bounce light off it as it travels past because light's not self-interacting but it's kind of crazy to imagine what the universe would look like if photons did actually bounce off each other we wouldn't be able to see anything because just the air or empty space would no longer be transparent you'd just see a jumble of like random photons everywhere and it would be a mess so it's a good thing that photons aren't self-interacting the w and z bosons are the force carriers for the weak force and have got very large masses which they get from interacting with the higgs field the z boson doesn't carry electric charge though only interacts with the weak force but the w boson can have either a positive or negative electric charge so interacts with the electromagnetic force as well as the weak force and finally we have the higgs boson which comes from the higgs field and particles get their masses from interacting with the higgs field when they move through it apart from of course the neutrinos the neutrinos get their mass from some other mechanism which we haven't worked out yet which is just one of our outstanding mysteries in particle physics the higgs boson itself has got mass because the higgs field is self-interacting and the higgs fields has got a different nature to the other fields and so doesn't result in a force like the strong weak or electromagnetic forces technically the higgs field is a scalar field and not a vector field like the others okay so that's my summary of the standard model of particle physics but before i go i want to talk about gravity when we learn physics we learn that the four fundamental forces are electromagnetism the strong force the weak force and gravity but i haven't mentioned gravity until now that's because we don't have a quantum description of gravity gravity is described by general relativity as the result of a curved space time people have predicted a gravity particle called a graviton which would be the particle associated with gravitational waves but unfortunately there's no way we can make a particle accelerator with high enough energies to probe this because gravity is so weak in comparison to the other forces even so physicists have been trying to squash the standard model and general relativity together for nearly a century now to try and make a theory of quantum gravity like string theory or a quantum theory of space time like loop quantum gravity i've done a previous video on all of this if you want to find out more now that's not an end to the mysteries in particle physics let's take a look at a few more a big question is the issue of baryon asymmetry according to the standard model at the big bang mata and antimatter particles were created in even amounts but if we look at the universe today everything we see is made of matter and not antimatter so where did all the antimatter go we don't know physicists have also made theoretical predictions of more fundamental particles beyond the standard model stuff we haven't discovered yet the most famous of these were the supersymmetric particles which should have been detected by the lhc but weren't so currently it's not looking good for the theory of supersymmetry also the most popular theory to explain dark matter is that it's made of some kind of particle we haven't detected yet although there are many efforts to do just this other questions are why are there exactly three generations of quarks and leptons what are the masses of the neutrinos what combinations of quarks are possible and what's up with the recent results on muon procession frequency protons are very stable particles but do they ever decay neutrons aren't stable but what exactly is the lifetime of a neutron do magnetic monopoles exist is there a planck particle why is gravity so weak what what is time as ever in science more research is needed so what does the future of particle physics look like historically we've built bigger and bigger accelerators to collide particles at higher and higher energies in order to recreate the conditions billionths of a second after the big bang but building new accelerators that are bigger than the ones that exist today would cost tens of billions of dollars which might be a tough sell so we'll have to see what happens there but there are many experiments we can still do with the current generation of particle accelerators and as well we've got several large detectors without particle accelerators which are mostly giant vats of liquid deep underground looking at high energy neutrinos or trying to detect dark matter particles here's just a quick sponsor message before i round the video up this bit sponsored by brilliant it's a website an app chock full of science and mathematics content in the form of question based courses watching youtube videos is great but to really learn something it takes practice and one excellent option is brilliant with its interactive questions which coach you bit by bit through a variety of courses in physics mathematics and computer science from the fundamentals like geometry or classical mechanics right through to cutting-edge subjects like cryptocurrency or quantum computing there's something for everybody i like it because i can pick it up at any time on my phone and actually do something useful instead of just doom scrolling twitter again and after a bit of phone time i've actually learned something which feels great if you're interested you can go to my special link brilliant.org dos or you can click on the first link in the description below and this helps me out because they know you've come from here and on top of that the first 200 people to sign up will get a 20 discount on the premium subscription which unlocks all of their content but there is a lot of content on there for free and they're adding courses all the time so it's worth checking out in any case ah so that's it we've reached the end of the video um i hope it's been helpful i'm aware that there's a huge amount of information to take in but i've summarized all in this image which you can study in your own time like all of my posters the digital version is free to download for your own use or educational uses but you can buy a physical poster as well from my dftba store and if you've got an inkling to buy it this is a fantastic way to support me and my channel to help me make more videos like this i obviously put a lot of work into them this was a huge amount of work for this video and i want them to be available as a free educational resource i've also got many kind people supporting me on patreon which is fantastic for me to uh surf the very unpredictable waves of the youtube algorithm um okay don't worry about subscribing unless you really want to but do check back every so often because i've got a lot of videos lined up and you can't necessarily rely on youtube actually showing them to you when they come out but if you check back on my channel every couple of months or so you should be fine also if you know someone who you think might be interested in this video share it with them that'd be cool otherwise thank you for watching and i'll see you soon you

Channel: DoS - Domain of Science

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Length: 31min 48sec (1908 seconds)

Published: Sat May 01 2021