Beyond the Atom: Remodelling Particle Physics

[Music] wow [Music] um [Music] the large hadron collider or lhc for short is one of the largest machines ever built thousands of scientists and engineers involved billions of dollars when it's running it can use as much power as a good-sized city what it does is accelerate particles called protons to very nearly the speed of light around a 27 kilometer underground loop actually there are two beams of protons racing in opposite directions that can be brought together so they can collide now everyone loves smashing things together but with all the time and effort that has gone into the lhc there must be a really good reason for building a machine that does that and there is it just takes a while to explain but if you can understand the reason for building this giant machine there's a very good chance you'll come to understand something quite profound about this the everyday world more to the point you'll understand something about the bits that make up the everyday world and how those bits stick together and by everyday world i mean not only what you can see out there but the entire universe and not only the universe at this moment but right back to the beginning of time so it's a big job mainly the job of these people particle physicists there they are in their natural habitat the things they're trying to discover and understand the particles that make up our universe are at least as small compared to the large hadron collider as the lhc is compared to our entire milky way galaxy that is mind-numbingly small so small it's incredible that we can know anything about these particles but we do know things through a process of experimentation and making theoretical models physicists discover what kinds of particles exist and how they interact it's a journey that never stops discoveries revise models and those models suggest new experiments when you're a kid you learn that the world is made up of atoms an atom is a kind of particle a basic piece of matter and you're told to picture them as round balls later you learn that there are over a hundred different kinds of atoms that can be organized into a periodic table of elements but it turns out that atoms aren't the very basic bits that make up everything way back when experiments showed that there were particles called electrons inside the atom and if electrons are one part what is the rest of an atom and how do those two parts fit together coming up with an answer that question meant creating a model when you talk about building a model it's something that physicists have been doing for hundreds of years one observes the world makes measurements then tries to find a pattern in them and a mathematical description a picture of what's going on that can explain those results and be used to to derive predictions for other scenarios that one can test what scientists came up with was this a raisin bun the raisin bits were the negative electrons and the bun was the positive part that balanced out the negative charge of the electron so the whole atom was neutral not a bad idea for what was known at the time but you'll notice that we don't picture atoms as raisin buns these days and you can thank this guy for that the raisin bun model of the atom was exactly what ernest rutherford was testing when he devised a very clever experiment that produced one of the great breakthroughs of particle physics here's what he did rutherford started with some radioactive material that he knew emitted particles with a positive charge called alpha particles next he chose a target a very thin gold foil and positioned this so it'd be hit by the alpha particles then to detect what happened to the alpha particles after they hit the gold foil he encircled the whole thing in a chemical screen that would create flashes when struck by the alpha particles this is what he observed almost all the particles flew right through the gold foil and struck the screen on the opposite side a small percentage were deflected a little to one side or the other but the biggest surprise was when a very few deflected backwards as so they hit something very hard and immovable now the raisin bun model of the atom would not have predicted that last result but what model does so if you take an object and you know its energy and it's basically moving at the speed of light and it bounces back you get a classical estimate for how much force that atom would have to apply and if if the length scale of that energy transfer is much much much smaller than the atom itself that's where you say aha the atom is not some billiard ball it's got to have some structure in there that's much smaller than the atom that's how he arrived at his his model that there's got to be some solid core that's much smaller in time the nucleus itself was shown to be made up of smaller particles called neutrons and protons in effect this model reduced the collection of around 100 elements to a more fundamental model of just three particles it's easy to overlook just what a breakthrough rutherford made with this experiment not just for what it discovered but for the technique of colliding particles together as a way to penetrate inside the structure of matter before we move on we need to get a few physics concepts straight when you throw a ball into the air the ball is interacting with earth's gravitational field as the ball rises we see it slow down as the gravitational field exerts a force on the ball a physicist pictures this as the energy stored in the motion of the ball moving into the field and then back into motion as the ball falls back down in rutherford's experiment the alpha particle and the gold nucleus were interacting in the electric field surrounding the gold nucleus as the alpha particle moved directly towards the gold nucleus it began to slow in the field its energy stored in motion moved into the electric field and then back into motion as the particle flew off in the opposite direction in a particle collision the more kinetic energy particles start out with the closer together the particles will get the history of particle collider experiments is one of designing larger and larger machines to create higher and higher energy collisions in order to see deeper and deeper into matter as particle accelerators became capable of focusing energy in such a microscopic volume of space something quite extraordinary happened new particles were created i know that may sound like magic but at the scale of fundamental particles the rules of physics not only allow for it they predict that it will happen the key rule is e equals m c squared and what that means is that mass and energy can be converted one into the other just like you can convert british pounds into dollars at a certain exchange rate only the exchange rate between energy and mass is c squared the speed of light squared mass and energy are just two face of the same thing you cannot differentiate between one versus the other one one way to interpret this formula is to think of mass as just a type of energy and consequently if you have a lot of energy you can also create mass or another word matter you can actually see particles popping into existence in the particle detector of an accelerator for instance in this image from a bubble chamber detector the tracks were created by charged particles passing through the detector the detector has a magnetic field so particles curve one way or the other depending on whether they have a positive or a negative charge look closely at these two spiral tracks that appear out of nowhere and curve off in opposite directions what you can't see is the photon a packet of energy that was traveling through the bubble chamber when it suddenly turned into these two new particles this track curving to the left is an electron which has a negative charge but this track curving the opposite way has a positive charge and turns out to be the anti-particle of the electron called a positron an anti-particle that has the same mass but the opposite charge and so when you collide things with lots of energy what typically happens is that there's lots of particles there and they will be pair producing new kinds of particles all the time particles antiparticles as long as you have enough energy to produce their mass which is related to energy through v equals mt squared uh in all these collisions the things that are coming out were not there to start with and and so you're getting a sampling of all possible particles that could be produced at the energies that you have access to which makes it much more efficient you don't have to you have to choose the initial thing carefully to have the right ingredients you can collide almost anything at high enough energies and you're going to get everything out so how much energy do you need to create a particle about this much not much but it's not only a matter of how much energy you create but how much energy you can put into a very very tiny space in fact when particle accelerators began operating at high enough energies all kinds of new particles began showing up in detectors these weren't the particles that make up the matter around us instead they were massive particles that would pop into existence and then decay quickly in a few decades physics had gone from three fundamental particles found in the atom to a multitude they called it the particle zoo and physicists had to find a model that explained it [Music] so in the 50s and the 60s particle physicists were discovering new particles at a very fast rate and it was very unclear whether or not there was some underlying organizing principle sort of tying together all these particles or if each new particle was just sort of a separate entity with its own properties sort of unrelated to the rest of the particles when physicists develop a model they're finding a pattern that explains what they are seeing in their experiments in the everyday world we're pretty good at detecting patterns but some patterns are more complex than others here's what i mean you know those music visualizers that you can find on digital music players [Music] the ones that use information from the song you're playing to alter and transform the animation the result is a complex animation but rhythmic and beautiful now imagine that you lived in a world of music visualizers but you couldn't hear the music [Music] as beautiful as it seemed it would be difficult to understand why your world looked the way it did but if you were the curious sort you could by careful observation start to find patterns in what you see and with some creative thinking you could even reconstruct the underlying music and with some clever modeling you discover little things called notes which combine in different ways to create your world the particle zoo presented a similar problem what was the pattern that connected these particles together physicists did eventually discover pattern by arranging groups of particles according to certain characteristics the pattern revealed that most of these particles weren't fundamental at all they were made out of a combination of more fundamental particles called quarks there are six quarks in all 12 if you include their antiparticles the up and down quarks are particularly important for us because they make up the protons and neutrons that we find in the everyday world each proton or neutron is a bundle of three quarks two up quarks and a down quark make a proton two downs and an up make a neutron while quarks seemed to solve the problem of the particle zoo they left physicists with a new one how do these quarks stick together in the way that they do the answer required coming up with the new fundamental force of nature called the strong force the strong force is a fundamental force like the electromagnetic force but it acts very differently a core can have one of three charges which are labeled as the colors red green and blue combine them together and you get a color neutral particle and while the electromagnetic force gets weaker the farther you get from a charge the strong force is weaker nearest the particle and actually gets stronger as you move away now think about that for a moment as long as the quarks are near each other they're bound together loosely but the moment they try to separate they experience a stronger and stronger force that keeps them confined a bit like an elastic leash that pulls back the further out you pull it that makes it nearly impossible to separate quarks from one another what we call a proton or a neutron is really the very very small area where a group of quarks is confined by the strong force that nucleus that rutherford discovered so long ago is actually a seething mass of quarks held together by the strong force in groups of threes which we call protons and neutrons and around those held more loosely are the clouds of electrons but the complete model of particle physics involves more than the atom it has to include all particles that exist in the universe the current model of particle physics is called the standard model and it describes all the fundamental particles that we know of and the forces that make them stick together it's one of the most successful models ever it can make predictions about particles to an amazing degree of accuracy if you take the particles that we know about you ask what's the most general way in which those can interact at low energies consistent with quantum mechanics and relativity the standard model is it i'll describe it briefly so that we cover some of the particles i haven't mentioned so far first there are the particles that make up everyday matter the electron the neutrino and the up and down quarks then there are what we call the second and third generation of these particles more massive and unstable but otherwise identical to their lighter cousins in the standard model there are also particles associated with each kind of force these force particles called bosons carry the information of a particular force for instance the photon carries the information of the electromagnetic force the gluon for the strong force and the w and z particles for another force called the weak force so these are the particles that make up our universe and that have been detected in particle accelerators so far now it's one thing to make a list of particles but a complete physics model must describe how all these things relate to one another for a given model model meaning a description of how the world works there are a set of equations and and things like particles and particle interactions and forces all of these concepts are in the equations so for physicists the model will look something more like this mathematical equations that spell out how all these particles relate to one another now if we look closer we might notice a term in here let me find it here it is an h [Music] the h is a particle called the higgs boson and it took physicists decades and billions of dollars to find so why were physicists so sure that it existed for one thing the standard model doesn't work without it imagine you're standing on a subway and it stops suddenly yeah you keep moving forward that property of matter is called inertia inertia is a resistance to a change in motion and it is caused by the property of matter called mass in fact objects have inertia because of their mass most of the mass of everyday matter comes from the energy involved in holding its fundamental particles together e equals m c squared at work again but fundamental particles can have a mass too all on their own so the standard model needs to explain where fundamental particles get their mass that is where the higgs comes in here's how it works first the higgs boson is associated with a field and it's the higgs field that is the really important idea the higgs field fills the entire universe it's everywhere you can't get away from it like a sticky fog that has settled around everything because it is everywhere every particle is traveling through it and most particles are interacting with it when a particle moving in the field tries to accelerate it experiences a resistance the idea is that the particles don't have a mass on their own but only through their interaction with the higgs field how much resistance the higgs field exerts depends on the particle in question if you're a particle like an electron you interact with the higgs field and experience a resistance every time you try to accelerate on the other hand if you're photon you glide right through the higgs field without interacting you're massless and you go about your business at the speed of light remove the higgs from the standard model and it would predict that all particles would be massless particles just like photons which they aren't and it's a good thing because our world would be very different without the higgs field if you look at the the world around us we know all the atoms are built out of protons and neutrons now each of these protons and neutrons is itself built out of three quarks but different quarks and the higgs interacts with the higgs field interacts with these different quarks slightly different amounts that actually makes the neutron a little heavier than the proton the fact that the higgs mechanism makes the neutron a little heavier allows you to have a stable proton stable hydrogen the fact that it's only a small difference lets you build up the entire periodic table of elements so that means we can have all of the materials that this chair has built up that the world has built up chemistry and stars every process that that you know that we know and love and that makes that makes this world possible is due to is largely due to this precise difference between the proton and neutron masses but to prove that the higgs field exists physicists would need the biggest particle accelerator ever built hold on they've built one of those because fields are associated with forced particles called bosons if you detect a particular boson you have evidence of that particular field so when the large hadron collider detected the higgs boson that was evidence that the higgs field existed the lhc generated collisions that involved 10 times more energy than any previous accelerator the lhc the way it reaches these very very high energies has a number of different uh components to it and the the first which many people don't think about very often is the is the kind of particle that's being collided we're colliding protons which are 2 000 times as heavy as for example an electron we accelerate protons by passing these protons through an electric field that is being generated by accelerating cavities it's much like a surfer that gets pushed by waves in the ocean these protons get pushed by an electric field as they pass through these accelerating cavities and in gaining speed they gain a lot of kinetic energy it's a combination of a large size and very very strong magnets which you use to bend these particles in a circle to keep them under control there are these quadrupole magnets which are used to squeeze the beam down and make it as narrow as possible make the particles as dense as possible so that when they collide you have those very very high luminosities to give you the most number of collisions imagine protons as fish bowls and the quarks is ice cubes tossing around inside and they are flying towards one another at high speed most of the time the balls would just smash together but every so often two of the ice cubes would also collide in the lhc it is the rarer collisions between quarks and the gluons that hold the quartz together that produce the really intense energy needed to create a higgs boson it's one thing to create a higgs event but you also have to be able to see it happen no particle accelerator is complete without its detectors our detector is made up of different layers of sensors and each of these sensors is designed to measure the trajectory the energy and also the different types of particles that emerge from this head-on collision we wouldn't directly see the higgs but rather the different types of particles coming from the disintegration of the higgs and by the pattern of those particles physicists knew a higgs boson must have been present so here's the problem though um so you have billions upon billions of particles flying into this detector every fraction of a second okay and you have to digitize all the information that represents their trajectories okay you got to digitize all that information you got to pump it up to computers to throw out most of it because at a raw level you have well over uh terabytes per second flying up into the main processors that are actually in the collider halls and that's got to be filtered down to a more manageable level which you can then pump out into a computing grid to reconstruct exactly what happens in these events at a billion or so interactions every second there is a lot of data to sort through and even though physicists knew what kind of signature a higgs event would produce in the detectors sifting through all those events and collecting enough data to find what they were looking for took years but when at last the discovery was announced on july 4th 2012 all those years of effort filled in a critical part of the standard model but the lhc is doing much more than looking for one particle it is delving into matter at the smallest scales ever allowing us to look into a realm of very high energies and it is recreating conditions that haven't existed since a split second after the big bang and if experience has taught us anything it's that when you explore deeper into matter there's a very good chance you'll discover all sorts of new things on what scale the lhc will be important we don't really know yet but what we do know is that it's answering questions that have been the driving questions of particle physics for 20 years what i would really love to see and what i've been working on is looking for substructure to quarks so this is looking for excitations and quarks this would be earth shattering in the sense that it would take us to the next level it would take us to something smaller than a quark using techniques that were developed 100 years ago but at much much higher energies i hope that they see a candidate for dark matter because that's one of the biggest puzzles as a cosmology is that we are facing with uh where is this what is this mysterious matter that is dominating our universe but we don't have a clue what it is so let's hope there is a candidate and unless they can see it that would be a big discovery for me i think and i truly believe that in 20 30 50 years from now in physics textbooks we'll be talking about the pre and and post lhe in terms of our knowledge and description of nature [Music] what i'm really curious about is how science will follow up on this experiment how deep can you explore until the little bits that make up our everyday world and what kind of models will come out of what is discovered i guess what i'm saying is that even this moment in science will pass and we will move on to new discoveries new models of the world and that is pretty exciting [Music] particle thistles [Music] you

Channel: Perimeter Institute for Theoretical Physics

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Keywords: teaching, pedagogy, education, science, particle physics, Higgs boson, Large Hadron Collider, CERN, LHC, Standard Model, quarks, momentum, energy, scientific models, process of science, subatomic, Perimeter Institute, physics, lesson plans, classroom resources, Canada, Ontario, STEM, classroom, students, teacher resource, teachers

Id: 5lqAvC46OjY

Length: 26min 46sec (1606 seconds)

Published: Wed Aug 25 2021