QCD: Visualizing the Strongest Force in the Universe: Quantum Chromodynamics

as far as we know right now there are four fundamental forces in the universe two of them act only at the quantum level so we don't experience them directly but two of them act at our macro level where we can experience them directly so for example when you jump up the only reason you come back down is because of gravity when you pick up magnets on a table or use your phone or any other electronic device it's because of the effect of electromagnetism electromagnetism is responsible for chemistry it's the force that keeps negatively charged electrons in a cloud around the positively charged nucleus of atoms but the force of electromagnetic repulsion is just as strong as the force of attraction so if we take the example of the helium atom which is composed of two protons and two neutrons in the nucleus if you do the calculations you'll find that the protons repel each other with a force of 20 pounds or about 10 kilogram weight if you consider that this is a macro scale force within something as small as an atom which is a million times smaller than a millimeter it is an enormous force on a quantum scale and this repulsive force is even higher in some bigger atoms so if the force pushing protons apart and the nucleus is so strong what keeps them glued together so tightly well it turns out that the force keeping them together is even stronger than electromagnetism in fact it is about a hundred times stronger it is the strongest force in the universe and physicists creatively call this force the strong nuclear force and amazingly we a-brained humans happen to have figured out how this force works pretty darn well it's really strong but works only at very short distances and it's also a very colorful force that's why its study is called quantum chromodynamics in other words it's the quantum theory of colors but as you'll see shortly these are not the kind of colors you're used to seeing what the heck am i talking about that's coming up right now before we talk about the fascinating science of the strong force i'd like to thank today's sponsor magellan tv for making this episode possible magellan is an ad-free documentary streaming service created by documentary filmmakers their collections are richer and broader than just about any other streaming service around you can dive deep into subjects like history culture and of course science and technology i'm excited to tell you that magellan is offering a buy one get one free gift card for the people in your life that you want to give the gift of knowledge click the link in the description to take advantage of this if you're new you'll get a free one month trial of magellan if you're already a member you'll get an extra month anytime you purchase a gift card there's never been a better time to spread the gift of knowledge i highly recommend magellan tv so be sure to click the link in the description in the 1920s it became clear that the atom was made up of negatively charged electrons and a positively charged nucleus this model although great in its simplicity left one big question if like charges repel each other how can you pack positive protons so close together in the nucleus it was obvious that there must be some extremely strong force which can overcome this repulsion one of the great minds that sought to answer this question was japanese physicist and later nobel laureate hideki ukawa he proposed the first significant theory of this strong force in 1934 he realized that since electromagnetic repulsion happens on large scales for example we can see its effects in our macro scale world when playing with magnets this strong force which opposes electromagnetic repulsion must occur on a very small range because we don't see the effect of the strong force in our macro world another clue that the range of this force has to be small is because there appears to be a maximum size of atomic nuclei any nucleus larger than the size of a lead nucleus is unstable suggesting that there is a point beyond which the strong nuclear force can't hold the nucleus together this also suggests that the range of the force must be smaller than an atom the analogy is like velcro if you put velcro on two similar poles of a magnet if they're close enough the velcro will overcome the repulsion and keep them together but if the magnets are not touching the velcro makes no difference yukaba proposed that there must be a massive mediator particle that transfers this force between protons and neutrons using einstein's energy equivalence principle equals mc squared and heisenberg's uncertainty principle which says that the uncertainty in energy times the uncertainty time is greater than or equal to h over four pi we solved the equation to find an approximate mass the range of this force can be found by multiplying c the speed of light times the time of the particle's existence time therefore is just the range divided by c we can set the range r naught as just the radius of the nucleus then by knowing the time we can find the energy or mass plugging in the numbers we get a mass of about a hundred mega electron volts or mev and this is remarkably close to what was found later to be the correct answer which was about 138 mbv yukawa coined the term meson to describe this particle meson means middleweight the leptons like electrons are lightweight and baryons like protons and neutrons are heavyweight as more experiments were being done in particle accelerators new mesons and other types of particles kept getting discovered dozens in fact this created a kind of crisis in particle physics because it was not thought that these particles could all be fundamental in 1964 murray gelman and george ziwig proposed the various mesons and other particles that interacted with the strong nuclear force could be explained if they were composed of even smaller but fundamental particles galman called these particles quarks from a term in a novel by irish author james joyce gelman proposed that there were three types of quarks now called up down and strange the up quark has a charge of two-thirds whereas down and strange have a charge of negative one-third all had a spin of one-half and they all experienced a strong force the quark model proposed that mesons were made up of quark anti-quark pairs but another way these quarks could combine is in groups of three quarks these became baryons or heavy particles such as protons and neutrons a proton is made of two up quarks and a down quark you'll notice that these charges add up to plus one the neutron is composed of two down quarks and one up quark you will notice that the charges add up to zero other combinations of these quarks form the other heavy baryons but this had a major problem and that is the idea of having three quarks within the same particles this according to quantum mechanics should be impossible because at least two of them would have the same quantum properties and according to the pauli exclusion principle two fermions like quarks with the same quantum properties cannot exist in the same location at the same time this principle is the reason you can have only two electrons in any atomic orbital because the electrons can have a spin of either plus one half or minus one half span if you put three or more electrons in the same orbital it would have to have a spin identical to one of the already existing electrons in that orbital but one of the detected particles called the delta particle was composed of three of the same quarks up up and up for example in that case since these particles all have a span of one-half we can have one particle that has spin plus one-half one that is minus one-half but the third one would have to have either a positive one-half spin or negative one-half spin making it identical to one of the other up quarks this would violate the pauli exclusion principle so how is it that a delta particle can exist with three of the same types of quarks this was quite a conundrum in 1964 american physicist wally greenberg proposed that quarks must have an additional property that gelman and ziweik did not consider this additional property is now called color this has nothing to do with the color that you can see with your eyes it's a metaphor for a kind of charge a color charge so if this was the case the three similar up quarks would not be identical because they would have different color charges and thus they would not violate the poly exclusion principle they could exist in the same place making up the delta particle there are three kinds of color red green and blue the color metaphor is apt because in order to get a neutral charge you have to combine colors just like when you combine the visible colors red green and blue you get a neutral white this is similar to the way electric charge is conserved in qed if you have two negatively charged particles coming in you must have two negatively charged particles coming out in qcd color must be conserved that is all the colors must combine to get a neutral white this means that any particles containing color charges must combine with others to form a neutral color three quarks if they form a particle must be red green and blue to be neutral they can't be red red and blue for example or blue blue and green now you might say what about the pi meson which is only composed of two quarks how do two quarks form a neutral color well it turns out that there are anti-colors called anti-red anti-green and anti-blue and when color anti-color charges combine they also form a neutral color charge so a pie meson for example can have a combination of a green color charged up quark and an anti-green colored upward such a particle would have a neutral color charge and a neutral electrical charge now while the predicted core combinations have been detected in particle accelerators providing credence to the quark theory model the problem was that no one had ever detected a quark by itself and because of this galman even thought that maybe his theory was just a mathematical construct and the quarks were not actually real particles scientists came up with an idea called quark confinement to explain that quarks must be confined somehow to within the nucleon which is another name for protons and neutrons this implied that there must be something strongly holding these quarks together within the nuclei some new particle must exist to confer this attractive force for quarks this particle was called the gluon because it acts like the glue that keeps quarks together how do these gluons work this is the crux of quantum chromodynamics similar to the way qed or quantum electrodynamics deals with electric charges and photons as the mediating particle for the electromagnetic force qcd deals with color charges and the mediating particle for it called gluons like the photon gluons are massless the difference is that the photons are electrically neutral so while they transmit the electromagnetic force they do not experience it nor do they interact with each other the gluon on the other hand not only transmits the strong force but also has a color charge itself so it experiences the strong force meaning gluons interact with other gluons this has other implications as well in qed when electron emits a photon it remains negatively charged but when for example a red quark emits a gluon since the gluon will carry the red color it will result in a change of color of the cork to green or blue the colors are constantly changing within the nucleons as gluons are being emitted and absorbed all the time as you pull two corks apart the strong force acts like a rubber band the further you pull them apart the more and more energy it takes this tends to pull the cork back inside the proton or neutron these gluon gluon interactions can strain the color fields to string like objects called flux tubes which exert constant force when stretched but if a cork is pulled with enough energy it pulls away until the flux tube breaks like a rubber band if you try to pull it apart it becomes increasingly difficult but if you put in enough energy the flux tube breaks just like a rubber band breaks but the energy expended in pulling the corks apart results in a surprising outcome at the instant the cork is pulled free from the nucleon a new cork is formed in its place and the quark that just pulled free will at the same instance be coupled with another newly formed anti-core this quark anti-corp pair is a meson more and more mesons can be created depending on the energy available it is these mesons that mediate the remnant of the strong force which binds protons to other protons and neutrons in other words residual pi mesons cause attraction between neutrons and protons the reason why it's so hard to pull the cork out of the proton is that you must use enough energy in the process to create at least two new quarks this is called confinement and it's the reason quarks remain bound to the nuclei this is also the reason why no free quarks have ever been detected note also that the mesons are a form of matter antimatter particle which do not last long and are unstable the longest lasts for only a few hundredths of a microsecond this effectively limits their range to within less than the diameter of a proton one of the most important aspects of the strong nuclear force is that almost all the mass of an atom is due to qcd not the higgs field how is this possible let's take a look at the proton for example a proton consists of two up quarks and a down quark the higgs field is responsible for giving rest mass to quarks in this case the combined mass of two up quarks and a down quark is 9.1 mbv but the measured mass of the proton is 938 mev so it weighs about 100 times more than its constituent particles where does this extra mass come from well you have to remember that quarks and gluons are moving very fast in a very tight space to keep that much energy in such a small volume requires a lot of binding energy it is this combination of binding and kinetic energy that makes up about 98 of the mass of an atom the mass comes from the energy of the quark gluon quark plasma the strong force inside the protons and neutrons as well as the force holding the nucleus of atoms together only about two percent of the mass actually comes from the higgs field which gives rest mass to the quarks so most of our rest mass and the rest mass of the universe is actually energy confined within the nuclei of atoms due to a force we can't directly experience hopefully this gives you an intuitive feel for how the strong nuclear force works in the next video i'm going to get more into some of the details of this as well as the other fundamental forces through findman diagrams so stay tuned you won't want to miss it and if you have a question posted in the comments below i will try to answer it i will see you in the next video my [Music] friend [Music] you

Channel: Arvin Ash

Views: 306,581

Keywords: quantum chromodynamics, strong interaction, quantum physics, strong force, particle physics, strong nuclear force, quantum mechanics, four fundamental forces of physics, strong force explained, hideki yukawa, murray gell-mann, george zweig atomic theory, wally greenberg, QCD, quarks explained, gluons explained, gluons and quarks, flux tubes, flux tubes quarks, gluon plasma, quark confinement, quark confinement in particle physics, where does mass come from, qcd physics

Id: KnbrRhkJCRk

Length: 15min 48sec (948 seconds)

Published: Tue Dec 08 2020

Reddit Comments

[u/Fewwordsbetter]

Great! Didn’t need the music bed, imho