How 2 Fundamental Forces Unite: Electromagnetism & The Weak force - Electroweak force

Transcript

as far as we know there are four fundamental forces of nature gravity responsible for keeping you planted to the earth and the moon in orbit the strong nuclear force responsible for keeping protons and neutrons bound together in the nucleus of atoms the weak nuclear force responsible for some kinds of radiation and electromagnetism which is responsible for light and chemistry but remarkably at the moment of the big bang all these four seemingly different forces were thought to be one and the same but as temperatures cooled and energy is lowered the force is separated into distinct interactions of their own gravity is thought to have separated first at some of the highest energy densities and temperatures then the strong force separated at cooler temperatures what remained after that at relatively lower energies and temperatures was electromagnetism and the weak force as one combined force called the electro weak force well it turns out that the energies at which the electroweak force exists is something we can simulate in particle accelerators such as the large hadron collider in geneva and modern particle physics has a fairly robust theoretical basis that describes the nature of this combined force but how could these two forces be one and the same at a fundamental level because at the temperatures and conditions we live in today they seem utterly different and utterly incompatible for example electromagnetism is all around us in the form of light that we can see in chemistry that we can observe it has infinite range and is relatively strong the weak nuclear force on the other hand is pretty weak and something we can't directly see it's effective at ranges smaller than the diameter of a proton how is it that these two seemingly incompatible forces are one and the same at a fundamental level it turns out that the key to uniting these two forces is something that was only recently confirmed to exist and that is the higgs boson how the heck did we figure this out the incredible story of the electro week theory is coming up right now [Music] before i talk about the electro week theory let me tell you what inspired me to make this video i saw a fascinating documentary on magellan tv today's sponsor called shock and awe the story of electricity it's a fascinating journey about how electricity has become the lifeblood of modern technology and how the electromagnetic spectrum is buzzing all around us magellan is a new kind of streaming documentary service that was founded by filmmakers and producers who bring you premium documentary content featured subjects include history nature space and science you can watch it anytime with no interruptions and an amazing 4k magellan has a special offer for arvind spears right now if you use the link in the description right now you're going to get a free one month trial i highly recommend magellan tv be sure to use a special link i think you'll be impressed the best theory we have about the makeup and function of the universe is represented by the standard model of particle physics think of this as the periodic table for elementary particles these particles along with their anti-matter partners represent all the elementary particles we know about i want to draw your attention to the red particles these are called gauge bosons and they represent the mediating particles that carry three of the four fundamental forces of nature gluons mediate the strong nuclear force photons mediate the electromagnetic force and the w and z bosons mediate the weak nuclear force gravity is the fourth force that does not currently fit into this model you should be aware that there are actually two w bosons combined into one on this chart there's a w plus and a w minus boson which represent their electric charges you might ask what is a mediating particle the exchange of virtual versions of these particles confer the appropriate force so for example when two electrons are near each other the repulsion is due to the exchange of virtual photons the simple analogy is illustrated here the basketballs represent virtual photons the boat represents similarly charged electrons the exchange of momentum is what results in a repulsion similar to the way the exchange of momentum of the basketballs results in two boats moving apart attraction is also mediated by photons but it is analogous to the exchange of boomerangs and brings the two boats closer together this is a simple analogy of how charged particles attract or repel each other in either case the mediating particle is virtual photons the weak nuclear force also works via the exchange of a virtual w or z boson let's take a look at perhaps the most important weak interaction and that is the beta decay of a neutron a neutron is composed of two down quarks and an up quark an isolated neutron that is not part of a nucleus is unstable on average within 15 minutes one of its down quarks turns into an up quark by emitting a w minus boson this turns the neutron into a proton this w minus boson almost immediately decays into an electron and an anti-neutrino so what we detect in this decay is the electron and the anti-neutrino the virtual particle is not detectable the weak force is unique in this respect in that it is the only force which can change the identity of an elementary particle in this case a down quark becomes an up quark resulting in a neutron turning into a proton this is called a beta minus decay another similar decay called the beta plus decay turns a proton into a neutron this kind of weak force decay is important in making atoms stable and plays a big role in nuclear fusion reactions that power the sun the sun wouldn't shine without it and we wouldn't exist let's go back to the chart of the standard model you'll notice that the mass of particles are expressed in terms of energy using the mass energy equivalence equals mc square championed by albert einstein you'll notice one big problem in uniting the electromagnetic force with the weak force and that is the fact that photons are massless but the w and z bosons are massive very massive their masses are about 80 and 90 giga electron volts or gev this is over 80 times the mass of a proton which has a mass of about one gev or 1000 mav so given the differences between these particles and the fact that the weak force appears so different than electromagnetism how did scientists realize they were controlled by the same guiding principles scientists including american physicist sheldon glacial had noticed some clues indicating that the weak force and electromagnetism could be related for one thing even though electromagnetism is millions of times stronger than the weak force at large scales the theoretical strengths of both forces appear to be identical at very small lengths about 1 1000 the width of a proton suggesting a point where the two forces may merge as one in addition theoretical models predicted an additional gage boson named the z boson this weak force boson would need to be neutral like photons are glacial showed that an acceptable theory for the unification of the weak and electromagnetic interactions could be found but the two forces had to be treated as one as an electro weak force and this model predicted four bosons that would mediate this force those bosons were the w1 w2 w3 and b there were two problems with this model first it predicted a third weak force boson now called the z boson which was not predicted by any theory at that time the first evidence of this particle only came much later in 1973 with the detection of something called weak neutral currents in addition glashow's model only worked if all four bosons were massless it put the weak force and electromagnetism on equal footing as both would have massless particles one can call this masslessness a kind of symmetry but ironically this was a big problem because this did not fit observations something had to break the symmetry because the weak force was weak and interacted at only very small distances indicating that its mediating bosons had to be massive glacial could not find a mechanism for making the four massless bosons from his electroweak theory into the three heavy weak nuclear bosons and the massless photon you might ask why do these gauge bosons have to be massive why does mass make a difference in the distance that the forces can act this has to do with the heisenberg uncertainty equation which says that for virtual particles the change in energy times the change in time has to be less than h over 4 pi if we substitute mc squared for energy we can see that the greater the mass the shorter the time of the virtual particle's existence so the short range of the weak force can be explained by the fact that massive virtual particles just don't exist for very long whereas for electromagnetism since photons have no mass they can exist theoretically for a long time and thus interact over long distances so how do we reconcile the idea that photons and the w and z bosons are ultimately similar and have a common origin if there's such a huge discrepancy in their masses this remained a mystery glashau is even quoted to have said it's a stumbling block we must overlook in other words he thought his idea was right even though it did not fit observational data three years later in 1964 robert brought and francois anglair in brussels and peter higgs at the university of edinburgh developed a mechanism whereby mass could be given to elementary particles while maintaining a meaningful theory the browd inglair higgs or beh mechanism employed the properties of a field what is now called the higgs field to break the symmetry and it predicted another massive particle the higgs boson in 1967 and 1968 pakistani physicist abdul salam an american physicist steven weinberg independently took the ideas of the higgs mechanism and combined them with glashow's ideas to show how glashaw's three weak force mediating particles could gain mass while the photon of electromagnetism could remain massless in the same theoretical framework this is the basis of what is now the electoral week theory glashow salam and weinberg were awarded the 1979 nobel prize for their efforts so now the question is how does the higgs mechanism give rise to masses in the three weak force particles but not the photon to illustrate this i'm going to show a diagram that depicts the higgs potential the higgs mechanism can in a very simplified way be understood by considering the higgs potential the he's potential gives the higgs field the possibility of changing its effects at energy scales above 160 gev the higgs potential looks like this this does not do anything because at the origin the higgs field interactions do not result in mass and all symmetries are intact however at energies below about 160 gav the point at which electro weak interactions breaks down into two distinct interactions the weak and electromagnetic the higgs potential begins to look much different it looks more like this graphic this is referred to as the mexican hat potential because it looks like a sombrero the z axis is energy and the x and y axes indicate the vacuum expectation value this vacuum expectation value becomes non-zero below 160 gav vacuum expectation value is like the value of the higgs field this non-zero expectation value is what gives particles that interact with the higgs field mass as i've said before all particles prefer to be at their lowest energy state at energies of more than 160 gav the minimum of the higgs potential is at the origin where the vacuum expectation value is zero in other words everything is massless but at energies below 160 gav the minimum vacuum expectation value is not at the origin it is not zero thus particles fall into the new minima in other words the obtain mass you might ask okay why aren't all the four particles including the photon massive if this mexican hat potential applies to all four glacial particles well to put it very simply this has to do with the way the four particles interact with the higgs potential the simplistic explanation is that the photon remains at the bottom of the groove of the mexican hat and does not have any other movement it can freely move around this group without needing any energy and as einstein's equation equals mc squared says zero energy means zero mass the other three particles however interact with the higgs potential differently they oscillate around the low point of the potential this interaction with the higgs potential requires energy as they try to climb the potential again energy is equal to mass and this is how they gain mass so why is the electro weak force important you have to realize that the reason we are where we are today in our understanding of particles and forces is due in no small part to the search for the higgs boson which was needed to explain the higgs mechanism that united the weak force with electromagnetism even though the two forces seemed utterly different and this gives us hope that the next step in the unification which would be to unite the strong nuclear force with the electroweak force may also be possible no matter how different these two forces appear to be right now this unification would be the grand unified theory and when we achieve this the only missing piece of the puzzle towards a complete understanding of all forces would be a quantum level understanding of gravity called quantum gravity and when we unite the grand unified theory with quantum gravity we will have the biggest prize in all of physics and probably all of science the holy grail the mother of all theories the theory of everything let's keep going until we get there i'd like to thank my generous supporters on patreon and youtube if you have a question leave it in the comments and i will make every effort to answer it i will see you in the next video my friend [Music] you