In this lesson, we will be discussing elementary forces between particles. And describe for the standard model of elementary particles really is. All forces around us can be identified as one of four basic interactions. So let's go over them. The first one is the electromagnetic force, it is responsible for the fact that positive and negative charges attract each other whereas same sign electric charges repel. It is the force that makes up atoms and molecules, and the whole structure of matter around us. In terms of elementary particles, a force corresponds to the exchange of a force particle, a boson. This means that when two electrons repel each other, this is due to the fact that they exchange a force particle. You can compare it somehow in the sense of playing football. The team players are the particles and the force between them, the way they move is predicted by the way that the ball is moving in the game. The ball is the force particle in this picture and makes the players interact. The fourth particle of the electromagnetic force is the photon light. A photon has no mass, but consists of pure energy. Electrons and protons fuel each other due to continuous exchange of zillions and zillions of these photons. Now the second force, the second force is called the strong force, it is a force that keeps the quarks together to form a photon or the nitron. It's also responsible for keeping protons themselves together, it's by the way it has a positive electric charge. But yet they stick together to form heavy atoms, and the strong forces responsible for that. The force particle of the strong forces called the gluon, coerce continuously exchange gluons and by doing so, are bonded together. And third, we have the force called weak force. This is a force responsible for radioactive decay and plays a crucial role in the burning of the sun, for example. The weak forces as its name suggests very weak, compared to the strong force and the electromagnetic force. There are actually three force particles, three bosons responsible for the weak interaction. These are the W plus, the W minus boson, and the Z boson. They're similar to the photon, but have one lies difference, they have a non zero mass. This W and Z bosons were discovered for the first time in 1983 in CERN near Geneva. And lastly, the fourth force perhaps the most fear familiar one is gravity. And it's the force that lets the apple fall down on the floor and keeps the moon and the Earth together. We don't understand gravity in terms of exchange particles as the matter of fact. The reason is that gravity is an extremely big force for an individual particle. And only with the whole set of particles like the whole Earth or the moon, you see the effect of gravitation. And a quantum model, in terms of elementary particles of gravity does not yet exist. And theoreticians are very busy in trying to find one. The three quantum forces, electromagnetism, the strong, and the weak force determine the dynamics of the interactions between the elementary particles. They provide besides gravity, the physics laws of our universe. These laws are written down in what we call the Standard Model of elementary particles. And the Standard Model of particle physics was developed during the late 1960s by Glashow, Weinberg, and Salam. However at that time, the mathematical description was only consistent, was not consistent, I have to say. And basically, the model at that time made ridiculous predictions when calculations were made. It produces infinities all the time. And this changed drastically with the insight of Martinus Veltman and Gerardus t'Hooft in 1970. These Dutch physicists were able to make the mathematical description consistent and the infinities in the calculations were gotten. However with one major consequence, the mathematical trick only works in the case that elementary particles don't have a mass. And here the hicks particles enters the scene. Because the solution, written down by Peter Higgs, Francois Englert and Robert Brout already in 1964 and in 1965 is to describe the mass of particles in an alternative way. And one way to look at it is the following. As I just said, the mathematical trick of Veltman and 't Hooft works for particles with no mass. And the trouble is that if particles don't have a mass, they fly away. They fly away through empty space with the speed of light. And this is what Einstein already predicted with his special relativity. You cannot create stable matter if the constituent particles have zero mass. And Higgs mechanism is very out and simply postulates that empty space is not empty at all. It's actually filled with continuous background field. We call this the Higgs field, and with this field all particles have to traverse it. And by traversing it, they interact with this background field. And they effectively they slow down, it's like they have to traverse it or or substance and they slow down. And in that way, the particles don't move anymore with the speed of light. They slow down, and we interpret this as the particle has obtained a mass. So the mass of the particles is not due to the fact that they have an intrinsic mass, or property called mass now. The mass of a particle is brought about by the interaction of the particles with the substance, with this background field, with the Higgs field. And the larger the interaction with that field, the larger the mass of the particle is. So for example, the top quark apparently, hasn't extremely strong interaction with this Higgs field and hence we say, it is heavy. And the up quark does not feel the Higgs background field very much. And therefore, it is light, it has a small effective mass. But how do we know that all makes sense? The Higgs theory predicts that the background field actually may have a fluctuation, a quantum fluctuation as predicted by Quantum theory. And these quantum fluctuations on top of the Higgs field corresponds to what we call the Higgs particle. So the discovery of the Higgs particle indicates the existence of the Higgs field. And that, that field is responsible for the mass of all the other particles and ultimately of all atoms of matter of you and me. And it was therefore a big time for particle physics when the Higgs particle was discovered in the Large Hadron Collider in CERN in 2012. It was found to have a mass of 125 gv, approximately equivalent to 125 proton masses. Currently, scientists are very busy in understanding if all the measured properties of the Higgs particle correspond to the predictions of the Standard Model. And only one tiny deviation from these predictions of the Higgs particles, means that understanding of where mass comes from fails completely.