Updated: Aug 4
Author: Krishnasri Gollakota
[Quanta Magazine. https://www.quantamagazine.org/a-video-tour-of-the-standard-model-20210716/]
What are the fundamental building blocks of the universe which make up you, me, the galaxies and everything else? “What is the matter made of on the most fundamental level" is the question we are in quest of for a long time! In this article, we will talk about the Standard Model of particle physics, viewed as one of the most successful theory of all time.
The Standard Model describes how everything in the universe is made up of twelve different types of matter particles, interacting with three forces, all bound together by a rather special particle called the Higgs boson.
Note that there is a fourth fundamental force, gravity, which is not included in the Standard Model. Therefore, either the Standard Model is an incomplete theory or gravity is an emergent force, i.e., not a fundamental force.
One can say that all matter is made out of three kinds of elementary particles: leptons, quarks, and mediators. As of now these can be viewed as the actors of a drama with peculiar characteristics.
There are six “flavors” of quarks (up, down, charm, strange, top and bottom) and six leptons (electron, electron neutrino, muon, muon neutrino, tau and tau neutrino). Everything we see around can be reduced to just three matter particles, the electron, up quark and the down quark. The proton and neutron each contain three quarks, the proton has two up quarks and a down, while the neutron has two down quarks and a up. Protons and neutrons together make up the nucleus, which with the electrons make up the atom. And atoms make up molecules, and so on...
Next, neutrinos. Neutrinos are not like the particles mentioned above; neutrinos are extremely light and barely interact with anything else. As you are reading this, around 100 trillion neutrinos are passing through your body. Therefore we have discussed the four matter particles. But Nature has a twist in the plot! There exists two heavier generations or copies of the above-mentioned particles.
We don’t see these second and third generation of particles in everyday life , because being unstable they quickly decay to the first generation of particles, but they do exist (from experimental observations).
Now, how could we miss talking about the forces, which are among the most important actors of this drama, and without which the drama would be quite boring to watch? In the Standard Model of particle physics, we have the strong force, the weak force and the electromagnetic force. There are particles called bosons, which act like force carriers.
Talking about the electromagnetic force, it acts on anything that carries the electric charge (it will not act on the neutrinos because neutrinos are electrically neutral.) The particle associated with this force is the photon. The strong force acts on quarks, and holds together the nuclei of atoms. The particle associated with it is the gluon. The weak force acts on subatomic distances, which governs the decay of unstable subatomic particles and also initiates nuclear fusion reaction that fuels the stars. The particles associated are the W boson and the Z boson. .
Next, there are some problems with the Standard Model. As quoted in Quanta Magazine:
The Standard Model has been a boon for physics, but it’s also had a bit of a hangover effect.
We discussed three fundamental forces, while there are actually four fundamental forces. The most obvious force, gravity is not a part of the Standard Model of particle physics (one of the reasons why we have no idea how to incorporate the general theory of relativity into the quantum world).
We are unable to account for several major features of the wider universe, including the action of gravity at short distances and the presence of dark matter and dark energy. Physicists would like to move beyond the Standard Model to an even more encompassing physical theory. But, as the physicist Davide Gaiotto put it, the glow of the Standard Model is so strong that it’s hard to see beyond it.
Yet we are in the quest of it!
1) Quanta Magazine:
2) Introduction to particle physics by David Griffiths