What is everything made of? This question led to the most successful scientific theory in history!

If we could divide an apple, or anything around you at this moment, into smaller pieces, what would we get in the end?

Ancient Greek philosophers were already thinking about this question more than 2,000 years ago. They believed that the four elements of water, fire, earth and air were the basic units that made up the world. But as physicists were able to explore smaller and smaller distance scales, they finally developed the most successful scientific theory of all time in the 1970s, called the Standard Model. This unremarkable-sounding name was probably first mentioned by Stephen Weinberg, who made great contributions to the Standard Model, in a lecture 50 years ago.

In short, the Standard Model describes the properties of the fundamental particles that make up everything (such as mass, charge, and spin), as well as the interactions between them. The fundamental particles here refer to particles that cannot be further divided. So, what fundamental particles does the Standard Model include?

We know that everything is actually made up of very small atoms. Atoms are familiar to most people. At the center is a nucleus, and around it are electrons. Electrons are the first fundamental particles discovered, and they play a vital role in physical phenomena such as electricity, magnetism, and heat conduction. But the nucleus is not fundamental. It is made up of protons and neutrons, which are about 1,800 times more massive than electrons.

Everything is made up of atoms, and atoms are made up of electrons and atomic nuclei. (Figure/Principle)

But later physicists discovered that protons and neutrons are not elementary particles either. They are actually made up of up quarks (u) and down quarks (d). Protons contain two up quarks and one down quark, while neutrons contain one up quark and two down quarks.

Protons and neutrons are made up of quarks. But this is a simplified picture. In fact, their internal structure is extremely complex. (Figure/Principle)

An amazing fact is that whether it is you, me, or your cat living on Earth, or distant planets, stars, or interstellar gas, they are all made up of up quarks, down quarks, and electrons. These three basic particles are mixed together in various ways to form everything we see today.

Everything we see today is made up of up quarks (u), down quarks (d) and electrons (e). (Figure/Principle)

Of these three particles, electrons are leptons. In addition to electrons, there is another very light and uncharged lepton called a neutrino. You may not be very familiar with this particle, and even feel that it has nothing to do with us, but in fact, trillions of neutrinos pass through our bodies every second, but we cannot feel them. They are also often called ghost particles because they hardly interact with matter. Neutrinos released from the sun can easily pass through the earth, so it is very difficult to capture these particles.

Electrons and neutrinos (ν) are both leptons. (Figure/Principle)

Now, we already have four fundamental particles, and it seems that our universe only needs these four particles. But physicists have discovered that these four particles are only the first generation of three generations of matter particles!

Three generations of material particles. (Figure/Principle)

The Standard Model also includes second and third generation matter particles. Each generation of matter particles is heavier than the previous one. For example, if we look at muons and tau particles, they are exactly the same as electrons, except they are heavier! The mass of a muon is about 206 times that of an electron, while a tau particle is more than 3,000 times heavier than an electron.

Except for their greater mass, muons and tau particles look exactly like electrons. (Figure/Principle)

But the problem is, in everyday life, we don't see fancy flowers formed from second and third generation particles. These heavier particles can be created, but they are very unstable and decay quickly into first generation particles. It seems that the Universe can function just fine without the heavier two generations of particles, so why are there three generations? This is one of the great mysteries in physics.

Fermions, or material particles, include quarks and leptons. (Figure/Principle)

OK, now that we know the fundamental particles that make up everything, how do they interact with each other? Now, we have to mention another type of particle in the Standard Model, the gauge boson, which is also known as the force-carrying particle. There are four known fundamental forces in nature, namely gravity, electromagnetism, strong force and weak force. The Standard Model describes the other three forces besides gravity, and these three forces are realized through the exchange of force-carrying particles.

For example, when two negatively charged electrons meet, they repel each other because of the electromagnetic force. In fact, they are exchanging photons, one of which is emitted and the other is absorbed. In other words, photons are the force-carrying particles of the electromagnetic force.

Photons (𝛾) are the force-carrying particles of electromagnetic force. (Figure/Principle)

We just mentioned that protons and neutrons are made up of three quarks. So what kind of force binds the quarks together tightly and prevents them from being separated? The answer is the strong force. Just as the electromagnetic force is transmitted by photons, the strong force is transmitted by a force-carrying particle called a gluon. Gluons carry "color charge" and can therefore interact with quarks that also carry color charge.

Gluons (g) were discovered in 1979 and are the force-carrying particles of strong force. (Figure/Principle)

The weak force, like the strong force, operates at very small scales. Inside a proton or neutron, the weak force can switch quarks. For example, the weak force can change a down quark inside a neutron into an up quark, which would turn the neutron into a proton. If the neutron is inside an atomic nucleus, this transformation would cause the atom to become a different element. The force-carrying particles associated with the weak force are called W and Z bosons.

In 1983, physicists discovered the W and Z bosons, which are the carriers of the weak force. The W boson is the only charged gauge boson. (Figure/Principle)

Now, one of the fundamental particles that we haven't mentioned in the Standard Model, and it was the last particle to be discovered in 2012, is called the Higgs boson. It's very special because it gives particles mass. Without it, electrons and quarks would be like photons, which have no mass.

The Standard Model consists of 12 matter particles, 4 gauge bosons, and the Higgs boson, a total of 17 fundamental particles.

The Higgs boson is the last particle discovered in the standard model. (Figure/Principle)

What we need to remember is that although I have just used the language of particles to introduce the Standard Model, it is actually written by a very complex quantum field theory. In quantum field theory, what is more fundamental than particles is the more abstract "field". Each particle has a corresponding field, such as quark field, electron field, Higgs field, etc. These fields permeate the entire space. The interaction between particles is actually the interaction between various quantum fields. For example, electrons gain mass by interacting with the Higgs field.

The Lagrangian of the Standard Model, which describes all the elementary particles in the Standard Model and the interactions between them. (Photo/THOMAS GUTIERREZ)

The Standard Model is arguably the most successful scientific theory ever. In some cases, its predictions agree with experimental results to an astonishing 12 decimal places, which is unimaginable in other fields.

Although the Standard Model has achieved great success, there are still many questions that it cannot answer. For example, it only explains three fundamental forces, but not gravity; it also cannot explain what dark matter and dark energy are; it also does not answer why the universe we live in is dominated by matter, not antimatter. Therefore, looking for experimental evidence beyond the Standard Model has become the top priority for many physicists. But this is by no means an easy task. What we can do now is to continue to be curious and grope forward in the dark.

This article is a work supported by Science Popularization China Starry Sky Project

Team: Principle

Reviewer: Zhang Shuangnan, Researcher, Institute of High Energy Physics, Chinese Academy of Sciences

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.