author: Mara Krause, 08.08.2025
Have you ever wondered where you end up when you keep zooming in on matter? What we are fundamentally made of? What particles your arm is made of? Or your brain?
This is a question already Greek philosophers theorised about.
To understand the world, we must understand the building blocks. We know that breathing keeps us alive. But what is air? What are oxygen atoms? What are protons?
So, scientists conducted experiments and proposed theories. Theories developed from atoms to smaller and smaller scales. Finally, ending up with the standard model of elementary particles.
A table of matter and force-carrier particles that cannot be broken further down.
The building blocks of everything you see, feel or have ever interacted with.
The universe is an orchestra of only 12 notes and four forces: the standard model. Somehow, from this minimal set emerges Beethoven, black holes, and your thoughts. (Note: this does not include dark matter or antimatter).
The Standard model even made its way in the high school curriculum and is widely known across the world. It is the most accepted theory at the moment. But why? How exactly does it explain our universe?
Background
Aristotle assumed that matter is infinitesimally divisible. Fire, water, earth, air are the fundamental elements.
As you might know, this turned out to be wrong. Although a classmate in year 7 brought up this idea again in our chemistry class with honest sincerity.
Other Greeks introduced the idea of atoms as indivisible building block of matter.
In 1911 Ernest Rutherford showed that atoms themselves have an internal structure and are not fundamental. This internal structure included protons, neutrons and electrons and was revolutionised multiple times.
From Dalton’s solid sphere model, Thomson’s plum pudding model, Rutherford’s nuclear model, and Bohr’s model, to the quantum model that describes electrons in orbitals around the nucleus.
School teaches the Bohr model. Electrons are on discrete energy levels around the nucleus, or shells. A few years later Schrödinger added the quantum touch and included the uncertainty of electron’s location. They have a probability area, demonstrated by orbitals.
A game-changer was the invention of a cyclotron, a type of particle accelerator, by Ernest Lawrence in 1929-1930. Protons were accelerated in a circular path through electric fields until they reach millions and billions of electron volts. Beams of particles are smashed into those fast protons which literally revealed a zoo of particles in the 1950s. There were so many, it was an embarrassment to the physics community. Robert Oppenheimer even said that the Nobel Prize that year should be received by someone who didn’t find a new particle.
Enrico Fermi said: „If I had known there would be so many particles with Greek names, I would have become a botanist rather than a physicist“.
This frustration and overwhelming flood of particles needed order. A new model. The Standard Model.
More protons were smashed, more theories proposed, more experiments conducted. And finally, we have come to a nearly complete model which I will present in the further article.
This is where we ended up:
Overview
The standard model consists of 12 matter particles and four force carriers plus the Higgs boson.
Matter particles are called fermions and force-carriers bosons. Physicists have never been great at naming things, but with the standard model, they truly outdid themselves.
Fermions can again be divided into quarks (build protons and neutrons) and leptons (like electrons). They all have a spin 1/2, an important property, while bosons have a spin 1.
Quarks and leptons have three generations (or categories) with similar properties, but only the first generation is stable.
Bosons carry one of the four fundamental forces:
- Strong nuclear force: hold nucleus together, carried by gluon
- Weak nuclear force: responsible for decay, carried by W and Z boson
- Electromagnetic force: responsible for electricity, chemical bonds, light…, carried by photon
- Gravity: attraction between massive objects like planets, no bosons known
Let’s start with fermions.
Fermions
As already mentioned, fermions can be divided into quarks and leptons.
Quarks
Quarks come in six flavours: up, down, charm, strange, top, and bottom. The up and down quark are the most important ones, called first generation. They are the lightest, most stable, and make up all the visible matter in the universe.
The second (charm and strange) and the third generation (top and bottom) are basically just heavier versions of the first.
- The strange quark is like heavier cousin of the down quark
- The charm quark plays the same role for the up quark
- The top quark is a massive version of the up quark, about 70,000 times heavier
- The bottom quark is the heavy version of the down
All quarks have spin, charge, and mass. But not only an electric charge, they also have a colour charge. Their colour is not related to actual colours but is just bad naming. It is a property of the strong force.
- Electric charge: quarks have only have a fraction of a whole charge. The up, charm and top = +2/3. Down, strange, bottom = -1/3.
- Colour charge: Quarks can be red, green, or blue. Those colors are not fixed, but change.
Quarks are never alone in nature. Either a pair of three quarks or a quark anti-quark pair (called meson).
Leptons
Leptons come in six types:
- Three charged leptons: electron, muon, and tau
- Three neutrinos: one paired with each charged lepton (e.g. electron-neutrino)
The most common is the first-generation electron, responsible for most of life’s processes and structures, and its electron-neutrino which occurs in beta decays.
The muon (second generation) is about 200 times heavier and decays quickly.
The tau (third generation) is about 3,500 times heavier and is rarely found in nature.
All three have electric charge of -1, but no colour charge.
Neutrinos
Neutrinos have tiny mass, no charge, and are incredibly hard to detect. In fact, they are able to travel through a light-year of lead without interacting with anything.
The number of neutrinos in the universe is unimaginably huge. Even more than photons in the cosmos, the particles of light.
About 400 billion neutrinos pass through each one of us each second.
But where do they come form?
During beta decay a neutron turns into a proton (or vice versa) emitting an electron (or positron) plus an anti-electron-neutrino (or electron-neutrino).
Nuclear fusion in the sun produces huge numbers of neutrinos. Additionally, radioactive materials in the earth’s crust emit neutrinos, probably 50 billion passing through you at this very moment. Even your bones emit neutrinos.
First generation matter: from particles to everything
Everything is only made of three elementary particles rearranged. Put them together and you get atoms. Put a few atoms together and you get molecules. Put a few molecules together and you eventually get a human being.
The first generation of fermions dominates everyday life: up and down quark and the electron. Nuclei of atoms are made of up and down quarks and electrons whiz around them.
Protons and neutrons are a combination of three quarks and are called baryons (another fancy name).
- Proton: 2 up + 1 down
- Neutron: 1 up + 2 down
Their inner structure explains their properties. Protons have positive and neutrons neutral electric charge. (up: + 2/3, down: -1/3)
- Proton: 2 x 2/3 + (-1/3) = +1
- Neutron: 2/3 + 2 x (-1/3) = 0
The down quark is slightly more massive than the up which is the reason the neutron is slightly heavier than the proton.
However, about 98% of a hadron‘s mass comes form interactions between the quarks and the force-carrier.
Second and third generation hadrons decay rapidly into protons and neutrons due to their greater mass.
Let’s have a look at the four fundamental forces and their bosons.
Forces and Bosons
As already mentioned, matter is hold together by four fundamental forces.
Electromagnetic force
The electromagnetic force interacts with electrically charged particles like electrons and quarks. Its boson is the photon.
It holds electrons around the nucleus and is the reason you don‘t fall through the ground.
The electric force is executed by exchange of virtual massless photons, or the particles of light.
Photons have no mass or charge and travel though empty space at the light speed of 299,792,458 m/s. Photons are quantum particles with discrete energy. According to the wave-particle duality light can be a particle and a wave, a photon or an electromagnetic wave.
Electric charges can be positive or negative. The force is repulsive between like charges and attractive between different charges. On the small scale of atoms and molecules, the electric force dominates and is responsible for the structure of nature. Only on a much smaller scale, it loses to the strong force.
Now, we encounter a paradox: How do protons stick together in a nucleus when a repulsive electric force acts on them?
Strong force
To solve the paradox physicists needed to shift their attention to a different force: the strong nuclear force. It holds quarks together and protons in a nucleus.
At the smallest level, quarks constantly exchange colours, a form of charge. They come in three varieties: red, green, and blue. Gluons, force-carriers of the strong force, carry the colour charge form one quark to another constantly near the speed of light.
At a larger level, gluons carry colour charges between protons and neutrons inside the nucleus. Precisely, they exchange pions, a pair of quark and antiquark formed form gluons.
This force holds the nucleus together and outweighs the electric repulsive force. But it is only stronger than the electric force in the range of 2.5 femtometers or 2.5 x 10^-15 meters. Beyond this scale, the electric force takes overhand.
This is why heavy elements with big nucleus decay. Likely charges protons lie too far for the strong force apart and repel each other.
In uranium, for example, the protons feel a bigger repulsion than attraction and decay.
Michio Kaku describes the importance of the strong nuclear force: „Without it, our whole world would dissolve into a could of subatomic particles.“
Weak nuclear force
The weak force is responsible for radioactivity. It is about a million times weaker than the strong force, hence its name.
The three force carriers are: W+, W-, Z. The W with positive or negative charge and Z with no electric charge.
A neutron is unstable and eventually decays into a proton or vice versa. This is called beta decay.
Let’s take the beta plus decay: proton decays to neutron. Or: two up + one down -> one up + two down. Logically, one up quark decays into a down quark.
This emits a W+ boson, one of the force-carriers of the weak force. This boson instantly decays into a positron and electron-neutrino.
The beta decay happens all the time in the sun during fusion, in your bones, inside earth…
However, the beta decay is a paradox. The mass of the W boson is approximately 80 times greater than that of a proton or neutron. How does a proton emit a particle way more massive than itself without violating conservation of energy?
Quantum mechanics gives the answer. Heisenberg’s uncertainty principle allows the W boson to ‘borrow’ energy for an extremely short period of time before it decays (therefore the weak force has a short-range nature and is called weak).
Gravity
Gravity causes attraction between all objects with mass. It is the only fundamental force not included in the standard model yet.
It is the weakest of the four fundamental forces by far. Gravity between a pair of protons is weaker than their electromagnetic repulsion by a factor of 1036.
We would not even notice it if it weren’t acting over large (and i mean large) distances.
Isaac Newton’s famous classical theory of gravity still plays a crucial role today. But in the 20th century, Albert Einstein revolutionised our understanding of gravity with the general theory of relativity. He proposed that gravity is the result of curvature of spacetime. Spacetime is comparable to a bed sheet. Put a ball on it and it curves the bed sheet. Anything else is attracted by this curvature of the ball.
This raises a huge problem: According to Einstein, gravity is is treated geometrically, not as a force in the standard model.
In other words, relativity and the standard model don‘t match.
But in the last few decades ideas have been proposed to include gravity. Scientists are now looking for a quantum particle of gravity, called the graviton. This would fit in the standard model to the other bosons.
Summary Bosons and Forces
In short, every force has force carriers portrayed as particles in the standard model. However, gravity leaves us with nothing but assumptions.
- Weak nuclear force: bosons are Z (neutral), W (positive and negative); interacts only with quarks over short ranges
- Strong nuclear force: boson is gluon in three colors; interacts with quarks and leptons over short ranges
- Electromagnetic force: boson is photon; interacts with charged particles and infinite range
- Gravity: has no boson yet; interacts with everything with mass and has infinite range
Higgs Boson: the God Particle
The Higgs Boson gives particles their mass. It is the boson of the Higgs Field, a particle-like manifestation of the field’s energy.
In 1964 Peter Higgs proposed the Higgs Boson. In 2012, the Large Hadron Collider (LHC) at CERN confirmed its existence.
You can imagine the Higgs field like a room of journalist. When Theodor Hänsch, Nobel prize winner, walks into the room, the journalist gather around him. He „gains“ mass that the Higgs field gives him. But if I walk into the room, nearly unnoticed, the Higgs field won’t give me as much mass. This explains why particles with different properties have different mass.
Complications with the Standard Model
Although the Standard Model is widely accepted, is has several shortcomings:
- It takes no notice of gravity. One could say that a theory that doesn’t even explain why we don‘t fly into space cannot be called theory of nearly everything.
- It cannot explain what happened before the Big Bang or inside a black hole. Hence, no theory of everything.
- It has a number of parameters that are undetermined. Why is the mass of quarks what it is? Why do heavier fermions than the first generation exist if they decay instantly? The number of constants without context is remarkable.
- It does not include dark matter. In recent years, this new mysterious type of matter was introduced. Physicists calculated that normal matter wasn‘t able to form the structures and galaxies that we observe today. So, they concluded there had to be a different type of matter that has a gravitational pull but does not interact any further with our matter. But where does dark matter fit into the standard model?
Conclusion
All in all, 12 matter particles, 4 force-carriers, and the Higgs boson build everything you know. Up and down quarks make up the atom with electrons, although most of the mass comes form the forces bwtween them.
The standard Model explains everything form how matter is built up to how complex structures form.
Especially in recent years it became very accepted with several measurements of the LHC in CERN. Particularly the prove for the Higgs Boson was a milestone for this theory.
Michio Kaku says: „Remarkably, the Standard Model could accurately predict the properties of matter all the way back to a fraction of a second after the Big Bang.“
But I have to clarify that it is just a convenient way to think of the fundamental building stones as particles. The standard model is actually written in a language known as quantum field theory: matter is not made of particles, but fields everywhere in space.
Going further
Do even smaller particles exist? We were pretty confident that the atom was indivisible. And than that the proton and neutron are fundamental. With the development of science, we constantly found smaller building blocks and „updated“ theories. So, why should we be right this time?
Stephen Hawking argues: „It is very certainly possible to find smaller particles, but we do have some theoretical reasons for believing that we have, or are very near to, a knowledge of the ultimate building blocks of nature.“
Also, the standard model raises many questions.
Heavy fermions like the top quark decay instantly, but why do they exist at all?
Why do quarks have fractional charges? Why a fraction of three?
It is certinly not the ultimate, all-explaining theory yet. But who knows what the future holds.
Sources
- Kaku, M. (2021). The God Equation : The Quest for a Theory of Everything. Doubleday
- Close, F. (2023). Particle Physics: A very short Introduction. Oxford University Press
- Hawking, S. (2015). A brief History of time. Bantam Press
- Quanta Magazine, July 2021. „The Standard Model of Particle Phyiscs: a Triumph of Science“ (Video). YouToube. https://youtu.be/Unl1jXFnzgo?si=4-Dstibqrq_HjVMA
- ScienceClic English, Feburary 2025. „The basics of Quarks and Chromodynamics“ (Video), YouTube. https://youtu.be/oA3Sbm2PIGI?si=oG_C5BaFwdo3S9mV
- U.S. Department of Energry. „DOE Explains… the Electromagnetic Force“. Office of Science. https://www.energy.gov/science/doe-explainsthe-electromagnetic-force
- U.S. Department of Energry. „DOE Explains… the Electromagnetic Force“. Office of Science. https://www.energy.gov/science/doe-explainsthe-weak-force
- Institute of Physics. „The Standard Model“. https://www.iop.org/explore-physics/big-ideas-physics/standard-model