The Standard Model of Particle Physics: A Triumph of Science

Summary

David Tong introduces the Standard Model, the most successful scientific theory explaining the fundamental building blocks of the universe. It encompasses 12 matter particles, 3 forces, and the Higgs boson, yet excludes gravity. The script delves into the mysteries of fermions, bosons, and the quest for a Grand Unified Theory, hinting at the unknown realms of dark matter and energy, and the pursuit of a theory of everything.

Takeaways

• 🔬 Galileo laid the groundwork for modern science, seeking to understand the fundamental building blocks of the universe.
• 🌌 The Standard Model of particle physics is the most successful scientific theory to date, explaining the structure of matter and its interactions.
• 🌐 The model includes 12 matter particles and 3 forces, but notably excludes gravity, which is described by Einstein's general relativity.
• 🤔 Quantum field theory, the language of the Standard Model, suggests that matter is composed of fields, not discrete particles.
• 🚀 Particles are classified as either fermions, which make up matter, or bosons, which mediate forces, with distinct quantum properties.
• ⚛️ Three fundamental particles—electrons, up and down quarks—combine to form protons, neutrons, and atoms, the basis of all visible matter.
• 👻 The neutrino is a nearly massless and weakly interacting particle, a 'cosmic ghost' passing through matter almost undetected.
• 🔀 Nature replicates the basic set of particles into three generations, with heavier versions of the electron, quarks, and neutrinos.
• 🔗 The fundamental forces—electromagnetism, the strong force, and the weak force—are mediated by bosons such as photons, gluons, W and Z bosons.
• 🎯 The Higgs boson and its associated field give mass to particles, a crucial component of the Standard Model confirmed by the LHC in 2012.
• 🔍 Despite its success, the Standard Model is incomplete, with open questions about unification of forces, gravity, dark matter, and the precise origins of particle masses.

Q & A

What is the Standard Model in the context of modern science?

The Standard Model is a theory in physics that describes the fundamental building blocks of the universe and their interactions. It includes 12 different types of matter particles and three of the four fundamental forces, excluding gravity, all bound together by the Higgs boson.

Why is gravity not included in the Standard Model?

Gravity is not included in the Standard Model for two main reasons: first, at the microscopic level, gravity is extremely weak and has negligible effects on subatomic particles; second, general relativity, the successful theory of gravity, is a classical theory and has not been successfully incorporated into the quantum framework of the Standard Model.

What is the significance of quantum field theory in the context of the Standard Model?

Quantum field theory is the language in which the Standard Model is written. It tells us that at the fundamental level, matter is not made up of particles but of fields that are spread throughout space. These fields interact to produce particles, which are the physical manifestations of the fields' interactions.

What is the difference between fermions and bosons?

Fermions are matter particles that obey the Pauli exclusion principle, meaning no two fermions can occupy the same quantum state simultaneously. Bosons, on the other hand, are force particles that do not obey this principle and can occupy the same state, allowing them to mediate forces between other particles.

What are the three matter particles that make up everything we know?

The three matter particles that constitute everything we know are the electron, and two types of quarks: up quarks and down quarks. Protons and neutrons are made up of these quarks, and together with electrons, they form atoms.

What is a neutrino and why is it considered peculiar?

A neutrino is a fourth type of matter particle that is extremely light and interacts very weakly with other matter, to the extent that trillions can pass through the human body without notice. It is considered peculiar because of its elusive nature and its origin from both the sun and the early universe.

Why does the Standard Model have three generations of particles?

The reason for the three generations of particles in the Standard Model is not well understood and remains a mystery. Each generation consists of heavier versions of the basic particles, and while they are unstable and decay into the first generation, their existence is confirmed in particle accelerators.

What is the role of the Higgs boson in the Standard Model?

The Higgs boson is associated with the Higgs field, which is responsible for giving mass to all fundamental particles in the Standard Model. Without the Higgs field, particles would be massless and would travel at the speed of light, making the formation of atoms and larger structures impossible.

What are the three fundamental forces described by the Standard Model?

The three fundamental forces described by the Standard Model are electromagnetism, the strong force, and the weak force. Each force has an associated boson: the photon for electromagnetism, the gluon for the strong force, and the W and Z bosons for the weak force.

What is the significance of the discovery of the Higgs boson in 2012?

The discovery of the Higgs boson at CERN in 2012 confirmed the existence of the Higgs field, which is essential for giving particles mass. This discovery completed the particle content of the Standard Model and was a significant milestone in modern physics.

What are some of the open questions and mysteries that the Standard Model does not answer?

The Standard Model does not answer questions such as why there are three generations of particles, why certain particles have the masses they do, or what constitutes the 95% of the universe's energy in the form of dark matter and dark energy. It also does not include gravity or explain the potential unification of the three fundamental forces.

Outlines

00:00

🔬 The Birth of the Standard Model

This paragraph introduces the foundational work of Galileo and the evolution of modern science, culminating in the development of the Standard Model. The Standard Model is described as the most successful scientific theory, explaining the fundamental building blocks of the universe through a complex formula. It encompasses 12 matter particles and three forces, with the Higgs boson playing a pivotal role. The paragraph also acknowledges the exclusion of gravity from the model due to its weakness at the microscopic level and the challenges of integrating general relativity with quantum mechanics.

05:01

🌌 The Matter Particles and Their Generations

The second paragraph delves into the specifics of matter particles, starting with the electron, up and down quarks, and the neutrino, which together form the basis of all visible matter. It then introduces the concept of particle generations, with heavier versions of these particles known as muons, tau particles, and heavier quarks, which are unstable and decay into the first generation. The existence of these particles is confirmed through particle accelerators, and the paragraph ponders the mystery of why there are three generations of particles, a question that remains unanswered.

10:01

🔗 The Forces and Their Mediating Bosons

This paragraph discusses the three fundamental forces within the Standard Model—electromagnetism, the strong force, and the weak force—and their associated bosons. Electromagnetism is described as the force governing chemical properties and technology, acting on charged particles and mediated by photons. The strong force, responsible for nuclear binding and fission, is mediated by gluons. The weak force, which allows for particle decay and is responsible for nuclear fusion and beta decay, is mediated by W and Z bosons. The neutrinos' unique interaction with the weak force is highlighted, and the paragraph emphasizes the need for these forces to create an interactive universe.

15:03

🌐 The Higgs Boson and the Mystery Beyond

The role of the Higgs boson in the Standard Model is explored in this paragraph, explaining how it confers mass to fundamental particles through the Higgs field, despite the equations of the model prohibiting mass. The discovery of the Higgs boson at CERN in 2012 is noted as a significant experimental confirmation. The paragraph also raises questions about the limitations of the Standard Model, such as the absence of gravity and the unexplained presence of three particle generations. It hints at the possibility of a Grand Unified Theory and the existence of dark matter and dark energy, which are not included in the model, leaving 95% of the universe's energy unexplained.

🤔 The Unanswered Questions of Particle Physics

The final paragraph focuses on the unresolved questions within the Standard Model, such as the varying masses of particles like the muon and top quark, and the exceptionally light neutrinos. It emphasizes the lack of predictive power regarding these masses and the potential underlying patterns that could reveal a deeper structure. The paragraph concludes with an optimistic outlook on the future of physics, suggesting that experimental findings and theoretical advancements may lead to a deeper understanding of the universe beyond the Standard Model, continuing the quest for a theory of everything.

Keywords

💡Galileo

Galileo Galilei was an Italian astronomer, physicist, and engineer who is often referred to as the 'father of modern science.' In the video, he is credited with starting to piece together the basic principles of reality, which we now recognize as the foundation of modern scientific understanding. His work laid the groundwork for the exploration of the fundamental building blocks of the universe.

💡Standard Model

The Standard Model is the theoretical framework that describes the fundamental particles that make up matter and the forces that govern their interactions, except for gravity. The video script refers to it as the most successful scientific theory of all time, despite its seemingly mundane name. It is central to the video's theme as it forms the basis for understanding the fundamental building blocks of our universe.

💡Fundamental Forces

In the context of the video, fundamental forces are the basic interactions that occur in nature. The Standard Model accounts for three of these forces: the electromagnetic force, the strong nuclear force, and the weak nuclear force. The video explains that while there are four fundamental forces in total, gravity is not included in the Standard Model due to its complexity and the fact that it operates on a different scale.

💡Quantum Field Theory

Quantum Field Theory is the theoretical framework within which the Standard Model is formulated. The video mentions this language as the medium through which we understand that matter at a fundamental level is not made up of particles but of fields that permeate space. These fields interact according to the laws of physics, giving rise to particles and their properties.

💡Fermions

Fermions are a class of particles that make up matter. They are subject to the Pauli Exclusion Principle, which means that no two fermions can occupy the same quantum state simultaneously. The video script explains that every particle is either a fermion or a boson, and fermions include electrons, quarks, and neutrinos, which are the building blocks of matter.

💡Bosons

Bosons are particles that carry the fundamental forces. Unlike fermions, bosons can occupy the same quantum state as other bosons, which allows them to mediate forces. The video script discusses bosons in the context of force particles, such as photons for electromagnetism, gluons for the strong force, and W and Z bosons for the weak force.

💡Higgs Boson

The Higgs boson is a particle associated with the Higgs field, which is responsible for giving other particles mass. The video script describes the Higgs boson as a special particle that ties the whole Standard Model together. Its discovery at CERN in 2012 confirmed the existence of the Higgs field and was a significant milestone in validating the Standard Model.

💡Neutrinos

Neutrinos are extremely light, electrically neutral particles that interact very weakly with other matter, making them difficult to detect. The video script mentions neutrinos as one of the four types of matter particles and describes them as 'cosmic ghosts' that pass through us, originating from the sun and other cosmic sources.

💡Generations of Particles

The video script refers to generations of particles to describe the pattern in which Nature has created multiple copies of the basic matter particles, such as electrons, quarks, and neutrinos, each generation being heavier than the last. This repetition is a key aspect of the Standard Model, although the reason for these generations remains a mystery.

💡Dirac Equation

The Dirac Equation is a relativistic wave equation derived by physicist Paul Dirac, initially intended to describe the electron. The video script points out that all known matter particles, including quarks and neutrinos, are described by the Dirac Equation or its variants, highlighting a mathematical unity in the description of particles.

💡Grand Unified Theory

The Grand Unified Theory is a hypothetical framework that aims to unify the three non-gravitational forces—electromagnetism, the strong force, and the weak force—into a single force. The video script mentions this concept as a dream of physicists, suggesting that the forces may be different manifestations of a single, more fundamental force, although there is currently no experimental confirmation of this theory.

Highlights

Galileo initiated the foundation of modern science 400 years ago, exploring fundamental questions about the universe's composition.

The Standard Model of particle physics is the most successful scientific theory, describing the universe's basic structure with remarkable accuracy.

Despite its success, the Standard Model does not include gravity, due to its weakness at the microscopic level and the challenge of integrating general relativity with quantum mechanics.

Quantum field theory underpins the Standard Model, suggesting that matter is composed of fields rather than discrete particles.

Particles in the Standard Model are classified as either fermions, which constitute matter, or bosons, which mediate forces.

Three fundamental particles—electrons, up quarks, and down quarks—form the basis of all known matter.

Neutrinos, extremely light and interacting minimally with other matter, are a peculiar fourth type of matter particle.

The existence of three generations of particles, including heavier versions of electrons, quarks, and neutrinos, is a mystery that the Standard Model does not explain.

All particles are described by the Dirac equation, highlighting a mathematical unity in the fundamental description of matter.

The three fundamental forces in the Standard Model—electromagnetism, the strong force, and the weak force—are mediated by force-carrying bosons.

The strong force, responsible for nuclear fission and fusion, is conveyed by gluons, which bind quarks together within particles like protons and neutrons.

The weak force, responsible for radioactive decay and nuclear fusion in stars, is unique in acting on all particles, including neutrinos.

The Higgs boson and Higgs field endow particles with mass, a crucial aspect of the Standard Model confirmed by the discovery of the Higgs boson at CERN in 2012.

The Standard Model, while successful, is incomplete, as it does not account for dark matter, dark energy, or the unification of forces.

Physicists seek an experiment that the Standard Model cannot explain, as a step towards understanding what lies beyond the current framework.

Unanswered questions in the Standard Model, such as the varying masses of particles, suggest an underlying structure waiting to be discovered.

The pursuit of a theory of everything continues, aiming to provide a comprehensive explanation for the universe and all its phenomena.

Transcripts

400 years ago, Galileo started piecing together the basic principles of reality—what we

now call modern science.

But the questions he was trying to answer are as old as humanity itself.

What are we made of?

What are the fundamental building blocks of the universe from which you, me, the stars,

and everything else is constructed?

In the centuries since Galileo, thousands of theories and experiments have peered into

smaller and smaller distances... converging on a single picture of the structure

of matter.

This somewhat daunting-looking formula is where we end up.

It gives the correct answer to hundreds of thousands of experiments, in some cases with

an accuracy that is unprecedented in science.

It is, by any measure, the most successful scientific theory of all time.

And yet for something so extraordinary, we give it a rubbish name.

We call it the Standard Model.

I’m David Tong, a theoretical physicist at the University of Cambridge.

And in this video, we’re going to build the Standard Model, piece by piece.

By the end, I hope you’ll have some intuition for how all of the parts fit together to create

the fundamental building blocks of our universe.

This is the Standard Model.

It describes how everything in the universe is made of 12 different types of matter particles,

interacting with 3 forces, all bound together by a rather special particle called the Higgs

boson.

Before we get going, some caveats.

First...I said “three forces”.

While there are actually four fundamental forces...

at play in the universe.

This means that there’s something missing from this picture.

That would be gravity, the most obvious force at play in the world around us and yet, in

some sense, the one we understand least.

We do have a theory of gravity, a very successful theory.

It was given to us by Albert Einstein and goes by the name of general relativity.

But there are two good reasons why it’s not included in the Standard Model.

The first is that, at the microscopic level, the force of gravity is so weak that it barely

has any effect on a single subatomic particle.

The second is that we don’t really know how to incorporate general relativity, which

is a classical theory, into the quantum world.

We have no idea how to peer into a black hole where quantum gravity effects are at work.

A second caveat is that the Standard Model is written in a language known as quantum

field theory.

This tells us that matter, at the fundamental level, is not really made up of particles.

Instead, it’s made up of fields: fluid-like objects which are spread throughout all of

space.

These fields are engaged in an intricate, harmonious dance to a music that we call the

laws of physics.

The interactions between the fields produce the physical world in the form of particles.

To understand the Standard Model, it’s more convenient to use the language of particles.

As we build up the Standard Model, we’re going to meet lots of particles with an array

of names that can very quickly become bewildering.

But there is one classification that is, by far, the most important: Every particle is

either a fermion, which is a matter particle, or a boson, which is a force particle.

The distinction between fermions and bosons lies in the quantum world.

Fermions must obey something called the Pauli exclusion principle.

Roughly speaking, this means that you can’t put two fermions on top of each other in space.

As such, these are the building blocks of matter.

Bosons, on the other hand, can pile on top of each other as much as they want because

they’re totally unconstrained by the Pauli exclusion principle.

Bosons are the particles that mediate the forces and we’ll talk about them more later.

For now, let’s start by looking at the fermions.

Everything that we’re made of can be reduced to just three matter particles: an electron

and two species of quarks, known as up and down quarks.

The familiar proton and neutron each contain three quarks.

The proton has two up quarks and a down, while the neutron has two down quarks and an up.

Put protons and neutrons together, and you have a nucleus.

Add electrons to the mix and you have an atom.

Put a bunch of atoms together and that’s what you’re made of.

All the beauty and complexity that we see in the world around us can be traced to this

same collection of three particles, rearranged over and over in different combinations.

Next comes the fourth type of matter particle.

It’s called the neutrino and it’s not like the others.

Neutrinos are extremely light and barely interact with anything else.

For example, in the time it took me to say that, something like 100 trillion neutrinos

passed through your body.

Most of them came from the sun, but many of them have been streaming through the universe

uninterrupted since the first few seconds after the Big Bang.

So, there we have it: four matter particles.

Three that make up you, me, and everyone we know, and one very peculiar cosmic ghost flowing

through us all.

Four is a nice simple number.

But here’s where things start to get weird.

Because Nature didn’t stop there.

For reasons that we don’t understand, she took this collection of four particles and

made two further copies.

This means that there are actually three different kinds of electron-like particles.

In addition to the original electron that we know and love, there are particles called

the muon and the tau.

The muon and the tau behave exactly like an electron, with one important exception: they’re

heavier.

The muon is about 200 times heavier than the electron, the tau almost three and a half

thousand times heavier.

The same generational pattern then repeats for the quarks.

There are two heavier versions of the down quark, called strange and bottom quarks, and

two heavier versions of the up quark, called charm and top.

And then there are also two more neutrinos: we become slightly unimaginative in our naming

and call the full collection the electron neutrino, the muon neutrino, and the tau neutrino.

We don’t see the second and the third generations of particles in everyday life.

We can create these heavier particles but they are unstable, which means that they quickly

decay to the first generation of particles that we’re made of: the electron, up, or

down quarks.

Nonetheless, we know they exist.

We can detect them in particle accelerators.

In some cases, we’ve even been able to take photographs of the tracks they leave behind.

So this is the collection of particles that makes up our world.

Three sets of four.

Now some of this we understand very well.

In particular, we understand why particles have to come in a set of four.

There is a mathematical consistency condition in the Standard Model that tells us that you

can’t have one particle without the other three.

In contrast, we have no idea why there are three generations rather than any other number.

That’s a complete mystery.

There’s a surprising aspect of mathematical unity here: all particles are described by

exactly the same equation.

This equation was written down in the 1920s by the physicist Paul Dirac, originally to

describe the electron.

But, as we discovered more and more particles -- quarks and neutrinos -- we realized that

they too are described by the Dirac equation, or variants of it.

In fact, we now know enough to be sure that if we found more matter particles, they too

must be described by the Dirac equation.

So, that’s the stuff we’re made of.

But we’re still missing something crucial!

And that’s the forces.

Without the forces, the universe would be boring.

All of the particles would wander around the cosmos like lost souls, never interacting,

never doing anything interesting.

There are three fundamental forces in the Standard Model and these allow us to explain

what we see around us in the universe: electromagnetism, the strong force, and the weak force.

Each of these forces comes with an associated particle.

These particles are bosons, the other half of our particle family.

Bosons are force-carrying particles.

In one way of viewing things, you can think of the fermions as constantly swapping bosons

between them, affecting their motion and giving rise to what we call a force.

Let’s begin with the most familiar of these forces.

Electromagnetism is responsible for the chemical properties of the elements and we’ve harnessed

it to create much of modern technology.

It acts on anything that carries electric charge.

That means that it acts on the electron-type particles and the quarks, but not on the neutrinos

because neutrinos are electrically neutral.

An electron sitting in space will give rise to an electric field which spreads radially

outwards, and attracts or repels any other electrically charged particle in its neighborhood.

But if you look more closely at that electric field, you will find that it’s comprised

of a collection of particles called photons.

The next is the strongest fundamental force in nature, aptly named the strong force.

This force acts only on quarks and, subsequently, on particles like protons and neutrons that

are made of quarks.

This is the force that holds together the nuclei of atoms.

It’s also this force that is responsible for nuclear fission and gives the energy that

is released in an atomic explosion.

Just as the photon is associated to electromagnetism, there is a particle associated to the strong

force.

We call it the gluon because it literally sticks quarks together.

Similarly, as the electron gives rise to an electric field, so a quark sitting in space

will give rise to a gluon field.

But something miraculous happens: unlike electromagnetism, the field doesn’t spread out radially.

Instead, the quark produces a thin flux tube, a string-like object, which can only end when

it finds a different kind of quark.

This is what makes the strong force strong.

Because the two quarks are joined by a flux tube, it takes more and more energy to pull

them apart.

This is why we never see quarks on their own.

They’re always bound together by the strong force inside bigger particles like protons

and neutrons.

This is a phenomenon known as confinement.

The third and final force is the most intricate and subtle of all the forces.

This is the weak force.

Like the strong force, the weak force only acts on subatomic distances.

But rather than bind particles together, the weak force is all about decay.

We just learned that the strong force binds quarks together to form protons and neutrons,

creating the atomic nucleus.

The weak force has the astonishing ability to allow quarks to switch their identity.

For example, a down quark can turn into a up quark, releasing an electron and neutrino

in the process.

This means that a neutron can morph into a proton.

This process is called radioactive beta decay.

In this way, the weak force is responsible for the nuclear fusion reactions that power

the sun and produce the energy required for life on Earth.

Finally, the weak force is also the reason that heavier matter particles, like the muon

and the strange quark, quickly decay into the three lighter and more stable fermions

that make up matter as we know it.

The weak force is the only one of the three forces to act on all the particles.

In particular, it’s the only force that neutrinos feel.

There are particles associated to the weak force and we call these the W and Z bosons.

It’s finally time for us to meet the last piece of the jigsaw: the particle that in

many ways ties the whole Standard Model together.

This is the Higgs boson.

To explain why the Higgs boson is special, I should first tell you a striking fact: none

of the fundamental particles in the world have a mass.

In fact, it’s worse than that: the equations of the Standard Model prohibit the particles

from having any mass!

Massless particles, like the photon are obliged to travel at the speed of light.

So why don’t matter particles fly around, massless, at the speed of light?

This is where the Higgs boson comes in.

Its role is rather dramatic: it endows all fermions with a mass.

The reason for this doesn’t have to do with the particle itself, but rather the Higgs

field that permeates the universe.

I’d love to be able to give you a clear explanation of why this happens, but sadly

it’s difficult to come up with good analogies for the Higgs field.

A so-so analogy is that you should view the Higgs field as something like a cosmic molasses,

trapping matter particles as they travel through space and giving them a mass.

We’ve known about the effects of the Higgs field for a long time.

But experimental confirmation came only in 2012, when the Large Hadron Collider at CERN

was able to smash protons together at high enough energies to cause a ripple in the Higgs

field—a ripple that’s the particle that we call the Higgs boson.

So this is the Standard Model: 12 matter particles, interacting with 3 forces and a Higgs field.

It’s a beautiful picture, the pinnacle of 400 years of science.

But it’s clear that this is not the last word in physics.

Since the discovery of the Higgs boson, physicists like me feel that in many ways the Standard

Model is too successful.

It gives the right answer for pretty much every experiment that we can do.

Our current hope is that we will eventually find an experiment that the Standard Model

gives the wrong answer to.

And there are some hints that this may be happening.

Because only then can we get some clues about what lies beyond.

One of the open questions about the model is whether the three fundamental forces are

actually different, or whether they are a manifestation of a single all-encompassing

force.

This is the dream of a Grand Unified Theory.

There are some theoretical signs that this may be the way things work, but no experimental

confirmation.

What Else Is Missing Of course, we’re also left with the obvious

force that’s missing: gravity.

At the beginning of the video, I talked about the problem of quantum gravity.

In recent years we’ve discovered gravitational waves, which are ripples of space and time

itself.

And, if we look closely, there are good reasons to believe that these waves are made out of

quantum particles called gravitons, just like light waves are made out of photons.

But we’re a long way from discovering individual gravitons experimentally.

There are other things missing from the Standard Model, too.

It doesn’t include the invisible realm of dark matter and dark energy which means that

we’re missing an explanation for a whopping 95% of the energy in the universe.

Dark matter, for example, is almost certainly made up of additional particles that don’t

interact with light.

Perhaps these particles have their own forces and their own messenger bosons.

Outro And there are still more questions about the

Standard Model that we don’t know how to answer.

Why is the muon 200 times heavier than the electron, while the top quark is almost 350,000

times heavier than the electron?

Why are the neutrinos a million times lighter?

We have no idea, and no way of predicting these masses other than by measuring them

in experiments.

But there are clearly patterns within these masses which strongly suggest that there is

some underlying structure just waiting to be uncovered.

The hope is that, with experimental results pointing the way, together with new theoretical

ideas, we will ultimately be able to reveal the next layer of reality and understand what

lies beyond the Standard Model.

Until then, we continue Galileo’s journey, with the ultimate goal, a theoretical framework

to explain the universe and everything in it: a theory of everything.