The Map of Particle Physics | The Standard Model Explained

Here’s a sanitized version of the provided YouTube transcript:

—

You know how sometimes kids ask the most difficult questions? If a four-year-old asks you why things exist, you might struggle to give them a decent answer because we don’t really know. If we peel away the layers of complexity from humans to cells to molecules to atoms to subatomic particles, we reach the Standard Model of particle physics, which is our best description so far of the fundamental machinery of the universe. However, it doesn’t really answer why anything exists; it describes what exists and how it behaves.

In this video, we’ll be discovering the fundamental rules of particle physics, which will help set the context for future discussions when people claim to have evidence beyond the Standard Model. It’s good to first understand what the Standard Model is, so that’s what this video is for. We know the Standard Model isn’t complete; there are many unresolved mysteries that I’ll address at the end of the video.

Particle physics is one of the main branches of fundamental physics. I’ve briefly covered it before in the map of physics and the map of quantum physics, but here we’re going to dig deeper. I’m going to tell you all the basics of the fundamental particles of the Standard Model, which I’ve summarized in a map available for free digitally and for purchase as a physical poster (links in the description).

Here are all the fundamental particles we’ve discovered so far. This set is what everything in the universe is made of. There are a few distinctions to point out: these are called fermions, and these are the bosons. Fermions make up the physical matter in the universe, while bosons mediate how those matter particles interact with each other, also known as force carriers or exchange particles. The key difference between them is a specific quantity called spin. Fermions have a spin of a half, while bosons have a spin of one or zero (for the Higgs boson).

Spin in quantum mechanics is a form of angular momentum, which generally refers to some kind of rotating motion. However, spin specifically exists within elementary particles and is also known as intrinsic angular momentum. Spin can be a confusing term because it doesn’t correspond to classical spin, like spinning a basketball. In quantum mechanics, every particle has a wave function, and the spin of a particle tells us how the wave function behaves under rotation in 3D space.

When we rotate a spin-one particle, we see it look the same after one full rotation. For a spin-half particle, you need two full rotations to see the same appearance. This gives us a decent picture of spin, though it’s a complex topic that I won’t be able to fully cover in this video. The important things for us are the consequences of these different kinds of spin.

The first consequence is that spin must be conserved; it’s one of the conservation laws of particle physics. There are rules about how particles can interact with each other, known as the conservation laws. These laws state that certain quantities, like spin, cannot be created or destroyed in a particle interaction.

For example, an electron and a positron (anti-electron) can come together and annihilate into a photon. This is allowed because the particles going in have a spin of a half each, which adds together to give the photon a spin of one. Thus, the same amount of spin comes out as went in, and spin is conserved.

There are other conservation laws we’ll encounter throughout this video, but the two most fundamental ones are energy and linear momentum. These apply to everything in physics, not just particles.

The second consequence of spin is even more dramatic. When you gather a bunch of fermions together, they behave very differently from bosons. Fermions obey a rule called the Pauli exclusion principle, which states that fermions cannot share the same quantum state. This principle is crucial because it allows electrons in atoms to occupy different energy shells. Without it, atoms would collapse into the lowest energy state, and the complexity of chemistry and biology, including us, would not exist.

Bosons, on the other hand, can share the same quantum state, leading to interesting phenomena like superfluidity, superconductivity, and lasers. All the forces we know of come from virtual bosons interacting with real particles.

Interestingly, these rules apply not just to fundamental particles but also to collections of particles. For example, helium is known as a composite boson because it has two electrons with spin-half each. When combined, these spins yield a whole number, allowing helium atoms to behave like bosons. When cooled down, helium atoms can inhabit the same quantum state and become a superfluid with zero viscosity.

Now, let’s take a closer look at the spin-half particles, the fermions. You’ll notice that fermions are split into quarks and leptons. Quarks cannot exist alone; they are always found bundled together. Up and down quarks make protons and neutrons, which comprise all elements. Quarks can also be combined in other configurations, like pions, which are made of a quark and an antiquark pair.

Quarks carry electric charge, but in different amounts: some have a charge of two-thirds, while others have a charge of minus one-third. These charges add together to give an overall charge. For example, a proton is made up of two up quarks and one down quark, resulting in an overall charge of plus one, while a neutron consists of one up quark and two down quarks, giving it an overall charge of zero.

Charge is another conserved quantity in particle interactions. Quarks interact with all fundamental forces. Each particle interacts with specific forces, including the electromagnetic force, strong force, and weak force. Quarks are the only particles that feel the strong force, along with the carriers of the strong force, the gluons.

Quantum field theory states that there is a field associated with each fundamental particle, and particles are excited states (quanta) of their field. The forces are carried by specific fields, which is what we mean when we say that bosons are force carriers. The Higgs field is also important; it doesn’t cause a force but gives mass to certain particles through interaction.

Now, let’s add some extra conservation rules to our chart: baryon number and color charge. A baryon is any particle made up of an odd number of quarks, with at least three quarks. Baryon number is plus one for each quark and minus one for each antiquark.

Next, let’s discuss antiparticles. Each fermion has an antiparticle partner with the same mass but opposite charge and other quantum numbers.

Color charge is a property of quarks and gluons and is essential for strong force interactions. There are three kinds of color charge: red, green, and blue. To create a proton or neutron, you need one quark of each color to achieve color neutrality.

Quarks interact with each other through the strong force via gluons, which come in eight types, each with different color and anti-color pairs.

Now, let’s move on to the other half of the fermions: the leptons. The most famous lepton is the electron, which is crucial for chemical bonds, electricity, and light interactions. The muon and tau particles are similar to the electron but have higher masses and are unstable.

Neutrinos have very small masses and do not carry electric charge, so they only interact via the weak force. There are three types of neutrinos: electron neutrino, muon neutrino, and tau neutrino. Each lepton has its own quantum number, known as lepton flavor.

Neutrinos are fascinating because they were once thought to be massless, but we now know they have a very small mass. They can oscillate between different lepton flavors, and we observe only left-handed neutrinos and right-handed anti-neutrinos.

Before we continue, let’s take a breather. Neutrinos are quite peculiar, and there’s ongoing research to understand them better.

Now, let’s discuss symmetries in physics. If we mirror the universe, would the laws of physics remain the same? The answer is yes, except for neutrinos, which break parity symmetry. Similarly, neutrinos also break charge conjugation symmetry. However, if we apply both transformations, we find that this combination is conserved, known as charge-parity (CP) conservation.

Historically, physicists have been frustrated by the need to relax conservation laws, making the Standard Model less elegant. The main takeaway is that different conservation laws apply to different forces.

Now, let’s review what we’ve covered. We’ve described fermions, which include quarks and leptons, and we’ve seen the conservation laws and field interactions for all particles.

Finally, let’s look at the bosons, or force carriers. The gluons we’ve already discussed in relation to color charge and the strong force. Photons carry the electromagnetic force and interact with anything with electric charge. However, photons do not carry electric charge themselves, so they don’t interact with each other.

The W and Z bosons are the force carriers for the weak force and have large masses from interacting with the Higgs field. The Higgs boson itself has mass because the Higgs field is self-interacting.

Before we conclude, I want to mention gravity. We learn that the four fundamental forces are electromagnetism, the strong force, the weak force, and gravity. However, we don’t have a quantum description of gravity. Gravity is described by general relativity as a result of curved spacetime.

There are predictions of a gravity particle called a graviton, but we currently lack the means to probe this due to the weakness of gravity compared to other forces.

There are still many mysteries in particle physics, such as baryon asymmetry, the existence of supersymmetric particles, and the nature of dark matter.

The future of particle physics may involve building larger accelerators, but that could be costly. Fortunately, there are many experiments we can conduct with current particle accelerators and large detectors looking for high-energy neutrinos or dark matter particles.

This video is sponsored by Brilliant, a website and app filled with science and mathematics content in the form of question-based courses. If you’re interested, you can check out my special link for a discount on the premium subscription.

Thank you for watching! I hope this video has been helpful. If you know someone who might be interested, feel free to share it with them. Otherwise, I’ll see you soon!

—

This version maintains the core content while removing any informal language or unnecessary filler phrases.