The Standard Model explained: A deep dive into modern physics – part II

Einstein’s theories of relativity successfully explain the behavior of gravity. However, it does not account for the other three fundamental forces.

The Standard Model of particle physics explains the electromagnetic, nuclear strong, and nuclear weak forces. It describes the forces and behavior of the Universe’s fundamental particles: quarks, leptons, and bosons.

It is a very comprehensive framework backed by extensive experimental evidence and is the foundation of modern particle physics.

The Standard Model extends the principles of quantum mechanics—which describes nature at the smallest scales—to provide a comprehensive picture of how the fundamental particles and forces work (except for gravity).

Together with Einstein’s theories of relativity, they form the pillars of modern physics.

Let’s start at the beginning.

The birth of quantum mechanics

Max Planck

One of the first contributions came from Max Planck, who introduced the concept of quantized energy . Planck showed that energy could only be exchanged in discrete units or packets, which he termed quanta .

Albert Einstein

In 1905, Einstein proposed the photon theory of light. According to this, light consists of discrete packets of energy called photons. His work on the photoelectric effect introduced us to wave-particle duality, where light can exhibit both particle and wave-like properties.

Bohr and de Broglie

In 1913, Neils Bohr proposed that electrons in an atom orbit the nucleus in discrete energy levels. The introduction of discretized electron orbits addressed the stability of the atom and explained the atomic spectra of materials.

Following this, in 1924, Louis de Broglie extended Einstein’s concept of wave-particle duality to the electron, proposing that it also has dual properties.

Schrödinger, Dirac, and beyond

In 1926, Erwin Schrödinger developed a mathematical framework to describe quantum systems and introduced the Schrodinger equation . Schrödinger provided a unified framework that accounted for both the wave and particle nature of quantum particles and systems.

In the same year, Paul Dirac extended Schrödinger’s work by introducing Dirac notation. He also introduced the Dirac equation, combining Einstein’s special relativity with quantum mechanics, which predicted the existence of positrons (antiparticle of the electron) and antimatter in general.

The Standard Model

Fundamental particles

Matter in the Universe is made of fundamental particles, essentially the building blocks. It was assumed that atoms were indivisible for the longest time, but we discovered electrons, protons, and neutrons.

Now we know that particles like neutrons and protons can be further divided into quarks, one of the three fundamental particles.

Quarks

There are six types of quarks: up, down, charm, strange, top, and bottom. Quarks combine to form hadrons. Hadrons can further be categorized depending on the types of quarks that make up the hadron.

We have baryons, which are made of three quarks, like the neutron and proton, and we have mesons, which are made of one quark and one antiquark.

Leptons

Apart from quarks, we have six leptons: electron, muon, tau and their corresponding neutrinos , electron neutrino, muon neutrino, and tau neutrino.

One of the main differences between quarks and leptons is that leptons do not interact via the nuclear strong force and ultimately do not combine to form larger particles like quarks do.

Quarks and leptons both fall under the category of fermions.

Bosons

Finally, we have bosons. There are gauge bosons (photon, W and Z bosons, gluons), which act as mediators of the fundamental forces, and the Higgs boson, which gives mass to the quarks and leptons.

Bosons are also known as force carriers because they mediate the interaction of these fundamental particles via one of the three forces: electromagnetic, nuclear strong , and weak forces.

Fundamental forces

The Standard Model accounts for three of the four fundamental forces, each with a carrier particle that helps mediate it.

Electromagnetic force

The electromagnetic force is responsible for many phenomena we see day-to-day, like a lightning strike, a bulb lighting up, or a magnet sticking to a fridge. It is the interaction of any two charged particles.

The mediator of the electromagnetic force is the photon, the particle of light. This is because light itself is an electromagnetic wave. Whenever two charged particles interact via the electromagnetic force , it occurs through the exchange of photons.

Nuclear strong force

The nuclear strong force, or strong force, binds quarks to form hadrons. The force carrier for the strong force is the gluon.

This force is responsible for holding the protons and neutrons together in the nucleus of the atoms against the repulsive electromagnetic forces. Therefore, the strong force is much stronger than the electromagnetic force.

Nuclear weak force

The weak force is seen in radioactive decay and neutrino interactions. As the name suggests, the nuclear weak force is weaker than the strong and electromagnetic force.

Its weaker strength compared to the other two is its short range. This means that its effects are less noticeable at the macroscopic level.

It is mediated by the W and Z bosons, which are massive particles. This affects the range of interaction of the nuclear weak force (discussed in detail later in the article).

However, the weak force is stronger than the gravitational force , albeit over shorter distances.

Quantum field theory (QFT)

The Standard Model is built on the mathematical framework of quantum field theory (QFT), combining the principles of quantum mechanics with special relativity.

QFT assumes the fundamental particles are disturbances or excitations (quanta) of their respective quantum fields . The notion is that the quantum fields are the fundamental entities that exist throughout spacetime.

Think of a stone being dropped in a pond or a lake, causing ripples. The waves are the particles, and the water is the quantum field. For instance, photons are quanta of the electromagnetic field, and the quarks and leptons are quanta of their respective quantum fields.

Then we have the Higgs field, which gives rise to the Higgs boson. The Higgs boson has a pivotal role for the fundamental particles.

Enrico Fermi, Richard Feynman, Hans Bethe, and Steven Weinberg are among the contributors to QFT.

The role of the Higgs field

The Higgs field plays a unique role compared to other quantum fields. While its excitations, like those of other fields, produce a fundamental particle—the Higgs boson—it is distinct in that it imparts mass to fundamental particles.

When the fundamental particles interact with the Higgs field , they gain mass. The more strongly they interact with the Higgs field, the heavier they are.

The W and Z bosons, the carriers of the weak force, interact very strongly with the Higgs field, which is why they are so massive. Conversely, photons do not interact with the Higgs field , which is why they are massless.

Because the photon doesn’t have mass, it is the fastest-moving particle in the Universe, traveling at the speed of light (c) in a vacuum. This is by Einstein’s theory of relativity.

Higgs boson

The Higgs boson, a physical manifestation of the Higgs field, was first theorized in 1964 through a series of three papers published in Physical Review Letters .

The particle was named after Peter Higgs , the author of one of the papers. Along with François Englert (co-author of one of the papers), he won the Nobel Prize for laying down the theoretical groundwork for the mechanism by which particles acquire mass.

The Large Hadron Collider at CERN was built to find the Higgs particle and test other predictions made by the Standard Model. In 2012, the ATLAS and CMS experiments confirmed the discovery of the Higgs Boson.

Predictions and limitations of the Standard Model

Apart from the Higgs boson, the model predicted the force carriers for the strong and weak forces long before their discovery. Two of the quarks (top and charm) were also predicted by the model, and the model has also predicted the behavior of particles in high-energy collisions .

Although the Standard Model successfully unifies three fundamental forces, it is nonetheless limited by several limitations. The most obvious one is its non-inclusion of gravity.

This hinders the efforts to develop a unified theory of everything , which some physicists argue may need to be more idealistic.

The Standard Model also struggles to explain dark matter and dark energy, which is one of its limitations.

Also unexplained is the dilemma about the neutrino’s mass. According to the Standard Model, neutrinos are assumed to be massless particles; however, experiments and observations have demonstrated that they possess a small, non-zero mass.

The Standard Model also fails to explain the abundance of matter over antimatter ( baryon asymmetry problem ).

Physics beyond the Standard Model

The challenges with the Standard Model have prompted the suggestion of several alternate theories. Among the more well-known are String Theory , Loop Quantum Gravity, Supersymmetry, and Grand Unified Theories.

However, they remain “theoretical” frameworks due to insufficient experimental evidence.