The Quest for a Fundamental Model

In this segment, we will provide a brief overview of atomic history to set the context.

The initial scientific breakthrough toward a simplified model of matter occurred when researchers recognized that all matter in the universe could be constructed from around 120 elements, resulting in what we now call the periodic table of elements.

This model, inspired by the structure of playing cards, was a significant accomplishment as it made chemical sense. However, it soon became evident that this was not the most fundamental representation of Nature.

The first scientific concept of the atom emerged in the early 1800s through John Dalton, but a true breakthrough came in 1897 when J. J. Thomson discovered particles significantly lighter than hydrogen, which were named electrons due to their ability to conduct electricity.

This discovery ignited a profound scientific interest in atomic structures, revealing the tangible reality of atoms. Ludwig Boltzmann mathematically explained entropy using atomic theory, while Albert Einstein demonstrated that Brownian motion results from tiny, invisible energy carriers.

In the 1910s, Ernest Rutherford conducted experiments demonstrating that atoms possess positively charged nuclei. Concurrently, Niels Bohr proposed a model where electrons orbit the nucleus. It was soon discovered that the nucleus itself was not fundamental, as it comprises protons and neutrons.

Later, in the 1970s, it became clear that even neutrons and protons could be subdivided into smaller particles known as quarks. Physicists designated these as the up-quark and down-quark without any specific rationale.

The encouraging news is that these three particles—the two quarks and the electron—are, as far as we know, fundamental in the sense that they cannot be further divided. Astonishingly, everything on the periodic table is made from just these three particles! Everything in Nature, from celestial bodies to terrestrial life, is composed of these three particles arranged in various configurations, leading to chemical diversity.

So, are we finished? Do we have a complete model based on these three fundamental particles? Not quite. While students learn that fundamental particles are the building blocks of Nature, our best and most precise models suggest that the most foundational elements of Nature are not particles at all!

What Is a Particle… Really?

Currently, it appears that the most fundamental constituents of Nature are what we refer to as fields.

What exactly is a field, and how does it relate to particles?

A field is something that exists throughout the universe and possesses specific values at every point in space, which can vary over time.

To illustrate, consider Earth's climate systems. We can measure parameters like temperature, pressure, and humidity. Each location has a defined temperature that changes over time, similar to how a field operates. This analogy, while not perfect, provides some insight into the concept of a field.

The notion of fields originated with the brilliant Michael Faraday.

Faraday, largely self-taught, conducted numerous experiments on electricity and magnetism. He envisioned space filled with electric and magnetic fields, which persisted even in the absence of detectable electricity or magnetism. This abstract idea remains challenging for many to grasp even today.

Around 1820, H.C. Ørsted found that electric currents generate magnetic fields. Faraday's significant discovery in 1830 demonstrated that moving magnetic fields produce electric currents, showcasing the interrelation of these fields. He proposed that even when it seemed there was nothing between interacting objects, electric and magnetic fields were indeed present.

Faraday illustrated that the world is not only composed of particles but is also permeated by fields.

In 1846, during a lecture, Faraday speculated that light might be the ripples in these invisible electric and magnetic fields he proposed. Fifty years later, Maxwell validated his hypothesis!

These waves in the electromagnetic field gave rise to what we now term photons.

The Quantum Reality

In the 1920s, physicists like Bohr, Heisenberg, and Schrödinger unveiled the mysterious nature of the quantum world, which differs significantly from the macroscopic reality we observe. Quantum mechanics emerged as a result.

The fundamental principle of quantum mechanics is that Nature is inherently discrete.

Specifically, energy is not continuous; it exists in small packets known as quanta, and there is a minimum amount of energy possible in the universe. It is nonsensical to consider smaller amounts because they simply do not exist. This principle also applies to distance and other intriguing concepts.

The endeavor to reconcile the discrete nature of quantum entities with Faraday's continuous fields is termed quantum field theory.

For instance, we understand that waves or ripples in the electromagnetic field represent light. However, when we examine it closely, we discover that light comprises small, discrete energy packets called photons. The debate over whether light is a wave or a particle is valid, as it can be characterized as both; the electromagnetic field oscillates, and the excitations within this field are photons.

Remarkably, this same principle applies to all fundamental particles in the universe. Fields such as the electron field and top-quark field extend throughout the space around you; the excitations in these fields manifest as energy packets we refer to as particles.

It is astonishing to think that all electrons in your body and mine are waves within the same fundamental field. We are interconnected through the electron field! This connection holds true for all fundamental particles.

Thus, particles are not the fundamental entities of Nature; fields are!

Fields are omnipresent… literally.

Empty Space Is Not Empty

Returning to the intriguing simulation of empty space presented earlier, it doesn’t truly seem vacant. That’s because it isn’t.

The explanation lies in the fact that, even when all particles are removed, fields persist and must adhere to quantum mechanics. In fact, due to their wavy nature, Heisenberg’s uncertainty principle applies to these fields. This principle indicates that it is impossible to precisely know both the momentum and position of a field simultaneously. As a result, a field cannot remain static; it must fluctuate, leading to what we term quantum vacuum fluctuations, which essentially are small particles appearing and disappearing.

It’s crucial to understand that these quantum fluctuations are real, and experiments have been designed to demonstrate how these virtual particles generate forces between two metal plates. Quantum field theory accurately explains this phenomenon mathematically.

I assert that this theory stands as the most successful in science. One reason is our comprehension of the standard model and the Higgs boson, but there is further evidence that we are on the right path. The experimental measurement of the electron's spin g-factor has been achieved with astonishing precision—better than one part in a trillion. This represents the best prediction in science!

Why are these quantum fluctuations significant?

One reason is that they elucidate a mystery that remains challenging to explain without them. Humanity has gazed into the distant past using powerful space telescopes, capturing images of the universe when it was merely 300,000 years old—a mere fraction of its current age.

This image is known as the cosmic microwave background radiation, often referred to as the universe's baby picture. However, scientists have grappled with understanding the temperature distribution observed in this image.

It turns out that when the universe was just a fraction of a second old, it underwent a phase known as inflation—an explosive expansion. During this stage, fields existed within the universe, and the temperature variations we see today stem from quantum fluctuations in a field that expanded so rapidly that it became "frozen" in this distribution. What we observe is essentially a snapshot of a vibrating field! Isn’t that fascinating?

A pertinent question arises: which field is it? The answer remains unknown. It could be the Higgs field or perhaps something entirely new. Physicists are tirelessly working to enhance our understanding of the cosmic microwave background.

The Standard Model

What fundamental framework does quantum field theory provide?

We have discussed that all elements are composed of just three particles: the electron, the top quark, and the bottom quark. Additionally, there exists another particle known as the neutrino, which interacts minimally but plays a role in our universe.

Thus, we have four fundamental fields corresponding to four particles. For reasons we do not yet fully comprehend, Nature decided to duplicate these four fields. Consequently, the electron has two heavier counterparts known as the muon and the tau. Likewise, neutrinos have two counterparts called the muon neutrino and the tau neutrino.

As for quarks, their counterparts are called charm, strange, top, and bottom. This comprises the entirety of matter's building blocks in our universe—just twelve fields!

As Ørsted and Faraday discovered, fields interact in various ways, facilitated through additional fields termed force fields. The corresponding particles are known as bosons.

We recognize four fundamental forces in Nature.

The electromagnetic field gives rise to the photon, the particle responsible for transmitting electromagnetism, commonly understood as light. The gluon carries the strong nuclear force, binding protons and neutrons within atomic nuclei. The W and Z fields govern the weak nuclear force, responsible for processes like radioactive decay.

Our world operates as it does because of the interactions among these sixteen fields.

Additionally, we have the Higgs field, which confers mass to particles. The Higgs field's discovery represents a significant accomplishment in quantum field theory, underscoring the remarkable capabilities of the Large Hadron Collider.

Lastly, there is the graviton, which is theorized to convey the force of gravity, but neither the field nor the particle has been detected yet.

The interaction mechanism involves ripples in one field inducing ripples in another, which can then affect yet another field, and so on. For example, the electric field can cause oscillations in the electromagnetic field, and vice versa (an electron can emit or absorb a photon), which can in turn influence a quark field.

The standard model is our best current theory of the universe and serves as an exemplary quantum field theory.

However, this does not imply we have unraveled all mysteries. Many aspects remain unexplained, and we know with certainty that something is absent from the model. In fact, we only comprehend a small fraction of the energy and matter that constitutes our universe. The remainder remains a mystery. Furthermore, even within our known parameters, enigmas lurk, such as the role of gravity in quantum physics.

Other fundamental models of Nature, like string theory and M-theory, also exist, indicating that our understanding is far from complete.

As we conclude this exploration, a profound philosophical realization emerges: we are all physically interconnected through vibrating fields, and as I pen this, I am in awe of the marvels and beauty of Nature.

This article draws inspiration from discussions and interviews with the esteemed explainer David Tong.

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