The Fundamental Forces of Nature

Podcast Transcript

Everything in the universe, from galaxies to the atoms in your body, is driven by just a few fundamental forces.

It took centuries for physicists to identify these forces, but once identified, they believed that some of these forces were merely different manifestations of the same underlying phenomenon.

Once they realized this, some physicists felt that all the forces in nature could be explained by a single theory of everything.

Learn more about the fundamental forces of nature and the quest for a grand unified theory and a theory of everything on this episode of Everything Everywhere Daily.

Before we get into the weeds on what the fundamental forces of nature are, I should explain what a fundamental force of nature is.

A force is a push or pull that acts on an object, causing it to move, change direction, or deform. There are many forces in the world. You can push or pull on an object and exert a force on it.

A fundamental force is one that cannot be explained as the result of any other force or interaction. It is a basic, irreducible interaction of nature that governs how particles and matter behave.

In the case of you pushing an object, you can explain it with your muscles, then the food you ate, and then the energy in the molecules released by changing molecular bonds.

All observed forces in the universe arise from combinations or manifestations of these fundamental forces.

There are generally considered to be four fundamental forces in the universe: gravity, electromagnetism, the strong nuclear force, and the weak nuclear force.

I know that some of you might be raising a technical objection right now, but I will be getting to that.

Gravity is the weakest of the four fundamental forces, but it has an infinite range and always exerts an attractive force. It acts between all objects with mass and is responsible for the structure of the universe on large scales, such as the orbits of planets, the behavior of stars, and the formation of galaxies.

To appreciate just how weak gravity is, lift your hand in the air. Your arm is able to counteract the entire gravitational pull of Planet Earth.

The discovery of gravity as a fundamental force evolved over centuries, culminating in Isaac Newton’s groundbreaking work in the 17th century.

In the ancient world, philosophers like Aristotle believed that objects moved toward their “natural place,” such as stones falling toward Earth. This was a qualitative idea, not a universal or mathematical one.

The more scientific approach to gravity began with the work of Galileo Galilei in the early 1600s, who conducted experiments that showed all objects fall at the same rate regardless of mass, contradicting Aristotle’s views.

The true formulation of gravity as a universal force came with Isaac Newton. In 1687, he published his Philosophiæ Naturalis Principia Mathematica, where he proposed the law of universal gravitation. Newton declared that every object in the universe attracts every other object with a force proportional to the product of their masses and inversely proportional to the square of the distance between them.

Although Newton described how gravity works, he didn’t explain what it is. That deeper understanding came with Albert Einstein in 1915, who showed in his General Theory of Relativity that gravity is the result of the curvature of spacetime caused by mass and energy.

The second fundamental force, electromagnetism, is much stronger than gravity and also has infinite range. It acts between charged particles and is responsible for electricity, magnetism, light, and most of the interactions you encounter daily, like friction and chemical bonding. Opposite charges attract, and like charges repel.

The discovery of electromagnetism as a fundamental force unfolded over centuries, beginning with separate observations of electricity and magnetism before their unification in the 19th century.

In ancient times, people observed that amber, when rubbed, could attract light objects, which was an early observation of static electricity. Likewise, natural magnets were known for their ability to attract iron. These phenomena were thought to be unrelated until much later.

The modern understanding began in the 18th century with scientists like Benjamin Franklin, who experimented with electricity and proposed the concept of electric charge. However, the turning point came in 1820, when Hans Christian Ørsted discovered that a current-carrying wire could deflect a magnetic compass needle. This was the first evidence that electricity and magnetism were connected.

This discovery launched a wave of research. André-Marie Ampère soon showed that electric currents produce magnetic fields and formulated mathematical laws describing this relationship. Michael Faraday then discovered electromagnetic induction in 1831, showing that a changing magnetic field could induce an electric current.

The unification of these insights came through James Clerk Maxwell, who in the 1860s formulated a complete set of equations, now called Maxwell’s equations, that described how electric and magnetic fields are generated and interact.

The third force, the Strong Nuclear Force, is the strongest of the four forces but acts only over extremely short distances, about the size of an atomic nucleus. It holds protons and neutrons together in the nucleus and binds quarks inside protons and neutrons.

The discovery of the strong nuclear force arose from the mystery of how atomic nuclei stay together despite the powerful repulsion between positively charged protons. Since like charges repel, something stronger than electromagnetism had to be at work inside the nucleus.

This problem became evident after the discovery of the atomic nucleus by Ernest Rutherford in 1911. His gold foil experiment showed that atoms have a dense, positively charged core. But if the nucleus was made only of protons, the repulsive electromagnetic force should blow it apart. Something had to hold it together.

In 1932, James Chadwick discovered the neutron, which helped clarify the structure of the nucleus. Physicists now understand that atomic nuclei consist of protons and neutrons, collectively called nucleons. But the puzzle deepened: since neutrons carry no charge, their presence didn’t explain how they could help bind the nucleus together.

To solve this, physicists proposed the existence of a new, powerful force that acts only at very short ranges of about 1 femtometer. In the 1930s, Hideki Yukawa developed the first theoretical model of the strong force.

The final force is the Weak Nuclear Force, which is responsible for radioactive decay and nuclear fusion in stars. It also operates at very short ranges and can change one type of subatomic particle into another, such as turning a neutron into a proton during beta decay.

The story begins in the early 20th century with the study of beta decay, a type of radioactivity where a nucleus emits an electron or positron. In 1896, Henri Becquerel discovered radioactivity, and soon after, beta radiation was identified as one of its components. But a puzzle emerged: in beta decay, energy and momentum seemed not to be conserved.

To resolve this, Wolfgang Pauli proposed in 1930 that an invisible, neutral particle must be escaping along with the electron. He called it a “neutron” but was later renamed the neutrino after James Chadwick discovered the actual neutron in 1932.

Enrico Fermi incorporated this idea in 1934 into the first full theory of beta decay, introducing the concept of a new fundamental interaction: the weak force.

Before, I mentioned that some of you might have raised an objection when I said that there were four fundamental forces. For the purposes of studying basic physics, four are usually given, but in reality, based on our current understanding, there are three.

This is because it turns out electromagnetism and the weak force are actually the same thing, now known as the electroweak force.

The unification of the electromagnetic force and the weak nuclear force into a single force was one of the major breakthroughs in 20th-century physics. This discovery came not from a single experiment, but from theoretical insights in the 1960s, followed by key experimental confirmations in the 1980s.

The starting point was that both the electromagnetic and weak forces shared some important characteristics: they act on leptons, such as electrons and neutrinos, they obey quantum field theory, and they are both mediated by subatomic particles known as bosons.

Photons for electromagnetism, and W and Z bosons for the weak interaction.

In 1967 and 1968, Sheldon Glashow, Steven Weinberg, and Abdus Salam independently developed what became known as the electroweak theory. They proposed that at very high energies, electromagnetism and the weak force are not distinct, but are two manifestations of a single underlying force.

In 1983, the W and Z bosons were directly observed at CERN in Switzerland using the Super Proton Synchrotron. Their observed properties of mass, charge, and interaction behavior matched the predictions of the electroweak theory almost exactly.

So, there are really three fundamental forces, then?

Well, many physicists believe that the electroweak force and the strong nuclear force might also be the same thing. This is a central goal of theoretical physics at the moment and is known as the Grand Unified Theory.

After the electroweak unification was confirmed in the 1970s and early 1980s, physicists sought a deeper unification that would combine all three of the known quantum forces: electromagnetism, the weak force, and the strong force.

At extremely high energies, trillions of times higher than those currently achievable by particle accelerators, it is hypothesized that the strong and electroweak forces become indistinguishable and are described by a single force with a unified set of fundamental particles and interactions.

If, at some point in the future, physicists can prove the Grand Unified Theory to be accurate, then there will be two fundamental forces. Could these two forces also be unified?

This is known as the Theory of Everything.

Merging gravity with the other fundamental forces is extraordinarily difficult because gravity, described by Einstein’s general relativity, is a classical theory based on the smooth curvature of spacetime, while the other three forces are described by quantum field theory, which operates with probabilistic particle interactions.

Attempts to quantize gravity leads to mathematical inconsistencies, particularly infinities that render the theory inoperable at extremely high energies.

If you recall my episode on the ultraviolet catastrophe, this type of mathematical problem was how quantum mechanics was initially founded.

Theoretical efforts, such as string theory and loop quantum gravity, remain unproven. This fundamental incompatibility between quantum mechanics and general relativity remains one of the greatest unresolved challenges in physics.

So, depending on what can be proven in the future, there might be three, two, or one fundamental force….

Except that might not be true either.

Putting aside the unification of forces that I’ve discussed, some physicists are proposing that there might be a fifth fundamental force, or maybe even more.

In recent years, a team of Hungarian physicists reported evidence for a possible new force-carrying particle, dubbed X17, based on anomalies in the decay of excited beryllium-8 and helium atoms.

Some cosmological models propose that dark energy, the mysterious energy driving the universe’s accelerated expansion, might be explained by a new, long-range force acting on cosmic scales. This force would be extremely weak and could interact with mass or spacetime in a novel way,

Some theories extend general relativity by adding a new force that interacts with matter and gravity. These models predict an extra “fifth force” that can vary and change with time or environment.

So, of the fundamental forces in nature that affect us every day, we are positive that there are at least four, which is really three, but might be two, or even one, but could be as many as five to seven.

Absent a breakthrough in theory, a major discovery, or the construction of a massive particle accelerator, the ultimate answer to these questions will probably have to wait.