The Standard Model Part 3: Enter the Forces

The Four Fundamental Forces and corresponding visuals. Source: Sci Myst

The Four Fundamental Forces

There are four fundamental forces in the universe: the strong interaction, the weak interaction, electromagnetism, and gravitation. All four have different strengths, respectively: 1) the strong interaction, as its name suggests, 2) electromagnetism, 3) the weak interaction, and 4) gravity. Both the electromagnetic force and gravity have an infinite range, while the strong and weak interactions work over very short ranges and operate only in the quantum world.

What does each force do? Gravity keeps you on the ground, and creates black holes, and keeps Earth orbiting the sun. Electromagnetism keeps your magnets stuck to your fridge, as well as electrons attracted to protons. More precisely, electromagnetism is involved in the interaction of electric and magnetic fields. The strong force binds quarks together and keeps atomic nuclei stable. The weak force initiates the decay of unstable particles through a process called nuclear beta decay, and initiates the nuclear fusion reaction that fuels the Sun and other stars. The weak force can also change the flavor of quarks, which changes a proton into a neutron, or vise versa. There are no relatable examples of the strong and weak force as they only operate in the subatomic world.


The fundamental forces are carried by force-carrier particles referred to as bosons. Three of the four fundamental forces were found to have bosons. Particles of matter transfer distinct amounts of energy by exchanging bosons between each other. Think of force-carrying particles as the visible manifestations of their corresponding forces. The particle is to the force like the tip of an iceberg is to its invisible base.

  • The Strong Force is carried by the gluon
  • The Electromagnetic Force is carried by the photon
  • The Weak Force is carried by the W and Z bosons

The only thing that’s left is gravity. Although the theoretical “graviton” has not yet been found, it should be the corresponding force-carrying particle for gravity. The discovery of the graviton would be unlike any other. It could potentially provide a long awaited solution to the disparities between Albert Einstein’s general theory of relativity and quantum mechanics. Of course, this would depend on it being proven mathematically compatible.

Higgs Boson

In the 1970s, it was realized that two of the four fundamental forces, the weak force and the electromagnetic force, have close ties and similarities. This means the forces could be unified and described within the same theory, which in turn forms the basis for the Standard Model. This “unification” of the two forces means that electricity, magnetism, light, and some types of radioactivity could all be categorized under a single, greater force referred to as the electroweak force.

The mathematics behind the unified theory accurately describe the electroweak force and its corresponding force-carrying particles, specifically the photon (electromagnetism), and the W and Z bosons (the weak interaction). Everything seemed to come together except for one major error. All the force-carrying particles emerge from the equation without a mass. While this is accurate for the massless photon, it is known that the W and Z bosons have mass that is nearly 100 times that of a proton. An answer was proposed by theorists Robert Brout, François Englert, and Peter Higgs to fix the error.

What is now called the Brout-Englert-Higgs mechanism gives mass to the W and Z boson when they interact with the Higgs field. The Higgs field is invisible and permeates the universe. The more a particle interacts with the field, the heavier the mass of the particle. For example, the photon does not interact with the field, and appropriately has no mass. Just like all fundamental forces, the Higgs field has a corresponding force-carrying particle called the Higgs boson. The proposed Higgs boson within the Standard Model is the simplest representation of the Brout-Englert-Higgs mechanism.

Simulated particle collision where a Higgs boson is produced. Source: CERN

However, this remained just a theory for many years, until July 4, 2012. That day, ATLAS and CMS experiments at CERN’s Large Hadron Collider (LHC) proclaimed they had each observed and recorded a new particle in the mass region of approximately 125 GeV. The particle that was recorded had characteristics consistent with the expected Higgs boson. It will take further experiments to confirm with 100% accuracy whether or not this was the Higgs boson said to exist by the Standard Model.


Additionally, there is an antimatter particle that corresponds with every particle. Everything about the two particles is identical except that they have opposite charges.

For example:

  • proton — antiproton
  • neutron — antineutron
  • electron — positron
Matter vs antimatter. Source: Science Notes

Every single particle has a corresponding antiparticle. There is no known remaining antimatter in the universe today. This is because when matter and antimatter meet they annihilate one another. A good way to think about this is using positive and negative numbers, such as -1 + 1. If you add them together, they would cancel out to give you a sum of 0. What’s misleading here is that when antimatter and matter annihilate, they produce something: pure energy.

It’s believed that during an early stage of the universe, all the antimatter annihilated during a process referred to as Baryogenesis. You may be wondering, if matter and antimatter are created and destroyed together, shouldn’t the universe contain nothing but leftover energy? Why is there still matter today? What is the cause of this asymmetry? The answer is, we don’t know. By studying the subtle differences in the behavior of matter and antimatter particles created in high-energy proton collisions at the LHC, physicists may be able to find out these answers.

What is known is that the energy left behind from matter-antimatter annihilation may be harnessed in the future. It could possibly be the worst weapon known to humanity, or be used to propel the future of space travel. The implications are widely varying.

To sum up the first three parts of the series:

All ordinary matter that we interact with in our daily lives, including every atom on the periodic table of elements, is made of only three types of matter particles: up and down quarks, and electrons. And all matter is held together by four forces. The forces are carried by force-carrying particles called bosons.

The last article of the series presents the Standard Model and synthesizes everything we’ve covered. See The Standard Model Part 4: Putting it All Together.

If you haven’t read the first two, here they are:

The Standard Model Part 1: What are Particles?

The Standard Model Part 2: Enter the Atom



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