When scientists care only about a single chapter of the rule book, they call the included rules a “field,” filtering out all but a certain set of related instructions for every point in space.
Fields are useful for describing gravity, the force with which you’re probably most familiar. When you put something into space, like a star, its mass changes the behavior of the field. In other words, the star’s presence sends a message at light speed, updating the instructions for everything in space. The main instruction for gravity is simple: If something is heavy, move toward it. Gravity is most prominently felt by big, heavy masses, as in the case of Earth and the sun.
Fields are also useful for describing electromagnetism, another of the four forces. Rather than mass, an innate property called “charge” determines how things should behave. This property can be a positive or negative number. If you create something with electric charge, that charge updates the electromagnetic field instructions: Positive charges start moving away from positive parts of the field, flocking to negative charges, while negative charges do the opposite.
Electromagnetism is more prominent than gravity at the lengths of space where humans experience life. Think about how electric charge can move up a wall to power your lights, even though Earth's gravity is fighting against it.
Electromagnetism has something else that scientists have never observed in gravity: a particle that carries the force’s instructions, called a photon, otherwise known as a boson.
Bosons, like fields, are things scientists use to explain phenomena in nature. Their effects are real; photons are the smallest piece into which we can break light, and we see photons with our eyes and in experiments. However, the laws of quantum mechanics say that below a tiny distance, we can’t measure how fast something is going and what it looks like at the same time — so you can’t take a video of a boson buzzing through space. When physicist Albert Einstein said light travels in photons, what he meant was something like: "The math that explains the universe makes the most sense if we treat the smallest packet of a force field’s effects like a particle delivering instructions as it moves between two things.” Those force particles even have properties and identities we can detect and measure.
With that caveat, we’ll treat the next two forces like the exchange of messenger particles, rather than as fields, even though they’re fields the same way that gravity and electromagnetism are. The difference is that the effects of these fields are felt only on tiny distances, such as those between protons inside an atom.
The weak nuclear force is probably the least intuitive; it isn’t in charge of sticking things together. Instead, it is a list of other instructions, including some relating to the identities of the quarks (the components of protons and neutrons) inside an atom’s nucleus. Sometimes the neutrons of radioactive atoms’ spit out W bosons, the particles responsible for weak nuclear force. Unlike photons, the W boson has mass, which means it travels only a tiny distance before decaying into an electron and another particle called an antineutrino. The effect of the force makes the original neutron’s quarks change flavor, if you will, and the neutron turns into a proton. The weak force has two other bosons called the W+ and the Z that play with quarks, too, changing neutron identities when the time is right.
If you’ve gotten this far, you might be scratching your head. Why on Earth does a force particle have mass when forces are just instructions? That didn’t make any sense to particle physicists, either. until 1964, when Peter Higgs (and other scientists) theorized that there was another secret chapter of instructions (and therefore another boson) that gave mass to the W and Z bosons. Scientists at the Large Hadron Collider announced their discovery of that particle in 2012.
After the weak nuclear force comes the strong nuclear force, which is a little easier to explain on the surface. Have you ever wondered why nuclei, rife with positive protons, don’t blow apart via the electromagnetic force? That’s because there’s a much stronger force that starts acting on these lengths, which glues the quarks inside each proton and neutron together, and helps secure the protons and neutrons in place. In fact, the strong nuclear force’s boson is aptly called a gluon.
Now that you know what a force is, you might be able to guess what a “fifth force” would be. Physicists observed what looks to be a completely new particle that might share some of the properties that earn gluons, photons, W-, W+, Z and Higgs bosons the title of “boson.” If these observations stand the test of time (and scrutiny), then physicists might need to draft a new chapter in the universe’s instruction manual.
Special thanks to my friends Kevin Crowley, graduate student in cosmology at Princeton, and Brandyn Lee, graduate student in physics at the University of Texas at Dallas, for contributing their expertise.