In physics, we often find ourselves describing the motion and properties of matter without explaining the core physical reasons why the universe behaves in these ways. The orbital motion of satellites is governed by gravity, the atoms and molecules that make up matter don’t just fall apart, stars produce energy through fusion. But what exactly is happening when we talk about gravity? Why do atoms exist with the structure that we know, and what is the reason nuclear fusion is possible to begin with?
The Fundamental Forces of the Universe
All of the previous examples are processes that can be explained with fundamental interactions or fundamental forces.
The fundamental forces are known physical interactions in our universe that cannot be broken down into any more basic processes. With current research, these forces are the descriptors of the physical processes of nature, including both structure and motion of all objects, at the most reduced level.
Let’s dive into the definitions for each of the four fundamental forces and look at some examples to understand just how important these interactions are to physicists.
Defining the Fundamental Forces
Though physicists are searching for more interactions that can describe the universe with a complete and single theory, we currently know of only four fundamental interactions.
The fundamental forces of our universe are:
- The gravitational interaction
- The electromagnetic interaction
- The weak nuclear interaction, also called the weak force
- The strong nuclear interaction, also called the strong force
To mathematically explain the fundamental interactions, they are frequently modeled as vector fields, which allow us to visualize the magnitude and direction of a given force at different points in space. We can use these models to help us build a physical understanding of the different forces and why the fields of forces govern all sorts of interactions in our universe.
Examples of 2D vector fields, where arrows represent the magnitude and direction of a force at various points in space, Wikimedia Commons CC BY-SA 4.0
An important aspect for understanding the fundamental interactions is that both distance and time scales change which forces appear to dominate in a given physical process — we’ll discuss why and some important cases for each one.
The Weakest Fundamental Force: Gravity
The gravitational force, first discovered by Isaac Newton, is the interaction you are probably most familiar with and is the weakest of the four fundamental forces.
The gravitational force, or gravity, is the universal attraction of all objects with mass to one another.
Whether an object is the size of a particle or a galaxy, it will experience the force of gravity. Your body has mass and is gravitationally attracted towards the center of the Earth, keeping us stable on the surface of the Earth. The spherical shape of stars and planets is thanks to gravity pulling mass towards the center, and our solar system is gravitationally bound together.
Gravitational Vector Fields
We can use plots of a gravitational vector field to visualize this invisible force. Let’s look at two different representations in the figure below.
A 2D gravitational vector field shows direction and magnitude for gravitational force vectors; 3D representation of gravity shows points closest to the central mass experience the attractive force strongest, Wikimedia Commons CC0 1.0
In the 2-dimensional plot on the left, imagine that the central point is a massive object like the Sun. These vector arrows represent the gravitational force exerted from the Sun’s field on some much smaller mass, at many different points in space. The attraction of masses means the force vector at all points will direct towards the center of mass. The 3-dimensional plot demonstrates the same concept. At the bottom of the well-shaped curve is some large mass like a star or planet, and small objects with the least distance will feel the strongest gravitational force.
Gravitational Dominance
A common misunderstanding is that the gravitational force is the strongest interaction in our universe, yet it’s actually the weakest fundamental force. Fields and forces of gravity have a dependence on mass and distance. This means a greater mass results in a stronger field of gravity, but a greater distance from the field results in a much weaker force exerted.
On an atomic scale, we’re dealing with particles with very small masses — take an electron for example, which has a mass ofat rest. Because of the dependence on mass, gravitational interactions on atomic and molecular scales are nearly negligible compared to those on a more massive scale like planets, stars, or entire galaxies. The other fundamental interactions are many times stronger than gravity but might not be noticeable until we study them at the atomic scale.
The Electromagnetic Force
The second-strongest interaction is the electromagnetic force, and is just as prominent in the structure and motion of our macroscopic selves and lives.
The electromagnetic force describes the interactions between particles with charges, like protons and electrons. Electromagnetic forces can be either attractive or repulsive, where opposite charges attract and like charges repel.
The electromagnetic force is stronger than the gravitational interaction by a factor of, an incomprehensibly large number!1
Electromagnetic interactions are also observable in our everyday lives at different scales. On a macroscopic scale, think of the static electricity from combing your hair or your clothes tumbling in a dryer machine, or a refrigerator magnet. We listen to digital music with speakers that use magnetic coils that convert electrical energy into mechanical energy — sound. Electromagnetic radiation, or light, is also a result of this interaction.
Do electric and magnetic forces seem unrelated to you? In a casual understanding they might, and for a long part of history, they were thought to be separate phenomena. Both electricity and magnetism are related to the electrical charges of atoms. Atoms are made up of positively-charged protons, neutrally-charged neutrons, and negatively-charged electrons. Because all atoms and matter are composed of electrically charged particles, atoms exert electrical forces on each other, and can become a charged ion by gaining or losing charge.
Electric charges result in attractive forces that hold electrons and nuclei together, and repulsive forces that keep us from passing through solid matter, Wikimedia Commons CC BY-SA 3.0
What about magnetic forces? Magnetic fields are just the result of charges in motion. Electric forces are from charges sitting still, and magnetic forces are from charges moving within a material, like a magnet or wire. Fundamentally, electricity and magnetism are the same interaction.
Electromagnetic Dominance
Although the electromagnetic force is stronger than gravity by an incredibly large factor, gravity still appears to dominate at most spatial scales — why? Electromagnetic charges can be canceled out by one another, but gravity can’t since there is no known version of gravity that is repulsive. Since atoms are neutrally charged more often than not, gravity continues to be the dominating force within stellar, galactic, and universal scales.
The Weak Nuclear Interaction
The weak nuclear force is the second weakest fundamental interaction and governs certain important atomic processes.
The weak nuclear force drives key nuclear interactions between particles on subatomic scales, mainly electrons, protons, and neutrons.
The weak nuclear interaction is the reason radioactive atoms experience radioactive decay over time. The weak force istimes stronger than the gravitational force, but only acts over very small distances — a subatomic scale, not even the diameter of a whole atom!
Radioactive atoms are unstable atoms that emit particles in a process called beta decay as they try to achieve a more stable state. If there are too many neutrons in the nucleus of an atom, a neutron can decay into a proton, releasing a high-speed electron or positron in the process. These high-speed particles emitted are what we know as radiation. The weak nuclear interaction is the driving force behind this decay.
The weak force is also what drives nuclear fusion, the process our sun and other stars go through as they burn gas and create increasingly heavier elements in their cores. In the core of a star, hydrogen atoms fuse together and create helium atoms. This releases very small particles and energy in the form of light and heat.
The Strong Nuclear Interaction
The strong interaction or strong nuclear force is the final and strongest fundamental force. Like the weak interaction, the strong interaction has a limited range to act on.
The strong nuclear force is the attractive force that holds atomic nuclei together. Protons and neutrons themselves are made up of even smaller particles held together by the strong nuclear force.
To get an idea of just how strong this force is, think back to the electromagnetic interaction earlier: like charges have a repelling nature and naturally don’t want to share a tight space. Protons are all positively charged and repel one another. Yet the nucleus of an atom is made up of protons sharing a small space with neutrons — the strong force completely dominates the electromagnetic force in order to hold the nucleus together.
The strong nuclear interaction also contributes to nuclear processes like fusion. When two nuclei are close enough, like in the core of a star, the strong force dominates and attracts the two nuclei together. The two nuclei fuse and create a new heavier element.
Examples of Fundamental Forces
Fundamental interactions are happening all around us, all the time. Let’s go through some more examples of fundamental forces.
Auroras like the Northern Lights are the result of charged electrons spiraling through Earth’s magnetic field. When these high-speed electrons crash into gas molecules in our atmosphere, energy in the form of light is given off.
Auroras result from charged particles traveling at high speeds through magnetic fields, Pixabay CC0 1.0
Electromagnetic forces drive a lot of processes and forces beyond the more obvious examples like household appliances and static electricity. The electromagnetic force not only helps holds atoms together but keeps solid objects from passing through each other as well.
Normal and frictional forces are really just electromagnetic interactions. As you sit in your desk chair, the electrons in the chair’s material repel the electrons in your body and clothing. Your chair and body are exerting forces that are equal in magnitude and opposite in direction, and because of this repulsive nature between charged particles, you’re able to rest on top of the chair instead of falling through it or getting pushed off.
Since it’s less common, weak nuclear interactions might be more difficult to identify at first. An important application of the weak nuclear force is using carbon dating to estimate the age of many objects.
Anything living on our planet absorbs some amount of carbon-14, a radioactive form of carbon with two extra neutrons. Over time, the carbon-14 left behind will experience beta decay due to the weak nuclear force. We can use carbon dating to estimate the age of materials and remains like skeletons, since we know the time it takes for carbon-14 to lose half of its original amount, and older specimens will have less carbon-14 remaining than newer samples.
By now, the importance of the four fundamental forces should be pretty clear — it’s safe to say a universe without the fundamental forces governing its composition and motion would look very different, if it were able to exist at all.
Fundamental Forces - Key takeaways
- The four fundamental forces of the universe are important descriptors of the physical processes that govern the motion and makeup of all objects.
- Gravity is the weakest fundamental force and causes the attraction of all masses at all distances in the universe.
- The electromagnetic force is both attractive and repulsive and explains the interactions between charged particles, like electrons and protons.
- The weak nuclear force drives important atomic interactions such as radioactive decay.
- The strong nuclear force is the strongest fundamental force and is the attractive interaction that binds atomic nuclei together, overcoming the charge repulsion of protons.
- The distance scales we study a certain process determine which fundamental force appears to dominate.
1. Jeff Vogtschaller, Stanford Solar Center, ‘Gravity is Really Weak?’, 2020.
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