The universe is big. So big that we don’t really know precisely how big it is. The most recent estimates, based on scientific observations, give a radius of about 46 billion light years. (A light year is the distance that light, with all of its speediness, travels in a year.) While the universe contains mostly empty space, the part we know about is also full of celestial gems like planets, stars, and about 170 billion galaxies, give or take. Each of these massive galaxies is controlled by gravity, which keeps all the planets, stars, dust, etc., bound together my mutual attraction.
For centuries, scientists have been trying to understand how things work from the ultra-big (e.g., the universe) to the ultra-small (e.g., an atomic nucleus). Astrophysics and cosmology focus on the origin and structure of the whole universe, while particle physics aims at understanding the atomic nucleus, whose size is of the order of 0.000000000000001 meters (or 10-15 m, also known as one femtometer). Given this vast range of scale, how many forces would you think are needed to explain all of the physics from one end of the scale to the other?
At first sight, it might seem countless. After all, there are forces responsible for the explosion of super-massive stars, for the rotation of Saturn’s rings, for the powerful winds of hurricanes, for the acceleration of bicycles, and for the movement of electricity to our homes. Forces are also felt every time a batter knocks a pitch out of the ballpark, every time friction brings a toy pinwheel to a halt, and every time you stick a little yellow note to your computer display. Forces are everywhere, and they seem to come in countless different varieties.
You may therefore be startled to learn that every one of these examples can be described by only two different forces, and that all of physics as we know of can be described by a grand total of four. That’s right–despite the apparent complexity of the world that surrounds us, physicists have managed to explain it all using only four fundamental forces. The four fundamental forces in nature are the electromagnetic, the strong, the weak, and the gravitational forces.
Here is a second look at the length-scale figure now compared with typical lengths on the galactic scale.
We’ll describe each of the four in turn, but first we need to take care of a little terminology. We have introduced in earlier the concept of “force,” and we have also described the “interaction” between objects or particles. In fact, physicists prefer to use the word “interaction” when discussing fundamental forces. How come? The reason is simple. The word force has the association of a simple pull or push, which is not enough to cover all the situations we want to describe. For example, we can talk about the electromagnetic “force” that holds an electron in an orbit around the nucleus of an atom, but the term force is less useful when we describe the emission of a photon from an excited atom.
We will be discussing below the four fundamental interactions, one by one. In some cases, we will describe typical “forcelike” behavior, such as changing the momentum of a particle. In other instances, though, these interactions are manifested in much different ways. As we shall find, the ranges and strengths of the four fundamental forces vary widely.
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