Originally, scientists believed there were four elements of nature: fire, earth, air, and water. Though this philosophy is long gone, scientists do believe that elements in the form of atoms are the building blocks of matter. Particle physics has helped us gain tremendous knowledge of the world around us, even discovering smaller particles than protons, neutrons, and electrons like leptons and hadrons. While we will not dive into the characteristics of these elementary particles now, it is important to understand the forces involved as these particles interact.
There are four fundamental interactions or forces that occur in nature—and, believe it or not, the force of gravity is the weakest. The strongest interaction is what keeps the neutrons and protons together in the nucleus of an atom. This is called the strong nuclear interaction. It is the glue that holds the nucleus together. It is very short-ranged, though, only reaching as far is the approximate size of a nucleus, 10-15 m. The next strongest is the electromagnetic interaction. It is only about 1/100th (10-2) the strength of the strong nuclear interaction. This interaction is what provides the attraction of oppositely charged particles and the repulsion of like charges. It is what holds the electrons in place around the nucleus of an atom as well as holds atoms together as they share electrons. The electromagnetic interaction is infinite in its reach, though, so it reaches very far distances much like gravity. The next weakest interaction is called the weak nuclear interaction. This interaction is not as easily understood as it requires an understanding nuclear decay of elements, however it is only about 10-13 of the strength of the strong interaction. It is also is very short-ranged. In fact, it only has a range of about 0.1% the radius of a proton. That's because it is mainly the interaction between subatomic particles called quarks. Finally, the weakest interaction is the gravitational interaction. It is the smallest in strength, only about 10-38 that of the strong interaction. It is infinite in its reach, meaning all objects everywhere exert a gravitational force on every other object everywhere.
| Interaction (Force) | Relative Strength | Range |
|---|---|---|
| Strong Nuclear | 1 | 10-15 meters |
| Electromagnetic | 10-2 | infinite |
| Weak Nuclear | 10-13 | 10-18 meters |
| Gravitational | 10-32 | infinite |
Question
If the electromagnetic forces and the gravitational force are both infinite in range, why do we not feel the forces from particles everywhere?
While the interaction or force is infinite, the strength of both are inversely proportional to the square of the distance between the two particles or objects (\(\small\mathsf{ \frac{1}{r^2} }\)). So, doubling the distance actually decreases the strength by \(\small\mathsf{ \frac{1}{(2r)^2} = \frac{1}{4r^2} }\).
