The Strong Force

There are four fundamental forces (kind of).

The first is the gravitational force. The second is the electromagnetic force. The third is the weak nuclear force. And the last is the strong nuclear force.

The strong nuclear force is, as one might expect, strong. At the scales where it applies, it dominates all other forces; at a distance of around a proton's radius, it overpowers electromagnetism by a factor of around 137, the weak interaction a factor of 10\(^6\), and gravity is a whopping 10\(^{38}\) times weaker than the strong force.

But if this is the case, then why don't we see it in day-to-day life? Well, you do, just not how you might expect. The strong force is what holds atomic nuclei together. You may have learned as a child that the nucleus of an atom consisted of positively charged protons and neutral neutrons, and wondered how that stayed together, despite electrostatic repulsion. Well, there's your answer.

As for why the strong force doesn't dominate at larger scales, well, that's a little more complicated. To understand the strong force, we need to make a distinction regarding scale.

When the strong force acts at the scale of around 0.8 femtometres (the insides of a proton), it is a force that is 'carried' by gluons, and binds quarks together. When it acts instead at the scale of around 1 to 3 femtometres, it's a force mediated by a category of particles known as mesons, which binds neutrons and protons together.

The important thing here is that the strength of the strong force is independent of distance between pairs of quarks, but when it's between trios of quarks, such as in neutrons and protons, it stops being so. Instead, when in trios, the quarks kinds of "cancel" each other out. This cancellation isn't perfect, through, and what happens is that we get a very quickly decaying force that's usually called the residual strong force.

This residual strong force, it turns out, is what's really keeping neutrons and protons together. And that also explains why it doesn't manifest itself obviously at the scale of our lives, like gravity does. But how does this relate to neutron stars?

Well, as you can see from the graph above, it's not strictly true that the residual strong force is always attractive. At very small distances, about 0.8 femtometres (hence the above statements), the residual strong force is actually repulsive. Thus, when the neutrons in a neutron star really are compressed very densely, the repulsive effects of the residual strong force start to be significant.

Additional Materials

https://profmattstrassler.com/articles-and-posts/particle-physics-basics/the-known-forces-of-nature/the-strength-of-the-known-forces/ http://webhome.phy.duke.edu/~kolena/modern/forces.html#005