The Four Forces of Nature

by bkanber

There are four "fundamental interactions" -- these are the four very basic types of forces that affect particles. They are the strong interaction, the weak interaction, the electromagnetic interaction, and the gravitational interaction.

Electromagnetic: we're most familiar with this interaction, and it has the most direct effect on our day to day lives. It is very, very strong -- many orders of magnitude stronger than gravity. The EM interaction dictates all of chemistry. If you've ever picked something up, or felt friction, or drank water, oranything that has nothing to do with radiation, nuclear forces, or gravity, then it's dictated by the electromagnetic interaction. The study of the electromagnetic interaction at the quantum level is called QED: [1] Quantum Electrodynamics, and is mediated by the photon. Richard Feynman made a lot of progress here.

Strong Interaction: if we look closely at the nucleus of an atom, we'll find that the strong interaction shows up in two places: it holds protons and neutrons together inside the nucleus, and it also holds quarks together to form protons and neutrons and other hadrons. The strong interaction is even stronger than EM--but its effects fall off very quickly with distance so we don't really experience it at the macroscopic scale. We discovered the strong interaction because we couldn't figure out how EM could hold things together inside the nucleus. The study of the strong interaction is called [2] Quantum Chromodynamics, and is very interesting.

Weak Interaction: This one dictates radioactive decay; the forces are mediated by the W and Z bosons.

Gravity: gravity is very, very weak -- many orders of magnitude weaker than the strong force. We don't see gravity at human scales; it only appears at galactic sizes (planets, stars, etc). Because it's so weak, it's exceedingly hard to study. When looking at subatomic particles, the EM and Strong forces are so much more powerful than gravity that it's nearly impossible to see the effects of gravity at a small scale. Because of gravity's weakness, we have not been able to study it closely at the quantum level. Gravity is "split off" because it's too weak to study at a quantum scale. It's hard to see and it's hard to study. Perhaps if we understood more of its characteristics at the quantum scale we'd get some more hints about how to reconcile the maths.

Now it turns out that some very smart people discovered that Electromagnetism and the Weak interaction are actually two aspects of a single interaction which we call "the electroweak". Electromagnetism and radioactive decay are therefore two facets of one "parent" interaction -- leaving us with only 3 fundamental interactions! We also have strong evidence to suspect that the Strong interaction can be combined with the Electroweak interaction, and I think we've made progress there, but I'm not up to date on this.

So there's evidence that the Strong, Weak, and EM interactions can be combined into one. Given that, whywouldn't we be able to bring gravity into the mix? We should be able to unify the four into one big theory, and show each one as a different facet of the "unified field theory". The main problem is that we don't understand gravity as much as we'd like to, because it's too weak to study. We haven't figured out the math yet -- because with our current understanding of gravity, the math doesn't work out correctly. If we could more accurately characterize gravity (perhaps there's something that's too small to see yet), our understand of gravity might change slightly and we'd be able to fit it in with the others.

Slight clarification: We don't understand quantumgravity as much as we'd like to. General relativity, however, gives us an excellent framework for macroscopic gravity. Our main issue is using what we know from relativity in conjunction with quantum physics. Einstein's relativity works so well that it's hard to imagine describing gravity any other way; this is what I mean when I say "we don't understand quantum gravity well enough"--we understand gravity excellently, but we don't understand it at the quantum level.