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.

When I turn off the lights, where does all the light go?

by Thecat5001

Light is a form of energy, but when you turn the light off, the light goes away, so where does the energy go?

The short answer is: it gets absorbed by the wall as heat.

The longer answer needs a bit of a more detailed mental picture. The wall is a solid, which consists of a (fairly) regular structure of atoms. Just imagine a grid of hard spheres laying against each other. This is the surface of the wall. At absolute zero, these atoms do not move and are simply at rest, one just touching the next. Having a temperature means that the wall contains thermal energy. This thermal energy is a random motion of the atoms around their equilibrium point, they're basically vibrating. Such a vibration can travel rather far through the lattice in the form of a wave. One ball pushes the next, which pushes the next, which pushes ... etc. Such a wave is commonly called a 'phonon', because it is also the way in which sound can move through solids.

Now think of the light. Light consists of tiny particles called photons, not to be confused with the phonons in the wall. Each photon is a tiny packet of electromagnetic energy and momentum. If such a photon hits (an atom of) the wall, its energy and momentum is absorbed. Since both these quantities need to be conserved, it means the atom will get a little "kick" from absorbing the photon. It will move, and kick against its neighbor, etc etc. So basically the photon has been converted into a phonon.

If enough photons get absorbed, this will result in the wall warming up slightly. So the light gets converted into thermal energy in the wall.

It's rather analogous to a stone falling into a lake. The energy of the stone will spread out over the surface of the water in the form of waves. The water itself doesn't move much, but the waves can carry the energy quite far. Likewise, the atoms don't move much, but the energy/momentum from the photons can carry rather deep into the wall.