Re: What are geons actually and why are they thought to exist?

Hello Teo, I've never heard of "geons" - but since you mention holding quarks together I suspect that you're talking about gluons. (If not ... my apologies, and try resubmiting your question!)

First off I'll refer you to The Particle Adventure where you can read a lot about particle physics, including gluons, quarks, and the strong force (quantum chromodynamics).

Normally when we think of forces we think of classical, mechanical influences that make things move. Two opposite charges attract one another, a magnet picks up a piece of iron, air pressure holds up an airplane. When you get down into quantum mechanics, a "force" acting between two objects looks like a particle exchanged between them. In the real world, two charged objects pulling on one another can be said to be exchanging particles back and forth, and the effect of emitting and absorbing these particles is to pull the objects together ... so the emission and absorbtion processes, which are complicated and quantum mechanical, average out to look like a classical electrostatic force. The particle they exchange is the photon - the same particle you're used to using to describe light. We say that the photon is the force carrier, or the gauge boson, of the electromagnetic force.

I'll get to gluons in a minute.

However, these particle exchanges don't have to result in classical forces that make things move around. Emitting and absorbing these gauge bosons can also change the identity of the emitter/absorber. The electromagnetic interaction is pretty simple - it doesn't do this - but imagine if it did. For example, think about two charged balls sitting there attracting or repelling one anotherby exchanging photons. Now imagine - what would happen if the photons were charged? Every time your ball absorbs a photon, the ball's charge would change. Can you figure out whether the balls still attract or repel one another? Neither can I.

It's worth thinking about though, because the other two important particle interactions, the "weak force" and the "strong force", both do this sort of thing. So, let's look at forces between quarks. The interaction is called the strong interaction, and it describes quarks (particles) exchanging gluons (force particles). OK, try to start off by thinking of it just like the electromagnetic interaction ... two strong-charged particles will exchange gluons and thus attract or repel one another (just like two electrically charged particles exchange photons and thus attract or repel one another).

There are three huge complications, though: Instead of there being two types of charge (electric charge is positive, or negative ... two choices), there are three (strong charge is "red" "green" or "blue"), so it's not immediately obvious who is attracted to whom. Secondly, every time you emit or absorb a gluon, your color-charge changes! And finally, to add insult to injury perhaps, the gluons themselves can absorb and emit more gluons ... and those gluons can emit more gluons, and so on.

Now if you looked at this and said, "Whoa! That doesn't look like it should work!" ... you'd be right. It doesn't work. Or rather, the process of emitting endless infinities of gluons is referred to as "non-perturbative", meaning that even our most powerful mathematical tools (and our fastest supercomputers) cannot compute exactly how two strong-charged (colored) particles interact. We can compute enough - a quality called "asymptotic freedom" makes the non-perturbative mess go away at high energies - to be very, very confident in this picture of quark interactions.

Finishing up your question: the only thing that a gluon can do, really, is go back and forth between two quarks. Since they never come out of their larger particles (hadrons) we don't talk much about them decaying or reacting on their own. They don't have anything in particular to do with black holes, although they have a lot to do with neutron stars, which may at their cores consist of "quark gluon plasma".

The question "What led scientists to begin thinking about gluons?" is an interesting one. The answer comes from a very old and very important idea called Noether's Theorem, discovered by mathematician Emmy Noether in 1905. This theorem states that: Any aspect of the universe that has a symmetry also has a conservation law. For example: if we write down the laws of physics such that they're the same no matter where you are in space (translation symmetry), then we can predict the law of conservation of momentum ("momentum generates translations"). This very beautiful idea translates into quantum field theory, which describes electrons and quarks and so on. Basically, if you write down the "quantum fields" that represent a particle, and the equations still work under a "gauge transformation", than there must exist a new particle which accompanies that gauge transformation and works like a force. I can't think of a clearer way to explain it right now - it's worth another few pages of its own. The upshot is this: Once we figured out how to write down the wave equations describing quarks, the symmetries in those equations immediately told us what the force-carriers should look like. The force carriers looked like color-charge, spin-1 particles, someone (whimsically) named them Gluons, and we've been studying them ever since.

I should add: None of these particles are ever seen quite on their own. The quarks are stuck, in a fundamental way, inside larger particles. The gluons are stuck in there as well, holding the quarks together, but everything we have seen so far in experiments is consistent with quantum field theory of the quarks, and its resulting demand for the gauge bosons we call gluons.

Again, I'd recommend The Particle Adventure as a great website to learn more. If you can get a hold of the magazine Physics Today, check out the August 2000 issue, with an article called "QCD Made Simple" by Frank Wilcek which gives a more physicsy perspective on this whole theory.

I hope this is what you were looking for!

Sincerely,

-Ben Monreal