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User talk:KSorenson (view source)
Revision as of 14:40, November 13, 2009
, 14:40, November 13, 2009→What about the other interactions?
:You, good sir, have just boggled my mind. A question I would like to ask about that thought experiment, though. Would magnetism, the weak force, and the strong force react the same way? [[User:RandolfH|RandolfH]] 08:38, 13 November 2009 (EST)
:You, good sir, have just boggled my mind. A question I would like to ask about that thought experiment, though. Would magnetism, the weak force, and the strong force react the same way? [[User:RandolfH|RandolfH]] 08:38, 13 November 2009 (EST)
It's "miss," technically, but my friends call me Kate. And thank you.
Before I try to answer your question, let me emphasize something: I'm ''not'' an expert on the Standard Model. I'm not an ''expert'' on general relativity either, by any means, but it is my primary field of study. To talk about other branches of theoretical physics, I'm mostly going to have to rely on my memories from grad school. So I apologize if I can't articulate everything perfectly or if I mis-remember some of the finer details.
Let's start with magnetism. Magnetism and the Coulomb force I described above are two sides of the same coin: a current flowing through a conductor creates a magnetic field, and a wire moving through a magnetic field will generate a current. Magnetostatic interactions, like the tendency for a north pole and a south pole to move toward each other, are mediated by photons as described by quantum electrodynamics. (In technical terms, the photon is the ''vector gauge boson'' of the electromagnetic interaction. I'ma come back to this terminology shortly.)
The strong interaction is exactly like the electromagnetic interaction, and completely different at the same time. It's alike in that it's also described by a quantum field theory in which the interaction is mediated by vector gauge bosons (told you I'd come back to that jargon), but the characteristics of the theory are different, and particles other than photons are involved. Whereas the theory that describes electromagnetic interactions is called quantum electrodynamics, the theory that describes the strong interaction is called quantum chromodynamics.
I'm not sure how much QCD you want to hear about, so I'll just give you the high points:
Quarks and gluons possess a property called ''color charge,'' which is almost entirely unlike both color and charge, but that's what it's called because all the good names were already taken. Electric charge comes in positive and negative varieties, and magnetic "charge" (pardon the expression) comes in north and south varieties, but the color charge comes in six different varieties: red, blue, green, antired, antiblue and antigreen.
If you put a red, green and blue quark together you get a baryon, maybe a proton or a neutron. (There are other kinds of baryons as well, but I'm stuffed if I can remember any of them right now.) If you put a red and an antired quark together, you get a meson.
The vector gauge boson of the strong interaction is called the ''gluon.'' (Get it? Glue? Particle physicists are hilarious.) Whereas photons possess neither mass ''nor'' charge, gluons are massless but have color charge. This is the most significant difference between the electromagnetic and strong interactions: the gauge bosons of the strong interaction ''participate in the strong interaction.'' Photons can only interact with charged particles, but gluons can interact with quarks and with other gluons. Because photons are electrically neutral, the electromagnetic interaction gets weaker as charged particles get farther from each other. But because gluons participating in the strong interaction, the strong interaction actually gets ''stronger'' as the distance between quarks increases. At a certain distance, it actually requires less energy to create a new quark-antiquark pair than it does to move the quark farther away. This leads to a phenomenon called ''quark confinement,'' which basically says that at normal energy levels (sub-Big Bang-type energy levels) there's no such thing as a free quark; quarks are always confined within baryons or mesons.
So in a sense, yes, the strong interaction works just like the electromagnetic interaction. But in another sense, it's really different.
As for the weak interaction: believe it or not, magnetism and the weak interaction are both aspects of the same scenario I described above. The Standard Model describes both the electromagnetic and the weak interactions as aspects of the same thing, creatively dubbed the ''electroweak interaction.'' Glashow and … urg. Two other people whose names I can't remember won the Nobel for it back in the late 70s.
The weak interaction is the least understood of all the fundamental interactions — at least by me! Whereas the electromagnetic and strong interactions can be described as vector interactions — that is, interactions between particles change the particles' momenta — the weak interaction is what governs particle decay. Under the weak interaction, for example, a neutron can spontaneously decay into a proton, an electron and an electron antineutrino. This is what causes radioactivity, but the exact mechanism by which a neutron decays wasn't understood until about thirty years ago.
The weak interaction is also described by a gauge theory, but this time the gauge bosons have mass. The W<sup>+</sup> and W<sup>-</sup> bosons have positive or negative electric charge, respectively, while the Z<sup>0</sup> boson is electrically neutral. In the example I gave above, one of the quarks inside the neutron spontaneously emits a W<sup>-</sup> boson. In the process, that quark changes ''flavor'' (an intrinsic property of quarks), and sheds one unit of electric charge, turning the neutron into a proton. The W<sup>-</sup> boson immediately decays into a highly energetic electron and an electron antineutrino, which skitter off to parts unknown. Because neutrinos are generally very weakly interacting particles — not in the sense of the weak interaction, just in the sense that they don't interact very much with matter — the electron antineutrino emitted during beta decay basically vanishes out into the universe. But the electron interacts via the electromagnetic interaction, which means it usually doesn't get very far (in cosmic terms) before it causes something interesting to happen. As it passes through matter, the electron sheds some of its energy by emitting energic photons — in other words, X-rays. If one of those X-ray-spectrum photons interacts with an atom, it can imbue one of the atom's electrons with enough energy that it's stripped free of the atom altogether, turning the atom into an ion. That's why the radiation from this type of proton decay is called ''ionizing radiation,'' and it can be pretty darned unhealthy in significant doses.
So getting back to your question, I guess we could say that no, the weak interaction wouldn't really behave similarly to the electromagnetic interaction between two electrons in a hypothetical empty universe. If you plopped an atom of potassium-40 down all by itself, though, eventually one of the quarks inside one of its protons would emit a W boson, and you'd get the scenario I just described. And if you plopped down three quarks, one each of red, blue and green, they would start furiously exchanging gluons (at the speed of light) and bind together into a baryon of some description. But that thought experiment is ''even wilder'' than any of the others I've proposed. To be honest, I'm not even sure it's valid. Free quarks are ''never'' found in nature at less-than-hellishly-insane energies, so I'm really not sure what a free quark would do if you willed it into existence in an otherwise empty universe. It might spontaneously decay into some other form of energy by some mechanism nobody's ever observed because the situation's never come up. A particle physicist might understand the Standard Model well enough to make a prediction, or at least a guess, but that's beyond me.
I hoped this helped, at least a little. Like I said, I'm not a particle physicist, unfortunately. Sorry.