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Half way across the bridge I stopped To gather in the prospect of either bank One before And one behind The one behind Beckons to me Autumn leaves glowing bright upon it Memories of distant summer Ahead a new world waits But I hesitate Not ready yet to move on Nor intending to go back So I stand above the river With the prospect of two banks One before And one behind |
| date | 2000-11-20:18:01 |
| Physics |
I have an urge to write about demonology, so I will. Most physicists have looked at the problem of Maxwell's Demon one time or another. It just seems weird that no such thing is possible, and the proofs I've seen that it isn't possible leave me unconvinced, particularly because there is at least one case where a Demon occurs in nature, which I'll talk about below. I wrote about temperature a while back. Temperature is a measure of the number of different ways you can re-organize the atoms in a piece of matter and not change it's energy. This is interesting because it means that temperature is a measure of the possible number of ways of distributing the energy, so it gives us a point of contact with the metaphysical question of what POSSIBLE refers to that I wrote about last week. Now here is the thing: according to the 2nd Law of Thermodynamics, it is impossible to get any useful work out of energy that has been "thermalized"--that is, stored as heat in the random motions of a collection of atoms. What James Clerk Maxwell (he was a Scott, so "Clerk" is pronounced "Clark") father of electro-magnetic theory, wondered why you couldn't do the following: Consider an airtight box with a thin partition separating it into two halves. There is a tiny door in the partition, just big enough to let a single atom through. At the door is a demon who only opens the door when a slow-moving atom is coming at it from one side, or a fast-moving atom is coming at it from the other. So in time, one side of the box tends to get more fast-moving atoms, and the other gets more slow-moving atoms, which means one side of the box heats up and the other cools down. You can use this temperature difference to get work done. The door and the demon act as a kind of valve that move heat energy around, but it depends on the demon being aware of what's going on. There are information-theoretic analyses of this situation that show that if we treat the motion of the gas atoms as storing information, the system with hot gas on one side and cold gas on the other stores less information, and the law of conservation of information implies that the demon must gain information to make it all work, which apparently messes things up. If such a thing existed, it would be useful, so physicists sometimes play with ideas. What is particularly interesting is that the outer reaches of stellar atmospheres consists of gas at something like a million degrees, whereas the denser lower regions consist of gas at a few thousand degrees, and the temperature of the medium around the star is cold. So it appears that Maxwell's Demon lives on the surface of stars, and we understand why. What happens is that in the outer reaches of the stellar atmosphere the mean free path of gas atoms, which is the average distance they can travel without wacking into another atom, gets very long because the gas gets thin. This means that gravity starts to dominate ballistics of atomic motion. Gravity is a weak force relative to the electro-magentic forces that mediate collisions between atoms, so the mean free path has to be really big--probably thousands of kilometers, but I don't know. There just happens to be a region in a typical star like the sun where the stellar atmosphere is dense enough to be interesting and thin enough that atoms typically travel far enough for gravity to bend their courses significantly. If we think about a gas atom coming up from the colder gasses in the lower atmosphere, it is pretty clear what's happening. Fast atoms have more energy, so they get pulled back by gravity less quickly, and have a better chance of making it up into the upper reaches of the stellar atmosphere. Thus, we get a layer that is wildly out of thermodynamic equilibrium with its surroundings, and we conclude that Maxwell's Demon exists. So why can't we build one? My guess is that we can, but that it won't be efficient enough to be interesting. What is required is a means of imposing either a strictly constant, or a velocity-dependent, force on an unionized gas (that's "un-ionized", not "a gas that is a member of a collective bargaining unit.") This isn't easy to do, although there are ways that might be considered. I'll speculate about them at some other time. Heat also manifests itself as thermal radiation: infra-red light. There is a class of Maxwell's Demon proposals that revolve around optical manipulation, but they all seem pretty unlikely as well. The most interesting one I've seen involves confocal ellipsoidal mirrors with different areas. An ellipsoidal mirror is such that light that leaves on focus radially always gets reflected to the other focus. By fiddling with the area of one half of the mirror, but keeping the foci the same, you can create a situation where it looks like there should be more thermal radiation going from one focus to the other than vice versa, and so one will heat up and the other cool down. The strange problem with this is that thermal radiation has a Lambertian angular distribution, so for any finite-sized object at either focus, the thermal radiation won't quite be purely radial, and this minor deviation happens to be exactly what is required to cancel the effect. This is one of those instances where the universe seems to be conspiring against us, as there is no particularly obvious reason why a Lambertian angular distribution ought to do this. But it is the only distribution that has this effect, so building such an apparatus with non-Lambertian emiters (which do exist, although usually at the cost of reduced emmisivity, which creates problems of its own) might allow one to demonstrate a demon in a desktop apparatus. This is a project for my spare time. |
| Humans |
The Boston salon was great, and I'm grateful to Kirez and crew for the opportunity to come and play with them. I hope I'll have the chance to do so again. For my first public outing as a poet it was really the perfect forum: welcoming, eclectic, thoughtful, entertaining. The most interesting person I met there was Carolyn Croissant, a dancer attending the Boston Ballet School, who is well-read, articulate, sensitive, ambitious and writes nice poetry. I have friends my age who have children older than Carolyn, yet there was an immediate sense that this was a person who is, in important respects, like me. When I meet people like this who are a lot younger than me, my impulse is to give them the advice and support that I didn't have at that age. Talking to her in particular made me very conscious of all I've learned from Caro in the past year, and how much beter I am for it. It was good to see Ted again, and to have a chance to talk at length with Bill about lyrics and song-writing. There were all sorts of interesting people there I barely got a chance to talk to: Hawver, Amanda, Spot (who brought a really cool videotape of some dynamic concrete poetry) and others. There'll be other times. The world is more full of opportunities for enjoyment than we know, and recently I've been feeling like my life had better last a really long time to give me the opportunity to experience them all. |
| Creatures |
Three bucks running about across the street this morning, dancing at each other, trying to make each other back off. They moved fairly rapidly off into the bush. There wasn't doe in sight, and they weren't really fighting, but I think this is probably mating season and they're getting into practice, showing the other guys what a big rack they have. Men can be like that too. |
| Poem | I'm hesitating right now over a lot of things, not sure what I should do next. This poem was a way of capturing one aspect of that, as well as being a concrete description of crossing over the Charles toward MIT on Sunday morning, walking in the sunlight. |
| Reading |
In my persistent attempt to escape Homer, I read David Lodge's Out of the Shelter over the past week. It is less overtly funny than Lodge's later novels, particularly Trading Places, which immediately followed it, but I found it a very good example of the Bildungsroman or whatever the German word is for a "coming of age" story. It follows the life of Timothy Young from the age of about five in the Blitz to the age of sixteen, when he visits his sister who is working for the American occupation forces in Germany. Timothy is exposed to a great deal in a very short time, and Lodge portrays his growing sense of being able to cope with the world very well. He starts out shy, uncertain, but willing to face the world, and ends up more confident, more ambitious, more adult. |