- - - - - Oct22/89 01:58 187:60) Brian Holtz: We'd be a tough species to extinguish. You'd have to make sure you got _all_ of us: Eskimos, Polynesians, Australian aborigines, Mongolian herdsmen, Tanzanian hunter-gatherers, Montana survivalists, Antarctic scientists, and anybody who happens to be in Earth orbit. The only mechanical way I could see to do it would be set off ground-level explosions of dirty nuclear weapons in mass quantities, and with enough geographic dispersion to make sure that no inhabited island _anywhere_ escaped enduring exposure to hard radiation. Biological would definitely be the way to do it, but you would still have to distribute your bug, or build it so that it would distribute itself, to every site of human habitation. A tall order. I can't think of any feasible climatological way to do the job; it's been a lot warmer in the Earth's past than it is now, so causing a Venus-style runaway greenhouse effect can't be too easy. On the other end of the spectrum, I can't think of how to leak the atmosphere and thus Marsify us. The most sure-fire way to pull this off is to pull the plug: take away the sun. If our solar system's galactic orbit anytime soon intersects that of a nearby, unseen black hole, we could find ourselves knocked out of orbit. Or maybe there's a massive enough asteroid in a low enough orbit to hit us and turn the lights thoroughly off. - - - - - Oct22/89 10:06 187:63) Brian Holtz: Mark, just because _you and I_ would be extinct does not mean Homo sapiens would be extinct. You'll note that none of the population groups I mentioned depend on McDonald's or Stop-n-Go. - - - - - Oct22/89 18:23 187:67) Brian Holtz: Mark, nothing I said contradicts anything you said. The text of the item is "How do people think the human race will go extinct?". Our species has spent most of its history _in_ stone age conditions -- we're built for such conditions, in fact -- and we already have proof that stone ages can lead to silicon ages. A cataclysm of the types I described would definitely bum me out, in that our modern culture and civilization would be extinguished, but I would be _far_ more bummed out if our _species_ were extinguished. It's a lot farther from chimps to men than it is from caves to the moon. - - - - - Oct22/89 23:17 187:72) Brian Holtz: How would that work, Jared? Mother nature has been experimenting with bacteria for over 3 billion years; how come she hasn't come up with any super-bacteria yet? What would be the difference between a super-bacterium and a regular one? - - - - - Oct24/89 21:56 187:75) Brian Holtz: Ah, you mean mechanical bacteria, like in "The Crabs Take Over the Island". Luckily, we're a lot farther from being able to build a self-replicating automaton from scratch than we are from editing nature's blueprints for the extant self-replicators on earth. Macro-technology "crabs" would be possible to suppress, but nanotechnology bugs would be a tougher nut to crack. I think they're a long ways away. Far scarier are Von Neumann machines -- interstellar crabs. They would prople themselves from star to star, parking in orbit next to asteroids or around planets, sending down robots to mine, build launch vacilities, and assemble a new Von Neumann machine in orbit. (Pioneer computer scientist John Von Neumann is credited with the idea for these nasties.) They would reproduce exponentially if released, and since they're a relatively obvious (though anti-social) thing to build, some people have taken the fact that the galaxies have not been reduced to a swarm of VNM's as an upper limit on the age of any technological civilizations in the universe. Then again, 90% of the theoretical mass of the universe is "dark" and "missing"; let's hope that's not a sign that the Von Neumann machines are 90% done. - - - - - Oct22/89 02:12 188:24) Brian Holtz: Over the long term, the most important constraint we face is the amount of solar energy incident on the earth and moon; over the longer term, the constraint becomes the net output of the entire sun. Our near-term problem is resisting the urge to use dirty non-solar energy sources after their dirtiness makes them counterproductive. Does anyone know how dirty (hot) fusion is? It just produces helium (which we are running low on, incidentally), right? - - - - - Oct22/89 18:30 188:32) Brian Holtz: They've been using lasers lately to to try to induce fusion in pellets of deuterium. Clean as a whistle. - - - - - Oct22/89 20:21 188:39) Brian Holtz: (Say, if the attraction between Galileo and Venus is the same when Galileo is coming and going, how does Galileo gain any speed from the flyby?) - - - - - Oct22/89 23:25 188:42) Brian Holtz: The key to your coal vs. nuclear comparison is "normal operation", Jared. The best thing you can say about coal is that you start paying for its pollution immediately. With nuclear power you're borrowing against our children's ability to keep nuclear wastes out of the biosphere for millenia to come. I loved the Kenyan proverb quoted in today's Times: "The earth was not given to you by your parents. It was loaned to you by your children." - - - - - Oct23/89 00:10 188:44) Brian Holtz: (I don't get the analogy, Shawn. If momentum is conserved and Galileo speeds up, what slows down? I'm not sure, but I think that the key to this is that both Galileo and Venus are in solar orbit; Galileo passes Venus on the outside and somehow steals some of its orbital velocity. In isolation, surely whatever momentum a small body gains in its last n seconds of approach to a large body it loses in its first n seconds of departure.) - - - - - Oct23/89 20:32 188:48) Brian Holtz: The sun puts out 4 * 10^26 watts; at one AU, that's 1400 watts per square meter. Man currently uses 10^13 watts, while at one AU 10^17 watts fall on a disk the size of the earth. Since energy is conserved, the 10^13 watts we use ends up as heat pollution. If our energy use continues to double every two decades or so as it has been, in a century or so the heat pollution will equal 1 or 2% of incident solar radiation, with results equivalent to the sun putting out that much more heat. Heat pollution makes the greenhouse effect seem like small potatoes. We will soon face the choice of halting the growth of our energy consumption, or finding a way to feed ourselves with a climate turned topsy-turvy by global warming. - - - - - Oct23/89 21:54 188:50) Brian Holtz: What are you saying, John? If you add to the sun's incident energy another 2% released on the earth's surfaced, you've got 2% more heat on your hands. The average temperature is going to rise until the extra heat radiated balances the extra heat generated. The earth isn't a cup full of heat that can casually spill off any extra heat poured into it. Sure, some places will still be cooler than others, but most places will be warmer than they were before. - - - - - Oct24/89 06:15 188:54) Brian Holtz: Yes, Dave, the nice thing about solar energy is that our use of it does not add to heat pollution. Tania, I don't care _how_ much out-of-equilibrium the atmosphere is; if you add 2% more heat to it, it's going to heat up. I only said that the average temperature will rise; I didn't say anything about heat distribution. - - - - - Oct24/89 22:15 188:57) Brian Holtz: Yes, Shawn; the problem is that we are releasing all at once energy that the earth spent eons storing up. Yep, windmills (and hydroelectric dams) are essentially solar energy collectors. Using geo-thermal energy doesn't add to heat pollution unless you dig waaay deep and loose heat that wasn't going to get out anytime soon. Yes, the figures I quoted were from space. The earth's albedo is pretty high (.39), but that only measures how much of the visible spectrum we reflect. Infrared is mostly absorbed and reradiated, instead of reflected. John, my "average temperature" corresponds precisely to the amount of heat energy in the earth's atmosphere and surface divided by the number of molecules in the earth's atmosphere and surface. Increase that amount of heat, and the second law of thermodynamics guarantees that the vast majority of those molecules will get their fair share of the new heat, resulting in a rise in "average temperature". Clearly, if you start introducing 2% more heat in the biosphere, it's not going to sit in the middle of the Pacific making pretty geysers as it rises up and out of the atmosphere. _Especially_ since we are releasing the extra heat where we live. - - - - - Oct25/89 18:52 188:59) Brian Holtz: Good point, John. San Jaun is warmer than Anchorage because a lot more of the earth's heat income arrives there. Similarly, most of humanity's heat pollution is released in the temperate zones where we live. The fact that the atmosphere tends to locally warm up and reradiate heat before it can globally redistribute it only means that we can't count on the colder climes to act as heat sinks for us unless we start doing our heat polluting up there. - - - - - Oct25/89 19:08 188:61) Brian Holtz: But that's just _worse_ news, isn't it, John? If heat stays where it's released, and we release our heat pollution where we live, then we're going to sweat even _more_ than if average temperature weren't "meaningless". At any rate, surely you don't think that adding arbitrary amounts of heat to the biosphere won't warm things up for us? Or do you think that there's a repulsive force between humans and human-created heat? - - - - - Oct25/89 22:21 188:63) Brian Holtz: Heat pollution by humans is doubling every seventeen years; in a century, Since the industrial revolution or so we've been doubling our heat pollution every 17 years. At that rate, in a century we'll be adding 2% more heat on top of earth's solar radiation; in two centuries, we'll be _matching_ our solar radiation. Something's got to give. - - - - - Oct25/89 22:33 188:64) Brian Holtz: Luckily, earth's population is projected to grow only to 10 billion by the middle of the next century. 10 billion people can each consume as much energy as the average American does today and still only release 1% more heat on top of our solar input. After that, though, we'll have to pretty much halt population growth and find a way to meet the expectation of rising living standards without consuming more energy per capita in the process, or we'll run into climactic trouble pretty quickly. - - - - - Oct23/89 00:17 197:1) Brian Holtz: What I find remarkable about tele-socializing is the self-selection available. If you're interested in topic X and want to talk about it, it's easier to find people with the same interest when you're navigating through a heavily populated conversational space than through a heavily populated physical space. - - - - - Oct23/89 22:18 204:4) Brian Holtz: Shawn and Ken, I don't think the throw-the-ball analogies are applicable (or even correct). They ignore the aspect of solar orbit. Surely when in an empty universe two bodies pass each other, neither is going to steal momentum from each other over the long haul. My guess is that as Galileo dives down (from a solar perspective) toward Venus, their mutual attraction will lift Venus up into a slightly higher (i.e., slower) solar orbit, while yanking Galileo down into a considerably lower (i.e., faster) solar orbit. The existing orbit that Galileo was boosted into from earth is probably already eccentric enough that its aphelion is well above earth; in my scenario the Venus encounter would push its perihelion closer to the sun and its aphelion out as far as Jupiter. So I guess I'm hypothesizing that you can steal orbital energy from a lower-orbiting body if you dive down past it; or, at least, you can make your orbit more eccentric that way. - - - - - Oct24/89 06:29 204:9) Brian Holtz: Well, for one thing, Ken, do they trade _both_ momentum and orbital energy, or just one of the two? Angular momentum is the tendency of a body spinning on an axis to keep spinning; I don't think angular momentum (of which Venus has very little, incidentally) has much to do with it. I just wanna understand this thing as well as I understand, say, regular orbits (which ain't much, admittedly). - - - - - Oct24/89 22:38 204:16) Brian Holtz: Thanks, Eric, now I get it. Sorry, Ken and Shawn; in your analogy I thought the ball caught and thrown was the gravitational force, the thrower was galileo, and the truck was Venus. Now I see where the sun enters that picture: the stationary observer's reference frame is the sun's. From both Venus's and Galileo's perspective, neither speeds up. And just to show I was _completely_ muddled on this: yes, it's angular momentum that's being conserved, the angular momentum of the solar system. So my guess was wrong; you don't have to swoop _down_ on a fellow orbiter to steal any of its share of the system's angular momentum. All you have to do is bounce off the front of it; that is, have it somehow deflect your velocity vector more towards the tangent of its orbit than it was before. Thanks guys; you only have to explain something to me six or seven times before I start to understand it. You see, I'm a graduate student... in science! - - - - - Oct24/89 22:47 204:17) Brian Holtz: Say, I got another question. In circular earth orbits, low ones are fast and high ones are slow, right? If so, then how come you have to _speed up_ to climb into a _slower_ orbit? Is that the impulse you use knocks you into a faster but more eccentric orbit, and then you retrofire at apogee to slow down and circularize your orbit? If so, which takes more energy, the speedup or the braking?