Article/Document:

The Fermi Paradox and Interstellar Flight

Bob Seitz

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Summary: What I'm first going to examine are some notions about what it will take to allow us to travel to the stars. Obviously, if we can travel to the stars, then others could come to see us. Then I'll take up the Fermi Paradox in a later installment.

I'm going to present my comments in several installments because they require a bit of thought and a bit of arithmetic. What I'm first going to examine are some notions about what it will take to allow us to travel to the stars. Obviously, if we can travel to the stars, then others could come to see us. Then I'll take up the Fermi Paradox in a later installment.

I'm going to assume three alternatives for the purposes of this discussion:
(1) We can someday somehow circumvent the light-speed barrier, and can somehow slip from one place to another far faster than light could make the trip,
(2) We can someday somehow transmit information faster than electromagnetic radiation could convey it, although we can't physically travel faster than light, or
(3) We are constrained to sub-light speeds long enough to implement sub-light-speed interstellar travel.

There are severe problems with circumventing light-speeds having to do with current models of the universe that I'll discuss in a later installment, but for now, I'm going to restrict myself to the conservative third option.

First, I don't think we'll be sending people to the stars any time soon. One of the reasons is the imminent explosion of robotics and artificial intelligence that I think is about to occur. We're just this year getting microcomputers powerful enough to support entry-level machine vision. After decades of hype, the period from 2000 to 2010 should finally see automatic carpet sweepers, floor polishers, and perhaps, automowers finally begin to appear in commercial applications. Once this process starts, I believe that it will be like personal computers or the Internet: it will feed upon itself. As the decades roll by, we'll see ever more capable robots and artificial intelligences. I believe that it is these that will pave the way to the Solar planets and to the stars. One of the great advantages of these robotic devices is that they won't be constrained by human time scales. I envision them preparing human habitats on Mars, and ultimately, on planets that circle distant stars. One of the ways that humanity may colonize the stars is by sending "seeds" to the stars, either literally or figuratively, that robots will "plant" in local environments. Alternatively, given later 21st-century biotechnology, it may be preferable to remotely construct genomes by locally sequencing DNA, using blueprints carried with robotic probes. We might even carry the process a step further and provide a local crêche (with robotic nursemaids) that simulated Earth, so that our distant "children" would grow up to be like ourselves. Mars and perhaps the outer planets will be our test-beds for robotic systems that can establish mini-industrial complexes on other planets. These robots will have to survey for raw materials, develop transport systems to bring modest amounts of raw materials together for refinement, and then fabricate whatever is needed to bootstrap their way to a full civilization capable of manufacturing additional robots. This might be a product of inorganically-based "life forms" (robots) or it might be instituted by a locally created human population that would use tools and equipment sent from Earth, plus whatever resources the robot(s) sent from Earth could provide.

A second development that I think will be important is the extension of human life-spans. If, in 2100, you could live ten times as long as you do now, a twenty-year round trip to Bernard's Star would correspond to a two-year absence today. (The February, 2000, issue of Popular Mechanics is predicting life-spans of 150 years by 2050.) In addition, you would be able to stay in touch with home and vice versa, albeit with multi-year propagation delays. I could imagine other manipulations that would be tantamount to personal immortality, through transfer of one's memories into "computers", in the broadest sense of the word, or into younger copies of one's self. Obviously, things would change during your absence(s) from Sol Prime, but given an exceedingly long life span, some people might be willing to go a-viking.

A third important consideration (I think) is that of progress and its paradigms. In 1850, the paradigm of choice was, I should think, the mechanical clockwork. Babbage's differential calculator was, perhaps, the flag-bearer of 19th century machine intelligence made flesh . And think of how far we've come within the last 100 years! In 1900, interplanetary travel must have seemed forever impossible. Even heavier-than-air flying machines were deemed impossible ("Darius Green and his flying machine"). Dr. Simon Newcomb, America's leading astronomer and aerodynamicist, had proven that passenger-carrying aircraft were aerodynamically impossible. Rockets were only a gleam in Robert Goddard's eyes. When I was a child, "He has about as much chance of doing that as he does of going to the moon." was a common expression. People don't say that anymore. Today, our paradigm is electronic computers. In 2100, it could, perchance, be something quite different, such as bio-engineered super-brains, or hybrid organic/inorganic combinations (computer-enhanced cognition). Or how about energy from a vacuum? Or how about popping through wormholes? And in 2,200.. ? 3,000.. ? 4,000.. ? 1,000,000.. ? 10,000,000.. ? Another perhaps-relevant factor is the steady rise in gross global product. Would anyone care to guesstimate how our gross global product today would compare with the GGP in Roman times, or in the days of Pharaoh? As Steve has observed, as our GGP rises, our ability to field great works rises with it.

I should think that one first step in considering interstellar travel would be to continue along our present course of examining nearby stars using long-baseline telescopes to determine whether or not they sport Earth-sized planets, together with any other information that we might glean. For example, given their reflective spectral signatures, we might be able to determine whether water vapor is present in their upper atmospheres. (Right now, we can't even see Earth-size planets around other stars, but that might change within the next one hundred years.) The Centauri system is the stellar system closest to us, but it's a ternary star system and might not be best-suited for indigenous life or for colonization. Wolf 359 and Barnard's Star are the next closest. Two promising farther stars might be tau ceti and epsilon eridani. These are stars about 10 light years distant(?) but are fairly similar to our own sun.

OK, how much could we do to design an interstellar probe within the foreseeable future?

First, it depends upon how fast we plan to get there. Using a solar-electric propulsion system that would swing in toward the orbit of Mercury (thereby increasing the power output of its solar cells) and then availing itself of gravity boosts from planets on the way out (with the solar-electric propulsion system pedaling as fast as it can all the way out), we might be able to muster an extra-Solar escape speed of the order of 30 kilometers per second or 1/10,000th the speed of light. This would require nothing exotic in the way of propulsion systems, and might lie within the range of present-day technology. At that rate, it would take ~43,000 years to reach the nearest star, and, perhaps, 800,000,000 years to arrive at the farthest point in our galaxy.

That seems a trifle slow. What would it take to reach 300 kilometers per second, or 0.1 % of the speed of light? At that speed, it would take our probe 4,300 years to reach our nearest neighbor. One possible way to proceed would be to somehow catapult it out of the Solar System. However, this would not be easy, and the probe would still have to be able to decelerate in deep space as it approached its target star. And of course, in deep space, it couldn't utilize solar-electric power. Right now, nuclear power is the only way I know to power something in interstellar space. In any case, since it's going to take 4,300 years to reach the nearest target of opportunity, we can afford to accelerate and decelerate very slowly. At an one gee, neglecting relativistic effects, it would take about one year to reach the speed of light (300,000 km./sec.), or about 1/1,000th of a year to reach our target speed of 1/1,000th of the speed of light. Since we're going to spend 4,300 years reaching the Centauri system, we could readily afford to take 1,000 years to reach or to kill our cruising speed of 300 km./sec. This would require an acceleration of only about 1/1,000,000th of a gee. We could achieve an acceleration of 1/1,000,000th of a gee if we provided 9.8 newtons (1 kilogram) of force to propel a vehicle with a gross weight of 1,000 metric tons (1,100 English tons). Now a rocket is most power-efficient when it is traveling at the speed of its own exhaust. When that happens, its reaction mass appears to be standing still with respect to the moving rocket. ("Standing still" in this situation refers to the same speed the rocket had when it was first launched.. zero, by difinition.) Since our coasting speed is 300 kms./sec., that's somewhere nearly the exhaust velocity at which we'd want to run our ion rocket. Now the power required to drive the ion rocket is given by the exhaust speed, 300,000 meters per second X the thrust (9.8 newtons)/2, or about 4,900,000 watts, or 4,900 kilowatts, or 4.9 megawatts. That's quite a bit of power, but the power-to-weight ratio is only about 5 watts per kg., which is low.

That's pretty slow. We might have trouble keeping our equipment running for 1,000 or 2,000 years. Supposing we look at the scenario in which we operate at 1% of the speed of light. Now it takes us 430 years to reach the nearest star. We can no longer afford to take 1,000 years to decelerate to arrive at rest in the Centauri system. Suppose that we try a deceleration time of 100 years. That will multiply our required acceleration by a factor of 10. However, since we want to cruise at 10 times our former speed, we'll have to increase our acceleration by a factor of 100 (to 0.0001 gees) in order to reach 3,000 kms/sec. in 100 years. So our thrust will have to increase by a factor of 100, upping the ion engine power requirements by a factor of 100. However, because we now have to operate our ion engines at an exhaust speed of 3,000 kms./sec., their power input has to increase by a factor of not 100, but 1,000. Consequently, the power-to-weight ratio of the probe would have to be 5 kilowatts per kilogram, which is a bit on the high side. Of course, there are ways to reduce this somewhat. For trips to tau-Ceti, 10.2(?) light-years away, we might take 500 years to decelerate, or 1,000 years, if the probe could be more rapidly accelerated in the neighborhood of the Solar System. That would allow a 5- to 10-fold reduction in the required power-to-weight ratio, to levels of the order of 500 watts per kilogram.. still pretty high. Still, we could probably operate at speeds close to 1% of c.

Another ploy that might take us as high as speeds of 0.1 c would be the Orion concept. Thermonuclear fusion converts mass into energy with an efficiency of about 0.7%. This is a little more than the 0.5% efficiency needed to achieve a cruising speed of 10% of the speed of light. At 0.1 c, a trip to Centauri would last a little over 43 years.. not too bad, given the life span improvements and robotics capabilities that are allegedly in the offing. At that rate, it would take about 800,000 years to reach the farthest outposts of the galaxy ( and 10,000,000 years to reach Andromeda). However, in the meantime, we would be skipping from star to star. To achieve 0.1 c, we could allow 20-50 years for deceleration, and would need accelerations of .002 to .005 gees. This would be obtained by exploding shaped-charge micro-thermonuclear devices behind a "pusher plate" of ablative material able to withstand the inordinate temperatures of even minute thermonuclear blasts. Since to be effective, the propulsion system would have to eject mass at a speed of 30,000 kms./second, it's anybody's guess whether such a system is even possible.

I mentioned that it would not be easy to catapult something out of the Solar System. Let's look at how that might be done. What we would need would be a linear accelerator aimed at our target star. We wouldn't want our accelerator to be orbiting around the sun if we could help it, although exquisite timing and orbital placement might allow each "booster" to be at exactly the right place at exactly the right instant. Fortunately, the sun's gravitational pull, although inexorable, is of the order of only 1/10,000th of a gee at the Earth's orbit. We could support our "accelerator stations" with slight rocket boosts or ion rockets to maintain their positions long enough to launch our interstellar probe. How might we accelerate our probe? One technique might be to explode a tiny focussed-charge thermonuclear device just as the probe whipped past each station. If we had 10,000 such micro-bombs along our way, each one would only have to impart 30 meters/second to the probe to accelerate it to 300 kilometers per second, or 300 meters per second to bump it to 3,000 kilometers per second. It would be similar to the Orion concept, only in some ways, a lot easier to implement. Of course, it would be Mr. Toad's wild ride, with 10,000 kicks in the after-quarters. unfortunately, the first probe to arrive at its stellar target couldn't use that technique to decelerate (although it might be a technique that could be used for subsequent vehicles.) One further note: at an acceleration of 1 gee, the linear accelerator would have to be 4,500,000 kms. long to accelerate a probe to 0.1% of c (300 kms./sec.), 450,000,000 kilometers long to accelerate a probe to 1% of c (3,000 kms./sec.), and 45,000,000,000 kilometers long to accelerate a probe to 10% of light-speed. Still since the stations merely have to be stretched out along the flight path, and since the accelerations could, perhaps, be boosted above one gee, this might not be as impractical as it sounds at first blush.

This pitcher-catcher technique might be used to achieve speeds as high as 50% of light-speed. Of course, if your catcher missed you at your destination, it could turn into a re-e-eally long trip!

I hadn't thought through or realized until this moment that this nuclear-powered catcher-pitcher concept, with launch facilities on both ends, might be a winner. We might be going to the stars sooner than at least one of us (me) had thought. It would still be a long trip, and it would still require a sufficient level of robotic "intelligence" that it could construct an industrial complex on the other end that could build a space-based "decelerator" on the far end. However, it's even conceivable that the initial payload might be made large enough to carry with it a decelerator that could be deployed to catch the next payload that came across. Hm-m-m. Food for thought.

There's one last element that we might need to examine: interstellar gases and dust. Obviously, there isn't much out there or we wouldn't be able to see so clearly billions of light years in all directions to the very edges of our universe. The molecular density number I've seen (years ago) for interstellar space was one hydrogen atom per cubic centimeter.

To run a reality check on this, we observe that the air pressure at sea level is 14.7 lbs./in2. or 1.034 kgs./cm2. That means that there is 14.7 lbs. of air stacked on top of each square inch of the Earth's surface.. a 1" X 1" column extending up to the edge of our atmosphere that contains 14.7 lbs. of air. Using the metric equivalent, 1,034 grams of air, there is 1,034 grams of air lying on top of every square centimeter of the Earth's surface.. Now the gram-molecular-weight of air is about 29 grams so there are 1,034/29 or 35.6 gram-molecular-weights of air on top of every square centimeter of our heads. Further, a gram-molecular weight of any material contains Avogadro's number or 6.023 X 1023 molecules. Since there are 35.6 of these gram-molecular-weights of air stacked on top of every square centimeter of our heads, it means that there are 35.6 X 6.023 X 1023 or 2.15 X 1025 air molecules on top of every cm2. of your hair. Wow! No wonder it's hard to hold your head up high! If we assume one hydrogen molecule per cm3. in interstellar space and we note the a light-year is about 1013 kilometers or 1018 cms., we arrive at a total molecular population of about 1018 gas molecules per square centimeter when we look into deep space. The edge of the universe is about 1010 light-years away, so the total number of molecules between us and The Rim would be about 1028 or about 1,000 times what we peer through when we look up at night. Since we see shimmer and other atmospheric scatter looking through our own atmosphere, it might be argued that the total number of molecules between here and the edge of space-time must be orders of magnitude less than 1028/cm2 or we wouldn't be able to see it so well..Of course, the molecular density in extra-galactic space may be considerably lower than it is in intra-galactic space, but even so, the estimate of 1 molecule per cm3. might be orders of magnitude too high. However, I'm going to use this one-molecule-per-cm3. bogey in my calculations.

Using the above numbers, the total molecular flux (of mostly hydrogen molecules) it traveling from here to the Centauri System would be about 4.3 X 1018 molecules per cm2. Traveling at 0.1% of c, the speed of light, they would strike our forward shields with 500 volts of energy--enough to cause some ion sputtering and some chemical changes, but no radiation dangers. If we were running at 1% of light-speed, they would impact us at 50,000 volts--enough to generate x-rays. At 0.1% of light-speed, they would strike with energies of 5,000,000 electron-volts, and this would be sufficient to cause some secondary radioactivity. And at 0.5 c, they would strike with energies of 250,000,000 mev.. high enough to create hard secondary radiation. (Damage to materials and to electronics would be the primary concern. However, the fastest, but somewhat expensive, semiconductor material, gallium-arsenide, happens to be quite resistant to radiation.)

With respect to interstellar dust particles, there would be visible fireworks if they impacted the shields, and they wouldn't have to be very many microns in diameter to be dangerous. They would pose their greatest hazard in the neighborhood of stellar systems. Of course, it's also possible that they would simply penetrate the whole vehicle without giving up significant energy, and in that case, the principal problem would be the pinhole they would generate. I'd like to think that particles more than a few hundred Angstroms in diameter would be rare in interstellar space, but that doesn't mean it's true.

It would seem as though the optimum form factor for an interstellar probe would be a rod, with a "shield" on its front end to minimize the ship's cross-section.

Bottom line: I believe we can expand throughout the galaxy at some rate. Within this century, without assuming any breakthroughs, we might be able to launch, at speeds somewhere between 0.1% and 1% of light-speed, and possibly, at speeds between 1% and 10% of c, robotic probes capable of constructing a far-star micro-industrial base. So I think it can be done, and, if we don't destroy ourselves, it will be done.

Two questions might be those of funding, and of the will to undertake such missions, given the time-scales involved. Two incentives for such missions might be curiosity and a desire to spread our seed in the event of a cosmic disaster. I would be willing to help pay for such a mission, and if I'm willing, there will probably someday be a hundred million others who would be amenable to ponying up $10 each a year to pay for such a beau geste.

One interesting fact is that, where Sol is located, out in the rural reaches of the Milky Way Galaxy "where the stars are scattered thinly and the cold of space seeps in", the nearest star is located about 8,000 times the radius of our solar system, or about 500,000 times as far away as Mars. However, in globular clusters, interstellar distances might be of the order of 0.1 light-years or less, with interstellar travel times at 1% of light speed requiring, perhaps, 10 years. I don't know enough about interstellar dynamics to know id this is possible, but it might be that there would be some stellar systems separated by 0.01 light-years, permitting thriving interstellar commerce either for their indigenes or for humanity if we should ever colonize such stellar systems. One slight problem might be that star clusters exist because they've "recently" formed from nebulae, and there would be a relatively high level of interstellar dust within them.
I'm proceeding slowly with this because I want to work through some consequences of the ideas that everyone is presenting and perhaps, to generate publishable results. (I plan to share the credit for any publishable articles that might derive from these lucubrations.)
The realization, stemming from these discussions, that we could, in principle, reach the stars (slowly) with existing technology has been, for me, an unexpected revelation. Coupled with what I am led to believe will happen in robotics over the next few decades confers a significance on these results that they wouldn't otherwise warrant. Advances in robotics should eventually reduce labor costs, lowering the cost to orbit. NASA's current programs to cut the cost-to-orbit to $1,000 a pound and then to $100 a pound should fit into this scenario independently of whatever further improvements robotics might afford.

Thanks for your comments. Martin, you mentioned that,

"Another thought just occurred to me. Rather than cryogenics, a more
practical way of populating the universe (both from the standpoint of
transportation requirements and population) would be to include a sperm and
egg bank in a ship almost expressly designed for delivery. It could run
around the universe, incubating babies and raising some of them to a
sustainable level, dropping them off on suitable planets. I'm not
necessarily thinking that's a wonderful idea, but it does solve my earlier
objections that "no one would want to go." :-) It also takes away the
necessity of cryogenics, although developing a diverse and self-supporting
community would still be a big task for the ship, in "growing" its children."

That was what I was thinking, too, when I wrote,
"One of the ways that humanity may colonize the stars is by sending 'seeds' to the stars, either literally or figuratively, that robots will "plant" in local environments. Alternatively, given later 21st-century biotechnology, it may be preferable to remotely construct genomes by locally sequencing DNA, using blueprints carried with robotic probes. We might even carry the process a step further and provide a local crêche."

The "seeds" were meant to include sperm and ova for humans and animals, as well as actual plant seeds. That way, refrigeration wouldn't be required (not that that should be difficult in deep space). (I first thought of sending human sperm and ova and then decided that we might want to ship plant seeds as well.) However, now that I think about it, we probably aren't going to be able to reconstruct cells by the end of the 21st century, even if we can generate a genome. Maybe sperm and egg banks would be the way to go, although that approach would also require an artificial womb and an artificial placenta, and I don't suppose that's exactly just around the corner. My idea, once they they got to their far star, would be that the resulting children might be reared as though they were on Earth. Then once that first generation had children of their own, the normal family sequence could be established. One of the thoughts I've had about this scenario is that there must be roughly the same resources and elements everywhere we go. It wouldn't make sense to transport materials between stars, unless it were rare earth elements. And even those might be more readily and cheaply synthesized locally (assuming you can remove all the radio-isotopes). The interstellar medium of exchange might be information. Given the blueprint, you could locally manufacture or synthesize whatever had been developed in the next star system over. Also, to allow someone to experience life in the Tau-Ceti System, it might be simulated in some vacation spot or space habitat in the Solar System. Virtual reality might be used to create the illusion of actual presence.

"There's a short story called "The Immortals" that looks at the
psychological implications of omnipotence. I think it's clearly incorrect
to assume that as we get older, and as we live longer, our perception
stretches to fit the timeline. In other words, standing in line at the
post office is still frustrating even if you know you're going to live to
150. And bedsores and the physical ailments of living in cramped space
develop at the same rate regardless of how long the trip to the next planet
is. It's a quality of life issue."

I didn't mean to imply that time would seem to pass faster, but that the bite that a long trip would take out of a lifespan should be relatively smaller. I could envision that if humans went, a small village might be making the trip (like the most luxurious cruise liner). (Of course, this would run counter to the desire for a small cross-sectional area.) It would certainly have to be large enough that it could rotate, to provide "centrifugal gravity". It might be like a long, long 18th century sea voyage, except that the passengers would be in constant (if delayed) virtual contact with Earth. Such a trip might be best-suited for authors, artists, philosophers or theoreticians who are immersed in creative work.

My fantasy about what might happen when we experience a major breakthrough in lifespan is that the bearing of children would be deferred until later in life, since there would no longer be any urgency about "biological clocks", and that children might be reared one at a time, with the entire family particpating in their upbringing. However, given Dr. Hans Moravec's forecast of sentient robots before 2050, I think it's a given that "humanity's children" are going to be reaching for the stars before we biologicals do.

My principal concern is not how we're going to eat ths pie once it's baked but how we're going to make it.

Having examined this topic for this forum, I think that:
(1) we currently have an energy source, in the form of themonuclear fusion, that, in principle, might allow us to reach some reasonable fraction (10%-30%) of the speed of light.
(2) we could possibly launch interstellar probes that might attain, perhaps, ~1% of light-speed using nuclear-electric propulsion systems. At that rate, it would take ~450 years to reach the Centauri System.

The problem with nuclear propulsion is that, at least so far, after 55 years of technological development, it delivers its energy only in the form of heat. What we need is a directed rearward flow of particles at speeds of 0.3% of c and up that won't vaporize whatever is in front of the "engine". If we could electromagnetically focus the ions that are generated by a micro-burst, we might be able to do better. The problem is that if even the minutest fraction of the blast energy is deposited in our structure, it will vaporize it. We can't include a blanket of thermal shielding material because it would weigh way too much. We have to be content with the fissionable or fusionable mass itself. Fortunately, at low accelerations, we could give the "pusher plate" time to cool down between each burst.

Matter-antimatter propulsion wouldn't work very well, either. When matter and antimatter annihilate, they generate extremely energetic gamma rays, and you can't do much with gamma radiation except convert it to heat. (You can't focus it or reflect it.) Of course, there would be some thrust from the gamma radiation if you could focus it or reflect it, but unfortunately, unless you're seeking to travel at speeds close to the speed of light, a photon rocket is inefficient from an energy standpoint.

Another way to skin this cat is to utilize nuclear-electric propulsion. Here we can go from the arm-waving stage to actual engineering problems. One of the first design tradeoffs with a nuclear-electric system is that that you're caught between the Carnot efficiency of your turbines and the need to dissipate waste heat solely through radiation. On the ground, we can simply harness a river to cool our reactor, but not so in space. A radiator can radiate thermal energy as the fourth power of its absolute temperature. At room temperature of 25 degrees C or 298 degrees Kelvin (absolute), it can radiate about 500 watts/sq. meter from each side. If we used steam/water as the working fluid in a steam turbine and ran the radiator at the condensation temperature of steam (100 degrees C or 373 degrees absolute), then we could radiate about 2 kilowatts per square meter. The reason for all this yakkety-yak about radiators is that this has been a crucial stumbling block to the weight of proposed nuclear-electric propulsion systems over the past 40 years. If we could make this radiator very light, and could make the whole power conversion system very light, then we do great things with low accelerations over long periods of time. Still.. Back in 1959, Dr. Manfred Raether and I discussed this subject of nuclear reactors in space over lunch with Dr. Luis Alvarez (the Nobel Prize winner) at the Livermore Radiation Lab cafeteria. Dr. Alvarez' point was that it's quite a project to keep a nuclear reactor running on the ground, let alone an ultra-lightweight nuclear reactor operating unattended in orbit for a period of a year. "Don't hold your breath," he said. He was so right. To actually run a reactor for ten, a hundred, or a thousand years is probably going to require a robotic maintenance and refurbishment sytem that can repair and rebuild a nuclear-electric power system over and over again on its long journey to another stellar system.

So what could we do with such a system? First, it seems to me that we could reach 1/1,000th of light-speed (300 kms./sec.) fairly straightforwardly. Stepping this up to 1% of the speed of light (3,000 kms.) might be especially feasible on trips beyond 10 light-years. An acceleration of 1/10,000 gee would require 100 years to bring our ship to its nominal cruising speed of 300 kms./sec. and 100 additional years to slow it down at its destination. An acceleration of 1/10,000th gee would require a thrust of one newton per metric ton, and if the ion rocket utilized an exhaust speed of 3,000 kms./second, this combination of acceleration and exhaust speed would require a power input of 1,500 kilowatts per metric ton. If that's too high, then, inasmuch as the travel time would be measured in centuries, one could cut the acceleration. At 1/50,000th gee, it would take 500 years to accelerate and 500 years to decelerate, and the power input requirement would drop to 300 kilowatts per metric ton, or 0.3 kilowatts/kilogram. Operating a turbogenerator at efficiencies approaching the Carnot efficiency for a heat engine, we might get away with radiating 300 kilowatts, thus requiring 150 sq, meters of radiator surface. I think that such a system might be realizable, or at least, approachable.

To make things really interesting, we'd want to be able to ratchet our speed up another order of magnitude, to 10% of light speed. At that speed, we could reach the Centauri System in less that 50 years. That would put the mission within most people's lifespans, particularly if they were to live longer than they do today. Travelling at 10% of c brings us up against total-energy limitations imposed by the 0.7% mass-to-energy conversion "efficiency" of deuterium-tritium fusion. It would also require a 100-fold increase in the power-to-weight ratio of the power supply... a 10-fold increase because the exhaust speed would have to 10-fold, and another 10-fold increase because, since the flight time would shrink by a factor of 10, the acceleration would have to increase by a factor of 10. That would demand a power-to-weight ratio of 30 kilowatts per kilogram, and that's beyond anything that I would forecast using 40-year-old technology.

The punch line is that I believe that we'll eventually be able to span the galaxy one way or another, and if we can potentially do it, so could someone else. So where are they? I would be surprised if we were the first. At 10% of light speed, it would take less than a million years for us to span the galaxy, and there are plenty of stellar systems that are over four billion years old. We now know that planetary sstems may be the rule rather than the exception. The planets we've discovered so far have been gas giants close to their primaries because the we've only been able to detect them planets that are large enough and close enough to their parent stars to cause their stars to visibly wobble.. until a few months ago. A few months ago, a team at U of C, Berkeley, discovered a planet about 60% larger than Jupiter that occults its star (HD 209458) once every 9 2/3rds years. So given this zoo of large, detectable planets, it looks as though lots of smaller planets are hiding out there, too. Seen from Centauri, the Earth would appear to be separated from Sol by about 1 part in 250,000 (~25,000,000,000/~100,000,000). Hm-m-m. The Palomar telescope can resolve about 1/50th arc-second or about 1 part in 10,000,000. It should be able to resolve planets in other stellar systems. I guess the fact that the Palomar and Keck telescopes are ground-based might play a role, but the 96-inch Hubble Space Telescope ought to be able to swing it. It must be a problem with light-gathering power. There may be other tricks, like filtering out all light but room-temperature infra-red. Anyway, I've read that it will eventually be feasible to detect planets by their reflected light, although it will make stingent demands upon future generations of space-based telescopes.

    The Earth is ~4,600,000,000 years old in a galaxy that's perhaps, 12,000,000,000 years old. As Steve Coy has observed, the Solar System is a later-generation Population I (2nd or 3rd-generation) star that was formed after some of the earlier Type II (1st-generation hydrogen/helium) stars had created heavy elements and then had exploded as supernovae. These heavy elements were then incorporated into Type I stars such as our sun. Presumably, there are other families of Type I stars that formed at somewhat different times than our Sun and its cousins. Intelligent life could have expanded throughout our galaxy in less than 10,000,000 and probably, less than 1,000,000 years. It would be logical to suppose that some earlier space-faring race or races would have colonized the galaxy before us. But if they haven't, we will. Assuming that we people the galaxy at an expansion rate of 10% of the speed of light, we would reach its farthest outpost in 700,000 years out of, perhaps, a-greater-than- 700,000,000-year window. Of course, if someone came by Earth more than a few million years ago, they wouldn't have found intelligent life. Digitigrade apes have existed for only about 4,000,000 years of that period, and tool users for 2,500,000 years. Still, there might have been portentous signs for experienced observers. You might expect them to do what Arthur Clarke portrays in 2001.. hiding a telltale that wouldn't trigger until we reached a certain level of advancement. Such a beacon could be hidden on the moon, on Mars, in the outer Solar System, or in some other haystack, waiting for us to find it. You might expect it to lure us with RF or light signals, but its designers might have wanted to wait until we were farther along toward their definition of "intelligent". Or it might be working now, signaling them but not us. They might want to have a look before they leaped. Why advertise their presence until they've checked us out? If interstellar transportation is constrained to sub-light speeds, they might be on their way from their nearest depot. And once they arrive, they might be very discreet about advertising their presence until they're checked us out. Or they might not want to disturb this new species while they learn its peculiarities. Do you think that would be the way we would operate at our present level of sophistication? Except that we're projecting a human conquest of an uncontested galaxy. As Steve and Martin have both suggested, why wouldn't that first race have colonized Earth?
    As always, I can concoct reasons why the aliens haven't sequestrated Earth, such as the idea that advanced races wouldn't feel our current territorial imperatives (particularly if constrained to sub-light transit) but only if we ourselves and all other sentient species, when the time comes, prove to be interested in merely exploring the galaxy, rather than in colonizing it. I'm hoping that future emissaries of Earth will not behave as crudely as our forebears. For what must be the first time in history, slavery is forbidden around the world. We would seem to be the first age in history to attempt to establish full parity between men and women. There may have been isolated enclaves here and there, and now and then, but throughout Asia, Africa, Australia, Oceania, Europe, Australia and the Americas, women have been subjugated until the 20th century. The establishment of a Society for the Prevention of Cruelty to Animals and a similar society for the prevention of cruelty to children are, the best of my knowledge, uniquely modern developments. (I'm sure it doesn't hurt that we're well-fed, comfortable, and largely pain-free.) Anyway, if Hans Moravec is right, it will soon be supremely intelligent robots--inorganic lifeforms--humanity's children--that will explore the galaxy instead of us.
    At the same time, I'm very happy that we haven't encountered any aliens. Even if they were benign, I believe that discovering one or more advanced civilizations could be the worst thing that could happen to us. Right now, we're on a wonderful voyage of exploration and discovery. If we knew that there were others out there who had already far surpassed us, what satisfaction could we take in our wondrous discoveries? And think how much worse it would be if they had "IQ's" of 400 or 500!
    I have written a short science fiction story about this, called "The Grecian Gift", that I'll post. (Beware of Greeks bearing gifts!)
    Kerry, I was very interested in your ideas about "Cities in Flight" Your idea that they would soon lose an interest in planet-bound existence struck a chord. I think we are soon going to have the power to guide our own evolution in seven-league strides, but even beyond that, I anticipate the melding of man/woman and machine, with, perhaps, other eventual vehicles for sentience, such as electromagnetic/plasma or some other computational technologies that we don't presently foresee.
    I enjoyed spinning out a few thought vis-a-vis your space habitats. Although such habitats would have to come up for air around a star periodically to refuel, they could thrive for a lo-o-ong time between soundings. Nuclear energy could sustain them for millenia between refills. However, they might also be led to starfalls in order to expand their habitats. I could imagine a habitat 1 km. in diameter by three kilometers long. It would have an internal surface area of about 10 square kilometers or 2,000 acres per floor, and could probably comfortably house 1,000,000 people, with 2,250 cubic meters or about 66,000 cubic feet per person. It could incorporate streams and waterfalls, sylvan glades, and "wilderness areas". It could be constructed in solar orbit using nickel-iron and stone from the asteroid belt. It wouldn't necessarily require all that much material, since it could be made light and strong. It would have to rotate about 5 times per hour. Given that centrifugal force (artificial gravity) would be proportional to distance from the axis, the tensile load on the radial cables would be about that of an 800-foot hanging structure. If located near the Earth's orbit, it could derive adequate heat and power from sunlight. If its inhabitants wanted to roam out-system toward another star, they could readily derive enough power from one or a few nuclear reactors to supply them for eons. Of course, to make the trip in centuries or decades, they would need major power sources far beyond those required for life maintenance. Of course, you can then fantasize the kind of expansion-and-population pressure that drove Asiatic nomadic incursions into Europe during Roman and medieval times. But humanity didn't have birth control in those days. A space habitat could voluntarily stabilize its population.
   Structures longer and larger in diameter might, perhaps, be technically feasible and if so, would probably be constructed. For example, a space habitat 2 kms. in diameter and 20 kms. long could probably very-comfortably house 25,000,000 inhabitants. (Imagine how many robots could inhabit a space habitat!) Robots, or teleoperated robots may well be the implementers of such space habitats.
    At $100 a pound, $1,000,000,000 a year could launch 10,000,000 lbs. a year into low earth orbit. The asteroid belt has the potential to provide us with virtually unlimited raw materials that don't have to be mined and can easily be refined and moved about. Solar powered on rockets can move payloads out there, and can move them about within the asteroid belt. Solar power, even at those distances from the sun, can be used to power tools, and to melt and shape nickel-iron into structural members, surrendering its heat to other ingots as the vacuum-cast shapes cool. One excellent, clean and simple way to heat metals is by using electron bombardment--the plate heating of vacuum tubes.) (Vacuum casting yields dense bubble-free objects.) Teleoperated or robotically operated steel mills will be constructed there in the asteroid belt, and may be used to build space habitats. Of course, even with light-weight construction, it would take a lot of steel to fabricate a large space habitat. However, the first efforts will undoubtedly focus on building additional steel mills, tools, solar arrays, etc., in order to bootstrap a space-based industrial base. It's even conceivable that the first equipment will be very small and will construct ever-larger equipment, until the mills begin to churn out products for other purposes.

Time to quit and get this on the wire. I'll defer the discussion of the consequences of faster-than-light communication or transportation until the next installment.

Best wishes to you all,

Bob

At 09:17 PM 1/14/00 -0600, Bob and Tommie Jean wrote:
> I'm proceeding slowly with this because I want to work through some
> consequences of the ideas that everyone is presenting and perhaps, to
> generate publishable results. (I plan to share the credit for any
> publishable articles that might derive from these lucubrations.)

Bob: If you actually do document this, I think that would be very
interesting. To preserve everyone's contributions, it might be interesting
both from a historical and a functional point of view to establish some
basic questions and refutations or confirmations provided by various people
contributing to the dialogue. Eventually could submit it to engineers at
NASA for thoughts and comments or rebuttal? That would be a fun project
with a serious and challenging goal. Are we up to the task? I'm
interested in participating to what extent I can afford the time.

Meanwhile, a few comments (I'm abbreviating some of your thoughts to save
space here--I'll do my best and my translations should be considered more
utilitarian than authoritative--apologies for the spartan digestification
but need to whittle your tome just a bit for the purposes of discussion):

> The realization, stemming from these discussions, that we could, in
> principle, reach the stars (slowly) with existing technology has been,
> for me, an unexpected revelation. Coupled with what I am led to believe
> will happen in robotics over the next few decades confers a significance
> on these results that they wouldn't otherwise warrant. Advances in
> robotics should eventually reduce labor costs, lowering the cost to
> orbit. NASA's current programs to cut the cost-to-orbit to $1,000 a pound
> and then to $100 a pound should fit into this scenario independently of
> whatever further improvements robotics might afford.

Increases in labor efficiency require substantive wealth and job creation
in order for improvements in productivity and the sustainable creation of
wealth. But for the moment I'll assume you are implying that, so let's
just say that you're right :-)

>to reconstruct cells by the end of the 21st century, even if we can
>generate a genome. Maybe sperm and egg banks would be the way to go,
>although that approach would also require an artificial womb and an
>artificial placenta, and I don't suppose that's exactly just around the
>corner.

Probably true.

>My idea, once they they got to their far star, would be that the resulting
>children might be reared as though they were on Earth. Then once that
>first generation had children of their own, the normal family sequence
>could be established. One of the thoughts I've had about this scenario is

That's also what I was thinking in terms of sending the sperm and ova banks
along... that there would be a first generation "raised" somehow. It's
almost inconceivable to imagine how any computer or multimedia program
could do that though. I mean, robot mommy changing diapers is one thing,
but when baby grows up and realizes that mommy is a robot, that could be
kind of disturbing. Also it's probably the case that even if utility
robots improve as you mention, that's a long shot from humanoid-looking
robots who appear to age normally or show the other signs of being human.

The sheer mechanics of the human body suggest that the best tools for
building a functioning human robot that can perform complex tasks like
taking a child for a walk, balancing itself when walking quickly up a long
flight of curling stairs, responding with the kind of nuance the child
would recognize in him/herself... would be human tissue itself. Which
brings us back to problem one of course, which is that we can't send that
kind of robot into deep space and expect it not to malfunction (die) at
some point. You'd have to start *it* in an egg and find it some artificial
parents ;-) Back to square one.

    Martin, it seems to me that the generation of conscious awareness in a computer shouldn't be hard to
do. I'll be glad to send you this separately, since it's sufficiently voluminous that it would make this
present tome look like a brief thank-you note. I pondered this for a year, and wrote enough for a small
(but meaty) book on the subject. My computer program would monitor and review its own actions and
"feelings", respond to competing "urges", select among them by temporarily inhibiting them, and in
general, attempt to emulate an animal brain rather than a computer. I quit this project because I was
working alone, and something else came along (first, the opportunity to choose and buy some
computer equipment out of Georgia Tech year-end money, and second, the building of the platform
that my computer would need to explore its surroundings). Afterward, I went on to something else. I
would love to have help in this area. It doesn't necessarily require any specialized background in
anything, since it involves contemplating how we ourselves process information, although perusals of
studies of thinking and learning were starting to look beneficial at the time I quit the project. This might
be a great and absolutely fascinating group project. The stumbling blocks that kept me from trying this
out on my PC were as follows:
(1) Tommie Jean had bought me a digital webcam for my 1996 birthday, but I couldn't find anyone
who could tell me how to open uncompressed digital frames in a programming language such as Visual
Basic or C++. It's done all the time, but I wasn't able to find anyone who knew how to do it. It seemed
to me that giving my desktop computer the ability to see was key to its experiencing the world. (It
could have been blind, I suppose, and relied totally on hearing, but that seemd like a more difficult
problem for a first attempt.) Also, I have some ideas about "predictive vision" and "contextual vision"
that I wanted to try in order to reduce the computational load to desktop proportions.
(2) I planned to use my Clodbuster-mounted vidcam to transmit its imagery to my computer. I finished
the remotely-controlled teleoperted Clodbuster but then realized that I would transmitting a video signal
back to the my computer rather than the full digital signal. The bandwidth requirements for an
unprocessed 30-Hz EGA signal with 24-bit color depth would 55.3 megabits per second, far beyond
the 4+ MHz bandwidth of a 2.3 Gz downlink. Also, I would have to build or modify the software and
hardware interfaces for my computer in order to accept this wideband digital input. And finally, when I
called Futaba, they advised that they had no computer interfaces for their Clodbuster-type controllers,
although I have seen such an interface built by students at the Mississippi State University.
Consequently, I was going to have to mount a high-end laptop on my Clodbuster to pre-process the
information streaming in from my digital vidcam, and at that time, prior to Firewire, it would have been
difficult to even get the information into a computer. In short, it was turning into a project where one
could very rapidly become bogged down with details.
    My recommended gambit today, if we take this up again, would probably be to try to collaborate
with Rodney Brooks and/or Hans Moravec. I explored this in 1996 with someone in our Georgia Tech
robotics group, but I came away with a very bad taste in my mouth. I felt very bad vibes with the
individual (whose name I don't even remember). (We talked only about his activities, rather than what I
was doing, so my impresssions weren't based upon his assessment of my work.)
    The part I can't imagine how to accomplish is that of how to make a computer feel pain or pleasure.
The signal is electrical, coming in from distal nerve-endings and we've locate the pain and pleasure
centers in the brain, but what exactly
    Of course, I certinly can't know how far we could someday go (or not go) in devising a surrogate
nursemaid for these hypothetical children.

>that there must be roughly the same resources and elements everywhere we
>go. It wouldn't make sense to transport materials between stars, unless it
>were rare earth elements. And even those

The one problem I see is that for likely destinations, it may be they're
too far away to really accumulate a useful understanding of its
compositional elements. That's a pre-flight problem for astronomers and
geologists, I understand, but just wanted to note that.

>might be more readily and cheaply synthesized locally (assuming you can
>remove all the radio-isotopes). The interstellar medium of exchange might
>be information. Given the blueprint, you could locally manufacture or
>synthesize whatever had been developed in the next star system over. Also,
>to allow someone to experience life in the Tau-Ceti System, it might be
>simulated in some vacation spot or space habitat in the Solar System.
>Virtual reality might be used to create the illusion of actual presence.

You cover a lot of territory there, some of which I think indicates several
departing possibilities.

> I didn't mean to imply that time would seem to pass faster, but that
> the bite that a long trip would take out of a lifespan should be
> relatively smaller. I could envision that if humans went, a small village
> might be making the trip (like the most luxurious cruise liner). (Of
> course, this would run counter to the desire for a small cross-sectional
> area.) It would certainly have to be large enough that it could rotate,
> to provide "centrifugal gravity". It might be like a long, long 18th
> century sea voyage, except that the passengers would be in constant (if
> delayed) virtual contact with Earth. Such a trip might be best-suited for
> authors, artists, philosophers or theoreticians who are immersed in
> creative work.

This path of thinking definitely plays up the difficult problems inherent
in providing a reasonably sane environment for travelers over long distances.

I once knew a guy who spent six months at a time underwater on nuclear
submarine patrol. Now, being underwater for six months is one
thing. Remembering him describe the joy of seeing the sky after those six
months... well, it reminds me of the following fact:

Traveling through space for a long time alone is one thing.
Traveling through space for a long time with enough others to keep from
being lonely is another.
Traveling through space for 20 years or more in a ship going near the speed
of light is another.

The view, the noise, the air quality (okay let's say all that can be
"simulated away")... the inability to travel outside and be "in the sun" --
I think these things might seem like "Well, I could live with that." But
for 20 years or more? I truly believe it would make most people unfit for
the destination. Would it not be severely unhealthy both physically and
mentally? I suppose environmental engineering *might* be up to the
task. But what if you run out of light bulbs... or something more
serious? It's not like you can abort the mission and go back that
easily. The distance from safety nets seems to me would present a fairly
major problem.

Since these are general cautionary thoughts, I'm not sure it has any
relevance to trying to come up with something anyway, but again just wanted
to get that off my chest :-)

> My fantasy about what might happen when we experience a major
> breakthrough in lifespan is that the bearing of children would be
> deferred until later in life, since there would no longer be any urgency
> about "biological clocks", and that children might be reared one at a
> time, with the entire family particpating in their upbringing. However,
> given Dr. Hans Moravec's forecast of sentient robots before 2050, I think
> it's a given that "humanity's children" are going to be reaching for the
> stars before we biologicals do.

I don't believe in sentient robots. I believe in robots that can imitate
consciousness, but not experience it. They haven't really even been
attempted yet, and I can tell you that :-) The reason? It's a philosophic
problem of known proportions, not a mechanical engineering problem. That
devices can be constructed to imitate human behaviors is already a
given. That those devices can experience conscious awareness (rather than
literal awareness) is subject to philosophic debate. And neither Dr. Hans
Moravec nor anyone else can predict paradigm shifts in philosophy as a
function related to advances in mechanical engineering :-)

As I've mentioned, I don't think that achieving a species of self-awareness in a computer need be all
that difficult. And of course, I may be very wrong about that. And how could we prove that a
computer had conscious awareness, as opposed to merely the appearance of conscious awareness?
But I'd first like to try my ideas about programming self-awareness in a computer to see what happens.
Right now, I guess my claims and pretensions are kind of like, "If I had a saddle, I'd ride my horse, if I
had a horse."

> My principal concern is not how we're going to eat ths pie once it's
> baked but how we're going to make it.

That's fine. "One step at a time" works for me for the purposes of this
project.

> Having examined this topic for this forum, I think that:
>(1) we currently have an energy source, in the form of themonuclear
>fusion, that, in principle, might allow us to reach some reasonable
>fraction (10%-30%) of the speed of light.
>(2) we could possibly launch interstellar probes that might attain,
>perhaps, ~1% of light-speed using nuclear-electric propulsion systems. At
>that rate, it would take ~450 years to reach the Centauri System.

I assume this would have to be a fusion reactor built in space. I'm not
sure about the physics of a fusion reactor. That sounds pretty difficult,
risky, resource-intensive, and expensive for a spacecraft, even given
advances in robotics engineering. Is that your best candidate for the
propulsion system? Any other suggestions (assuming something we can
actually bring beyond debate into actual analysis for the purposes of
building our ship :-) )? (and I see what you've written below, now,
although I hate to delete this paragraph I've rather patiently thought out).

>square meter. The reason for all this yakkety-yak about radiators is that
>this has been a crucial stumbling block to the weight of proposed
>nuclear-electric propulsion systems over the past 40 years. If we could
>make this radiator very light, and could make the whole power conversion
>system very light, then we do great things with low accelerations over
>long periods of time. Still..

Very interesting. Sounds well, like quite a stumbling block.

>Back in 1959, Dr. Manfred Raether and I discussed this subject of nuclear
>reactors in space over lunch with Dr. Luis Alvarez (the Nobel Prize
>winner) at the Livermore Radiation Lab cafeteria. Dr. Alvarez' point was
>that it's quite a project to keep a nuclear reactor running on the ground,
>let alone an ultra-lightweight nuclear reactor operating unattended in
>orbit for a period of a year. "Don't hold your breath," he said. He was so
>right. To actually run a reactor for ten, a hundred, or a thousand years
>is probably going to require a robotic maintenance and refurbishment sytem
>that can repair and rebuild a nuclear-electric power system over and over
>again on its long journey to another stellar system.

Using what materials? Now you're talking about a pretty *huge* ship... if
this were to become common, how might radiated materials be disposed of
safely? If held onto in a cargo bay, you're talking even *bigger*
problems. If you chuck them out into space, and this occurs along regular
flight paths, well, a lot of ships coming later at near-light speeds may
run into some larger-than-particle objects than they ever expected.

If's the ship's crew is robotic, maybe radiation won't be as important as it would be with a human
crew. The "ashes" could be chemically separated from the plutonium in the fuel rods, and the rods
replaced. Then, since this is interstellar space, the radioactive material, perhaps emitted as a vapor,
could certainly be ejected. If another ship ever encountered it again, it would be in the form of one or a
few individual radioactive atoms.
I don't necessarily envision a huge ship, although I should that a huge ship might be desirable if it
were to carry human passengers.

A side question: Is there any reason to think that spaceships shouldn't or
couldn't or wouldn't need to dispense waste into space? Are there any
materials which *must* be disposed of in a spaceship ecosystem? (I
understand there are a lot of obvious recyclables, so would prefer to ask
the question by exception than by rule).

If there were humans on board, it might be desirable to evict radioactive waste. Of course, the
power supply would probably be far away from the passenger compartment, with various supplies
serving as a radiation shield. Otherwise, everything else is, supposedly, recycleable.

> To make things really interesting, we'd want to be able to ratchet
> our speed up another order of magnitude, to 10% of light speed. At that
> speed, we could reach the Centauri System in less that 50 years. That
> would put the mission within most people's lifespans, particularly if
> they were

I would say that's key, because who can say that the children of
spacefarers would share the enthusiasm of their parents' grand adventures?

That's for sure, isn't it? Generational ships don't sound like anything I'd want to ride. Suspended
animation might be a way to sleep from star to star.

>to live longer than they do today. Travelling at 10% of c brings us up
>against total-energy limitations imposed by the 0.7% mass-to-energy
>conversion "efficiency" of deuterium-tritium fusion. It would also require
>a 100-fold increase in the power-to-weight ratio of the power supply...

At traveling, yes. But what about the extraordinary power required for
acceleration?

>a 10-fold increase because the exhaust speed would have to 10-fold, and
>another 10-fold increase because, since the flight time would shrink by a
>factor of 10, the acceleration would have to increase by a factor of 10.
>That would demand a power-to-weight ratio of 30 kilowatts per kilogram,
>and that's beyond anything that I would forecast using 40-year-old technology.

Ah. And you answered my question :-)

> The punch line is that I believe that we'll eventually be able to
> span the galaxy one way or another, and if we can potentially do it, so
> could someone else. So where are they? I would be surprised if we were
> the first. At 10% of light speed, it would take less than a million years
> for us

You might be surprised, but until we even see real evidence of even just
one, we're pretty much up the creek as far as hoping for arrivals. Here's
a deeper question, actually. Given that there are systems much older than
ours, and given the "probability" of life developing in any given place
(which I myself believe is not a statistically sound inquiry, working from
just one piece of data and having no other control groups to compare with),
and given that you'd be surprised if we were the first, and given that it
would take roughly a million years to span the galaxy, and given that such
technology would likely be replicated, then why *haven't* we seen any
strong evidence of life arriving from other planets yet? Does anyone care
to do the math on this? If I have time I'll sit down and work on it, but
if anyone wants to give it a start I think that would be sort of interesting.

Martin, I wouldn't even know where to start to arrive at a probability. If you can see any way to do
it, go for it! And as to why they aren't here, I haven't the foggiest idea. However, one (rather-unlikely)
thought that just crossed my mind. I just finished reading about black holes that have just been detected
in interstellar space (through effects upon starlight). I also just read about the comets that wander off
into interstellar space. (these are being suggested as possibly accounting for some of the "dark matter"
that's thought to be somewhere out there. What if there are enough of these hazards to navigation that
you can't go whistling along blind-in-the-dark? Maybe interstellar traffic isn't as feasible as we think it
is.

Given assumptions about the likelihood of interstellar travel, and given
the assumptions about the probability of genesis of life on other planets,
and given the age of various parts of the universe in proximity to ours and
within our own galaxy, what are the odds that our planet *should* have been
visited by interstellar travelers by now, if in fact assumptions about not
being unique are correct? :-)

>Palomar telescope can resolve about 1/50th arc-second or about 1 part in
>10,000,000. It should be able to resolve planets in other stellar systems.
>I guess the fact that the Palomar and Keck telescopes are ground-based
>might play a role, but the 96-inch Hubble Space Telescope ought to be able
>to swing it. It must be a problem with light-gathering power. There may be
>other tricks, like filtering out all light but room-temperature infra-red.
>Anyway, I've read that it will eventually be feasible to detect planets by
>their reflected light, although it will make stingent demands upon future
>generations of space-based telescopes.

Seems this would be vitally important in scoping out a destination. Kind
of hard to change your mind once you get there.

> The Earth is ~4,600,000,000 years old in a galaxy that's perhaps,
> 12,000,000,000 years old. As Steve Coy has observed, the Solar System is
> a later-generation Population I (2nd or 3rd-generation) star that was
> formed after some of the earlier Type II (1st-generation hydrogen/helium)
> stars had created heavy elements and then had exploded as supernovae.
> These heavy elements were then incorporated into Type I stars such as our
> sun. Presumably, there are other families of Type I stars that formed at
> somewhat different times than our Sun and its cousins. Intelligent life
> could have expanded throughout our galaxy in less than 10,000,000 and
> probably, less than 1,000,000 years. It would be logical to suppose that
> some earlier space-faring race or races would have colonized the galaxy
> before us. But if they haven't, we will. Assuming that we people

Bob, it's amazing how each question I come up with next, apparently has
crossed your mind also in following the same path of thinking... slightly
different angle but I'm right with you here. As for assessing the
likelihood of life on other planets, see my question in the prior paragraph
for more fun and giggles.

I think the "If they haven't, we will" is key to pointing out that we
*could* be the first... although I'm not sure that the genesis or
simultaneous development of life on other planets really has any serious
theological consequence as some arguments might suggest (nothing you've
said, just wandering off in my thoughts here).

>the galaxy at an expansion rate of 10% of the speed of light, we would
>reach its farthest outpost in 700,000 years out of, perhaps,
>a-greater-than- 700,000,000-year window. Of course, if someone came by
>Earth more than a few million years ago, they wouldn't have found
>intelligent life. Digitigrade apes have existed for only about 4,000,000
>years of that period, and tool users for 2,500,000 years. Still, there
>might have been portentous signs for experienced observers. You might
>expect them to do what Arthur Clarke portrays in 2001.. hiding a telltale
>that wouldn't trigger until we reached a certain level of advancement.
>Such a beacon could be hidden on the moon, on Mars, in the outer Solar
>System, or in some other haystack, waiting for us to find it. You might
>expect it to lure us with RF or light signals, but its designers might
>have wanted to wait until we were farther along toward their definition of
>"intelligent". Or it might be working now, signaling them but not us. They
>might want to have a look before they leaped. Why advertise their presence
>until they've checked us out? If interstellar transportation is
>constrained to sub-light speeds, they might be on their way from their
>nearest depot. And once they arrive, they might be very discreet about
>advertising their presence until they're checked us out.

My main problem with the idea of a secret horde of aliens hiding outside of
earth's view is just that I don't care how technically advanced a culture
is, if it's a culture that empowers people enough to accomplish amazing
things, and puts powerful tools in the hands of its citizens, well...
someone is bound to drop by and say hello.

I tend to think if "they" are out there, they are separated by the sheer
physical distances we've discussed. Until a new astrophysics is proved, I
think I'll have to stick with that assumption for the sake of fruitful
investigation.

The only better counterargument I can think of here, an argument as to why
aliens would *not* visit, is that maybe "Earth is boring!" :->

> Or they might not want to disturb this new species while they learn its
> peculiarities. Do you think that would be the way we would operate at our
> present level of sophistication? Except that we're projecting a human
> conquest of an uncontested galaxy. As Steve and Martin have both
> suggested, why wouldn't that first race have colonized Earth?

Except in the sense that children often dream that their "real" parents are
kings and queens in some fairy tail fantasyland, I can't understand why the
answer would have to be that we came from somewhere else. I say this not
to defend a parochial idea of genesis, but rather, to unseat the
presumption that holier and more pure forms and original sources must come
from "outside." Such ideas, taken for granted, lean toward an arbitrary
opposite of xenophobia.

> As always, I can concoct reasons why the aliens haven't sequestrated
> Earth, such as the idea that advanced races wouldn't feel our current
> territorial imperatives (particularly if constrained to sub-light
> transit) but only if we ourselves and all other sentient species, when
> the time comes, prove to be interested in merely exploring the galaxy,
> rather than in colonizing it. I'm hoping that future emissaries of Earth
> will not behave as crudely as our forebears. For what must be the first

I also don't buy the idea that with technological advancement comes an
implied advancement of culture or civilization. I went into this
considerable with the SETI guy so haven't the energy to repeat it just now,
but if you'd like I'll post pieces of that conversation (perhaps another
time :-) ). I wouldn't worry too much about emissaries. The first guys to
enter a new world are usually the nicer ones, full of wonder and
delight. It's all the tourists who come later that kinda ruin the party !!!

>time in history, slavery is forbidden around the world. We would seem to
>be the first age in history to attempt to establish full parity between
>men and women. There may have been isolated enclaves here and there, and
>now and then, but throughout Asia, Africa, Australia, Oceania, Europe,
>Australia and the Americas, women have been subjugated until the 20th
>century. The

These things go on. I wouldn't be too proud of what exists today, in the
year 2000. If we'd hold ourselves today responsible for all of the
optimistic-oriented science fantasy concocted over the last 50 years
regarding the year 2000, well, I think we'd have to be rather ashamed of
ourselves.

>establishment of a Society for the Prevention of Cruelty to Animals and a
>similar society for the prevention of cruelty to children are, the best of
>my knowledge, uniquely modern developments. (I'm sure it doesn't hurt that
>we're well-fed, comfortable, and largely pain-free.) Anyway, if Hans
>Moravec is right, it will soon be supremely intelligent robots--inorganic
>lifeforms--humanity's children--that will explore the galaxy instead of us.

I'm not sure where you get the idea that these are uniquely modern
developments?
Also I'm not sure where you get the idea that we're well-fed, comfortable,
and largely pain-free?
Good grief man... I *am* happy for you if you have a comfortable life, but
I don't know if you realize just how tough it is for some of us out
here. Actually I don't mean to sound like you would not know that, I'm
sure you do -- but your phrase "(I'm sure it doesn't hurt that we're
well-fed, comfortable, and largely pain-free.)" -- I would not describe
that as the sum of my experience.

Martin, you're right You're reminding me how fat, dumb, and happy I am in retirement, now that I'm
no longer in the asphalt jungle. I'm remembering the nights I spent walking the floor, facing impossible
situations at work. And I'm remembering the impossible men to whom I sometimes had to report. And
at that, I probably had it better than most. I know by looking around us that there are terrible hardships
in the world. Thoreau's remark that most men (and most women) live lives of quiet desperation is
probably at least as true now as it was in his day.

> At the same time, I'm very happy that we haven't encountered any
> aliens. Even if they were benign, I believe that discovering one or more
> advanced civilizations could be the worst thing that could happen to us.
> Right now, we're on a wonderful voyage of exploration and discovery. If
> we knew that there were others out there who had already far surpassed
> us, what satisfaction could we take in our wondrous discoveries? And
> think how much worse it would be if they had "IQ's" of 400 or 500!

I would like to attempt to answer that. It would be a horrible experience
for people whose ego is attached to the "satisfaction of knowing something
others don't" rather than the "satisfaction of learning and pure
discovery." For example, I believe that much of what we're discussing here
has certainly been discussed in the past, but on some points it's new to
me, so that in itself is rewarding. I don't think of whether I came here
knowing everything or not, that's not the issue to me.

No, I think I must respectfully express a different opinion about the idea that finding far-advanced
and far-smarter aliens would affect just the intellectually egotistical. I think knowing that all the answers
have already been found would take the wind out of our sails as a civilization. We would be in the
position of any backwater stone-age society suddenly discovering the modern world, only it would be
far worse if we were mentally incapable of understanding the tenth part of what they could understand.

If they had an advanced intelligence as you suggest, we could only hope
that they are gifted with an advanced spirituality to match. Advanced
intelligence in the hands of the spiritually poor is a weapon in the hands
of the ruthless. If they come with a spiritual balance, then no difference
in intelligence should prevent them from being smart enough to know how to
put their knowledge to good use.

Just as I should hope anyone would, encountering differences amongst ourselves.

Amen to that.

>soon lose an interest in planet-bound existence struck a chord. I think we
>are soon going to have the power to guide our own evolution in
>seven-league strides, but even beyond that, I anticipate the melding of
>man/woman and machine, with, perhaps, other eventual vehicles for
>sentience, such as electromagnetic/plasma or some other computational
>technologies that we don't presently foresee.

My test for that will be whether I am conscious in any of those
machines. I predict I won't be. If they function only as sensory input,
it's not enough for me.

>Time to quit and get this on the wire. I'll defer the discussion of the
>consequences of faster-than-light communication or transportation until
>the next installment.

Looking forward to it.

I had an idea last night. Yes, really I did! I got to thinking that
volatiles must be few or non-existent in the asteroid belt. With several
billion years to outgas and evanesce, you'd at least have to blast open
asteroids to get at water-of-crystallization. And then it occurred to
me: maybe we could mine small comets when they zip by. There might be a
lot of very small comets that don't show up in telescopes. You wouldn't
have a huge window in which to decelerate them, or more properly, to
decelerate relatively small slivers of cometary ice. One possibility
might be something involving controlled nuclear micro-bursts to slow
small comets into lower orbits, or even to bring them onto trajectories
where they might graze the atmosphere of, e. g., Venus, long enough to
slow them down without capturing them. Another possibility would be to
dissociate water with solar power and then burn it in rocket engines to
decelerate modest water-ice payloads. We need water and other volatiles
for reaction mass for chemical rockets, for ion rockets, and certainly
for oxygen and water. We also need other light or volatile elements such
as the alkali metals, nitrogen, mercury, etc. But above all, we need
hydrogen and oxygen, and that should be available from comets. And it would be great if we could secure all the water we needed for the moon, Mars, and space habitats right out there in space.