Home > Clement's Game, Fermi Problems, Your Turn > Your Turn, Round 2-1: Fermi Problems

Your Turn, Round 2-1: Fermi Problems


In 1950, while sitting at lunch, Enrico Fermi is said to have asked “Where are they?“.  Fermi liked to do order-of-magnitude physics, and had worked out that if intelligent life is common in the universe and can spread through space at a significant fraction of the speed of light, extraterrestrial intelligence should already be here.  So, where are the aliens?

There are many proposed resolutions to this paradox.   One is that there are very few places for life to evolve – the Rare Earth hypothesis.  That becomes significantly less likely the more we know about extrasolar planets.  At the same time, SETI has advanced to the point that we can say that there are no other Earth-like civilizations within a few tens of light-years of here and no Dyson Swarms within several thousand lightyears.  So even if planets are common, perhaps life is not.

And even if life is common in the universe, perhaps intelligence is not often selected for.  This may be contradicted by life on Earth, but it depends on the definition of intelligence.  Apes are intelligent.  Dolphins and whales are intelligent.  Elephants are intelligent.  Dogs and cats and raccoons are intelligent.  Taking non-mammalian examples, consider parrots and ravens.  Octopi and cuttlefish are pretty smart too.  But if by intelligent we use the pragmatic SETI definition of being able to build a radio or other technology readily detectable over interstellar distances, intelligent life has only appeared once on Earth.

Given all of that, there are still obstacles to a civilization or culture spreading across interstellar space.  There has already been a lot of speculation on this theme, both in the scientific literature and in science fiction, but I decided to design a hard sci-fi setting playing around with it anyway.  I’ve used the results in a couple of variously-bad short stories and one never-completed NaNoWriMo novel (it reached 50,000 words, but would need an incredible amount of editing and a lot more material to flesh out the plot).  A couple of years ago, I put the then-current version of the setting online here, but I’ve decided on some changes since then.  Name-dropping, I call the setting “Fermi Problems”.

I’ll talk about the cultures in the setting more briefly here, one at a time.  As last time, I ask you to take them to pieces – both the ways to get around the Fermi Paradox and the details of each group.  I apologize in advance for a general lack of appropriate graphics, and beg for fan art.

People Who Can’t Get To Space

At the risk of making each culture in the setting a Planet Of Hats, I assigned a single primary reason why each of them had not spread across interstellar space in a detectable way before now (both actual ‘now’ and the several-hundred-year future ‘now’ of the setting).

For the first culture I designed, I decided that they had not spread into space because they couldn’t get off their planet.  That meant a steep gravity well, which meant a gas giant planet.  Fortunately, the idea of life on a gas giant isn’t entirely absurd.  Carl Sagan and Ed Salpeter studied the possibility in some detail in 1978.  Organic chemistry in the clouds, driven by lightning and UV and particle bombardment, becomes self-catalyzing.  Microbial life can survive as long as it reproduces fast enough to outpace the droplets it is reproducing in falling down to points in the atmosphere where they get baked dead.  Lift is better, so bags of heated gas and floating clots of aerogel come to dominate the biomass.

The floaters look kind of like Cloud City from Star Wars, except biological and filled with gas bags and aerogel bricks.  The cloud decks would also be shorter vertically under Ursa's gravity.

The floaters look kind of like Cloud City from Star Wars, except biological and filled with gas bags and aerogel bricks. The cloud decks would also be shorter vertically under Ursa’s gravity.

Sagan and Salpeter were considering the possibility of life in the atmosphere of Jupiter, which probably does not exist.  So I put my fictional biosphere in the atmosphere of 47 Ursae Majoris b, one of the first extrasolar jovian planets to be found that wasn’t a hot Jupiter.  For the purposes of the setting, 47 UMa b is ‘Ursa’ and 47 UMa the star is ‘Big Bear’.

Once there is a large population of floaters, life could evolve into niches that don’t have lifting power of their own.  Some are parasitic plants.  Others are animals, using heavier-than-air flight to get from one floater to another – powered by muscle power and biological chemical rockets (based on oxygen or methanol burned with hydrogen, as if a bombardier beetle secreted methane and burned it with oxygen) .  One species of those animals eventually evolved intelligence and a technological society.  Call them the ursians.

The ursians have had a technological society for hundreds of thousands of years.  Artificial islands of aerogel blocks; blimps and jets to travel; genetic engineering of crops and of themselves; biochemical computers.  But they have no silicon, no iron, no aluminum, no titanium, no uranium.  The only source of those materials on the planet is grains of meteorite dust sieved out of the atmosphere.  The ursians cannot build rockets capable of getting off of Ursa. 

Orbital speed for the planet is ~75 km/s.  No chemical fuel provides sufficient impulse.  A gas-core fission rocket might be able to do the job, but requires uranium.  A fusion rocket could do it, but that would mean setting off an H-bomb without a fissile trigger since steady-state fusion devices do not have sufficiently high power density to provide enough thrust.   I had a large comet impacting the planet throw a floater into orbit, but the ursians on it did not survive.  They are quite efficiently confined at the bottom of a very deep hole, and not able to build high-power radio transmitters either.

Does this work as a way to have extraterrestrial intelligence that has been around for a very long time and maintains high technology without being obvious across interstellar distances?  Have I missed some way to throw objects into the sky or to generate monochromatic radio waves?

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  1. 2012/12/25 at 3:56 am

    I think it depends on how determined the Ursans are and how well they can bootstrap. They have carbon, which means they could make conductors and semiconductors from carbon nanotubes. Jupiter’s atmosphere also contains tiny traces (~10^-9) of germanium, which could be used for semiconductors. Semiconductors and conductors would allow large-scale photovoltaic electricity production.
    Given large-scale electricity production, various alternative launch methods might become possible. A railgun/launch loop, organic laser powered light sail, or a very light magnetic sail could conceivably work, subject to drag forces. Once they can get something into orbit, they could bootstrap up to a tether propulsion system capable of moving large payloads.

    • 2012/12/25 at 1:11 pm

      Assuming the germanium carries over to a different formation environment.

    • michaelbusch
      2012/12/25 at 7:11 pm

      Good catch! I had forgotten germanium and arsenic, and their wonderfully-named hydrides germane (GeH4) and arsine (AsH3). There isn’t any silane (SiH4) in the atmosphere of Jupiter because SiH4 reacts very readily with CO2 to form SiO2 and methane, and the SiO2 promptly precipitates.

      I will have to think about the possibilities for the germanium, arsenic, and nanotube-based power grids that you’re suggesting. It may be that they can build radio transmitters after all, but the literature on germanium and arsenic electronic components in the absence of silicon or gallium is very limited. But the Ursians can’t have sufficient electrical production for the other launch systems that you mention, because there is a size limit to any floater or island.

      The maximum cross-sectional area of a solid object in Ursa’s atmosphere is ~10 km^2. Any larger and it will be sheared apart by turbulence (the wind field is not uniform). So that sets a limit on the size of any power plant: 10 km^2 of solar cells, assuming that they can make usable cells at all. Call it 1 GW to allow for Ursa’s distance from Big Bear, the latitude, the day-night cycle, and the efficiency of the solar cells; and to make the math easier.

      1 GW of light onto a reflective sail provides <7 N of force, sufficient to loft ~0.1 kg in Ursa's gravity. Given that that mass is too small for anything useful in orbit, there isn’t any possibility for boot-strapping. And drag forces _are_ very important. With a cross-sectional area of <0.01 m^2 (10 cm on a side), there would be enough light hitting the payload’s rear surface to vaporize it. Even if the reflectivity of the mirror were perfect, the atmosphere along the beam path would promptly turn into opaque plasma. This doesn't work for sending anything anywhere. With a much greater cross-sectional area, to bring the power density down to something reasonable, the drag coefficient becomes ~1 and the maximum velocity at 1 bar becomes ~40 m/s before drag evens things out. At 40 m/s, in theory the craft would rise up to a lower-density and lower-drag region on a timescale of several minutes. But to get the power density down below ~100 Sun equivalents, the craft would need to have a cross-sectional area of ~10^4 m^2 and a surface density of 0.01 g/m^2. That’s too thin to cope with the turbulence in the atmosphere at the 1 bar altitude, and the craft gets ripped apart. They could try launching a sounding rocket to a hundred kilometers above the laser array, deploying the sail above most of the atmosphere. But even then the acceleration would be too little: transverse velocity takes the sail out of sight of the lasers long before it gets enough speed to go into orbit (75 km/s orbital speed, 70 m/s acceleration, ~1000 s to get up to speed, as compared to 10-100 s in view of the array).

      Magnetic sails are out for a similar reason – drag is too high. They have the added problem of even if nanotube/graphene based superconductors can be manufactured in bulk, both the planetary magnetic field and any field source on the launch station are too weak.

      A railgun has the same drag problem, made even worse with the drag forces scaling as the square of the velocity. The shock front set up in front of such a projectile would vaporize it. Tungsten projectiles are used for railguns on Earth, limited to less than 10% of Ursa's minimum orbital speed. Increasing the heating by a factor of a hundred would destroy any possible bullet. Lofting the launch platform to higher altitude would decrease the heating, but that's limited by the mass of the power plant and the gun. The exit velocity from the gun would also need to be considerably higher than the minimum orbital speed (a back-of-the-envelope calculation gives me 95 km/s), making these problems even more severe.

      And even if some of the ursians manage to get into a stable orbit after having survived a comet fall, they can't make a tether down to the planet. Even carbon nanotubes aren't strong enough to reach down to Ursa's atmosphere from orbit; and there are ring particles and the radiation bombardment from the inner magnetosphere in the way. If and only if a comet-thrown vessel can make it to and survive indefinitely on one of Ursa's moons would bootstrapping be possible. When I worked out the numbers on that it was impossible to send up a sufficiently large starting infrastructure.

      So the ursians are still tightly confined to the bottom of their hole. But as I said, I'll have to think about the implications of having some germanium and arsenic. They won't have that much, when it takes a cubic kilometer of air to provide 5 kilograms, but it may be enough for some interesting electronics. Again – good catch!

  2. michaelbusch
    2012/12/25 at 11:32 pm

    Okay. After spending part of the afternoon reading up on solid-state physics and some old papers on jovian atmospheric chemistry while Rachel read her Christmas present, I have the following conclusions:

    1. The total amount of arsenic and germanium in the habitable altitudes of Ursa’s atmosphere is ~10^12 kg. If the ursians could capture one one-thousandth of that and use it for solar cells, they could produce ~1 TW of power spread out over >1000 sites. But there is a problem: there is a lack of the most effective doping elements (gallium and indium) as well as silicon. I found some discussion of the possibility of gallane (gallium hydride) in the jovian atmosphere, but it has never been observed.

    2. Organic solar cells based on materials without metallic ions held in coordination complexes have low efficiency, but can still convert ~1% of incident visible light into electrical power. So if carbon nanotubes can be formed into usable wires, then a 10 km^2 island could produce perhaps 20 MW of usable electricity.

    3. The total ursian population has been fixed at roughly 10 million, spread out over more than ten thousand sites. This means that a large island with a population of a bit over 10,000 could have a kilowatt of electricity per ursian.

    So the question that is relevant for the Fermi Paradox now becomes: is it possible to build a narrow-band radio transmitter only from things present in the atmosphere of a gas giant?

    Thinking about this some more, I conclude that it isn’t possible. A narrow-band and high-efficiency radio transmitter requires at least two microwave resonator cavities – one for the transmitter and one for the frequency standard it is referenced to. Microwave cavities require a high-reflectivity metallic coating on the inside surface, and gallium and arsenic aren’t notably good reflectors at microwave wavelengths (carbon compounds are very efficient absorbers, so they don’t work either). Working through some wider-bandwidth and lower-efficiency designs, the best radio transmitter that the ursians would be able to assemble would have a total radiated power of ~10-100 KW, bandwidth ~1 MHz, transmitter frequency ~1 GHz, and gain of 10^4-10^5. Using the formulae worked out here, signals from such a transmitter would be undetectable by any built or planned radio telescope near Earth. A detector with effective area of >>10 km^2 would be necessary to see it.

    The ursians also wouldn’t have much use for such signals: without satellite relays, they can only go long distances over the horizon with wavelengths that are long enough for effective ionospheric bounce and so have lower gain and don’t significantly escape into space (but not so long that Ursa’s magnetosphere produces too much noise). The result will be <100 MHz radios with ranges of a few thousand kilometers, useful for relaying messages from one island/inhabited floater to the next, and lower-power high-frequency devices used for local communication. Heliographs with organic lasers are most efficient for high-bandwidth communication, but only when there is a clear line-of-sight.

    So the ursians are tightly confined to the bottom of their potential well, and undetectable by current radio SETI and even by the few-AU flybys by interstellar probes that I had humans build in the larger setting. But they can have more electronics than I had thought.

    • 2012/12/26 at 10:03 pm

      Well, I can’t think any other ways to get large objects out of Ursa’s gravity well. Indeed, it would be very difficult even with access to metals. The usefulness of small objects depends on how optimistic you are about nanotechnology.
      As for radios, if they have electricity, it would be trivial to build a transmitter with an RLC circuit. The trick is amplifying it, which requires vacuum tubes or transistors.

      • michaelbusch
        2012/12/26 at 10:26 pm

        Amplifying the signal is one thing. Controlling the transmission frequency into a narrow enough channel to be detectable over great distances is another. Low-efficiency high-frequency amplifiers can be constructed without a microwave cavity, but the low efficiency limits the transmit power and a non-resonant system will have a wide inherent bandwidth.

        Result: radios that only work locally, and assorted other electronic/biomolecular devices (the distinction is blurry).

        Ursian electronics _will_ be more expensive/valuable than their equivalents in real-life current human society. They don’t have the same returns to scale.

  1. 2012/12/29 at 4:59 am
  2. 2013/01/02 at 6:27 pm
  3. 2013/02/26 at 7:12 pm
  4. 2013/05/25 at 12:35 am
  5. 2015/11/22 at 8:26 am

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