Cysts of schistosoma in a patient's bladder. Aggressive alien colonizers could conceivably genetically engineer something similar to invade the human gut and extract nutrients - effectively starving the person to death. Sent in hollow robot probes to fresh water lakes and sources on Earth - the human population might be removed in 3-8 months.
On a new Discovery Channel series, the world famous theoretical physicist made bold speculations about the possible encounter of the human species with a colonizing alien intelligence. Hawking didn’t mince words or soothe fears that the encounter likely wouldn’t go our way, and compared it to what transpired to the native populations -mainly the Arawaks- after Columbus alighted on their beaches and islands. Not to put too fine a point on it, but within a few years the native population was decimated.
In a 1962 high school research paper, long before Hawking’s alien warning emerged, I showed how any encounter with a truly advanced alien intelligence would likely be disastrous for the human race. While scifi films (e.g. 'Earth vs. The Flying Saucers') up to then often painted the meetings or altercations in cartoonish terms, I reasoned a hostile future encounter was almost inevitable and that our (putatively) lesser civilization would fall to a much more advanced, alien one. It was impossible to avoid, and this may have been the basis of a 1960 Brookings Institution REPORT to that effect. (Which I cited in my paper)
The paper was innocuously entitled, "Peaceful Uses of Outer Space", but the Brookings Institution recommended prohibition of disclosure, with a warning against the revelation of the existence of extraterrestrial life to the people of America or the world. Brookings Institution thinkers feared social, economic and religious upheaval would result. The Brookings Institution's conclusions were largely based on the public reaction to the infamous Orson Wells' radio broadcast of "The War of the Worlds" and the local panic that ensued in 1938.
Less well known, buried deeper in the 1960 report, the authors cautioned that an actual encounter would likely lead to the end of human civilization since, “whenever a more technically advanced civilization encounters an inferior one, the latter is either absorbed or destroyed.” That, of course, is what Hawking was referencing in his invocation of the Columbus example – which has by now become iconic in a negative sense to all native peoples.
In the wake of the Hawking warning, however, what did we find? Well, mainly jiggles and giggles from the punditocracy with very few serious treatments. Even Keith Olbermann – generally known for presenting sober views on science issues- more or less treated this with a dismissive tone and played it mainly for laughs or levity. (Perhaps as needed balance for his preceding material on the odious Arizona immigration papers law, and the Goldman Sachs pillaging of investors by shorting mortgage securities sold to them). His astronomer guest, if I can call him that, was totally clueless, somehow convinced we humans had nada to worry about because any such encounter was “hundreds of millions of years off” if ever. I had to wonder what planet this character was on, further when he based advanced aliens’ knowledge of us from our diluting radio signals – which would only be “60 light years out by now”.
However, the man is living in a fool’s paradise. The diffusion of an advanced alien civilization based on applying known diffusion wave front equations to their spread (together with very pragmatic assumptions) was the basis for the paper ‘Galactic Civilizations: Population Dynamics and Interstellar Diffusion’ by William Newman and Carl Sagan, in Icarus, Vol. 46, June 1981, page 293.
The authors began with a standard diffusion equation, treating the spread of any colonizing civilization similarly to any medium that diffuses – for example, viruses, or general infections, or even human populations (say in the early colonizations of the New World).. The basic diffusion equation used was (p. 301, eqn.12):
d(rho)/dt = DIV(D(x,t,rho) x grad rho(x,t)
where, rho(x,t) describes the population density at time t, and position x, and D is the diffusion coefficient in terms of x, t and rho. The preceding equation is then tweaked and used as the basis for future refinements.
Rather than weary the reader with the dozens and dozens of equations leading to the Results section (page 314), I will simply commence at that section and then go from there.
The authors' first major computation is of N’, the steady state number of extant advanced civilizations in the Milky Way. This is essential to obtain because it is one of the key variables used to compute the mean distance between advanced civilizations in the Milky Way:
L_m = (2.5 x 10^11/N’)^1/3
Where the numerator refers to the number of stars estimated in the galaxy. The result is in parsecs, assuming the mean separation between stars in the galaxy is 1pc = 3.26 light years. (Bear in mind while our region near the outer rim is sparse with stars, the interior third of the Milky Way is teeming with them, very densely packed)
Based on a star –planet formation factor, f *~ 1, and a mean lifetime for an advanced civilization of 10^6 years, the authors obtain: N’ = f(10^6) = 10^6, or one million advanced civilizations in the Milky Way alone.
(* Note: my inclination is actually to increase f to ~ 2, based on the discovery of more than 370 actual extra-solar planets, which were unknown at the time Newman and Sagan published their paper. This would yield double the number of advanced civilizations, but we will retain the more conservative estimate)
Then, the mean distance between advanced civilizations in the Galaxy is:
L_m ={ (2.5 x 10^11/ (10^6)}^1/3 = [2.5 x 10^5]^1/3 = 63 pc = 205 Light years
Readers may well not appreciate this, but this is literally “next door neighbors” in terms of the galaxy!
The authors’ next task is to obtain the velocity of the colonization wavefront which they give as (Eqn. (79), page 316):
V = (v_k)(D g)^1/2
Here, (v_k) is a dimensionless constant of order unity(1), and g ~ 0.1 (based on the rate of migration of human populations today (Newman and Sagan estimated 0.01 /yr, but that was nearly 30 years ago before the age of globalization). The diffusion coefficient, D, they (very) conservatively estimate at: D ~ (2 x 10^-8 pc^2/yr).
Thus, the colonization wave velocity would be:
V = (1)[ (2 x 10^-8 pc^2/yr)(0.1 /yr)]^1/2 = 4.4 x 10^-5 pc/ yr
Which would imply 1.4 x 10^6 or 1.4 million years before the colonization wave reached Earth, assuming a 63 pc distance to the nearest advanced colonizers.
Now, before anyone gets too ecstatic, bear in mind:
1) Sagan and Newman based their diffusion coefficient on relatively low travel speeds (v much less than c) since anything near v ~ c would be enormously expensive in terms of shielding, propulsion (page 312). They opted then for speeds far below relativistic (e.g. ~ 40,000 km/h).
2)They deliberately assumed a “random walk” diffusion with directional bias “away from population centers".
Now, I personally believe the first is way too conservative and ignores the sort of ingenuity and enterprise that may well apply to a truly advanced civilization which is also space faring. And again, just because we can’t imagine humans attaining relativistic speeds, doesn’t mean advanced aliens couldn’t. So, just a shift (reduction) of the base travel time to about one ten thousandth of what the authors use enhances the diffusion wave speed, V to 0.004 pc/yr.
This reduces the time to encounter to 1.57 x 10^4 yrs. or just over 15,700 years. A blink of an eye.
Thus, if the alien colonizers commenced a journey in our general direction (the next thing I will deal with in (2)) from about 3ky before the Holocene geological era, then they’d be roughly 700 years away from finding us. (Give or take 1000 years in terms of uncertainties). This means the colonizers may well be “right around the corner”.
As for the authors’ assumption (2) that the directional bias is away from population centers, or populated planets, I suspect this is based on their beneficent view of colonizing aliens. Their take is that a true spacefaring civilization would have had to oust its aggressive tendencies for the most part, otherwise they'd not be able to have their civilization reach the stage of interstellar travel. They’d have destroyed themselves long before.
However, many other astronomers and physicists, e.g. Freeman Dyson in a 1972 speech, have expressed a less sanguine view: that some alien civilizations for whatever reason, may have discovered the engineering basis for practical interstellar travel long before their aggressive and conquistadore dispositions were bred out of them. In that case, to use Dyson’s words, they’d be “like a malignant, technological cancer spreading across the galaxy” and we’d do well not to wish for any meeting.
In 1978 the British Astronomer Royal, Sir Martin Ryle, pleaded for extreme caution in restricting any and all types of electro-magnetic signals and noise emanating from our planet. He warned that it would be foolhardy to be anything but passive listeners given we don't know anything about alien intentions - should they receive a calling card from another world. Of course, Sir Martin likely based his cautions on a paper in the journal Science two years earlier, which showed how an advanced alien civilization could put together a detailed picture of life on Earth, especially of our defense capabilities.
And if they came our way would we see an armada of alien ships hovering or landing, ray guns zapping us, and our missile defense reacting? Hardly! More plausibly, if they're intelligent enough to make it this far, and as aggressive as I suspect they'll be, they'd surreptitiously enter from the outer edge of the solar system (cloaking all EM signals). Next, they'd probably set up a standby base on Pluto's Moon, Charon, then dispatch robot probes laden with special, genetically engineered parasites to Earth, to remove us. They could probably send out a thousand or so hollow probes laden with billions of such creatures, and land them in fresh water lakes on Earth wherein they'd have the maximum effect.
The best design would be for the alien-engineered parasite to essentially deprive the human host of all key nutrients, leaving the cells to basically starve. (Or, maybe neutralize all electrolytes in the blood.) Using diffusion equations (or something similar) to what was already shown, they could then estimate or even calculate (given say 10 billion parasites deposited) how long before all humans would starve to death. This may be anywhere from three to eight months, depending on a number of factors -- but the patient aliens would simply bide their time and wait us out. No fuss, no ray guns, no ships in the skies. Just dead humans...eventually. Then the clean up crews enter, and the way is rendered clear for new inhabitants.
Grisly? Perhaps. But much more realistic than the shoot 'em up idiocy that now passes for the first encounters with an aggressive, intelligent alien predator species.
Even more grisly, humans wouldn't have a clue what was happening to them, until possibly too late. All they'd recognize is something that appeared to be a massive new plague. Eventually, some wizard might detect or locate the source - the actual parasite- but by then it might be too late, especially if capable of concealing itself in a secondary host - like the schistosomiasis fluke in the snail.
Laugh at Hawking's alien speculations if you will- giggles and jiggles. I will take them seriously in the spirit he intended.
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