Susser.pasa@Xerox.ARPA (07/27/85)
From: Josh Susser <Susser.pasa@Xerox.ARPA>
All this talk about why aliens would come to Earth raises a pretty big
question: Why haven't any aliens contacted Earth? I know there's a lot
of "evidence" that Earth has had visitors, currently and in the past,
but this is far from conclusive. To all appearances, we are alone in a
galaxy that should be teeming with life and sentience. And so, we are
left with the mystery of what has come to be called The Great Silence.
Those of you who read "Analog" know that David Brin has a semi-regular
column on science-fact and conjecture. Recently, he has done a few
pieces on the question of The Great Silence. I only caught his most
recent one, entitled "Just How Dangerous Is The Galaxy?" In this essay,
Brin reviewed a number of hypotheses that could explain The Great
Silence. In his previous column, he had asked for input from his
readers responding to the above big question. In "Just How Dangerous Is
The Galaxy?" he presented the responses, along with arguments for or
against these hypotheses.
For those of you who are interested in this topic, I reccommend Brin's
column. The essa is to long to summarize here, but I will present (in
no particular order) some of Brin's hypotheses for an explanation of The
Great Silence.
1) We are truly alone.
2) Sentient live is just appearing in the galaxy, and
there are no civilizations significantly more advanced
than we are.
3) There is a galactic interdict prohibiting contact with
immature civilizations.
4) We are fundamentally different from other sentients, and
thus have little chance of contact. For example, we might
not be noticed by beings who lived on gas giants or were
ethereal energy blobs.
5) Interstellar travel and communication are impossible.
6) Berserkers or other hostile galactics are killing off our
frindly neighbors.
7) Civilizations with the agressive tendencies necessary to
drive one to interstellar expansion kill themselves off
before they mature (cf. Nucear Winter), and the surviving
galactics are mellow enough to expand slowly, so they just
haven't found us yet.
The tone of Brin's essay was rather depressing, and had me feeling that
we really are alone in the galaxy. But, realizing this, Brin stuck in
his own pet hypothesis:
8) Most habitable worlds are water worlds, so most other
galactic sentients would be aquatic and incapable of
building spacecraft.
I haven't presented Brin's arguments for the above hypotheses; I would
like to leave that for the net to discuss. I would also be interested
in hearing any other hypotheses that could explain the mystery of The
Great Silence.
-- Josh Susser
<Susser.pasa@Xerox.arpa>
...to boldly split infinitives no man has split before...bnw@crash.UUCP (08/07/85)
From: <crash!bnw@Nosc>
Perhaps the problem is simply that there is no reason why anyone should have
found us. This is an excerpt from _Cosmos_ by Dr. Carl Sagan.
". . .If a great many years ago an advanced interstellar spacefaring
civilization emerged 200 light-years away, it would have no reason to think
there was anything special about the Earth unless it had been here already.
No artifact of human technology, not even our radio transmissions, has had
time, even travelling at the speed of light, to go 200 light-years. From
their point of view, all nearby star systems are more or less equally
attractive for exploration or colonization."
". . .A sphere two hundred light-years in radius contains 200,000 suns
and perhaps a comparable number of worlds suitable for colonization. . ."
Why the silence? We're just one little regarded blue-green world at
the unfashionable end of a spiral arm in the Milky Way galaxy.
/Bruce N. Wheelock/
arpanet: crash!bnw@ucsd
uucp: {ihnp4, cbosgd, sdcsvax, noscvax}!crash!bnw
franka@mmintl.UUCP (Frank Adams) (08/09/85)
In article <3157@topaz.RUTGERS.EDU> bnw@crash.UUCP writes: >From: <crash!bnw@Nosc> > >Perhaps the problem is simply that there is no reason why anyone should have >found us. This is an excerpt from _Cosmos_ by Dr. Carl Sagan. > ". . .If a great many years ago an advanced interstellar spacefaring >civilization emerged 200 light-years away, it would have no reason to think >there was anything special about the Earth unless it had been here already. >No artifact of human technology, not even our radio transmissions, has had >time, even travelling at the speed of light, to go 200 light-years. From >their point of view, all nearby star systems are more or less equally >attractive for exploration or colonization." > ". . .A sphere two hundred light-years in radius contains 200,000 suns >and perhaps a comparable number of worlds suitable for colonization. . ." > > Why the silence? We're just one little regarded blue-green world at >the unfashionable end of a spiral arm in the Milky Way galaxy. One more time ... an intelligent race with interstellar flight doesn't go to just a few places, it goes everywhere. All nearby star systems are more or less equally attractive, so you colonize all the nearby star systems. A few dozen generations later, you have filled up all the nearby star systems, so you colonize the next layer out. In a few hundred thousand years, you have filled the galaxy. All of it. A race from a planet where life started at the same time as on Earth, which evolved to a technological civilization one percent faster, has had thirty million years or so to spread out since. That is time enough to fill the galaxy about a hundred times over. "They just haven't found us [yet]" is just not an adequate explanation. Most plausible explanations for why they aren't here are variations on three themes: (1) they aren't there, (2) they are deliberately leaving us alone, and (3) war prevents permanent settlement of planets. And remember the time scales involved for (2) -- they have to have decided to leave "us" alone while "we" were dinosaurs (or perhaps earlier; I'm not quite sure of the evolutionary time scale (if you don't believe in the evolution of species, send comments to net.origins where I won't have to read them)). Now this does not mean that SETI is hopeless. They may be just waiting for us to contact them to welcome us into the Galactic Federation. But it seems about equally likely to me that as soon as they detect us, they will come by to sterilize our planet, before we can do the same to them. But we should not *expect* SETI to succeed.
msb@lsuc.UUCP (Mark Brader) (08/11/85)
I've cut down the included part as much as possible, but I thought the complete list was worth presenting again. Josh Susser writes: > ... Why haven't any aliens contacted Earth? > ... some of [David] Brin's hypotheses ... > > 1) We are truly alone. > 2) Sentient live is just appearing in the galaxy ... > 3) There is a galactic interdict ... > 4) We are fundamentally different from other sentients ... > 5) Interstellar travel and communication are impossible. > 6) Berserkers or other hostile galactics are killing off our > frindly neighbors. > 7) Civilizations with the agressive tendencies necessary to > drive one to interstellar expansion kill themselves off ... > and the surviving galactics are mellow enough to expand > slowly, so they just haven't found us yet. > 8) Most habitable worlds are water worlds, so most other > galactic sentients would be aquatic and incapable of > building spacecraft. I'm surprised not to have seen any followups to this item. Here's an obvious next one: 9) Interstellar travel is impractical, and we don't know how to listen to the communications method used by anyone else. For instance, maybe they modulate their star's neutrino flow. Mark Brader
mangoe@umcp-cs.UUCP (Charley Wingate) (08/11/85)
In article <579@mmintl.UUCP> franka@mmintl.UUCP (Frank Adams) writes: >One more time ... an intelligent race with interstellar flight doesn't go >to just a few places, it goes everywhere. All nearby star systems are >more or less equally attractive, so you colonize all the nearby star systems. >A few dozen generations later, you have filled up all the nearby star >systems, so you colonize the next layer out. In a few hundred thousand >years, you have filled the galaxy. All of it. Really? Isn't this article just a little anthropocentric? To do this, a race needs a whole list of things: lots of energy, lots of technology, and (most importantly) motivation. A race which doesn't have a severe population growth problem doesn't need to colonize. Resources for thousands of years are to be had in one's own solar system. A race which is just starting interstellar travel has enourmous constraints. Generation ships are SLOW. Assuming that FTL travel isn't possible, the energy needed to travel at reasonable speeds is tremendous. Since probes in the EM spectrum are relatively cheap, it makes some sense to pick and choose. If all Bernard's Star has is big gas planets, then it's going to take a lot of energy to make something livable there for a race like us. Detecting terrestrial planets over interstellar distances is enourmously difficult. They emit essentially no thermal radiation, they are too small to occult anything or influence the obvious body's orbits. The only way one could detect Earth from interstellar distances is to detect man-made radio emissions; we've already discussed reasons why these are difficult to detect and have reached only a few stars anyway. This brings us to the question of why one would want to visit a planet which is apparently inhabited. >A race from a planet where life started at the same time as on Earth, which >evolved to a technological civilization one percent faster, has had thirty >million years or so to spread out since. That is time enough to fill the >galaxy about a hundred times over. "They just haven't found us [yet]" is >just not an adequate explanation. This baldly assumes that they have a pressing desire to do so, and the technology to accomplish it. >Most plausible explanations for why they aren't here are variations on >three themes: (1) they aren't there, (2) they are deliberately leaving us >alone, and (3) war prevents permanent settlement of planets. And remember >the time scales involved for (2) -- they have to have decided to leave "us" >alone while "we" were dinosaurs (or perhaps earlier; I'm not quite sure of >the evolutionary time scale (if you don't believe in the evolution of >species, send comments to net.origins where I won't have to read them)). And 4) they haven't looked hard enough or had the time to. Assuming you had the power to search the entire galaxy for life, it's still going to take a long time to find any. Also 5) we aren't very interesting to them. >Now this does not mean that SETI is hopeless. They may be just waiting >for us to contact them to welcome us into the Galactic Federation. But >it seems about equally likely to me that as soon as they detect us, they >will come by to sterilize our planet, before we can do the same to them. >But we should not *expect* SETI to succeed. Likelyhood here is essentially meaningless. We are on an obscure planet whose civilization could only be apparent to those within 20 light years of us. Therefore the only real explanation must be that whatever civilization is on the planets within that range either a) hasn't noticed, b) doesn't care, or c) hasn't had enough time to act. (Remember, only those within 10 lightyears have had time to get back to us.) Or d) has acted, but we didn't notice. C Wingate
peter@baylor.UUCP (Peter da Silva) (08/13/85)
You're assuming there's only a few intelligent races in the galaxy. Besides:
why would a reace with a low exploratory/reproductive/etc. drive acquire
interstellar travel? And so what if they can survive for 1000 years on a
single solar system: that still leaves time for 3,000 iterations of the
explore/colonise/fill a solar system/explore cycle if they're only 1%
ahead of us. And do they have to fill the solar system before they want to
go for the next one?
--
Peter da Silva (the mad Australian)
UUCP: ...!shell!neuro1!{hyd-ptd,baylor,datafac}!peter
MCI: PDASILVA; CIS: 70216,1076herbie@watdcsu.UUCP (Herb Chong - DCS) (08/22/85)
>You're assuming there's only a few intelligent races in the galaxy. Besides: >why would a reace with a low exploratory/reproductive/etc. drive acquire >interstellar travel? And so what if they can survive for 1000 years on a >single solar system: that still leaves time for 3,000 iterations of the >explore/colonise/fill a solar system/explore cycle if they're only 1% >ahead of us. And do they have to fill the solar system before they want to >go for the next one? you're assuming there are many. maybe this will help somewhat. this is excerpted from a physics project of mine from many years back. this is not to say that one can't assume that there are many and go on from there to write a damned good SF novel, but one must account for the large number somehow if the story is to have a reasonable scientific basis. Herb Chong... I'm user-friendly -- I don't byte, I nybble.... UUCP: {decvax|utzoo|ihnp4|allegra|clyde}!watmath!water!watdcsu!herbie CSNET: herbie%watdcsu@waterloo.csnet ARPA: herbie%watdcsu%waterloo.csnet@csnet-relay.arpa NETNORTH, BITNET, EARN: herbie@watdcs, herbie@watdcsu --------------------------------------- extraterrestial communication "empty space is like a kingdom, and earth and sky are no more than a single individual person in that kingdom. upon one tree are many fruits, and in that one kingdom, many people. how unreasonable it would be to suppose that, besides the the earth an sky that which we can see, there are no other skies and no other earths." (Teng Mu, scholar of the Sung dynasty, c. 1000 AD) "[i have] a deep conviction and foreknowledge, that ere long all human beings of this globe, as one, will turn their eyes to the firmament above, with feelings of love and reverence, thrilled by the glad news: 'brethen! we have discovered a message from another world, unknown and remote. it reads: one...two...three...'." (Nikola Tesla, Jan. 7, 1900) "those who feel that the goal justifies the great amount of effort required will continue to carry on this research, sustained by the possibility that, sometime in the future, perhaps a hundred years from now, or perhaps next week, the search will be successful." (Frank Drake, 1960, radio astronomer who was first to seriously try -- unsuccessfully -- to detect signals from intelligent extraterrestials) 1) how hard is it to find another intelligent life form that we can talk to? no successful systematic search for other intelligent life can be made if we don't look far enough out into the galaxy away from us. how far we have to look depends upon how many there are out there listening for other intelligent life like us. for the moment, let us assume that there are N stars with planets in the galaxy which currently have intelligent life capable of interstellar communication. let N_star be the total number of stars in the galaxy. the ratio N/N_star, which we will define to be p, gives the probability that any single star is currently supporting intelligent life capable of communicating with us. the probability of examining exactly k stars before finding one such planet is given by p(1-p)^(k-1), the familiar geometric distribution. the average number of stars that must be examined to find one can be easily shown (by using the definition of the mean of a discrete random variable) to be 1/p. to be 50 per cent sure of finding a planet for a given value of p, the smallest value of k where the cumulative distribution sums to greater than or equal to 0.50 represents the average number of stars that have to be examined. a little manipulation of the infinite sum of the pdf gives a value for k at this critical point, k_50 = ln(2)/p. following are values of k_50 for various values of p. p k_50 10^-2 70 10^-4 7000 10^-5 70,000 (Sagan) 10^-6 700,000 1/3*10^-6 2,000,000 (von Hoener) 10^-8 70,000,000 if we assume the local density of stars, rho_star = 10^-3/ly^3, is constant for an appreciable distance away from us, the volume of space that must be examined is given by V = n/rho_star. for Sagan's estimate, V = 7*10^7 ly^3 while for von Hoerner's estimate, V = 2*10^9 ly^3. assuming that a sphere of radius r enclosing this volume is an appropriate shape, r is approximately equal to (V/4)^(1/3), or 260 and 790 light years respectively for Sagan's and von Hoerner's estimates. to be 90 per cent sure of finding a star system containing a planet with intelligent life that can communicate with us, the radii are 390 ly and 1200 ly respectively. these distances from Earth are still within the spiral arm that the sun is a part of so the stellar density is approximately constant. the stars that look the most promising for finding life similar to our own are the familiar G3 stars very much like the Sun and other types that are spectrally similar. 2) how many extraterrestial intelligences are there out there? the following equation has been proposed by many astronomers to estimate the number of intelligent civilizations in the galaxy that are capable of communicating with us. N = R_star f_p n_e f_e f_i f_c L the different terms in this product vary in reliability from relatively precisely known to wild speculation. the terms are defined as follows: R_star: rate of star formation in the galaxy - about 10/yr assuming it has been constant since the beginning of the universe f_p: fraction of stars with planets - still debated, but may be as high as 0.40 n_e: fraction of planetary systems capable of supporting life - no consensus, why not use our's as an example, 0.2 f_e: fraction of planets capable of and developing life - speculation, be optimistic, say 0.3 f_i: fraction of planets with life that develops into intelligent life - more speculation, 0.3 f_c: fraction of intelligent life capable of communications - your guess is as good as mine, 0.1 L: average lifetime of such a civilization - be optimistic, 10,000 yrs using these numbers, we get N = 72 in our galaxy at this time, so that p is about 10^-8 and V and r are about 10^11 ly^3 and 3000 ly respectively. a total of 10^8 stars need to be examined to be 50% sure of finding one that harbors an intelligent civilization capable of communications with us. at one star per hour, it would take a single telescope more than 10,000 years to examine that many stars. note that none of this takes into account spread of a civilization capable of interstellar travel. many may not. most that will probably will colonize only a few stellar systems in a 10,000 year civilization lifetime unless faster than light travel can be achieved. making another conjecture, double the number of planets with intelligent life capable of communicating with us. this changes p, V and r by neglible amounts. 3) radio communications consider a radio telescope of area A that can emit a radio beam in a narrow cone of angle alpha radians. if it emits power at the rate of P_e, the power received by a similar telescope, P_r at a distance R is given by P_r = P_e * (A/a), a is area covered by cone at distance R = (P_e * A)/(R^2 * d_sigma), d_sigma is solid angle of cone if we assume that the weakest signal that can be detected is comparable to that of the remnants of the Big Bang, whose spectrum is that of a black body at a temperature of 3 K, an radio telescope of area A will receive a power of E(nu,T) * d_nu * A from space where E is the power per unit frequency per unit area. d_nu is the bandwidth of the telescope, nu is the frequency, and T the absolute temperature. the amount that is actually reflected into the receiver is the part that comes from within the solid angle d_sigma. since only the front half of the telescope is capable of receiving a signal, the fraction of power actually recieved is (d_sigma/2 * pi) for a total power of P_r = E * d_nu * A * d_sigma / (2 * pi) equating the two and solving for R, we get R = 1/d_sigma * ((2 * pi * P_e)/(E * d_nu))^(1/2) the Aricebo radio telescope has a diameter of 300m and a radiating power of about 500,000 W. it's angular resolution is about 10 seconds of arc. if we assume the transmission bandwidth is 1 Hz, a signal transmitted on the 21 cm hydrogen band (10^9 Hz) can be detected to a distance of 3*10^21 m, or about 10^6 ly. for comparison, our galaxy is only about 10^5 ly across. no account is made here of attenuation of the signal by interstellar dust. 4) a communications attempt on saturday, november 6, 1974, a signal was sent from Aricebo toward the globular cluster M-13 in the constellation Hercules, 20,000 ly away. the signal is such that the entire cluster would just be covered by the angle of transmission. M-13 contains about 5*10^5 stars. based upon Sagan's and von Hoerner's estimates of p, there should be between 5 and 0.17 stars with intelligent civilizations in the cluster capable of receiving the signal and perhaps communicating with us. using my more pessimistic estimate, there is less than 1 per cent chance of there being one. the power received, P_r, in M-13 by a telescope comparable to Aricebo would be about 10^-22 W. the power from the background radiation of the Big Bang, P_b, would be about 10^-24 W. the signal to noise ratio is about 20 dB. 5) sending information we can rearrange the expression for R to solve for the bandwidth of a channel required to successfully transmit information to M-13 using the Aricebo radio telescope. d_nu = (2 * pi * P_e) / (R^2 * d_sigma * E) substituting appropriate values, we find that the required signal bandwidth is about 2*10^3 Hz. from information theory, the maximum information transfer rate on a noisy channel is given by R_d = d_nu * log_2(1 + P_r/P_b) = 10^4 bits/s assuming an error correction code of some kind for more reliable communications, this cut to about 3*10^3 bits/s. a typical book contains about 10^6 characters. if we assume that the book is ASCII encoded, this represents about 10^7 bits. a typical book then is transmitted in about 3*10^3 seconds. assuming a billion (10^9) books can contain all the current knowledge of mankind, 3*10^12 seconds are needed to transmit all this information. this is equivalent to about 2.5 million years.
peter@baylor.UUCP (Peter da Silva) (08/29/85)
> >You're assuming there's only a few intelligent races in the galaxy. Besides: > >why would a reace with a low exploratory/reproductive/etc. drive acquire > >interstellar travel? And so what if they can survive for 1000 years on a > >single solar system: that still leaves time for 3,000 iterations of the > >explore/colonise/fill a solar system/explore cycle if they're only 1% > >ahead of us. And do they have to fill the solar system before they want to > >go for the next one? > > you're assuming there are many. No I'm not. I'm just assuming that colinisation is possible. It certainly looks like it is. There are no physical laws that prevent it... they may make it difficult (it may take 1000 years to travel to the next star in a converted asteroid), but it's pretty unlikely to be possible. Let's say that we have a race with the capability to keep a small ecosystem going for 1000 years. I don't see that we won't be able to by the time we get around to chucking asteroids around. Let's say that they're inquisitive and dedicated... like us. They send out half a dozen of these ships. 1000 years later they arrive at the next system and start building a civilisation. Being acclimitised to life in space they're not likely to need planets, just energy and matter: a common resource in the vicinity of stars. I'm sure that within 500 years they're ready to send out another one. Likely the crew will just want to plant a colony and keep on going (who wants to live near a star? They're dangerous! Useful for fuel but dangerous!), but maybe thhey'll want to hang around & have a bunch of kids. Let's assume an average distance between usable stars of 7.5ly. This may be high or low, but it will do for a first approximation and is certainly about the right order of magnitude. OK. Pretty soon you'll have a sphere of these ships expanding at .5% of the speed of light. In a million years that sphere will have filled a pretty nice chunk of the galaxy. In 12 million years it will have filled the whole thing. Probably less, as advances in science speed things up and heavy duty radicals and dissidents head for the REALLY distant parts of the galaxy. -- Peter (Made in Australia) da Silva UUCP: ...!shell!neuro1!{hyd-ptd,baylor,datafac}!peter MCI: PDASILVA; CIS: 70216,1076