Sections
Dante
was being allegorical: I don't think he really believed the
lowest class of the saved were on the Moon.
Perhaps, the earliest science fiction story depicting
extraterrestrial intelligence (ETs)
was Johannes Kepler's (1571--1630)
Somnium in which the hero visits
the Moon and among other things
describes astronomy from the point of view of the
lunar inhabitants, the Subvolvans (near-side beings) and
Prevolvans (far-side beings)
(Ca-351--353;
Ko-420--425).
In the late 19th century, the falsely-detected Martian canals convinced some people, notably Percival Lowell (1855-1916), the founder of the Lowell Observatory in Flagstaff, Arizona that there was an intelligent civilization on Mars (e.g., IAL 14: Mars: the Red Planet). See the two figures below (local link / general link: percival_lowell.html; local link / general link: percival_lowell_mars_map.html).
Actually, in calling them canali in Italian, Schiaparelli was NOT implying that they were artificial: his meaning was probably closer to the English channels (Se-477--478).
But Martian canals they became in popular conception and some became convinced they were constructions of intelligent beings: the Martians.
A prominent believer in the Martian canals and the Martians??? was Percival Lowell (1855--1916). See the two figures below (local link / general link: percival_lowell.html; local link / general link: percival_lowell_mars_map.html).
They attacked with robotic tripods. See the figure below (local link / general link: h_g_wells_war_of_the_worlds.html)
When you did meet up with
Wells's
Martians,
they turned out
to be giant octopuses: an idea that seemed more novel in the
19th century
than after
Amazing Stories, etc.
See
John Walker's Fourmilab web The War of the Worlds (1898).
In fact, many/most astronomers never believed in the Martian canals: they were quite aware that the eye at the limit of visibility could play tricks.
The Mariner probes of the 1960s gave the first close up views of Mars and showed a barren, low-pressure-atmosphere, cratered world (No-578). At first sight sterile---but there might have been LIFE there once and even possibly subsurface today (e.g., IAL 14: Mars: the Red Planet)---but I've lost the FAITH.
I remember the disappointment of those first barren, cratered images---where were the Martian canals---where was Thuvia?
But let us concentrate on
extraterrestrial intelligent life
which we are
NOT likely to find in our
solar system.
Fermi
was one of the
20th century's great physicists.
Among many other things, he was team leader for development of the first nuclear reactor Chicago-Pile 1. See the figure below.
Fermi
was probably one of many who wondered how in the enormous
observable universe
we seem to be alone and have never been contacted:
there are NO credible
UFO stories where the
UFOs were beings NOT of
this world.
The instructor has spoken ex cathedra on that.
For example, Venus.
Unresolved flocks of birds---me in Oklahoma sometime in 1988--1991.
Breaking up Russian satellites---me in Oklahoma sometime in 1988--1991.
Ball lightning---if it really exists.
Earlier ages tended to attribute such things to Zeus or Thor.
The Drake equation, formulated by Frank Drake (1930--) in 1960, is a formula for estimating the number of technologically advanced civilizations (TACS) in the Milky Way or, if one wishes, in broader domains.
It can be described as a way of analyzing our ignorance.
Various versions can be given, but a typical version for the Milky Way alone is
N=RL , where N is the number of TACS,
R is their mean birth rate,
and L is their mean lifetime.
This equation can be understood simply.
Consider the TACS of lifetime L_i that are born at
rate R_i.
If one just turns on the
Milky Way,
in the first period L_i
R_i*L_i TACS are born, but none die.
After the first period L_i, the rate of births
and deaths are equal and cancel exactly.
Thus, the number of TACS N_i of lifetime L_i stays constant
at R_i*L_i.
Now just sum on i to get:
sum_i N_i = ( sum_i R_i ) * L
where L = ( sum_i R_i*L_i ) / (sum_i R_i) is the mean lifetime.
Thus
N=RL
where N = sum_i N_i and R = sum_i R_i .
R is can be parameterized by
R = R_star * f_planet * n_habitable * f_life * f_intelligent * f_TAC .
The factors can be explained and estimated.
If stars are too massive then their lives may be too short for life or intelligent life to evolve or their UV flux may sterilize any planets.
If the stars are too small, then the orbital region about them where liquid water can exist (if starlight is the only relevant temperature factor) may be too small for it to be likely that planets exist there. Life as we know it requires liquid water.
This region is called the habitable zone (CM-479). Smaller stars have smaller habitable zones.
The definition is too restrictive really since the conditions for liquid water can exist outside of the formal habitable zone.
For example, on Jupiter's moon Europa liquid water probably exists below a frozen surface. The interior of Europa is heated by Jupiter's tidal force flexing Europa.
Nevertheless, we'll assume for the sake of argument that only stars with masses NOT too different from the Sun are likely to have habitable planets.
CM-480 suggest only FGK type stars, which have masses of order 1.6 to 0.5 solar masses (Cox-480), are suitable.
The estimated birth rate of such stars is about 1 per year in the Milky Way (CK-454).
So we take R_star=1. This number is probably order of magnitude accurate.
Observations suggest planets and planet formation are common. Perhaps nearly all single stars have planets (CK-454). Double star can have planets too, but their orbits can sometimes be complex.
We estimate f_planet=1. This number is probably order of magnitude accurate.
They have to be habitable enough for TACS to evolve.
There could be more than 1 habitable planet per solar system, but there are so many ways a planet could be unhabitable that many people think n_habitable could be orders of magnitude smaller than 1.
We are clueless---in more ways than one actually.
Let us set n_habitable=1. This number could an overestimate by orders of magnitude: I don't think anyone would say it was a gross underestimate.
Well Earth is such a planet and it may have evolved life very early.
The first fossil evidence for life dates back 3.5 Gyr (CM-475; HI-500).
Recall that the Earth formed 4.6 Gyr ago and the heavy bombardment by early-solar system bodies only tailed off after 3.8 Gyr ago (Se-422, 446, 447).
Impactors probably disrupted life initiation and the largest ones probably caused shocks that ejected entire proto-atmospheres???.
Thus life on Earth may have started as early as it possibly could.
On the other hand, some believe the evidence for life is NOT strong before 2.7 Gyr ago (e.g., Wikipedia: Origin of Life).
We estimate f_life=1. This may be a great overestimate, but it is plausible.
Multicellular life developed only about 1 Gyr ago (CM-481): maybe 2.5 Gyr after life itself developed.
Intelligence---of the human level---is almost a novelty.
Anatomically modern humans have only been around for a relatively brief phase (i.e., since about 160,000 years for sure [Gibbons, A. 2003jun13, Science, 300, 1641]; since about 195,000 years with some probability [Fleagle, J., et al. 2005, Nature]).
The emergence of multicellular life and intelligent life may have been unlikely flukes that just turned up after long waits.
Both multicellarity and intelligence have survival advantages, but there are many survival advantages and their interplay is immensely complex and perhaps chaotic.
NATURAL SELECTION in evolution only recognizes advantages that are advantages in the local environment. An advantage that doesn't help locally is NOT selected for.
And some advantages may be rather marginal.
The great apes are rather intelligent animals, but they are NOT particularly numerous or successful: this was true even before human activities started endangering them.
Human intelligence together with human hands---those
poor dolphins---and certain key
environmental factors (see below) have enabled
humans to
become the dominant large animal on
Earth in numbers and in
the use of biosphere resources.
But quite obviously this advantageous nature of intelligence
is unstable.
If the local environment contains abundant
nuclear weapons
and the capability of irreparably damaging the
biosphere,
intelligence may slip over to being a disadvantage.
To pick up the thread of our argument,
intelligence may be a fluke development of
evolution.
There is no evidence that it strongly selected for.
But on the other hand maybe intelligence is common.
Maybe it was a fluke that intelligence developed only lately and once
on Earth.
So giving intelligence the benefit of the doubt,
we estimate f_intelligent=1. This may be a wild overestimate.
Here we define a TAC as one with
radio communication
since this is the most plausible way for interstellar signaling
Humans have only had a TAC since
Heinrich Hertz's (1857--1894)
discovery of radio
in 1887: so NOT much over a hundred years.
No.
Recall anatomically modern
humans have been around for
about 160,000 years for sure
(Gibbons, A. 2003jun13, Science, 300, 1641) and
about 195,000 years with some probability
(Fleagle, J., et al. 2005, Nature).
Almost all of that time, we were just hunter-gatherers with
stone-tools and fire.
But another factor was climatic: the ICE AGE of last
2.5 Myr including
the Pleistocene (0.01--1.64 Myr before present)
and Holocene (∼ 11,700 BP--present) has been
ongoing---and its NOT over yet probably
(WB-78;
Cox-248,251).
During the glaciations much of Eurasian and North
America was under glaciers and much of the rest cold and
barren.
Moreover, most of the modern human period has been
subject to rapid climate variations.
The last 10,000 has been remarkably stable
(WB-79).
Before 12,000 or so years ago, the conditions were NOT
suitable for agricultural societies to develop or
to become permanent.
Without the food surpluses of agriculture, it is probably
impossible to develop the specializations needed for
advance beyond typical hunter-gatherer technology.
Even in the stable period when agricultural societies
could and did develop, prolonged
cold spells and droughts
have brought about collapses of civilizations and retreats
to simpler economies
(Fa-xiiiff).
The upshot is that
humans
probably only did have a reasonable
chance of going beyond the hunter-gatherer stage in
the last 12,000 years when, in fact, they did
(Fa-xiii).
But even having a warmish steady climate is NOT
sufficient for technological progress.
One also needs domesticatable plants and animals.
The Australian Aborigines have been in Australia since about
40,000 years ago at least
(Daimond,
1997, p. 300),
but they never got beyond being hunter-gatherers until the
Europeans arrived---well invaded.
But the Aborigines really couldn't
have: there are almost no native flora or fauna of Australia
that are domesticatable
(Daimond,
1997, p. 308--309) and the Aborigines only brought
our oldest friend, the dog, with them.
The large and connected continental areas had the advantage
of more domesticatable species and an easier time spreading
them.
Some intelligent species may never get beyond the hunter-gatherer
stage before they become extinct.
But given massive food production available from agriculture
and the absence of any worldwide catastrophes (e.g.,
a large, planet-devastating impactor) perhaps
most worlds with intelligent life (of our sort) would
go on to develop into a TAC.
Physics and chemistry are the same all across our
observable universe,
and so technology might follow a
common course.
So although it is certainly an overestimate let us
set f_TAC=1. Maybe it is NOT a bad overestimate; on the
other hand, it could too large by orders of magnitude.
We've been technologically advanced only since 1887
by radio discovery definition.
Since circa 1950,
we have the capability of extincting ourselves
with nuclear weapons.
But it is certainly possible for TACS to destroy themselves
on the time scale of 100 years after becoming a TAC.
But on the other hand, it may be possible for a TAC
to become quasi-immortal.
Of course, the quasi-immortal TAC BEINGS
might transform themselves into a
hyper-intelligent pan-dimensional beings along the way---or simply reach
a quasi-perpetual steady-state with no further progress.
So L could be anything from of order 100 years to of order
10 Gyrs: i.e., the order of the age of
the
observable universe.
Of course, there does seem to be a take-off point with intelligence.
Hertz also
invented
Country and Western stations and
talk radio---fact.
But did we inevitably have to discover radio?
Even with hands, aquatic intelligent life
would probably have a hard time getting going
technologically. Another bad break for the
dolphins.
Perhaps, we weren't psychologically modern for a long time.
We don't know when language was invented for example.
Our current ICE AGE is
punctuated by interglacials about every about 100,000 years
(WB-76).
The current interglacial stabilized about 10,000 years
ago
(Fa-24) and
the one before that ended about
120,000 years ago
(WB-78).
Many of the circumstances that allowed technology to develop
are NOT universal givens.
Our exploitation of the biosphere
continues to do quasi-irrevocable
damage. We may be able to destroy it's capacity to support us,
although I don't think we will.
N = 1*1*1*1*1*1*L = L , where L is the mean lifetime of a TAC.
Remember most of our estimate factors could be off by
orders of magnitude.
In fact, many factors may be too high by orders of magnitude.
But if we take our estimates at face value, then the number of TACS in the Milky Way is of order L.
So if L=100 years, there are 100 TACS.
So if L=10**6 years, there are 10**6 TACS.
How easily could we receive signals from TACS?
That depends on distance among other things.
Remember radio communication can only be at the speed of light: so the greater the distance, the longer for a signal to reach us.
Assume the
Milky Way
is just the disk which is a quasi-two-dimensional
space.
The planar density of TACS is then
n = N/(pi * r**2) , where r is the disk radius.
We can also set
n = 1/d**2 ,
where d the length of the side of
a typical square in which one finds one TAC.
d is also the typical NEAREST NEIGHBOR DISTANCE among
TACS.
Then
N/(pi * r**2) = 1/d**2
and one can solve for
d=sqrt(pi/L)*r, where N=L recall.
The radius of the disk of the
Milky Way
is of order 50 klyr.
So
d = about 100 klyr / sqrt(L), where we estimate sqrt(pi) by 2.
We can present a small table.
L (yr) d (lyr)
______________________________
100 10,000
10**4 1,000
10**6 100 but if we get this small we again
have to worry about the finite thickness of
the disk.
If L is low (and thus d is high), we might receive signals from a TAC, but communication on our ordinary societal time scale is impossible.
Communication is imaginable if L is high.
But maybe TACS of order 10**6 years old have no interest in us or may NOT want to communicate with us: they may only observe us---there is the Prime Directive after all.
They may hide themselves from us, protect us, experiment on us.
All of science fiction is possible.
---Scotty in The Thing from Another World (1951)
He looked for radio signals from Tau Ceti (a G8V star 11.90 lyr away) and Epsilon Eridani (a K2V star 10.49 lyr away) (CK-452). These are nearby and NOT so unlike the Sun.
Since then there have been many searches as well as people willing to jump on any likely signal in the course of non-SETI radio astronomy.
The most famous SETI
organization is the
SETI Institute,
a private, non-profit organization founded in
1984.
NASA got out of SETI rather definitively in 1992 (FK-689). (Or so it used to seem: maybe NASA will jump back in.)
Given that SETI is rather controversial---many think it is just nonsense---this is probably a good thing. SETI probably should be carried out and supported by enthusiasts.
The SETI Institute also sponsors astrobiology in general: unkindly called a field without a subject (HI-496): but there is a subject: we just havn't found any samples yet.
Currently/Pastly, the SETI Institute has been carrying out Project Phoenix: this project is surveying about 1000 Sun-like stars within 200 lyr of Earth in millions of frequency bands.
Other SETI projects by other organizations are also underway.
One other interesting search is SERENDIP IV (Search for Extraterrestrial Emissions from Nearby Developed, Intelligent Populations) run by scientists at University of California, Berkeley (FK-689).
This project has a subsidiary project SETI@home in which millions of personal computer users participate.
They load a data analysis program on their computers which analyzes SERENDIP IV data and reports back to Berkeley.
Aside from its use for SETI, SETI@home, has been an example of massively distributed computing for many other fields. Some people may think SETI@home is the most valuable thing ever done by SETI.
Often anomalous signals are discovered and a list of best candidates is kept. Sometimes a signal catches media attention: e.g., one from source SHGb02+14a.
But no signal has really caused terrific excitement yet---we'd know it if it happened---unless there was a government cover-up---hey, maybe it has happened---there was Roswell---don't get me started.
I'm betting on answer 3.
I'm betting on answer 3.
Question: What would the discovery of
an artificial extraterrestrial signal mean?
Question: When are we likely to get a signal?
But maybe ... see the figure below
(local link /
general link: guinea_pig.html).