[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
starship-design: WHERE ARE THEY?
WHERE ARE THEY?
What follows is a commentary on several recent theories regarding life in
our galaxy and my ideas on the results of these theories when applied to the
Drake Equation in attempt to answer Fermi's Paradox.
Drake's Equation (see http://www.seti.org/drake-eq.html for an explanation)
has been a past topic of conversation here many times. I first came across
this equation many years ago and have been fascinated with it ever since.
For those of you not familiar with it, it basically an algebraic
representation of the probability for the existence of life, and in
particular, intelligent life in our galaxy. This was in response to Fermi's
famous query, "Where are they?", now called Fermi's Paradox.
I am always interested in any ideas or theories that would help to answer
Fermi's question and recently two new theories have been propounded that
bear directly upon the parameters for Drake's Equation. The equation itself
is fairly straight forward. Choosing values for each of the variables
however, is not so easy. Many of these variables must be no more than
educated guesses. Different values for some of the variables can produce
wildly different results, hence a better understanding of the conditions
which define those variables can make an enormous difference in the accuracy
of the equations solution.
GAMMA RAY BURSTS
The first new theory is from James Annis, an astrophysicist at Fermilab near
Chicago. He thinks cataclysmic gamma-ray bursts often sterilize galaxies,
wiping out life forms before they have evolved sufficiently to leave their
planet (Journal of the British Interplanetary Society, vol 52, p 19). GRBs
are thought to be the most powerful explosions in the Universe, releasing as
much energy as a supernova in seconds. Many scientists think the bursts
occur when the remnants of dead stars such as neutron stars or black holes
Annis points out that each GRB unleashes devastating amounts of radiation.
"If one went off in the Galactic center, we here two-thirds of the way out
on the Galactic disc would be exposed over a few seconds to a wave of
powerful gamma rays." He believes this would be lethal to life on land.
The rate of GRBs is about one burst per galaxy every few hundred million
years. But Annis says theories of GRBs suggest the rate was much higher in
the past, with galaxies suffering one strike every few million years -- far
shorter than any plausible time scale for the emergence of intelligent life
capable of space travel. That, says Annis, may be the answer to Fermi's
question. "They just haven't had enough time to get here yet," he says. "The
GRB model essentially resets the available time for the rise of intelligent
life to zero each time a burst occurs."
Paul Davies, a visiting physicist at Imperial College, London, says the
basic idea for resolving the paradox makes sense. "Any Galaxy-wide
sterilizing event would do," he says. However, he adds that GRBs may be too
brief: "If the drama is all over in seconds, you only zap half a planet. The
planet's mass shields the shadowed side." Annis counters that GRBs are
likely to have many indirect effects, such as wrecking ozone layers that
protect planets from deadly levels of ultraviolet radiation.
Annis also highlights an intriguing implication of the theory: the current
rate of GRBs allows intelligent life to evolve for a few hundred million
years before being zapped, possibly giving it enough time to reach the space
faring stage. "It may be that intelligent life has recently sprouted up at
many places in the Galaxy and that at least a few groups are busily engaged
The second theory deals with a surprising connection between the conditions
required for a total eclipse and for the emergence of intelligent life.
Guillermo Gonzalez of the University of Washington in Seattle points out
that our distance from the Sun is a necessary condition for us to be here.
"If we were a little nearer or farther from the Sun, the Earth would be too
hot or too cold and so uninhabitable," says Gonzalez. At the same time our
existence depends on an unusually large moon since its pull stops the Earth
wobbling around too much on its axis and causing wild and catastrophic
swings in climate like those on Mars. Our Moon, which is unusually large
compared to those in almost all other planet-moon systems, probably formed
from molten material blasted from the Earth during the impact of a giant
body more than 4 billion years ago.
In the current issue of Astronomy & Geophysics (vol 40, p 3.18), Gonzalez
points out that the way the Moon formed means it started off very close to
the Earth and has taken several billion years to move far enough away until
it precisely covers the Sun during an eclipse. "The timescale is very
similar to that of the appearance of intelligent life," he says. "It is
therefore not such a big coincidence that we are around at the time when it
is possible to see total eclipses."
Because tidal effects cause the Moon to slowly recede from the Earth,
perfect eclipses have been visible only for about 150 million years and will
continue for only another 150 million years, about 5 per cent of the current
age of the Earth. Furthermore, Earth is the only planet in our Solar System
where a perfect eclipse is visible, although there are 64 other moons.
If Gonzalez is right, then all extraterrestrials, wherever they are, are
likely to live on planets like ours that experience total eclipses. But
since an unusually large Moon is rare, he says, this suggests that both ETs
and total eclipses are very rare indeed.
WHAT IT MEANS
Taken together, these two factors enormously reduce the values of at least
one and probably as many as three variables to practically zero offering at
least one possible answer to Fermi's query, there aren't any...
If you consider all of the factors you can write a simple relation, using
Drake's Equation, for estimating the probability. I use the word "estimate"
intentionally because our knowledge of most of the factors is so poor that
we are really only guessing.
Drake's Equation, involves 7 factors as follows:
|Number of | |Rate | |Fraction| |Number | |Fraction|
|civilizations| |of | |of stars| |of | |of |
|in our galaxy| |star | |with | |planets | |planets |
|capable of | = |formation| X |planets | X |per star| X |on which|
|communication| |(per | |with | |life |
|now | |year) | |suitable| |appears |
|Fraction| |Fraction | |Longevity |
|of life | |of | |of each |
|bearing | |intelli- | |technology |
x |planets | X |gent | X |in |
|on which| |societies| |communi- |
|intelli-| |which | |cative |
|gence | |develop | |mode (years)|
|emerges | |communi- |
Suppose you guessed that stars in our galaxy form at the rate of one per
year (probably not a bad estimate), that 1/5 of the stars have planets (no
one knows), that there are 0.0005859375 planets with stable environments
(length of time between GRBs divided by the age of our solar system times
the fraction of planets with suitable moons in our solar system ), that life
appears on each (fraction = 1), that intelligence emerges on each of these
(fraction= 1), that 1/10 of these develop communication capability and that
these remain in this state for 1000 years. Then, it works out that the
number is 0.01171875. In other words, in one hundred thousand years, only
one intelligent, communicative civilization would appear, far less than most
Note that other than the value for suitable life bearing planets, all of the
rest of the values were heavily in favor of intelligent life developing.
Reducing either of the next two factors to reasonable levels further reduces
the odds by several orders of magnitude.
If you accept these two theories (and remember, they are just theories),
then in all likelihood Earth is the only planet in the galaxy that currently
harbors intelligent life.