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by Ray P. Norris
CSIRO Australia Telescope National Facility
Epping, Australia
from
AustraliaTelescopeNationalFacility
Website
ABSTRACT
This paper considers the factors that determine the probable age of
a civilization that might be detected in a SETI search. Simple
stellar evolution considerations suggest an age of a few Gyr.
Supernovae and Gamma-ray-bursters could in principle shorten the
lifetime of a civilization, but the fact that life on Earth has
survived for at least 4 Gyr places a severe constraint on such
factors. If a civilization is detected as a result of a SETI search,
it is likely to be of order 1 Gyr (Gigayears, or
billion years) more advanced than us.
1. INTRODUCTION
When we conduct searches for extra-terrestrial intelligence, we
often make implicit assumptions about the age of the civilization
that we are trying to find. For example, our strategy for searching
for a life-form of a similar age to us is likely to be different
from that for a civilization billions of years more advanced than
us. Similarly, in the event of a confirmed detection, the way in
which we plan our response will also depend on how advanced that
civilization may be. In this paper, I estimate the likely age of the
civilization that we are most likely to detect, should we be
successful in our searches.
The two key factors that determine how old a detected civilization
is likely to be are
(a) the length of time since intelligent life
first appeared in our Galaxy
(b) the median lifetime of a
civilization
The second of these is more problematic, since the
development of a civilization can be cut short by a wide range of
events, including disease, war, global mismanagement, asteroids,
supernovae, and gamma-ray bursters. We should also acknowledge the
possible existence of other hazards, of which we are not yet aware.
For example, the devastating effect of gamma-ray busters has only
been appreciated in the last 2-3 years, and there are probably other
phenomena yet to be discovered.
Events such as disease, war, and
global mismanagement are almost impossible to quantify, and so in
this paper I concentrate on those events that we can quantify:
-
asteroids
-
supernovae
-
gamma-ray bursters
But in the first
section of this paper, I consider what the maximum lifetime of a
planetary-bound civilization might be.
Throughout this paper, I make a very conservative assumption that an
extraterrestrial civilization (ET) resembles us in most significant
respects (other than age and evolution). In other words, ET lives on
a planet orbiting a solar-type star, and has taken as long after the
formation of their star to evolve to "civilization" as we have,
which is ~5 Gyr (Gigayears, or billion years). I therefore estimate
the longevity of ET by looking at the hazards that confront the
Earth.
2. THE NATURAL LIFETIME OF A
CIVILIZATION
I assume that stars like our Sun have been forming since the
formation of the Galaxy some 10 Gyr ago. Observed changes in
metallicity since then are not sufficient to alter this simple
assumption significantly. Our Sun is now about 5 Gyr old, and has an
expected total lifetime of 10 Gyr.
For the first 5 Gyr of the life of the Galaxy, there would not have
been enough time for a civilization to develop, and so ET did not
exist. Between 5 and 10 Gyr, assuming a constant rate of star
formation, the number of civilizations would increase linearly until
the present day. At around the present time, some of those first
solar-type stars will be dying at the same rate as others are
forming, and so, assuming their civilizations die at the same rate
as they do, the number of civilizations is then level from now on.
The median age of a civilization is therefore the median age of
those civilizations that started between 5 and 0 Gyr ago, which is
1.7 Gyr. Therefore, in the absence of other factors, any
civilization that we detect via SETI is likely to be 1.7 Gyr more
advanced than we are.
3. THE EFFECT OF SUPERNOVAE
A supernova results from the explosion of a high-mass star after its
hydrogen and helium fuels are used up, at the end of its lifetime. A
supernova exploding within 50 ly (light years) of the Earth will have a
catastrophic effect. The 1040 J of energy produced in the first few
days bathes the earth in a total amount of ionization some 300 times
greater than the annual amount of ionization from cosmic rays.
Surprisingly, little of this radiation reaches Earth.
Instead, most of it
ionizes atmospheric nitrogen, which reacts with oxygen to form
nitrous oxide, which in turn reacts with ozone3.
The effect will be to reduce the amount of ozone in the Earth's
atmosphere by about 95%, resulting in a level of UV on the Earth's
surface some four orders of magnitude greater than normal, which
continues for a period of 2 years. This will certainly result in
almost 100% mortality of small organisms and most plants.
The effect on mammals
is not clear, and some might survive. However this 2-year period is
followed by a longer (80 years) period of bombardment by the cosmic
rays from the supernova, which have similar, although slightly
reduced, effects. It is difficult to see how anything other than an
advanced civilization could survive such an extended holocaust.
A supernova such as this goes off in our galaxy roughly every 5
years, and we expect one within 50 ly (light-years) of the earth
roughly once every 5 Myr. We expect one even closer (within 10 ly)
every 200 Myr (million years). Therefore all life would be expected to be destroyed
at this interval. Clearly this has not happened, since we are still
here, and I will return to possible reasons in a later section.
4. THE EFFECT OF GAMMA-RAY-BURSTERS
Gamma-ray bursters (GRB) are a recently discovered phenomenon, in
which some 1045 J of energy are released in a few seconds. The ones
that have been observed on earth appear to be distributed uniformly
across the observable Universe. Their power is such that we are able
to detect GRB right up to the edge of the observable universe. The
mechanism is still not known, but is likely to involve the merging
of two neutron stars, possibly resulting in the formation of a black
hole.
A GRB is some 5 orders of magnitude more energetic than a supernova,
and could occur even at the Galactic centre,
25 000 ly away from us, and have a similar effect as a supernova
within 50 ly. However, in this case there is an even more deadly
effect, in that, should a GRB go off in the Galactic centre, the
immediate blast of ionizing radiation is followed by an intense
blast of cosmic rays lasting perhaps a few weeks4.
These cosmic rays
will initiate a shower of relativistic muons in the Earth's
atmosphere, causing a radiation level on the surface of the earth
some 100 times greater than the lethal dose for a human being. The
muons are so energetic that they would even penetrate nuclear
air-raid shelters to a depth of perhaps hundreds of metres2.
We expect such a GRB roughly once every 200 Myr, and it would almost
certainly result in the extinction of all life on earth other than
that deep in the ocean. Again, clearly this has not happened, since
we are here.
5. MASS EXTINCTIONS ON EARTH
The geological and biological record shows a series of mass
extinctions of life on Earth. The most famous is that at the
Cretaceous-Tertiary (KT) boundary, which was almost certainly caused
by an asteroid hitting the earth about 65 Myr ago. The KT mass
extinction wiped out the dinosaurs, and paved the way for the
emergence of mammals as the dominant species on Earth.
Less well known are a series of similar, and in some cases even more
extreme, mass extinctions every few tens of Myr, and many smaller
extinctions, the last of which was only 11000 yr ago. The cause of
most of these is unknown. It is likely that a range of causes
including asteroids, distant supernovae, and climatic changes are
responsible for them.
All these mass extinctions are on a much smaller scale than the
catastrophic events we expect from a nearby supernova or a gamma-ray
burst in the Galactic centre. In each of these cases, a number of
species (sometimes as many as 50%) were extinguished, but a
sufficient range of diversity remained for the biota to recover in a
relatively short time.
6. WHY ARE WE HERE?
I have identified two causes that should wipe out essentially all
life on Earth roughly every 200 Myr, and yet we are here.
Two
possible explanations are:
In the first case, simply multiplying the timescale by a factor of a
few is insufficient. We have been evolving for at least 4 Gyr, and
so the interval between catastrophes must be at least 4 Gyr for us
to survive so far. Presumably the precise interval will vary
randomly around this figure, and so any surviving civilization can
look forward to a lifetime of between zero and a few Gyr. In this
case, if we detect ET, then ET will have a median age of perhaps 1
or 2 Gyr, which is similar to the 1.7 Gyr derived from simple
stellar evolution arguments. Thus, in this case, the supernovae and
GRBs have not significantly changed the median age of ET.
In the second case, we have already survived for some 20 times the
mean interval between catastrophes, which is very lucky indeed.
Whilst it is not possible to quantify this without more detailed
knowledge of the frequency distribution of supernovae and GRB, it is
likely that the probability is so low that we are alone in the
Galaxy. Apart from providing a solution to the Fermi paradox1, this
implies that the median lifetime of ET is meaningless, as we will
never detect ET!
7. CONCLUSION
Conventional models imply that supernovae and gamma-ray-bursters
will extinguish life on planets at intervals of about 200 Myr. Since
this has not happened on Earth, either these conventional models are
wrong, or else life on Earth is probably unique in the Galaxy. The
first case predicts a median age of ET as being of the order of 1
billion years. The second case predicts that we will never detect
ET. Thus, if we do detect ET, the median age is of order 1 billion
years. Note that, in this case, the probability of ET being less
than one million years older than us is less than 1 part in 1000.
Therefore, any successful SETI detection will have detected a
civilization almost certainly at least a million years older than
ours, and more probably of order a billion years older.
REFERENCES
1. Annis, J., 1999, JBIS, 52, 19.
2. Leonard, P.J.T., & Bonnell, J.T, 1998, Sky & Telescope, 95, 28.
3. Rudermann, M.A., 1974, Science, 184, 1079.
4.Thorsett, S.E., ApJ, 444, L53.
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