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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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