Colonization and Panspermia

1

• Colonization and Panspermia
• If we assume, as discussed, that there
• there have been no vists of extraterrestrials
• to earth then the following argument has
• been proposed
• An advanced civilization would migrate
• from star to star, colonizing as it went, and
• would quickly colonize the galaxy
• 2) We observe no visits from extraterrestrial
• civilizations.
• 3) Therefore advanced extraterrestrial
  civilizations
• do not exist.

2

• Here I will analyse this argument.
• How easy is travel between stars?
• Current human technology The Voyager space
• Craft, launched 1977 are traveling at about
• 16,000 m/s 0.5 x 10-4 c
• 3x108m/s
• The nearest star is Proxima Centauri, about
• 4 light years away. Therefore it would take the
• Voyagers about
• 4/ 0.5 x 10-4 80,000 years
• to reach it

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What is physically possible for interstellar
travel? There is a universal speed limit
of c3 x 108 m/s. Near c, the energy cost of
acceleration grows as mc2(1-v2 /c2)-1/2 so
travel at exactly c is impossible (requires
infinite energy). Engineering estimates suggest
a practical upper limit of about c/10 at enormous
expense.
5
How would a civilization migrate? Not known, but
one possibility is that is would spread the way
species spread through habitats on the surface of
the earth. Biologists have studied the species
migration problem mathematically. Models are
complex but they are elaborations of a simple
diffusion model which I will use to illustrate
the possibilities.
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Diffusion Walk on a square lattice
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Choose a direction at random each hop time thop
and take a step of length l. On average, the
distance from the origin R increases as
()1/2 (t/ thop )1/2l
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Estimate filling time l R/Ngal1/3 tfill
thop(R/l)2thopNgal2/3 Whats thop ? Distance
between stars is of order a few light years. At
a maximum imaginable speed of c/10, this gives a
minimum time of order 10 years. Human technology
now gives times of about 105 years so
10
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With Ngal 1011 this gives 2x1082x1011 years The upper limit exceeds the age of
the universe. How should we interpret this? The
likelihood of seeing another civilization nearby
is proportional to the local density of
civilizations
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The likelihood of seeing another civilization
nearby is proportional to the local density of
civilizations nciv . This is (determined by a
balance of birth and death rates ngal/tb
nciv/td- nciv/tfill0 if td (td/tb)ngal (Drake equation) and if td tfill
then the diffusion (colonization) rate determines
the local density.
13
From SETI
N R fp ne fl fi fc L Where, N
The number of civilizations in The Milky Way
Galaxy whose electromagnetic emissions are
detectable. R The rate of formation of stars
suitable for the development of intelligent
life. fp The fraction of those stars with
planetary systems. ne The number of planets,
per solar system, with an environment suitable
for life. fl The fraction of suitable planets
on which life actually appears. fi The fraction
of life bearing planets on which intelligent life
emerges. fc The fraction of civilizations that
develop a technology that releases detectable
signs of their existence into space. L The
length of time such civilizations release
detectable signals into space.
From Hart NcivNgalfstarfplanetflife Here
N(td/ tb)Ngal
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The fs in the last part of the SETI form have a
slightly different meaning
In Drakes form
flfifc is
the probability that life will ever appear on
the planet during the stars lifetime
Correspondences are flife
flfifc(td/tstar) NgalfstarRtstar
fplanetfpne
fstarfplanetflifetd/tb
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Conclusion (within a diffusion model) For
civilizations with lifetimes more than 10 million
years, we may be more likely to see them because
they drift in (colonization) than because they
appeared and grew in their original
environment. Thus the argument that they havent
colonized us therefore they dont exist could
only apply to very long lived civilizations (
100 million years).
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Other models for colonization dynamics Nonlinear
diffusion models. For example article by Jones
in the book. Gives similar filling times. Self
avoiding random walk. This is like the
diffusion model but the walker never steps where
he has already been. The result is that the
distance from the starting point is, on
average (t/thop)6/5 l2 (6/5 replaces 1 in
the diffusion model.) This makes quite a
difference in the filling times.
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As you might expect, the filling occurs
faster With the self avoiding diffusion model.
Nevertheless, the filling times are at least 10
million years, and by the same argument as
before, only the local density of civilizations
living longer than this will be affected by
colonization and migration.
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So how likely is it that civilizations would last
1 million years or more? Well of course we dont
know but there is an argument which suggests
that the human example may not be very
encouraging. Consider two events Human
civilization continues for sometime td longer
than it has lived so far and then dies. Call
this event H. Human civilization survives
sometime between its present age, call it t, and
its beginning. Call this event E. This event
has occurred.
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Now consider these probabilities P(E), the
probability of E P(H) the probability of H P(HE)
the probability of H given E P(EH) the
probability of E given H P(E,H) the probability
of E AND H For any pair of events these are
related by P(E,H) P(EH) P(H) P(HE)P(E)
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H Human civilization continues for sometime td
longer than it has lived so far and then dies.
E Human civilization survives sometime
between its present age, call it t, and its
beginning.
P(E,H) P(EH)P(H) P(HE)P(E)
What we want to know is P(HE). Solve for it
P(HE) P(EH)(P(H)/P(E)) Clearly P(H)there is NOTHING SPECIAL ABOUT THE PRESENT
P(EH)t/td So P(HE)td
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P(HE)
Now the number we assign to t depends on what we
mean by civilization. The SETI people, who are
looking for Electromagnetic signals from a
civilization, Would say that our electromagnetic
lifetime Is about 100 years. (Weve been
sending Out radio signals that long.) If we
put t100years then we get P(HE)td So the probability of surviving 10,000
years is less than 1 and the probability of
surviving 1 million years is less than 1 part
in 10,000 (.01)
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A few comments about this result It refers to
human civilization and makes no specific
statement about other civilizations. However,
since human civilization is the only one we know,
assumptions about others which are inconsistent
with what we know about humans are
questionable. There is one other perspective
which strengthens the plausibility of the
result If our civilization is to live 1 million
years With probability 10-4 or greater then it
is easy to show that the probability per year of
civilization destruction must be about 10-5 or
less.
Ln(1-e)N -Ne ln 10-4 -4ln 10 e 4 ln 10/N
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Can we assume that annihilation probabilities are
as small as that? This is hard to quantify but
one can make a start. Some annihilation risks
that have been discussed include Impact by a
near earth object (asteroid) see
http//neo.jpl.nasa.gov/risk/ . For example
object 2008 AF4 is given a probability of about
4.3 x 10-5 of impacting the earth during the next
century (most probably in 2089). That would be
about 4.3 x 10-7/yr from this object. Another
object, 2007 VK184 is given a probability
of 3.4e-04 (that would be in 2048) or 3.4 x
10-6/yr . These are the objects currently listed
as most hazardous. But asteroid
impact probability alone seems to bring us to
within the range suggested by the Carter
argument.
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There are numerous other ways in which human
civilization could die. For an example of a
discussion see Existential Risks Analyzing Human
Extinction Scenarios and Related Hazards   Nick
Bostrom, Journal of Evolution and Technology,
Vol. 9, March 2002. Assessing quantitative
probabilities of most of these is very
difficult, but they can only increase the
likelihood of extinction.
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Conclusion The human example gives no grounds
for optimism concerning the likelihood
that extraterrestrial civilizations will live 1
million years or more.
28
Panspermia Here we are concerned with the
transport of presumably unintelligent or
uncivilized life.

• A similar Hart-like argument could be made in
• this case. It would have the form
• Simple forms of life can easily and quickly
• (on the relevant time scales) move from star to
• star
• 2) Simple forms of life have not arrived on earth
  in
• this way.
• 3) Therefore simple forms of life do not exist
  elsewhere
• in the galaxy.

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Actually this argument is not made very often
in this way. One reason is historical
Arrenhius first proposed panspermia as a way to
understand the prebiotic evolution problem in the
late 19th century. There are at least two
problems with the proposal that life started on
earth by transport of microorganisms to earth on
meteorites. It does not solve the prebiotic
evolution problem life still has to get started
somewhere within less than 14 billion years and
It is not clear if microorgansims can survive
the trip on a meteorite. This also bears on
point 1) of the Hart like argument in the
preceding slide.
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Another reason that the Hart-like argument is not
usually made for panspermia is because it is less
clear in the case of microorganisms that point 2
is correct. In fact scientists continue to
suggest that it has occurred or will occur. Now
we consider these issues for panspermia one at a
time.
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Can microorganisms easily move from planet to
planet or from star to star? When meteorites hit
the earth they can knock bits of the earths
surface off and accelerate them to escape
velocity. It is Conceivable that such bits will
carry microobes with them. So a plausible
mechanism exists for launching microobes on
meteorites into space from earth by natural
(nonhuman) processes. The same processes could
occur on other planets on which microorganisms
existed.
32
There is strong evidence that meteorites get
here from Mars in that way. (They compare the
isotopes found in the meteorites with the
distribution of isotopes found on Mars.) In 1996
there was a report that structures found in those
meteorites appeared to be fossils of organisms.
(picture on next slide) However subsequent work
has shown that these structures are probably the
result of nonorganic crystallization processes.
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We conclude that transport of meteorites
between planets (and possibly also between stars)
is possible. If these meteorites were to carry
microbes, would the microbes survive the
trip? Not if they were ordinary organisms. The
radiation levels in space are about 6
times higher than they are on the earths surface
and most organisms would be killed by them on a
trip of more than a few years. However,
recently microbes have been discovered on earth
which can survive much higher radiation levels.
35
A few things about radioactivity. It arises from
changes in the structure of nuclei of atoms and
the resulting radiation is of three basic types
all carrying a lot of energy and capable of
damaging living organisms. The biological harm
depends on the total energy deposited in the
organism (not on the rate at which it is
deposited) . The units of radiation dose
include the rad (1/100)joules/kilogram And
the rem Quality factor x rad
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The quality factor takes account of the
different effects of different types of
radiation. For relevant types, QF is always
larger than 1. On earth the radiation levels are
around 1/2rem/yr whereas in the solar system they
average around 3 rem/yr. Any life dose above
30rem is judged to be a serious risk for humans,
but some extremophile bacteria can tolerate more
than 3 million rem. Though no bacteria have
yet been found which simultaneously tolerate the
extremes of temperature, dessication and
temperature which they would encounter in an
interstellar journey on an asteroid, it seems
possible that some may exist.
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We conclude that undirected panspermia may be
possible and could have occurred at least once on
earth. It is unlikely to have occurred often
because if if had, we would see more biochemical
diversity in the biosphere than we do. How much
would undirected panspermia affect the
likelihood of finding life on any particular
star or planet? We can consider a diffusion
model, as before. It is more likely to be a good
model here than in the case of civilizations.
What hopping times should we use? Relativistic
speeds are unlikely. Thus the filling times are
likely to exceed the age of the universe.
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Recall that diffusion is irrelevant if filling
times exceed lifetimes. Actually bacteria are
immortal under favorable conditions. However we
dont know if their lifetimes can exceed 100
billion years. If so, then diffusion, that is
panspermia may be determine the likelihood of
finding bacteria on a habitable planet. If the
lifetime of bacterial colonies is shorter than
that then local evolution (that is the Drake
equation) will determine the probability.
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Directed panspermia Here one imagines that a
civilization, about to die (say because its star
is about to supernova or become a red giant) ,
sends a packet of essential biochemical
material (seeds) to a nearby hospitable
star. Speeds could be relativistic depending
on postulates about the nature of the
civilization. Transport would not need to be
diffusive because the civilization could aim
the probe toward one or a few selected
targets. Material might be radiation protected.
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Conclusions concerning directed
panspermia Most of the previous arguments fail
(diffusion model). The only evidence against
it is the failure to observe biochemical
diversity. This puts a limit on the frequency
with which it could have occurred here. This
is appears to be a practically unfalsifiable
hypothesis
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Summary Colonization The failure to observe
colonization is evidence that civilizations that
live more than about 10 million years do not
exist. The exact age depends on the model used
to describe their transport but relativistic
limits in this argument are unlikely to change.
Nothing is learned about more short lived
civilizations. Other arguments that civilizations
will not live that long were given. Panspermia
Undirected panspermia is probably possible and
may have occurred once on earth. It is unlikely
to be the dominant factor determining the
probability of observing life on
planets. Directed panspermia is conceivable but
appears to be an unfalsifiable hypothesis.