Download There are numerous other ways in which human civilization could

Colonization and Panspermia
If we assume, as discussed, that there
there have been no visits of extraterrestrials
to earth then the following argument has
been proposed:
1) 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.
Here I will analyse this argument.
1) 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
What is physically possible for
interstellar travel?
There is a universal ‘speed limit’ of
c=3 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.
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.
Diffusion:
Walk on a square lattice:
Choose a direction at random each hop time
τhop and take a step of length l.
On average, the distance from the origin
R increases as
(<R2>)1/2 = (t/ τhop )1/2l
Estimate filling time:
l = R/Ngal1/3
τfill = τhop(R/l)2=τhopNgal2/3
What’s τhop ?
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< τhop < 105 years
With Ngal = 1011
this gives
2x108< τfill < 2x1011 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
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/τb – nciv/τd- nciv/τfill=0
if τd << τfill then
nciv =(τd/τb)ngal (Drake equation)
and if τd >> τfill then the diffusion
(colonization) rate determines the local
density.
Forms of the Drake Equation (Appendix 1.1)
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: Nciv=Ngalfstarfplanetflife
Here: Nciv =(τd/ τb)Ngal
The f’s in the last part of the SETI form
have a slightly different meaning
In Drake’s form:
flfifc is the probability that life will ever appear
on the planet during the stars lifetime
Correspondences are
flife= flfifc(τd/τstar) , L=τd
Ngalfstar=R*τstar
fplanet=fpne
fstarfplanetflife=τd/τb
Conclusion: (within a diffusion model)
For civilizations with lifetimes more than
100 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 haven’t colonized
us therefore they don’t exist could only apply
to very long lived civilizations (> 100 million
years).
Results of a more detailed model (Appendix 6.2)
kR α (τhop /τd )1/2 Ngal1/3
Other models for colonization dynamics:
Nonlinear diffusion models. For example
article by Jones in the book 'Extraterrestrials
Where are They? (reference 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
<R2>=(t/τhop)6/5 l2
(6/5 replaces 1 in the diffusion model.)
This makes quite a difference in the
filling times.
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.
The hop times only take account of
travel times.
In fact it is likely that the need to settle
around a new star will greatly increase the
minimum hop time and therefore the
minimum fill time. Correspondingly,
the minimum lifetime of civilizations
excluded by the argument will be increased
by this effect.
So how likely is it that civilizations would
last 1 million years or more?
Well of course we don’t 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
τd 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.
Now consider these probabilities:
P(E), the probability of E
P(H) the probability of H
P(H|E) the probability of H given E
P(E|H) 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(E|H) P(H)= P(H|E)P(E)
H: Human civilization continues for sometime
τd 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(E|H)P(H)= P(H|E)P(E)
What we want to know is P(H|E). Solve for it
P(H|E)= P(E|H)(P(H)/P(E))
Clearly P(H)<P(E). If there is NOTHING SPECIAL
ABOUT THE PRESENT P(E|H)=t/τd
So
P(H|E)< t/ τd
P(H|E)< t/ τd
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. (We’ve been sending
Out radio signals that long.) If we put
t=100years then we get
P(H|E)< 100years/ τd
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%)
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-ε)N ≈ -Nε = ln 10-4 =-4ln 10; ε = 4 ln 10/N
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.
The asteroid 2007 VK184 which is given about one chance
in 3000 of impacting the earth probably in 2048. Its diameter is
about 0.13km and its mass is 3.3x$10^9$ kg. If it hits the earth,
the estimated impact velocity would be 19.19 km/s.
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.
Conclusion:
The human example gives no grounds for
optimism concerning the likelihood that
extraterrestrial civilizations will live
1 million years or more.
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
1) 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.
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.
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.
Can microorganisms easily move from
planet to planet or from star to star?
When meteorites hit the earth they can
knock bits of the earth’s 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.
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.
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 earth’s 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.
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
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.
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.
Recall that diffusion is irrelevant if
filling times exceed lifetimes. Actually
bacteria are ‘immortal' under favorable
conditions. We don’t know if
their lifetimes can exceed 100 billion
years but that is irrelevant because the
maximum time available for diffusion is
14 billion years. It is possible that lifetimes
of some bacteria might exceed 14 billion
years. However that time is less than
the estimated filling time, so in any case
diffusion is irrelevant to estimation of
the likelihood of finding
bacteria on a habitable planet.
Thus local evolution (that is the Drake
equation) will determine the probability.
‘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.
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 appears to be
a practically unfalsifiable hypothesis
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.