are the odds?
Let us go back to Drake's equation and discuss the factors one by one.
- The average rate of formation of stars suitable for the development
of intelligent life.
Note. By convention, the average rate (stars
per year) is used rather than the total number of suitable stars. This
leads to an estimate of the average number of new civilizations per
year, which, when multiplied by their expected "lifetime",
gives the number of civilizations available for detection.
We have a fairly good estimate of the age and the total number of stars
in the galaxy. The Big Bang occurred about 14 billion years ago, and
the Milky Way is one of the oldest galaxies among those that have been
studied. It probably formed within a billion years after the Big Bang.
It has some 200 - 400 billion stars. (The range of uncertainty reflects
the effects of dust in the center of the galaxy, but also, I think,
the number of red dwarves and the exact definition of a star.) Not all
stars are suitable for the development of complex life. Some burn out
too quickly. Some are deficient in heavy elements. Recent studies
indicate that only certain regions of the galaxy are suitable for the
development of terrestrial planets (assumed to be necessary for the
development of complex life). This is due to radiation from supernovae
in the dense inner regions of the galaxy, gravitational disturbances,
bombardment from comets, too little or too much metal content, etc.
Perhaps there are 100 billion stars that could potentially support (or
have supported) the development of life?
If we assume that 100 billion suitable stars have formed in 10 billion
years, we arrive at a star formation rate of the order of 10 per year.
(This rate is probably lower today, but our interest centers on stars
with an age comparable to that of the sun - some 5 billion years.)
- The fraction
of those stars with planetary systems.
The Hubble Space Telescope's edge-on view of a planetary system
being formed in the Orion nebula.
NASA/Space Telescope Science Institute
This is one parameter where great
progress has been made in just the last decade. Before the 1990s,
there was only one recorded planetary system - our own. At the time
of writing, 156
extrasolar planets have been discovered. Due to limitations in the
sensitivity of the measurements, nearly all are Jupiter-sized planets
in close orbits around the central star - quite bizarre objects in the
light of our previous understanding. It is expected, however, that as
the sensitivity of measurements improves, smaller planets will be discovered,
as well as giant planets in more "normal" orbits. In particular,
satellites will be launched in the next decade with the aim of detecting
Earth-sized planets. They may even be able to detect signs of biological
activity through spectroscopy.
Recently, there have also been observations of planetary system formation
in progress (the Hubble and Spitzer space telescopes).
Based on our present understanding, the fraction of stars with planetary
systems is significant, perhaps as high as 50 percent.
- The number of planets, per solar system, with an environment
suitable for life.
This parameter is more difficult to assess. First of all, we do not
know what is required for life to exist. For all we know, exotic lifeforms
based on a completely different chemistry than ours may be possible.
We do not even know for sure that the surface of a terrestrial planet
is the only suitable habitat. Perhaps life could arise in the atmosphere
of a gas giant or in the interior of a comet?
To have some basis for speculation, however, it seems natural to look
at what has made life possible on our planet. Here carbon-based organic
chemistry and liquid water seem to have been key ingredients. In addition,
conditions need to be reasonably stable over geologic time scales for
life to take hold. This means, inter alia, a rocky planet in
a stable circular orbit in a temperate zone neither too close nor too
distant from the central star (or stars - we should not exclude double
stars, which are very common in the galaxy). The planet needs to be
the right size to maintain an atmosphere as a shield against ultraviolet
radiation and energetic particles, and to regulate temperature.
Not very long ago, many scientists thought that the odds were good
that there should be at least one terrestrial planet hospitable to life
in a typical planetary system. After all, in our own system there is
a distinct possibility that subsurface life exists on Mars (although
any life on Mars may have originated on Earth - or perhaps even more
likely the other way around - through contamination via meteorites),
and perhaps on Jupiter's moon Europa.
ESA/DLR/FU Berlin (G. Neukum)
Water ice in a Martian crater.
A deep ocean is thought to exist beneath the water ice covering
the surface of Jupiter's moon Europa .
More recently, however, several aspects have been pointed out that
may make it less likely that our solar system is a typical "run-of-the-mill"
planetary system. It is believed that water on the inner planets comes
from comet impacts, and that Jupiter has had a key role in this process.
A gas giant in the right place and with the right mass may thus be required
but absent. Conversely, gas giants in the wrong places may preclude
a stable circular orbit in the temperate zone (liquid water) due to
gravitational effects. The surprising finding that out of the extrasolar
planets detected up to now, a fair number are in a highly elliptical
orbit, contributes to the pessimism.
The central star must also be the right size. If it is too large, it
will burn too quickly and emit too much harmful radiation. If it is
too small, the temperate zone will be too close to the star and the
planet will experience tidal locking, always turning the same side towards
the star. The energy output must be stable over eons, otherwise the
liquid water will freeze or evaporate.
For these and other reasons, it is now commonly believed that the number
is much smaller than 1, perhaps more like 0.001. This still leaves us
with some 50 million planets in our galaxy where life could potentially
- The fraction
of suitable planets on which life actually
Once conditions are right, what is the probability that life will actually
occur? Quite high in the opinion of most scientists. Experiments carried
out by Stanley Miller and Harold Urey in 1953 demonstrated that many
of the building blocks of life could arise in a "soup" of
chemicals thought to be present on young planets, in a water solution
when energy was added. However, the road to more complex molecules remains
obscure, and in particular there is no credible detailed scenario for
how DNA evolved. What we do know is that life developed fairly quickly
on Earth once the conditions were right. The oldest fossils are 3.5
billion years old, and the solar system itself is 4.5 billion years.
There was heavy bombardment of the Earth until 3.8 billion years ago,
so life may have emerged just a few hundred million years after conditions
made it possible.
It may well be that the emergence of life is a statistical certainty
once all ingredients are in place. It is of course also possible that
this event is so rare that it takes a miracle for it to occur (figuratively
or literally - take your pick). The prevailing opinion among scientists
seems to be that there should be millions of life-bearing planets in
the galaxy. And once primitive life takes hold, it could be extremely
difficult to stamp out.
fraction of life bearing planets on which intelligent
This parameter is probably where mainstream scientists most strongly
distance themselves from SETI enthusiasts. Many modern biologists do
not see evolution as an inevitable march from "lower" to "higher"
organisms with intelligence an important attribute for genetic success.
To them mankind is an "unforeseen" byproduct of environmental
adaptations that have been going on for billions of years. To the unbiased
eye, insects have been at least as successful as vertebrates - at least
until humans came along and invented pesticides. Insects are likely
to still be around long after mankind goes extinct. If history were
repeated from a few hundred million years back with a new roll of the
dice, in the opinion of many scientists, it is quite unlikely that another
intelligent species would evolve.
Evolution from the most primitive lifeforms occurred in several big
leaps. It took two billion years for life to evolve from simple anaerobic
bacteria to single-celled eukaryotic organisms. Multi-cellular algae
and sexual reproduction were established 1.2 billion years ago, and
fossils of multi-cellular animals have been found from 600 million years
ago. See timeline.
Reconstruction of organisms from the very end of the Precambrian.
(Diorama at the Smithsonian Institution.)
Then came the big breakthrough about 540 million years ago: the Cambrian
explosion, when multi-cellular animals evolved at a tremendous rate
within a very short period. (At least their morphology did, it now seems
that considerable genetic diversification
took place earlier - but without great fanfare.) Within as little
as ten million years a spectacular variety of body plans appeared. As
many as twenty basic designs evolved, of which only a few remain today.
Today we have many more species, but they are all variations of these
few basic themes (vertebrate, exoskeleton, jellyfish?). Thus, a rich
macroscopic fauna has only existed for the past 500 million years or
To the undiscerning eye, not much appears to have happened during the
first three billion years of evolution. (This reminds
me of a historian who tongue-in-cheek claimed that the 8th century had
never taken place - it was a construction. It turns out that it is not
all that simple to disprove his thesis.) All that Nature had
been able to come up with until 600 million years ago was what most
of us would regard as very primitive life forms: bacteria, algae, perhaps
some worms and sponges. It is tempting to think of the eons of pre-Cambrian
evolution as "wasted time" that could have been much reduced
with some luck. However, one should not underestimate the complexity
at the molecular level of the genetic machinery needed to evolve multi-cellular
animals. By the time of the Cambrian explosion, an impressive arsenal
of biochemical building blocks had been assembled. - If we look at a
newborn baby, not much seems to be going on during the first few weeks,
but in fact its mental capacity is evolving at a tremendous rate. It
is building an internal map of the world, learning to interpret sounds,
smells, colors, shapes, faces, motion, gravity. In a similar way, complex
life could not arise without a lot of prior basic development at the
molecular level. In the absence of a Designer, it should not come as
a surprise that billions of years passed before the Cambrian explosion.
It is not well understood how the Cambrian explosion was triggered.
It may be that the slow buildup of atmospheric oxygen passed a critical
threshold, or some catastrophic event "reshuffled the cards",
or some other environmental change occurred, or perhaps some mutation
affected the genetic machinery in a way that facilitated the evolution
of new organisms.
About 248 million years ago, the greatest mass extinction in history
occurred, when over 90 percent of all marine species disappeared. The
dinosaurs emerged 230 million years ago and went extinct 60 million
years ago, most likely as a result of the famous meteor impact in the
Yucatan peninsula at that time. Since then, mammals have taken over
as the dominant form of animals. Primates have been around for some
20 million years, hominids for some 4 million years, recognizable humans
for 300 000 years. Our own Cro Magnon brand of humanity appeared just
40 000 years ago, and we have not evolved in any significant way since
then. Einstein, Mozart and Shakespeare could easily have been born and
raised in an ice age hunter society!
What can we learn from all of this? There does not seem to be anything
inevitable in the rise of intelligence. Life emerged rapidly, once conditions
permitted, but then 3 billion years passed before anything more interesting
than bacteria, algae and tiny worms appeared on the scene. Contrary
to popular belief, the history of life has not been a steady, gradual
progression from "lower" to "higher" forms. The
modern view is - or was until recently - that there have been long periods
of equilibrium with minor fine-tuning of life forms, interspersed with
the rapid evolution of new life forms when the environment has changed
- sometimes as a result of catastrophic events (asteroid impacts, volcanism,
glaciation), sometimes when gradual change has crossed a certain threshold
(build-up of oxygen, plate tectonics rearranging the continents). -
It is interesting in this context that humans evolved during an atypical
epoch of repeated glaciations during the past 4 million years.
This theory of "Punctuated equilibrium", by
the way, does not fundamentally challenge Darwin's views on evolution.
Darwin was no stranger to the concept:
But I must here remark that I do not suppose that
the process ever goes on so regularly as is represented in the diagram,
though in itself made somewhat irregular, nor that it goes on continuously;
it is far more probable that each form remains for long periods unaltered,
and then again undergoes modification. (Darwin, Ch. 4, "Natural Selection,"
"It is a more important consideration ... that the
period during which each species underwent modification, though long
as measured by years, was probably short in comparison with that during
which it remained without undergoing any change." (Darwin, Ch. 10, "On
the imperfection of the geological record," p. 428)
The main driver of evolution seems to be environmental
change. Periods of environmental stability do not seem to correlate
well with rapid evolution. Catastrophic events causing mass extinctions
may well have been the key factor leading to intelligence, by allowing
a new "roll of the dice" from time to time. It is noteworthy
that, at least as far as we know, the dinosaurs did not evolve intelligence
over a period of 170 million years. If intelligence by itself had been
important for survival, there is no obvious reason why they should not
have been able to do so. (On the other hand, among mammals, dolphins
have evolved a certain level of intelligence, and they are not that
closely related to our species.) Besides, the possibilities of hard-wired
specialized "intelligence" should not to be sneezed at. Consider
the social insects, or the web-weaving and nest-building capabilities
of spiders and birds. - The Earth may well be unusual in having had
the right mix of environmental stability and rapid change to create
favorable conditions for intelligence to finally evolve.
The Earth and the Moon. The Moon may have played an important
role in the evolution of complex life on Earth.
Reasons for pessimism on the chances for intelligence,
or even complex life, to arise have been forcefully expressed by Ward
and Brownlee in their book "Rare
Earth". (Another link
with 94 reader reviews, some of them quite interesting.)
At the heart of the matter, there are different
views of evolution itself. Gould
argued that evolution has no inherent direction. It just adapts organisms
to fit the environment. Evolution could just as easily lead to decreased
complexity as the opposite. It proceeds by random mutations as a "drunkard's
walk" (Markov chain) with no preference for any particular direction.
There is a "wall" on one side (life cannot go below a certain
level of simplicity), and purely by chance some life forms at the other
end will become increasingly complex, so there will be a slow drift
toward increased complexity on average, but there is no reason why in
the long run those life forms should be more successful than simpler
The opposite view (religious beliefs aside)
is that evolution has an inherent component in the direction of increased
complexity. Even in a stable environment there is room for improvement
through the development of new capabilities based on increased complexity.
- There is some evidence for this view in the form of simulations
of artificial life forms.
To me it would seem very optimistic to assign a value
close to 1 to the factor fi,
meaning that every life-bearing planet will ultimately evolve intelligent
life. Even fi
= 0.001 may be optimistic.
fc - The
fraction of civilizations that develop
a technology that releases detectable signs of their existence into
This is another very uncertain number. Surely, a portion of intelligent
species would find it impossible to develop the capability to signal
their presence. Not only would tool-making be required, which puts certain
demands on the body design; it must be possible to extract and shape
metals and other materials and assemble them into devices, and to extract
and manage energy from suitable sources. We are fortunate to have had
easy access to fire, for instance.
Even given our own mental capabilities, it does not appear inevitable
that we should have succeeded to move from stone age technology to broadcasting
electromagnetic signals before our species became extinct. And our form
of intelligence may, purely by chance, be especially well suited to
develop science and technology. Most theorists believe that the development
of our intelligence had to do with language and social interaction,
and perhaps tool manipulation. Our ability to discover Maxwell's equations
was an unforeseen side effect, so to speak. Other forms of intelligence
may develop in other directions.
On the positive side, we managed to move from our present level of
intelligence to radio technology in just 40 000 years. As long as an
intelligent species does not go extinct, there is no particular reason
for hurry from the perspective of Drake's equation.
Still, it appears optimistic to believe that a large fraction of "intelligent"
(self-awareness, intelligent reasoning, language) species would be capable
of signalling their presence into space. (Think of dolphins again.)
L - The
length of time such civilizations release detectable signals into space.
This is perhaps the number about which there is the widest divergence
of views. The argument has centered on how long our own technological
civilization may be expected to last. There are extremely optimistic
views: "We will evolve into a stable society that could last
for millions of years. Human history has known many civilizations. Even
a thermonuclear war will not mean the end. Humans are resilient. After
a dark age, another civilization will inevitably arise.", just
as there are equally pessimistic views: "Mankind is on the brink
of extinction less than a century after starting to emit electromagnetic
signals. In addition to war, there are the threats of a global plague
- man-made or natural - or of a global environmental disaster. Any technological
civilization will be unstable and short-lived."
Even if a technological civilization should achieve a good long-term
perspective, it is far from obvious that it will "leak" electromagnetic
radiation into space, except for the specific purpose of trying to contact
other civilizations. Our own radio and TV transmissions are already
being switched to more efficient means of distribution (cable and short-range
It is quite possible that an advanced civilization would want to announce
its existence to the rest of "the galactic community", but
is is equally probable that it would prefer to remain inconspicuous,
listening rather than transmitting. As Fermi pointed out, any sufficiently
advanced civilization should be able to spread across the galaxy. A
corollary might be that any advanced technological civilization represents
a potential threat to other galactic civilizations.
Therefore, even an optimistic estimate of the average lifetime of a
technological civilization may need to be tempered with a more cautious
estimate of the length of time when such a civilization will emit detectable
Another factor that should perhaps be considered, is that a technological
civilization may become capable of creating artificial intelligence
(not limited to smart chess programs, but actually able to pass the
test), perhaps including an artificial evolution process millions
of times faster than any natural process, which could lead to a "robot
civilization" that might be more stable than the originating civilization.
As a final observation, already during the present century, mankind
will probably start tinkering with its own genetic machinery. Initially,
research will be directed toward curing a variety of genetically influenced
diseases, but as knowledge is gained (also from non-human genetic engineering)
and confidence increases, it is quite probable that ambitions will rise
despite ethical considerations and religious objections. This suggests
that mankind may evolve (or devolve) substantially in directions that
are difficult to foresee, even in a time perspective of just thousands
of years. - It is anyway a myth that human evolution has come to a standstill
just because the struggle for survival has become more lenient in modern
society. For example, I am convinced that strong selection for parental
instincts is going on right now, as child-bearing has become facultative
in Western society. - All of this may not have any direct bearing on
Drake's equation, but it should reinforce the need for humility when
we indulge in speculation. Not even our own species may be as stable
(in the sense of retaining all its present characteristics) as we like
to think over the course of the next few thousand years (assuming we
survive that long).
Based on the discussion above, let us finally propose a set of probabilities
that appear "reasonable" and see where that leads us. I have
picked numbers that seem plausible to me, perhaps on the optimistic
N = Rs*
fp * ne
* fc *
If I have counted the zeros right, this would give us a number of just
0,00025. This is very far from even a single civilization in our galaxy
emitting detectable signals.
Of course, I could easily be off by a factor of a billion or more,
either way. That would still only leave 250000 detectable civilizations
at best, spread out over a hundred thousand light years. We would in
that case have to inspect on the order of a million stars for each positive
In the next 30 years, we should gain a much better understanding of
the first four parameters through direct observations from space
telescopes. Atmospheric measurements of extrasolar planets indicative
of biological processes (presence and ongoing replenishment of gases
such as oxygen and methane) would be of great interest and might form
the basis for a statistical assessment of the chances for the emergence
Unfortunately, the last three parameters are likely to remain
very uncertain unless the SETI program turns out to be successful, something
that is entirely possible, of course. But a negative result even after
picking the "low hanging fruit" - searching the most promising
and accessible one million stars, say, in the next few decades - would
be discouraging, even though they would just represent a sample of one
in a hundred thousand.
The reason is Fermi's paradox. It would seem reasonable to believe
that a large portion of all detectable civilizations should also be
able to develop interstellar space flight. If there were many detectable
civilizations in our galaxy, we would expect at least one of them to
have expanded across the Milky Way. In that case, surely at least one
planetary system in a million should host a detectable civilization.
My conclusions, no strike that, my tentative opinions until
I talk to the next persuasive scientist with a different view, are:
1) There are probably millions of planets in our galaxy teeming with
simple life forms.
2) There is probably only one detectable technological civilization
in our galaxy at present - our own.
3) There are probably millions of advanced technological civilizations
in the universe.
pale blue dot
case for human expansion into the galaxy
A man half asleep at an astronomy lecture suddenly became
wide awake and asked: "Could you repeat what you just said, please?"
- "Certainly. Our sun will expand and engulf the Earth five billion
years from now." - "Thank God. I thought you said five million!"
At a distance of 15 million light years, Messier 83 has
approximately the same size and structure as the Milky Way.
It gives a good idea of what our own galaxy may look like from
the outside. We live in a spiral arm about 28000 light years
from the center of our galaxy.
European Southern Observatory.
Back in 1969, when Apollo 11 landed on the moon, even jaded journalists
remarked: "Incredibly, after billions of years of evolution, it
is within our own short life span that creatures from our planet have
for the first time visited another world!"
Even though the same event may have occurred many times in the history
of our galaxy, it may very well be that at present we are the only space-faring
civilization among billions of worlds in the Milky Way.
I do not know which vision fills me with more awe - the Milky Way with
many advanced civilizations, or the one where we are unique. But undoubtedly
the latter scenario places the heaviest responsibility on our shoulders,
for whether we are here by design or by pure chance, our demise would
mean the loss of something very precious, not only to Mother Earth,
but to the whole Milky Way.
As a species of mammals, we could normally look forward to an existence
of a few million years before replacement by a new species better adapted
to the environment. But ordinary rules do not apply to us. We are ourselves
modifying the environment at an unprecedented rate. In the long run
- if there is one - this capability could work to our advantage, but
in the near term (the next few centuries) we may well trigger irreversible
processes that spell our doom. A run-away greenhouse effect is just
one of the possibilities. The rapid spread of diseases made possible
by modern modes of transportation is another worry. Ecological disaster
as a side effect of genetic engineering is yet another possibility.
All unintended disasters seem less threatening than our tendency to
willfully inflict damage on ourselves, however. Even if my generation
has enjoyed a long period without a major war, it would be naive to
think that wars are a thing of the past. A thermonuclear war could well
bring us back to the stone age. A "nuclear winter" causing
famine after a major war remains a possibility. What worries me even
more is the potential for creating dangerous pathogens. This would not
necessarily mean that biological warfare is a strong threat, but the
technology is being developed by governments as a "precautionary
measure", and once the technology exists, there is always the danger
that it may fall into the hands of extremist groups "on a mission
from God" or whomever. I fear that the technological barriers to
developing means of mass destruction may well be lowered in the future.
There are people who regard mankind as a "cancer" on Mother
Earth and would not hesitate to perform an operation if given the opportunity.
Overall, I think that there is a significant probability that Man,
"king of ashes", will selfdestruct within the next few millenia,
if not before. To safeguard our cultural heritage, to facilitate a comeback
after a possible collapse, and perhaps to protect our very existence
as a species, we should avoid "putting all our eggs in the same
basket". This, in my opinion, is a strong argument for establishing
self-sufficient outposts in the solar system with some urgency (although
it will not
be easy), or perhaps O'Neill type space
colonies. But it is not yet a compelling argument for expansion
into the galaxy due to the time scales involved.
In the long run, however, I can think of no better way to ensure the
survival and the success of humanity than to establish a human presence
in other planetary systems than our own. The major disadvantage of having
to bridge the huge gaps that separate the stars in space, and therefore
also in time, is also the great virtue of interstellar colonization.
The colonies would be much more isolated from us than America and Australia
ever were from Europe. Trade and war and imperialism would become impractical,
but we could still share all the elements of culture - science, novels,
poetry, films, music, art, games - albeit with a huge time lag. Our
descendants on Earth would in effect become observers of many human
cultures, without the opportunity to interfere in their affairs, for
better or for worse.
In time, colonies would evolve into human civilizations in their own
right, launching new waves of colonization. Within a few thousand years,
the risk that our race will expire could be dramatically reduced. Within
a few million years, each of a million human civilizations could enjoy
the knowledge and the creative accomplishments of all the others. The
benefits would be literally unimaginable. - Of course, by then the different
populations might have developed traits that would make them seem quite
alien to us - but so might any civilization left on Earth.
Diversity is beneficial to the longterm prospects for success of any
species or grouping of species (phylum). I would argue that cultural
diversity is important too, and has played an important role in the
development of our civilization.
A star field in Sagittarius.
NASA/Space Telescope Science Institute
Yet, as technological advances have led to global trade and instant
communications, our culture seems to be growing more uniform. A hotel
room looks the same in Sydney as in Berlin or Buenos Aires. The cars
look the same, the fast-food restaurants look the same, the music and
TV shows and films played on radio and TV and in movie theaters are
largely the same. "Do you have what we Americans call pizza?"
Breakthroughs such as the Internet or mobile telephony sweep across
the world virtually overnight. True, a lot of this is less obvious to
the poor in large parts of the world. Still, our technological society
seems to be heading toward convergence rather than diversity. We are
becoming more like an ant hill and less like a meadow. - All of this
may be a good thing if it promotes international understanding and economic
development worldwide, but it still seems a little worrisome to me.
One of the attractive features of the idea of a human expansion to
other planetary systems is that cultural diversity would be guaranteed
and could grow beyond imagination. Thanks to the built-in time lag between
widely dispersed civilizations, they would all develop in different
directions, without however being completely cut off from the rest of
Sense of purpose
The final point to be made in favor of galactic expansion is the sheer
grandeur of the concept. - There is a story about two stone cutters
living in the 12th century. One of them was swearing and generally indignant.
"I have been given the [expletive
job of cutting this [expletive
deleted] stone into a square shape."
The other one was whistling. "I am building a cathedral."
- It is hard to imagine a more glorious undertaking than the settlement
of the galaxy with its hundreds of billions of stars, and it is one
that could give a sense of purpose to all future generations. - It is
also in line with religious teachings (Genesis 1:28).
What about cost?
What about implementation? Is it even feasible?
I do not believe that such an undertaking will be realistic
for several centuries. We must learn to walk before we can run, and
the exploration of our own solar system should keep us busy for quite
a while. Before this century is out we may be able to send unmanned
probes to the nearest promising stars. More likely, we will build giant
space telescopes allowing us to examine neighboring star systems in
much greater detail than will be possible initially.
Another thing to consider is when our technology is
good enough to aim for the stars. - There is an
amusing science fiction story by A. E. van Vogt, where humans travel
to our nearest star system Alpha Centauri for 500 years in suspended
animation. When they arrive there, they are welcomed by an advanced
human society! - It turns out that technology has progressed to the
point where faster-than-light travel has become possible. The target
planet was reached many years before our heroes arrived! -
I consider it extremely unlikely that faster-than-light travel will
ever become possible, but obviously it will make a great deal of difference
just how fast an interstellar ship (or robotic probe) will be able to
Another possibility when we think in terms of thousands
of years is planetary engineering ("terraforming").
We may be able to develop bacteria that can provide a planet with a
breathable atmosphere within a "reasonable" amount of time,
or at least an atmosphere that will moderate temperatures. We may also
want to seed sterile planets in order to "jump start" biological
evolution. If our scientific understanding becomes sufficiently advanced,
we might be able to guide biological evolution of extant life in the
direction of increased complexity. Something for an ethics committee
So, for the foreseeable future, human expansion into
the galaxy must remain just a vision. But it is a vision that can give
us hope for the long-term survival of our species, and give us hope
that great things await our descendants in the distant future!