Once an alien civilization has reached asymptotic technology, and has made the decision to spread their species or life itself to other solar systems, they wouldn’t just build a rocket and launch it. There would be some specific experiments they would need to do before they felt confident enough to launch. The cost of an interstellar voyage has to be very high, and so there would be the necessity to prepare for it, including testing as much as possible, before any commitment was made to a single chance.
Estimates made
elsewhere in this blog indicate that an alien civilization might
last, living at the highest technology levels and a high individual
standard of living, for about a million years on the resources
available within a single solar system. That is an order of
magnitude number, meaning somewhere between a third and three million
years, and the estimates are even rougher than that. The accuracy of
such as estimate is irrelevant, in that what is important is the
comparison of the time the civilization will last to the time
necessary for a space voyage.
If we take the
distance to their target solar system to be related to the density of
stars in a spiral arm, something like a hundred to a thousand light
years is the range. This estimate might be very low if they are
looking for a planet which already has a burgeoning ecology, where
they would be able to land and fit in immediately. If it is true, as
has been indicated in this blog on other posts, that life is hard to
originate, then looking for another planet with life already evolved
up to vegetation and animals might take them across the galaxy. If
they are only looking for a planet where they could seed life, and
have it evolve on its own for a billion or two or three years, then
the shorter distances would be relevant. Their goals can be either
of these. For the goal of starting life, they would realize that
they would become extinct or at least reduced to a primitive level,
very, very long before they could migrate to the seeded planet, but
their goal would include the expectation that evolution on the target
planet would result in something like them, given enough time.
Travel speed is a
question, but given the monstrous amount of propellant and energy
that are needed, assume an upper limit of 0.01c. This means they
will have to prepare a ship to travel without breaking down for ten
to a hundred thousand years, and then function in a very intricate
way to drop their cargo off on the chosen target planet. We on Earth
have a few things which still work after a hundred years, but our
learning technology is so recent, we haven’t had enough time to
have built something that would be tested for a thousand years.
Things go wrong over
long periods of time that do not go wrong over short periods. Things
go wrong in space that do not go wrong on a planet. Consider the
simplest thing, the hull. It is going to have erosion at the front
end. Going at 0.01c means that every piece of dust hitting the hull
is going at that speed, relative to the ship. This is a vaporization
level of energy. There is very little dust outside of solar
systems, but there is some, and dust clouds may exist that the aliens
do not know about. Can they map all the dust clouds, and if they
could, can they steer a ship moving 0.01c around them without insane
amounts of additional energy and propellant? Even individual atoms
and molecules will abrade the front surface of the hull, and there
are clouds of hydrogen in the galaxy just as there are dust clouds.
What does ten
thousand years of exposure to galactic cosmic rays do to a steel
hull? A single cosmic ray, meaning high energy proton, will most
frequently splash some electrons around inside the first millimeter
of hull. There is a lesser chance it will displace an atom or two in
the steel. The degrades the crystal structure, and the outer layer
of steel will grow weaker and weaker. Small nanoparticles may come
off, especially if the angle of penetration is far from
perpendicular. This is simply another avenue for erosion of the
hull, and it works in combination to dust particle erosion by
weakening the hull prior to the impact of a dust particle, which can
then remove more mass.
Besides erosion,
what goes on in a material like steel if you just let it sit still
somewhere for ten thousand years, in a non-corrosive environment? Is
there very slow migration of non-iron atoms, so that the material
becomes less homogeneous, and therefore less strong?
To figure out
erosion, experiments with a high rate of dust and particle flow could
be done in a laboratory. It should be possible to do these
experiments over a relatively short period of time. To do the
chemical segregation question, temperature might be used as a
surrogate for long periods of time, but just how accurate this is is
not clear. Certainly, observing some metals, semiconductors,
amorphous compounds and other single materials for a thousand years
at different temperatures might completely solve this problem, and
allow the alien civilization to extrapolate to ten or a hundred
thousand years.
This sounds
straight-forward, and with the alien civilization having a million
year horizon, a thousand year experiment doesn’t sound too bad. If
they found that certain materials failed after a few centuries, then
a series of experiments might have to be done to try various
solutions to the problem. There goes another few thousand years. It
is hard to see how the requirements to evaluate reliability and fix
problems in this arena would take more than some tens of thousands of
years. This means that the experiments to prepare the materials and
equipment for the ship might be done in the first ten percent of the
expected time of their existence, and that the need for
experimentation would not prevent an alien civilization from
traveling to another planet and at least seeding it with life.
What might overturn
this simplistic conclusion is the finding that there was some very
difficult engineering problem that was related to extremely long
periods of time. For example, there will have to be some embodiment
of intelligence in the ship, in order for it to decelerate correctly
to the target planet, go into orbit, and prepare its cargo for
descent. It might be guessed that nothing organic would last that
long, so DNA and some entire cells would have to be created by
scratch once the ship gets to the target solar system. Can a thinking automaton be built that will continue to
correctly function for that length of time? We think of automation
as involving semiconductors used in computation and data storage.
These could be shielded from cosmic rays to a great extent, but there
would have to be something connected with the ship’s systems that
the shielded computation system would have to manage. How would the
external system survive? If a mechanism is conceived which will
unpack the automaton after a hundred thousand years, how is this
mechanism supposed to last? What system will be monitoring the
location of the target star and tell the ship when it is time to do
the final deceleration?
Thus, the very first
guess at what would be a very difficult engineering system might be the external
ship sensors and data processors for them. Can the ship be run as a
dead lump, with the entire automation apparatus enclosing itself
inside a suitable shield, and then emerging after exactly the right
amount of time? In other words, would it have to be engineered so
that the first part of the trip, the acceleration, and the final part
of the trip, the deceleration to orbit, would be done with the
automaton exposed, but for the large majority of the trip, inside a
shield? This means the propulsor would have to be much larger than
for a continuously operating trip, as all the acceleration would have
to be done in a short time.
How is energy going
to be stored for thousands of years and then become available? If
the automaton hides inside a shield for almost the whole trip, it
must have some energy inside to at least run the clock and figure out
when to open the shield, and then to run the mechanism to open it?
Or should the shield be a labyrinth instead of a box, with no direct
path to outer space which does not pass through a thick wall, but
plenty of ways to snake connections out?
It seems that
building a ten thousand year ship can be considered today, and that
our knowledge of science and engineering might be enough to produce
some ideas that will help us determine if such a ship could be built
– a sort of feasibility thought-experiment. Such a
thought-experiment would help alienology determine better if there
are any very long term experiments that would have to be run.
Another interesting challenge...
