Friday, March 23, 2018

Experiments Before Star Voyages

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...

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