Last modified: 22/4/2006
The Search for ExtraTerrestrial Intelligence (SETI) project makes the assumption that an advanced civilisation had something to say in a similar sense that someone trapped on a desert island might have had something to say when putting a message in a bottle to float on the ocean. The probability that another intelligent being intercepts that bottle before it perishes on a shore line or sinks is small to say the least. The probability of receiving a bottled reply in the lifetime of the sender makes the former probability look inevitable.
In the words of the late Douglas Adams, "Space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the drug store, but that's just peanuts to space." To put that into context with objects we are familiar with say, stars (the Sun is about 1.4 million kilometers in diameter and is an average sized star), they have a density in our galaxy of about one in every 16 cubic light years or about 300 trillion trillion trillion cubic kilometers. They also have a finite lifetime. The Sun for example is expected to have a total life expectancy of 10 billion years. So although they are somewhat sparse, stars do last for a significantly long time, and are obvious amongst all that black making them a most suitable luminescent bottle to put a message in. What that message would be is beyond the scope of my feeble mind but if they had a sense of humor it might just be, "Hi!" or if they're not expecting a friendly reply but an invasion, "Trespassers will be obliterated." In any case, stars represent a natural lantern to broadcast from by modulating the light (Electromagnetic Radiation) emitted. This is suitable for sending a message around a galaxy in a reasonable amount of time with the hope of receiving a response, for example our galaxy is about 100 thousand light years wide, so in other words it would take 100 thousand years to send a message by EM from one side to the other. Along a given line of sight, many stars would be obscured and their signal distorted for many reasons. So for a determined broadcaster, as many stars as possible need to be converted. Never the less, it is perhaps reasonable to suppose that the galactic center be impassable to even stellar lanterns capable of focusing their signal unidirectionally.
Galaxies however, are in the order of millions to billions of light years apart and the time taken to send a signal between them might start infringing on the expected lifetime of a civilisation if indeed advanced civilisations have a lifetime. A message sent across these distances might say something like, "By the time you receive this, we'll have long been extinct. The following message is a compendium of all our knowledge, with any luck you'll last a little longer than us..."
The rest of this discussion is concerned with a feasible mechanism by which an advanced civilisation would construct a giant semaphore array and has been organised into phases.
There is a universal speed limit and that's the speed of light (in a vacuum) or 299,800 kilometers per second. For an object with mass (this is the usual definition of an object) to approach this speed takes a extraordinary amount of energy. This is because of the effects of relativity: as an object is accelerated and given more and more kinetic energy, it's velocity and it's mass both increase. But while the velocity will never quite reaches 299,800 km/s, the mass can continue to increase without limit. The higher the velocity, the more disproportionate the distribution of input energy: more goes into making the object more massive and less to make it go faster. That's the best explanation in English I can give. The point being, going to a stellar system quickly is like going to the toilet at a rock concert: it is a process of diminishing returns for the effort exerted. Ultimately one just has to wait unless the object can be made massless; in which case they would no longer be objects that we are familiar with.
To convert stars in the neighborhood of a civilisation into a semaphore, that civilisation needs to send stellar semaphore construction devices (SSCD). These devices are assumed to be objects with mass. Since the nearest stars are likely to be up to tens of light years away, to have them arrive say, about a century after launch they would need to travel at a practical speed of about one tenth the speed of light. In calculating the arrival time, the time taken to accelerate and decelerate the SSCD to a nominated velocity needs to be taken into account also. Now the greater the acceleration, the less time is taken by an object to reach the desired velocity but the greater the force experienced by the object. That's why driving a car fast won't kill you, it's the force exerted on you that's associated with the rapid deceleration that does. It is assumed that an advanced civilisation would be able to achieve high velocity in an insignificant quantity of time: one might imagine a launch device based on a principle not unlike that of a cannon. In contrast however, if the mechanism for propulsion was an on-board propellant that happens to exhibit the property of mass (and all currently known to humans do), it is easy to see that this would be another case of diminishing returns. So unlike a colony spaceship that contains fragile cargo that would need to accelerate gently and then decelerate gently into the relatively slow orbital velocity about a planet or star, a SSCD is assumed to slam directly into a planet and convert its considerable kinetic energy into obliterating the planet. It is assumed that the advanced materials that make up the SSCD will survive the impact or that the canister described in phase 2 would be jettisoned shortly before impact.
There are two benefits for such a violent strategy. The first is that the time taken for the SSCD to match velocity with the designated star system is small. The second is to disperse the material that makes up the planet over a larger area of space such that the resultant density approximates the gravity-well needed for phase 3 to be successful.
The progenitor contains miniature automatons or biological robots perhaps on a micrometer scale or microbiots for short. It is assumed that almost immediately after impact the progenitor releases the microbiots as they are expected to cope with the extreme temperatures of their surrounds.
'Feeding' on the planet debris, the microbiots replicate whilst assembling themselves into a dendritic structure. This structure helps to capture more planet debris to continue replication and would be similar in function to a spider web. The energy resources required for this phase abound: there is the radiation given out from the nearby star as well as the kinetic and thermal energy of planet debris.
Not all atoms in the planet debris would necessarily be suitable for replication. What would effectively be microbiot excrement, would not go to waste but help to form the dendritic structure and help protect the microbiots from the damage caused by high velocity meteors (debris). It is perhaps a subtle distinction but an important consideration that the molecular bonding arrangement of the microbiot need only exhibit (1) a high degree of elasticity and have (2) a high tensile strength in this low density environment as opposed to requiring (3) a high compression strength if in the case the planet were not destroyed and the microbiots attempted phase 4 inside the gravity-well of the planet. Note that impact ejecta would continue to rain down on the planet for centuries as a consequence of phase 1 so the bonding arrangement of the microbiots would require that they possess all three properties. A good analogy is an Earth-bound tree which has a chemical structure that must exhibit all three properties if only to a modest extent.
The main objective of the dendritic structure is to eventually envelope the star in a shell concentrically. The shell would be largely transparent, letting through a modulated light signal that would constitute an omni-directional transmission of a message. Slightly oval in shape and rotating to create a balance between the centrifugal force and gravitational pull of the star as well as assisting with stability, the shell would orbit only slightly further away than that of the parent planet. Stability is also assisted due to the micrometer thick shell behaving similar to a balloon: pressure from stellar wind helps to keep it smooth and inflated. To encode a message on the stellar electromagnetic spectrum would not necessarily require that the shell form a continuous membrane. No doubt there would exist an optimum porosity that factors in the shell structural integrity, message signal strength, star type and phase 5.
As amazingly sophisticated as microbiots need to be to accomplish the tasks described, it is still likely that high energy stellar radiation would damage or destroy individual microbiots from time to time. After all, I am not proposing any new laws of physics: merely a possible highly sophisticated use of them although well beyond the technology of present day Earth. Thus, as complex as the molecules that make up the microbiots would need to be, they would still be susceptible to incident high energy particles that can potentially break chemical bonds. While this might disable a microbiot, it may also cause a mutation that might be termed 'cancerous' in that it prevents or distorts the desired outcome: a transmitter that transmits a specific message. Anything from consuming resources such that the shell never gets built to transmitting the wrong message could occur. This is the reason why I named them biological robots: they would need to perform biological processes such as error checking; replication and implement information redundancy; but they would also be robotic in that they would need to be engineered to prevent speciation. Presumably this would be a balance between information redundancy, the mutation rate and available resources.
Apart from microscopic damage, from time to time, meteors would exceed the collision threshold of the shell and punch large holes through it. It is assumed that the collision threshold is such that there is more meteor material captured and used to repair the shell than there is material lost from the shell due to impacts. Indeed, there should be a sufficient net gain of materials over time to repair damage from high energy particles.
The choice of star system for A SSCD is imagined to be far from arbitrary. Obviously the size and composition of an orbiting target planet is important but the presence of other orbiting bodies would prove influential as well. Within a sufficiently close proximity and being sufficiently massive, a spectator planet could warp or capture the growing or finished shell. Alternatively, a Lagrange point or some other gravitational topology caused by a spectator planet could be utilised help construct the shell. An asteroid belt could assist in providing additional resources. The age, size and Type of the star would be crucial to success.
Point your telescopes at stars! In the spectral fingerprint of a star might be a message something like: "Hi. We're extinct right now, but if you would like to leave a message orbiting a Type G2 star, please study the following crash course in advanced materials telecommunication."More essays by this author...