Friday, June 26, 2015

Interstellar Communication

With distances between possible habitable planets or even between resource-rich planets used for stopovers or as some other type of outpost being of the order of tens of light years, communication between planets in different solar systems is of a very different character than communication within a solar system.  In a local environment like a solar system, travel times are a few hours at most, and may be only minutes for close-in planets.  In this environment, communication back and forth is little different than communication via certain media on a single planet.  For us, it would be like getting emails.  But with, say, 20 years between messages, communication like this is absurd and therefore, only a few special types of situations would involve it.

One situation that has already been discussed is exploration and colonization.  The least expensive way to get initial close-in information about a potentially useful exo-planet is to do a high-speed fly-by.  Then the fuel needed to decelerate the probe is not needed, and this drops the weight and size requirements considerably.  But unless the probe can communicate back to the sending planet, and transmit quite a lot of data, the probe would have to decelerate and return home with its collected information.  No fly-by means a lot more expense, including design work, testing, equipping and so on.  So it would be important to see if communication between different star systems is possible, and if it is, is it possible at a data rate high enough to be useful.

A consequent question is how big and massive a communications package would have to be.  For an sufficiently intelligent autonomous probe, there would be no need to make a receiver on the probe, only a transmitter to send the collected data back to the originating planet, and perhaps some status signals to let them know that things are going as planned and that there is no need to send a replacement.  If the equipment, including pointing control, antennas or integral transmitters, signal generators, power supplies and fuel, and anything else an advanced civilization would build into the system, is more massive than the burden involved in deceleration and return of the probe, then communication is not necessarily a good option.  It does cut down the time, as a probe that decelerates starting half-way through its voyage will have a longer travel time than one which accelerates all the way there or which cruises for the second half of the voyage, and getting the message back by speed-of-light communications is much faster than having the probe make the return voyage home.  The difference in time is measured in centuries. 

It is hard to make analogies between Earth’s situation and that of an advanced civilization, past the asymptotic technology transition.  Nevertheless, here we have grown accustomed to waiting years for an interplanetary probe to reach its destination and communicate back pictures and scientific measurements of the planet.  The Pluto probe, New Horizons, has a nine-year flight trajectory.  Perhaps an advanced civilization, with probably markedly greater longevity, could tolerate a 400 year probe return with no repercussions.  Such a civilization should be completely stable, so questions of whether an organization to manage the probe or even a nation might exist at the return time of the probe are not relevant.  If the civilization is one with an expansion meme, there might be some pressure to take the fastest solution, but the launch would likely be after a long period of stability, and so social pressure for anything may be almost non-existent and the best solution for the probe design, measured in reliability, data capture capability, total mass, consumption of resources, or whatever matters, should be the basis for the choice of design.  In other words, nothing much changes in the society so there is no hunger for faster change.

The other situation previously discussed that could make very good use of interstellar communication is that of deterrence.  The timing of this is unlikely, but it could be possible that there are two interstellar civilizations in proximity, and at least one of them is predatory, by which we mean that it would like to eradicate the other and occupy its home world or collection of colonized planets.  The other civilization, either being predatory and in conflict with the first, or non-predatory and wanting to be left alone, might want to depend on deterrence to prevent the first from carrying out the eradication.  It seems that one likely type of weapon for interstellar conflict would be large explosive devices, and the second civilization would want to detect the attack on itself with enough warning so it could do what it could for passive defense, and what it could for active defense, but also counterstrike.  With a counterstrike capability, some deterrence exists. 

Two different situations can be distinguished.  The two civilizations could exist in about the same numbers, meaning worlds occupied and inhabited, or one could be substantially larger than the other.  Remember that there is not likely to be any trade whatsoever between the different worlds with the same origin planet.  The economics are prohibitive for resources transfers, and all the worlds have the same background, and asymptotic technology has penetrated art and anything else that more primitive cultures might trade, so that there is nothing traveling between the worlds with the same origin, those of one civilization entirely, and they are roughly independent.  There may be traditions, but governing a planet from 20 light years away is hard to envision.  

The situation where one civilization is on the edge of the expanding wave of colonized planets of the other, perhaps older, civilization is more intriguing.  The simpler case should be discussed first.

If there is only one planet or solar system each for the two civilizations, deterrence might be done by having surveillance posts somewhere in each other’s solar system, monitoring for the launch of attack vessels in the direction of the other world.  The surveillance posts can be covert, if possible, as destroying them would disable deterrence and allow an attack to proceed with much less warning.  Alternatively, they could be known and built under some agreement.  But agreements can be broken and they could be destroyed.  Thus, there are some unique requirements for a surveillance station.  It should, first and foremost, be designed to provide a warning signal with a very high confidence.  The signal is something that says “Attack” when an attack is detected.  The station needs to be defended or at least its neighborhood monitored for incoming vessels or beamed weapons so that an attack on the station cannot be done without the “Attack” signal being issued with high reliability and high confidence that it will be received.

Since the attacks take decades to conclude, while the explosive devices travel to the other civilization’s planet, there is plenty of time to act.  Similar explosive devices could be launched toward the other civilization’s planet.  This is the essence of what we used to call, during the Cold War here on Earth, Mutual Assured Destruction.  If it works, no one attacks.

To guard against the possibility of a surprise attack being successful on the surveillance monitor station, some things might be done.  One is to have several, each monitoring the others.  Another is to have ‘Peace’ messages send out from each surveillance station to the origin world.  There would have to be sufficient encryption on both the ‘Peace’ message and the ‘Attack’ message that they could not be duplicated, and of course, high degrees of security so that the cryptography could not be compromised.  This also provides a defense against a cloaking attack, where the predatory nation detonates something to make a cloud of absorbing particles on the line of sight from the surveillance station to the origin world.

The requirements for interstellar communication between a surveillance station and its home world are quite different than for that of an expeditionary probe.  A coded message must be sent, perhaps several times a year, only containing a single binary bit of information: ‘Peace’ or ‘Attack’.  The power transmission requirements might be the same, or perhaps of higher power for more assurance with surveillance.  A probe can be done over.  A defense cannot.

The alternative situation described above, where one civilization is outnumbered by the colonized worlds of the other, calls for some discrimination.  Which of the worlds of the larger civilization does the threatened civilization point its deterrent weapons against?  Can it provide surveillance stations on all of them?  Is there some distance limit, so that it monitors those within 40 light years but not the other further ones?  Does it need to provide deterrence against a simultaneous attack by multiple worlds of the larger civilization? 

These two scenarios provide us with some ideas of what the requirements for alien interstellar communication are.  Is it possible, with the limited knowledge of technology we have, that some conclusions can be made about what is feasible for meeting these requirements and what is not?  It remains to be seen, but not in this post.  However, if we knew, we might have a better idea of what to look for in the galaxy as a signature of an advanced civilization and therefore this is an interesting area to explore.  

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