Friday, November 23, 2012

Searching for Bracewell Probes - part 1

Ronald Bracewell
Bracewell Probes were first discussed by Ronald Bracewell in a 1960 Nature paper. The basic idea is send robotic probes to solar systems that seemed promising, and these probes would sit and wait for millenia looking for signs of intelligent life in those systems, and should those intelligent creatures ever reach a certain measure of sophistication, then either radio home for instructions, or simply provide a library of advanced knowledge that would help these lowly beings achieve a kind of galactic citizenship.
that the "Superior Communities" would

Bracewell Probes still seem like a sensible way for superior communities to reach out in their galactic neighborhoods, whether their intentions are benign or defensive. Rather than guessing when to send probes to check out a suspect solar system, they simply strike out for all of them in the neighborhood, and then wait. Would their purpose be, as Bracewell conjectured, to enlighten us, or simply to alert the rest of the superior community that here there be uppity apes?

If the probes were also self-replicating as I would expect, they could spread throughout the galaxy, and there could be one or more such probes in every promising solar system. Moreover, there could well be multiple probes originating from each of several home worlds; some defunct, others active, and possibly unaware of each other's presence. The likely arrival window would be any time in the last 2.5 billion years - roughly since the time Earth had detectable oxygen in its atmosphere, and possibly sooner, depending on what was happening on Mars or Venus. Early arrivers would probably have been pounded into inoperability by now from meteor hits, radiation, and just good 'ol fashioned entropy.

But, as always, the interesting question is how do we find them if they are here?  We need to know both where and how to look.  The solar system is larger than we intuit.  So large, that just poking around is overwhelmingly likely to fail.  We need to define possible observables for Bracewell probes primarily in terms of the electromagnetic spectrum in order to make a search at all practicable.

There are so many variables in how they would be implemented that it is a daunting problem to estimate the parameters of a search. Here are just some of what we need to consider:
  • How many probes are there in our solar system? The answer could be anywhere from zero to a large number that would depend upon the other variables below.
  • Are they diverse, or essentially all the same design?
  • Are they self replicating and repairing? If so, they would need materials. A low energy cost for rendezvous with small bodies may drive them to the asteroid belt.
  • Whether self-repairing or not, how long would they be designed to last?
  • Do they harvest local energy, or bring their own power supply with them?
  • How large are they? The could be tiny if they were just dropped off here, or, if they still have their propulsion systems attached, very large - perhaps kilometers in size, or even larger.
  • How far from the sun would they need to orbit? Would they want to get close to the terrestrial planets (billions of years ago, Venus or Mars may have looked more viable than Earth). Or, perhaps they would park themselves in a high elliptical orbit, only swooping by the inner solar system from time to time to check on things?
The Basic Scheme of Influence Diagrams
No doubt you can think of a few other things I omitted. These tangled hierarchies we encounter in real life.  I prefer influence diagrams, which show how variables are related without assuming a false hierarchy.   The arrow semantics in influence diagrams are simple and intuitive - the target is influenced by the source. These diagrams are used to communicate decision framing and structure, but in this case the decisions have already been made, and we are trying to reconstruct what they were based upon our best understanding of the inherent logic.
questions suggest something like a decision tree of sorts, although I generally resist decision trees because they are either too simple or too complicated and in any case, not robust to the

Here are a few things I think we can take as working assumptions until we know more:
  • We haven't yet found a Bracewell Probe (please correct me if I'm wrong about that!).  This places weak constraints on their size and how "noisy" they are in the electromagnetic spectrum.
  • The probes would not need to land or even enter the atmospheres of planets, although there could be a secondary mission to do that.  Everything you would need to know about whether a planet harbors technologically sophisticated organisms can be learned by remote sensing from orbit.
  • It would not be necessary to visit each planet frequently.  Even one per century flybys might yield sufficient information to tell what is happening on each planet.  However, once it became clear that Earth was the only good candidate (probably billions of years ago), it might make sense to redesign for more frequent surveillance of the single planet.
Notice that we haven't even explored the question of how and when a functional Bracewell probe or probes would decide to contact us, if at all.  That's an extremely interesting question, but the first question I want to ask is how we find them.  If we can get that far, the decisions after taht are good problems to have.

In the next entry we'll use the influence diagram to tease out some reasonable assumptions (with big error bars) to come up with at least one set of observational parameters that suggest where to look.  In all likelihood, this will be wrong, but the process of looking will teach us much that we can't anticipate.

Let me know what you think in the comments.

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