#bracewell probe

First Addendum on “A Hyperbolic Trajectory Through the Drake Equation” 

In The Unseen Podcast episode, “A Hyperbolic Trajectory Through the Drake Equation” touched on the topic of Bracewell probes, an interstellar communication strategy due to Ronald Bracewell (cf. "Communications from Superior Galactic Communities," Nature, 186, 670-671, 1960). I said that the motivation for the Bracewell probe idea is to get around the time delay of communication by radio or optical SETI. Most SETI pioneers believed that interstellar travel would always be impossibly difficult, so that the only realistic chance for communication between civilizations separated by interstellar distances was through long distance communication.

Long distance (interstellar) communication, however, involves a time delay due to the finite speed of light (which is also the speed of radio waves, as both light and radio are forms of electromagnetic radiation). If another civilization were in the Andromeda galaxy, we would have to wait five million years for a single back-and-forth communication. This is obviously a non-starter for a technological civilization that has become accustomed to instantaneous communication.

Bracewell’s way around this was to suggest an interstellar probe with a rudimentary level of AI.The probe could be sent to other promising planetary systems, there to lurk unseen until a local civilization made itself known as a technological civilization, capable to communicating. The Bracewell probe could then hold some kind of limited conversation in real time with an intelligent species having reached what the probe designers programed as the technological threshold for communication. This threshold might be set as the first detection of an artificial radio signal, the first use of a nuclear weapon, or the first SETI message sent from a radio telescope. 

In Radio Technology and Existential Risk I referred to “Sagan’s Thesis” as the view that radio technology and nuclear technology are tightly-coupled, so that any civilization that develops the one will likely also develop the other, and moreover that these developments are likely to be close in time. From a “big picture” perspective, it isn’t much of a relaxation of the scope of tightly-coupled technological developments to include Bracewell probes in that cluster of technologies that emerge at about the same time: radio telecommunications, nuclear technology, radio telescopes, and Bracewell probes, inter alia.

This follows because we are close to being able to build a Bracewell probe -- maybe within a few decades, and a hundred years at most -- and we are not far removed in historical terms from the origins of radio technology. Thus all these technologies are likely to occur within a window of 100-200 years, which from the perspective of cosmological scales of time, is about the same time, in other words, these developments are tightly-coupled.

It would take hundreds to thousands of years for the first rudimentary Bracewell probes to filter through our galaxy, and longer for later generations of Bracewell probes to arrive at other galaxies, but the process of real-time communication with other civilizations (by proxy, through AI) would have been initiated. This can add further weight to Sagan’s Thesis, as the technologies that become available shortly after the invention of radio technology and the science that goes along with radio, rapidly cascade to become radio telescopes and Bracewell probes,and in a universe in which these technologies are tightly-coupled, it is very difficult to explain why we have seen to trace of other civilizations. 

A Bracewell probe is a hypothetical concept for an autonomous interstellar space probe dispatched for the express purpose of communication with one or more alien civilizations. It was proposed by Ronald N. Bracewell in a 1960 paper, as an alternative to interstellar radio communication between widely separated civilizations.

The Black Knight Satellite Conspiracy

Von Neumann machines or Von Neumann probes are a particular class of self-replicating spacecraft, which, sent into the cosmos, arrive at planetary systems and use the resources of each target planetary system to replicate themselves and send out further Von Neumann probes in turn, which would then expand exponentially through the universe. A Von Neumann probe might have additional imperatives beyond self-replication and expansion, such as communicating with any civilizations found in planetary systems at which probes arrive (in which case the Von Neumann probe is also a Bracewell probe), or carrying life in some form in order to “seed” target planets, possibly after terraforming them (in which case the Von Neumann probe is also a seeder ship, and may play a role in anthropogenic panspermia).

In my post Cosmological Niche Construction I suggested the possibility of Von Neumann machines that could have the additional imperative (beyond replicating themselves) of transforming each target star into a Shakdov thruster, which could move the star (including the planetary system associated with the star) to some predetermined location.  

The idea of a Shkadov thruster was introduced in “Possibility of controlling solar system motion in the Galaxy” by L. M. Shkadov (IAF, International Astronautical Congress, 38th, Brighton, England, Oct. 10-17, 1987), and now the idea is placed among other “stellar engines.” I haven’t yet gotten a copy of the original Shkadov paper, but the abstract is available online:

“The possibility of developing a thruster for the solar system motion control in the Galaxy is considered. It is shown, that if a screen reflecting solar rays is positioned stationarily at some distance from the sun, the central symmetry of solar radiation in the sun-screen system will be violated and a force disturbing the sun motion will arise. The evaluation of screen surface density providing a stationary position of the screen is obtained on the assumption that the attractive forces and light pressure are equal. A disturbed motion of the solar system under the action of the above force is considered. It is shown that during one orbital period of the sun a radial deflection of the sun from its reference orbit by the value of some 10-12 parsec is possible. Lateral deviation of the sun by 4.4 parsec from its orbital plane is also possible, when the screen axis is normal to the orbital plane and has a constant orientation. The possibility of the earth transfer from the solar system onto a circular orbit of another star is shown.” 

Given the exponential nature of Von Neumann machines, a relatively small initial swarm of Shkadov-producing Von Neumann probes sent out into the galaxy could, within a relatively brief period of time (”brief” according to cosmological scales of time), re-structure the galaxy -- within limits. Small and dim stars would be moved more slowly, and black holes could not be moved in this way (though they might be moved by transforming them into engines employing the Penrose process). Moreover, as noted in the abstract above, there are limitations to the amount of deviation possible even for bright, sun-like stars.

Nevertheless, a galaxy could be significantly reshaped over cosmological scales of time, and any supercivilization surviving over cosmological scales of time could more efficiently manage its stellar resources in this way. The imperative programmed into the initial swarm of Von Neumann probes could include instructions for the redirection of the target stars in relation to other stars (or other astronomical features, such as pulsars) so that a coordinated action was produced spontaneously across all affected stars, without the need for direct communication between Von Neumann probes. Once a coordinated motion of stars emerged over some billions of years on the basis of exclusively visible tracing of starlight, a more robust and sophisticated network of EM signals could be set up if more complex coordination were desired. But the EM signals would move no faster than starlight, so there may be no need for this.

While the above speculation that black holes might also be moved by a stellar engine powered by a Penrose process is an attractive idea, it is also a very limited idea. One can imagine the entire universe being reshaped by taking control of the supermassive black holes at the center of galaxies, which hold together entire galaxies by their gravity, but this would have to take place over a scale of time that would exceed the scale of time of the disappearance of other galaxies over the cosmological horizon (i.e., the “end of cosmology”), so this would be useful only if the power harnessed by the Penrose process could overcome the force propelling the galaxies apart.

(I found several similar ideas to the above in Possible modification of the Shkadov thruster and its prospects to solve SETI problems. by Dmitry Novoseltsev.)

Assuming that it would be no easy task to re-engineer the universe, re-engineering the galaxy -- and eventually the local group of gravitationally bound galaxies that will coalesce -- remains a possibility, and any supercivilization surviving over cosmological scales of time would want to fashion its remaining galaxy -- which will then be an “island universe” -- so as to most effectively and efficiently exploit the resources available to it.

What I have described here is the antithesis of the perspective of “rewilding” extrapolated to a cosmological scale. Arguably, cosmological rewilding would coincide with an idea that I found in Karl Schroeder: “Any sufficiently advanced technology is indistinguishable from Nature.” Rachel Armstrong has also written about this, as has Kevin Kelly. 

The dramatically different visions of a possible human future offered by re-engineering the universe we inhabit, on the one hand, while on the other hand a civilization so advanced that its footprint is not even visible on the universe, points to our own recent history, and especially lessons we have been forced to learn because of industrialization. Industrialization has dramatically transformed the terrestrial environment, and as the most technologically advanced civilizations make the transition to sustainable and renewable resources, their footprint upon the biosphere will become less evident than it was during the dirtiest phases of industrialization.

We can imagine a future in which the human presence upon the planet and upon the biosphere becomes even more subtle, and the only sign of us would be a few cities that take up relatively little of the Earth’s surface, with most of the planet (and not just half of it, as E. O. Wilson urges) returned to wilderness. We can also imagine a future in which the entire planet is returned to wilderness, while humanity and our industry and technology move into space. Under this scenario, Earth is a vacation spot, but no longer a locus of industrial production. Also under this scenario, our presence might be as masked to the rest of the universe as we cared to make it. And there is also the scenario in which all wildness is extirpated from Earth, and our presence here is painfully obvious.

Each of these scenarios has a parallel for a supercivilization of cosmological scale. The universe might be mostly wild, with only a few obviously inhabited planets, or intelligent agents might leave planets to evolve on their own, entirely abandoning the model of planetary endemism, or inhabited worlds might be very obvious wherever they occur.

While I personally think that the scenario in which planets are entirely wilderness is optimal both for any and all planetary biospheres, as well as for civilization, I do not think that this is likely. It seems much more likely that the civilization that managed to survive over the long term would retain a robust connection to its origins in planetary endemism, which would at best converge on the first scenario, in which an intelligent species is able to limit its footprint on the biosphere of its homeworld, but would not eliminate it entirely. Such a civilization would be difficult, but not impossible, to discover, unless it actively took steps to conceal its existence.

But it would take only a single successful technological civilization launching a single initial swarm of Shkadov-producing Von Neumann machines in order to significantly alter the shape of a galaxy or galaxies into something obviously artificial. As we have not yet identified a single obviously artificially reconfigured galaxy, we can infer that not a single civilization of this kind has preceded us in the universe within our lightcone (qualified for our ability to discern actual galactic structure; many galaxies are for us only a single point of light, so that even if they possessed an obviously artificial structure, we would not know it).

While we have not yet seen an obviously artificial galactic structure, the possibility suggests a research program: what might obviously artificial galaxies look like? How would a supercivilization want to manage its stellar resources for its own use? In each case of an artificially reconfigured galaxy, there would have to be a pathway from one of the well known naturally-occurring galactic structures -- elliptical, spiral, and barred spiral -- to an artificial structure. Given the limitations to the kind of stars moveable by a Shkadov truster, and the amount by which they can deviate, naturally-occurring galactic structures impose a strong constraint upon artificially reconfigured galaxies. Anyone with the appropriate physics background could work out a range of configurations that could be identified as obviously artificial.

It was a pleasant little planet. Third of nine, orbiting a yellow star a thousand light years spinward. They’d barely made their way into space; a thin flotsam of satellites surrounded the planet, and not much more.

When our probe made orbit, there was panic, consternation, all the usual reactions. Using their radio frequencies, we answered their questions. Yes, we were from another star. Our probe had travelled thousands of years to reach them. No, as far as we knew, traveling faster than light was impossible. 

But communicating faster than light was possible.

Would they like to join our network?

Then we waited. 

We waited as their planet orbited their star four times. We watched, and saw the atmosphere darken with the smoke of their burning cities.

Eventually their answer came. The survivors would join our network.

Bandwidth undreamed lit their world like a second sun. We gave them the knowledge they needed to rebuild. 

Now we live together, them and us and sixteen other species. 

They live their lives amongst our machines, in cities built on our designs, watched over by simulacra of our minds. Through the network, we share a consensual reality so many times more real than life on their planet could ever be. A fading atavistic underground still clings to their benighted old ways like hunter-gatherers protesting against writing, in the shells of their ruined cities. But on the whole our union is happy.