Interstellar Archaeology on the Galactic Scale

The European Planetary Science Congress ends today in Rome even as scientists and engineers on the astronautical side of things head for Prague, where the International Astronautical Congress convenes on Monday. I’ll be keeping an eye on events in Prague and wishing I could join the gathering of Tau Zero practitioners that will be taking place there — Marc Millis will be presenting four papers, and many of the Project Icarus team members are also making the journey, so we should be getting regular updates on matters interstellar.

Nor do I want to neglect the Royal Society meeting on extraterrestrial life, coming up early in October in Buckinghamshire in the UK. Emails from James Benford (Microwave Sciences) and Richard Carrigan (Fermilab) tell me both will be speaking at the session, which reminds me that it was way back in April that I promised more on Carrigan’s notions of interstellar ‘archaeology,’ a form of SETI that makes no assumptions about the originating civilization. It’s time to honor that pledge by looking at the kind of artifacts an advanced civilization might create that could be detectable. Today I want to focus on an extreme possibility, a civilization that spans its galaxy.

Kardashev Type III and Its Traces

What would happen if a true galaxy-spanning civilization went to work on the kind of astroengineering it would be capable of? We call this a Kardashev Type III civilization, one that could exploit the power resources of an entire galaxy, and my assumption has always been that such a culture would be blindingly obvious, its projects so vast that our astronomers would be able to detect them by noting anomalies that could hardly be natural occurences. Imagine, for example, a galactic culture that encloses each individual star in a Dyson sphere.

A Dyson sphere or ‘shell’ would absorb all of the visible light from a star, re-radiating stellar energy at infrared wavelengths. A Dyson ‘ring’ would use planetary materials that would mask only part of the star’s light. Dick Carrigan used a list of interesting infrared sources from the Infrared Astronomy Satellite (IRAS) in his own search, coming up with no strong Dyson sphere candidates. But Dyson spheres remain interesting, vastly increasing the habitable area around a star. What would a Type III civilization do with technologies that could create Dyson spheres not only in one place but across the galaxy?

Image: M81, a spiral galaxy in Ursa Major. A ‘wavefront’ Dyson sphere culture might spread across such a galaxy, causing stars to drop out of visible light entirely, one by one, to be detected in the infrared. Credit and copyright: Giovanni Benintende.

Whatever the answer, you would think it would be clearly noticeable. Freeman Dyson himself has said that “…a type III (Kardashev civilization) in our own galaxy would change the appearance of the sky so drastically that it could hardly have escaped our attention.” In a recent paper, Carrigan also quotes James Annis, who has studied anomalous galaxies in quest of signs of a Type III civilization. Annis’ view: “It is quite clear that the Galaxy itself has not transformed into a type III civilization based on starlight, nor have M31 or M33, our two large neighbors.” But Carrigan wonders whether we should take these statements as definitive:

…what would happen for a civilization on its way to becoming a type III civilization, a type II.5 civilization so to say? If it was busily turning stars into Dyson spheres the civilization could create a “Fermi bubble” or void in the visible light from a patch of the galaxy with a corresponding upturn in the emission of infrared light. This bubble would grow following the lines of a suggestion attributed to Fermi… that patient space travelers moving at 1/1000 to 1/100 of the speed of light could span a galaxy in one to ten million years. Here “Fermi bubble” is used rather than “Fermi void”, in part because the latter is also a term in solid state physics and also because such a region would only be a visible light void, not a matter void.

A Wavefront of Dyson Spheres

This is long-term thinking in the richest sense. We currently have spacecraft moving out of our Solar System at a speed of roughly 0.0001 of the speed of light. If we stay well within physics as currently understood, we can expend 100 times the energy to raise that velocity from 42 kilometers per second to 400 kilometers per second, in the neighborhood of 10-3 c. Now we’re talking galactic travel times on the order of a hundred million years and less, half the time the Sun takes to circle the center of the galaxy. A patient, long-lived civilization could, as Carrigan notes, envelop a galaxy on a time-scale comparable to or shorter than the rotation period of the galaxy.

Turning stars into Dyson spheres as it went, such a culture should leave vast Fermi ‘bubbles’ whose infrared signature would flag their existence. But detection might still elude us. Carrigan points out that we see M51, the Whirlpool galaxy, face-on at a distance of 30 million light years. We can say with some confidence that we see no unexplained voids larger than about five percent of M51’s area, but any void features below this level would be hard to identify because of spiral galaxy structure. Elliptical galaxies might be better places, says James Annis, to look for Fermi bubbles, because they display little structure, and potential voids should be far more pronounced.

Image: The elliptical galaxy Centaurus A. Are ellipticals the best place to search for Type III civilizations? Credit: NASA/CFHT.

Even so, separating artificial structure from natural phenomena is not easy. I was fascinated to learn from Carrigan’s paper that we have identified several relatively dark dwarf galaxies, including Virgo HI21, the Ursa Major dwarf spheroidal UMa dSph and others. Anyone making the case that these objects show evidence of Dyson sphere activity runs into more plausible natural explanations. Virgo HI21, for example, may be showing the effects of tidal shredding. These so-called ‘dark galaxies’ demand a thorough understanding of star populations and types and exquisitely precise radio astronomy measurements to help fix the rotation rate. What might have seemed an easy search for a Dyson infrared signature becomes exceedingly complicated.

When Annis ran his search for anomalous galaxies, he took advantage of the fact that galactic optical brightness can be measured against the maximum rotation velocity and radius of the galaxy. The relationship between these factors is relatively consistent, allowing the scientist to look for sources below the normal trend line in a sample of 137 galaxies. Annis found no candidate galaxies in this admittedly small sample, but the work allowed him to set a limit on dark galaxies below one percent of the population. Larger-scale searches will doubtless be attempted one day.

The Uses of Interstellar Archaeology

Even though we have only begun to tune up our methods for detecting an extraterrestrial civilization, there is much we can learn. Our existing SETI methods and near-term observations of exoplanets may be the best chance at finding a genuine life signature elsewhere, but we should keep the possibility of a truly advanced civilization in mind. Says Carrigan:

Breaking up a planet to build a Dyson sphere completely transcends the will, ability and long term dedication of contemporary civilization. There is not even a mildly plausible technique for stellar engineering. Surprisingly the step from a Dyson sphere to a galaxy filled with Dyson spheres only involves a strengthened capability for space travel. On the time scale of Dyson Sphere construction this type of relatively slow space travel can be discussed even now. To recapitulate, SETI and the search for planetary signatures of life and intelligence currently appear to be the most accessible signatures. For Dyson spheres and stellar engineering work should mostly be directed toward investigating engineering techniques and limitations.

I come back around to the premise behind interstellar archaeology, that unlike conventional SETI it does not require a civilization to have any intention of contacting us. There are numerous ways to proceed, involving the kind of Dyson sphere search Carrigan has himself conducted within our own galaxy, or looking at planetary atmospheres in hopes of finding not only biosignatures but the markers of an advanced industrial or post-industrial culture. As we continue the SETI hunt, keeping in mind how planetary change or deliberate decisions to expand into the galaxy could leave visible traces allows us to hunt for things advanced intelligence might do.

How many civilizations in our galaxy, for example, have already faced the end of their main sequence star’s lifetime? If the number is high, it may be that we can find evidence of their response in the form of planetary or stellar engineering, making stars of this description interesting targets for future searches. In any case, our model of SETI is changing as not only our technologies but our assumptions become more sophisticated, leaving us to ponder a universe in which the need for expansion or simple survival may have left its own detectable history.

The paper is Carrigan, “Starry Messages: Searching for Signatures of Interstellar Archaeology” (preprint). The Annis paper is “Placing a limit on star-fed Kardashev type III civilisations,” JBIS 52, pp.33-36 (1999).