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SETI and Stellar Drift

It was natural enough that Richard Carrigan would come up with the model for what he called ‘Fermi bubbles,’ which I invoked in Monday’s post. A long-time researcher of the infrared sky, Carrigan (Fermi National Accelerator Laboratory, now retired) had mined data from the Infrared Astronomical Satellite (IRAS) in 2009 to mount a search for interesting sources that could be Dyson spheres, entire stars enclosed by a swarm of power stations, or conceivably wrapped entirely by a sphere of material presumably mined from the planetary population of the system.

Carrigan’s work on infrared sources goes back well over a decade, involving not only data mining but theorizing about the nature of truly advanced civilizations. If we were to find a civilization transforming a galaxy by gradually building Dyson spheres to exploit all the energies of its stars, we would be witnessing the transformation from Kardashev Type II (a culture that uses all the power of its star) to Type III (a culture that exploits its entire galaxy’s energies. Carrigan reasoned that areas of such a galaxy would gradually grow dark in visible light, the signature of the civilization’s activities becoming traceable only in the infrared.

Both Carrigan and the researchers in the Glimpsing Heat from Alien Technologies (G-HAT) project at Penn State point out that there are natural phenomena that could mimic the Fermi bubble. In a recent paper, the G-HAT team led by Jason Wright mentions a kind of galaxy known as a flocculent spiral as a case in point. Unlike the classic spiral with well-defined structure, these are galaxies with discontinuous spiral arms. What might be perceived as a ‘bubble’ structure here would almost certainly be a natural feature.


Image: NGC 4414, a flocculent spiral galaxy in an image taken by the Hubble Space Telescope. It would be tricky business to find the signature of a Fermi bubble here given the lack of definition in the spiral arms. A bright foreground star from our Milky Way Galaxy shines in the foreground of the image. Credit: Olivier Vallejo (Observatoire de Bordeaux), HST, ESA, NASA.

Galaxy in Motion

But I think the G-HAT critique of the Fermi bubble idea truly gains strength when we consider the motion of stars in the galaxy vs. the times needed for galactic colonization to occur. For we have to remember that when we’re dealing with a galaxy of stars over billions of years, we have to set the galaxy in motion. In a 2014 paper cited on Monday, Wright and company note this:

The static model of stars, in which a supercivilization can be said to occupy a compact and contiguous region of space, is a reasonable approximation for short times (≲ 105 years) and in the case of fast ships (with velocities in significant excess of the typical thermal or orbital velocities of the stars, so ∼ 10−2 c). In such cases, the stars essentially sit still while the ships move at a significant fraction of c and populate a small region of the galaxy in some small multiple of the region’s light-crossing time.

Remember that the shorter the period for colonization, the briefer the ‘window’ for finding a Fermi bubble. But would such bubbles be apparent even assuming the slowest possible expansion?

The G-HAT team’s work makes a compelling case that they would not. For longer times, and assuming slower ships, the static model fails and fails badly. Stars do not stay in one place, and galactic rotation muddles the works. The G-HAT paper considers what it calls ‘conservative timescales’ for a ‘slow’ colonization of the Milky Way. We can use this work to consider a maximum galaxy colonization time to give us a sense of how apparent galactic colonization would be. It also has ramifications, and significant ones, for Michael Hart’s view that we are alone in the galaxy, but I’m not going to stray from the Fermi bubble question in this post.

Imagine that a single spacefaring civilization emerges that uses colony ships traveling at 10-4 c, a speed not so different from our own interplanetary probes. Also assume a very slow launch rate, so that a ship is launched every 104 years, with a maximum range of 10 parsecs. This is slow travel indeed: The travel time to the nearest stars in this scenario is roughly 105 years, a time during which 10 more ships will be launched. The paper explains that this travel speed is comparable to the velocity dispersion of stars in the galactic midplane, a fact that brings new stars into range of the colony ships.

This is an expansion without pause because as the stars mix locally, a stellar system can continue to populate the ten nearest stars every 105 years:

To first order, the stellar system can thus continue to populate the 10 nearest stars every 105 years, without immediately saturating its neighborhood with colonies or the need to launch faster or longer-lived colony ships to continue its expansion. Further, arrival of the colony ships at the nearby stars should not be modeled as a pause in the expansion of the civilization. Rather, the colonies themselves will continue to travel at these speeds with respect to the home stellar system, and themselves encounter fresh stars for colonization every 105 years, during which time they can also launch 10 colony ships.

Using halo stars, which have high relative velocities in relation to the disk, for gravitational assists can provide a boost in cruise speed that allows higher speeds. We wind up with the capability of crossing the galaxy on a galactic rotational timescale. Here is a model of slow expansion that is anything but the uniform growth imagined in a static field of stars:

The slow expansion of an ETI should thus be modeled not as an expanding circle or sphere, subject to saturation and “fronts” of slower-expanding components of the supercivilization. A better model is as the mixing of a gas, as every colonizing world populates the stars that come near it, and those stars disperse into the galaxy in random directions, further “contaminating” every star they come near. If halo stars are themselves colonizable, then those that counter-rotate and remain near the plane will provide even faster means of colonization, since they will encounter ∼ 10 times as many stars per unit time as disk stars.

Here again we note the key fact that this stellar motion obscures any well-defined Fermi bubble:

Non-circular orbits also provide significant radial mixing, and Galactic shear provides an additional source of mixing that is comparable to that of the velocity dispersion of the disk stars once the colonies have spread to vrotv ∼ 1/10 of galaxy’s size, or ∼ 1 kpc from the home stellar system.

Remember, these are extremely conservative assumptions, and they still show that when a civilization begins to colonize its nearest stars, it will populate the entire galaxy in no more than 108 to 109 years. The maximum timescale for galactic colonization is found to be on the order of a galactic rotation (108 years) even for present-day probe speeds. This has implications for the detectability of Fermi bubbles, for on a rotational timescale, such bubbles will be subject to rotational shear and thermal motions that disperse and ‘mix’ them. Or as Centauri Dreams regular Eniac put it in a comment yesterday, “Such bubbles would be sheared into streamers in relatively short order. The spread of civilization would look more like milk stirred into coffee than a clearly delineated expanding bubble.”

The upshot here is that it will be only during a fairly brief period of a galaxy’s history that a spacefaring civilization will have populated only a single contiguous part of that galaxy. The length of that time depends upon how fast the civilization is capable of expanding — the faster the expansion, the shorter the time to observe the interim period between Kardashev Levels II and III. The transition between this era and the galaxy-spanning civilization to follow is, by galactic standards, relatively brief. And if we assume the slowest possible expansion, our Fermi bubbles would be quickly obscured by natural stellar motion within the galaxy. Fermi bubbles, if they do exist, are going to be exceedingly hard to find.

The paper is Wright et al., “The Ĝ Infrared Search for Extraterrestrial Civilizations with Large Energy Supplies. I. Background and Justification,” The Astrophysical Journal Vol. 792, No. 1 (2014), p. 26 (abstract / preprint). I consider the SETI work that Jason Wright and his colleagues Matthew Povich and Steinn Sigurðsson are doing with the Glimpsing Heat from Alien Technologies project to be ground-breaking, and plan to check in with it often.


Comments on this entry are closed.

  • John May 13, 2015, 9:43

    If I might take a different tack on that logic: The lack of visible K3 civilisations, combined with the lack of Fermi bubbles puts some limits on the maximum speeds of propogation for any Dyson sphere building intelligence, as well as their maximum energy use. That might be hinting to us something about the limits of interstellar travel speeds…

  • Alex Tolley May 13, 2015, 10:29

    This model seems to fit really well with 2 ideas raised in the past:

    1. That stellar drift opportunistically will bring stars closer together to shorten travel time.
    2. Slow worldships spreading through the Oort clouds are a possible route to interstellar travel. If ships will only leave every 10^4 years or so, they will need to be large enough to carry the full requirement to reestablish a high tech civ at the newly close target star[s].

    I also like this model as it doesn’t implicitly assume a cohesive KIII civ, just a lot of independent cultures slowly populating stars, and engineering Dyson swarms of habitats around those stars, some of which will become the next generation of worldships to migrate.

  • Alex Tolley May 13, 2015, 13:38

    The Wright paper is pretty comprehensive and invokes the “monocultural fallacy” to suggest that at some point, colonization will happen (and that looking for artefacts is worthwhile).

    But what if that is incorrect? It may be that colonization is done by seeding worlds with life and letting it evolve. In which case, almost all worlds will be non technical and we are back to the Drake equation with relatively few stars supporting a technological civilization. We could do that well before we launch worldships, if only to establish terraformed worlds in advance of [post] human colonization.

    Their other suggestion is that ETIs become invisible to us, which is also plausible if they become transcendent or their technology is”like magic” (Clarke’s 3rd Law).

    Good paper, well worth reading over coffee.

  • lepton May 13, 2015, 14:20

    Generation ships with an average journey of 10^5 years to colonize the hosting galaxy?

    There is no evidence that a civilization with advanced technology* can last longer than 10^5 years on a planet, let alone on a generation ship.

    * “advanced technology” means one psychopath can easily wipe out a population.

  • Marshall Eubanks May 13, 2015, 14:40

    Until recently, it was assumed that (much like the planets in the solar system) disk stars were not likely to migrate much radially (i.e., that the Sun was always or less 27,000 light years from the galactic center). Now, radial mixing seems to be more likely. That would mean that a Fermi / Dyson bubble wouldn’t just stretch out in galactic longitude, but radially as well. See

    http://arxiv.org/abs/1101.1202v1 for more.

  • Larry Kennedy May 13, 2015, 16:14

    “one psychopath can easily wipe out a population”
    Darwin in action. Those segments of populations that don’t find a solution to this problem, don’t advance.

  • Roy Clymer May 13, 2015, 17:08

    An ignorant question: from my understand of our solar system, there is no where near enough mass to build a shell surrounding the Sun. What am I missing?

  • kamal ali May 14, 2015, 3:39

    Larry writes “Darwin in action. Those segments of populations that don’t find a solution to this problem, don’t advance.”

    what if the probability of a civ surviving 10^5,10^6..10^8 years is very low. ie the prob of a civ remaining space-faring for more than 10^5 years is say < 10^-5. then for 100K civs rising, only 1 would make it to 10^5 age (in expectation). more generally, maybe not enough opportunities exist in 4.10^11 stars for a space-faring civ to survive to 10^6 years etc.

  • Brett Bellmore May 14, 2015, 6:20

    I would say that, while assuming chemical propulsion speeds for interstellar journeys is a propulsion-conservative assumption, given the demands this makes on the probe’s longevity, it’s not a net-conservative assumption. A genuinely net-conservative scenario would have to balance the difficulty of propelling the probe against the difficulty of it surviving to it’s destination. Given that an almost trivial use of nuclear propulsion, 1950’s style Orion, would allow the probes to travel about 100 times faster than your estimate, that becomes the genuinely conservative assumption.

    “there is no where near enough mass to build a shell surrounding the Sun. What am I missing?”

    There’s not enough mass to build a super-massive shell around the sun, such as described in some works of fiction. There’s many times more than enough mass to build a statite array that completely converts sunlight to human use.

  • Tim May 14, 2015, 8:57

    I’ve always had a problem with the idea of Dyson Spheres. Not just from the obvious problem of there not being enough resources to hand, as mentioned by Roy above ^^^ but that it just feels like an antiquated idea, with it only being one of many possible outcomes for a T111 civilisation. Looking at our current civilisation development, using the last ten years of data as an example, population has increased about 16%, computing power continues to double every two years (look for that to speed up dramatically with the development of quantum computing) and there has been a huge increase in personal technology usage (ie tablets and smart phones). Yet, over all, our energy usage has dropped 10% in the last 10 years. Play out those trends over the period of 100 years, then 1000 years. A cosmic blink as we’re constantly reminded! Not only that but another issue I have with the dyson sphere idea, is that the natural development of technology seems to push it to get smaller and more energy efficient. I therefore theorise that there are probably many different outcomes for a civilisation reaching type 111 status that more than likely they wouldn’t be detectable due to such low amounts of waste energy (which would probably be close to zero), even using the equivalent energy output of an entire galaxy. Not only that, but in order for more and more complex computations to be carried out at a quicker and quicker rate, the technology would need to be super cooled, thus eliminating the method of detecting this type of technology from vast distances in the infrared light band that has been discussed. I definitely think there is credence in the theory of an advance civilisation harnessing the power of black holes, and more than likely in a way we can’t even begin to imagine. We don’t even know what 95% of the universe is made of and what gravity is and where it comes from, so until we can begin to start answering some of those questions, I fear we wouldn’t be able to grasp what that would look like. One exciting possibility, is that some of these deep an answered questions could be routed in ETI (for example, Dark Flow). But that’s just a thought.

    Pushing the Kardashev scale further could also be fun. Maybe a level 1111 (4) civilisation could exist, maybe they created this universe in lab and are God like. What we measure as dark matter could be the framework on which they have set our universe upon and dark energy is some kind of experiment control or a force they are exerting as part of the experiment. As rediculous as that sounds, at this stage that remains a possibility (wouldn’t that be a turn up for the books!). I don’t however, find that easier to beleive (although I don’t) than the religious of our world!

  • Harold May 14, 2015, 9:13

    Roy Clymer “An ignorant question: from my understand of our solar system, there is no where near enough mass to build a shell surrounding the Sun. What am I missing?”

    A solid Tyson sphere is likely impossible and is more a product of scifi than Dyson himself. Dyson spheres, as Dyson envisioned them, are comprised of independent solar collectors in orbit around a star. Overlapping the orbits could result in collecting 100% of the stars output. Such a Dyson swarm sphere is possible, but perhaps not common.

  • ljk May 14, 2015, 9:24

    Roy Clymer said on May 13, 2015 at 17:08:

    “An ignorant question: from my understand of our solar system, there is no where near enough mass to build a shell surrounding the Sun. What am I missing?”

    Start with this:


  • Roy Clymer May 14, 2015, 9:40

    Thanks. But such arrays wouldn’t lead to high IR-no visible light “voids” around single stars, let alone around expanding bubbles in galaxy arms. Are these advanced civilizations assumed to have fleets of super-scooper ships gathering up interstellar dust clouds?

  • Harold May 14, 2015, 9:51

    The amount of speciation, cultural and technical diversification possible over so many millions of years is mind boggling and could make galaxy scale artifacts even more difficult to find. In our own history, a powerful driver for leaving the parent group and colonizing is the desire of the daughter group to be independent, different, or to experience new adventures. If this is true for star-faring species, each colonized system is likely to spawn colonizers that differ from them; a KII system would send out colonizers that want to return to nature.

    Rather than looking like cream swirled in coffee, galactic colonization may always look like a rainbow of colors swirled in coffee. (Sorry, couldn’t think of a multicolored drink or dessert.)

  • Larry Kennedy May 14, 2015, 10:22

    This relates somewhat to the problem of our using the term civilization to these branches. Civilizations have had for the most part very limited times of existence here on earth. True that a civilization’s downfall sometimes has major impacts on progress, but overall progress has continued. This may be even more so once self sustaining systems are spread through the vastness of space. If we do Sagan style self annihilation before we get there our chance is over.

  • Alex Tolley May 14, 2015, 13:11


  • Alex Tolley May 14, 2015, 13:15

    @Tim Yet, over all, our energy usage has dropped 10% in the last 10 years

    What do you mean. Globally CO2 emissions are rising as global energy use increases. Do you mean per capita energy use in the post industrial countries?

    Bear in mind that energy use cannot be reduced much further unless we radically change our use of materials, e.g. steel and concrete. Similarly, if we use nitrogen fertilizers, we cannot reduce the energy needed to create them.

  • Alex Tolley May 14, 2015, 18:36

    per capita energy consumption. Chart 7.

    Only US appears to have had a decline.

    World bank data for countries. Looks like post industrial countries mostly seeing decline.

  • Mark Zambelli May 15, 2015, 10:22

    @Roy Clymer

    “…such arrays wouldn’t lead to high IR-no visible light “voids” around single stars, let alone around expanding bubbles in galaxy arms.”

    The logic behind the search for Dyson Spheres (a single shell of orbital collectors/habitats, or a multi-shelled ‘Swarm’, the pinnacle of which would be the Matrioshka Brain… kinda) is simply that each shell of collectors heats up and gives off waste heat. The starlight is blocked but the surrounding shell(s) give off this waste heat and it would therefore appear as an IR-excess with no optical component for a 100% efficient collecting swarm. A large volume of numerous spheres in, say, a spiral arm would resemble the Fermi Bubbles mentioned but only in a static model (which is why these Bubbles would distort to become nigh-on undetectable). That’s why the G-hat survey looked at all those galaxies to see, first of all, are any of them already KIII? Can’t wait for follow-ups in this Dysonian-SETI approach.

  • Eniac May 16, 2015, 23:12

    Larry Kennedy:

    This relates somewhat to the problem of our using the term civilization to these branches.

    Exactly. The word “civilization” in this context is a complete red herring. What we should be talking about is “our/their descendents”. “Species” even would be too narrow, because our descendents may well turn out to be machines not genetically related to us at all.

    When you think about it this way, postulating a finite lifetime becomes dubious. After all, the descendants of the first living organism on Earth are still around billions of years later, despite being confined to a single planet.

  • ljk May 29, 2015, 11:54

    The problems with talking to aliens

    By Marissa Fessenden on May 17th, 2015

    In the constellation Boötes, a star a bit smaller than our sun flares sporadically, dimming and brightening, but still too faint to be seen with the naked eye. Scientists have named this red dwarf Gliese 526 and categorized it as a star that just might have a habitable planet or two. No one has spotted a planet yet, but the abundance of planets orbiting other, similar stars make the chances look good. If life exists in that star system, we would be neighbors: Gliese 526 is a mere 17.6 light years away.

    If that life is intelligent, and listening, they just might be the first non-human beings to receive a message from Earth. In the summer of 2013, a small team of scientists and entrepreneurs announced the start of a project they called “Lone Signal.” Using a recommissioned radio dish in Carmel, Calif., the group, led by scientist and musician Jacob Haqq-Misra, would send messages to Gliese 526. Conceivably, some of the senders could get a reply in their lifetimes.

    Full article here:


    So where did Lone Signal disappear to? The article never says, neither does their Facebook page which pretty much frittered away over a year ago. The Web site no longer exists.

    In any event, this article is pretty useful as far as popular level pieces on this topic go, including some good links to other sources.

  • ljk June 19, 2015, 8:55

    Legal implications of an encounter with extraterrestrial intelligence

    In the event that humans detect a signal from an extraterrestrial intelligence, or the more unlikely event of a physical encounter with them, how would the legal system be prepared to deal with repercussions?

    Babak Shakouri Hassanabadi discusses how existing treaties and interpretations of international law might apply in such scenarios.

    Monday, June 15, 2015


  • ljk July 14, 2015, 17:21



    TODAY, NASA’S NEW Horizons spacecraft will finally let us see what the surface of Pluto looks like up close. And thanks to a new interactive website called Lightyear.fm, we already know what Plutonians are listening to: either “Trap Queen” by Fetty Wap or “Sugar” by Maroon 5.

    The caveat, to that, of course, is it’s what they’re listening to if they can pull in radio broadcasts from Earth. Lightyear.fm essentially takes the opening scene of the 1997 film Contact and makes it a functional interactive experience. Once you load up the site, your eyeballs are treated to a flight through space while your ears learn how long it would take radio broadcasts from Earth to make that same journey.

    You start out in Earth’s orbit, then push back through the cosmos with a running ticker of how many light-minutes, -hours, or -years you are from our planet. Depending on how far you are from Earth, you’ll hear a chart-topping song from the corresponding month or year. You can either just kick back and enjoy the ride, scroll your mouse wheel to activate hyperdrive, or manually scrub through time and space using the timeline on the left of the site.

    Full article here:


    Here is the Lightyear.fm Web site: