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On Galactic Migration

Yesterday I looked at the prospect of using technology to move entire stars, spurred on by Avi Loeb’s recent paper “Securing Fuel for Our Frigid Cosmic Future.” As Loeb recounts, he had written several papers on the accelerated expansion of the universe, known to be happening since 1998, and the resultant ‘gloomy cosmic isolation’ that it portends for the far future. It was Freeman Dyson who came up with the idea that a future civilization might move widely spaced stars, concentrating them into a small enough volume that they would remain bound by their own gravity. This escape from cosmic expansion has recently been explored by Dan Hooper, who likewise considers moving stellar populations.

Image: Harvard’s Avi Loeb, whose recent work probes life’s survival at cosmological timescales.

I gave a nod yesterday to the star-moving ideas of Leonid Shkadov, who suggested a ‘Shkadov thruster’ that would use the momentum of stellar photons to operate, but Loeb pointed out how inefficient the process would be. Better to harvest stellar energy more directly, as Hooper was proposing. This reminds me of Fritz Zwicky’s own ideas about moving stars. In his book Discovery, Invention, Research through the Morphological Process (Macmillan, 1969) the physicist developed ideas he had first presented in lectures at Oxford University in 1948 on how to reach Alpha Centauri.

The fiercely independent Zwicky coined the term ‘stellar propulsion’ at Oxford and went on to describe using the matter of the Sun itself as nuclear propellant. In his later work, he followed up on the idea, the plan being to turn our planet, pulled along with the Sun, into the ultimate generation ship. I have to pause to quote Zwicky on this, from a June, 1961 article in Engineering and Science called “The March Into Inner and Outer Space”:

In order to exert the necessary thrust on the sun, nuclear fusion reactions could be ignited locally in the sun’s material, causing the ejection of enormously high-speed jets. The necessary nuclear fusion can probably best be ignited through the use of ultrafast particles being shot at the sun. To date there are at least two promising prospects for producing particles of colloidal size with velocities of a thousand kilometers per second or more. Such particles, when impinging on solids, liquids, or dense gases, will generate temperatures of one hundred million degrees Kelvin or higher-quite sufficient to ignite nuclear fusion. The two possibilities for nuclear fusion ignition which I have in mind do not make use of any ideas related to plasmas, and to their constriction and acceleration in electric and magnetic fields.

Like Loeb, Zwicky (1898-1974) liked to think big. The discoverer of 122 supernovae, he came to be interested in galactic clusters, and in particular the Coma cluster. Here his reputation for being ahead of his time is on full display, for he discovered that the mass of the cluster was far too little to produce the gravitational effects observed. In other words, something was keeping the cluster together beside visible matter. This anticipation of what we today call ‘dark matter’ was one Zwicky suggested could be studied by another cutting edge idea, gravitational lensing.

I wish I had known Zwicky, who surely would have jumped into the ideas in Avi Loeb’s paper with gusto. Loeb argues that moving stars to concentrate them into smaller regions is ultimately not necessary. If we want to avoid the cosmic fate awaiting us, with galaxies winking out in the distant future as they move beyond the visible universe, we should think in terms of locating the places where stars are already the most concentrated, the huge galactic clusters. Zwicky, the force behind a six-volume catalog of 30,000 galaxies based on the Palomar Observatory Sky Survey, was just the man to appreciate this insight, and doubtless to add a few of his own.

The Coma cluster that was the subject of Zwicky’s observations on ‘dark matter’ is about six times further away than the Virgo cluster, but both are laden with resources. Given that accelerated cosmic expansion should be detectable by any sufficiently advanced civilization, these galactic clusters — massive reservoirs of fuel, as Loeb calls them — should be desirable places for migration, just as in our own history civilizations settled around rivers and lakes.

The benefits of moving a civilization into a galactic cluster are numerous, writes Loeb:

Once settled in a cluster, a civilization could hop from one star to another and harvest their energy output just like a butterfly hovering over flowers in a hunt for their nectar. The added benefit of naturally-produced clusters is that they contain stars of all masses, much like a cosmic bag that collected everything from its environment. The most common stars weigh a tenth of the mass of the Sun, but are expected to shine for a thousand times longer because they burn their fuel at a slower rate. Hence, they could keep a civilization warm for up to ten trillion years into the future.

As Loeb goes on to point out, nearby red dwarfs like Proxima Centauri and TRAPPIST-1 have already been found to have rocky, Earth-sized planets around them in or near the habitable zone. If this is the case with nearby stars we have just begun to examine, the implication is that planets are likely around most. This kind of star, the M-dwarf comprising up to 80 percent of all stars in the Milky Way, appears made to order for civilizations dependent on liquid water. Note the vivid image above, by the way: A civilization harvesting energy output like a butterfly hovering over flowers. Like fellow astronomer Greg Laughlin, Loeb is an uncommonly fine wordsmith.

Image: Almost every object in the above photograph is a galaxy. The Coma Cluster of galaxies pictured here is one of the densest clusters known – it contains thousands of galaxies. Each of these galaxies houses billions of stars – just as our own Milky Way galaxy does. Although nearby when compared to most other clusters, light from the Coma Cluster still takes hundreds of millions of years to reach us. In fact, the Coma Cluster is so big it takes light millions of years just to go from one side to the other. Most galaxies in Coma and other clusters are ellipticals, while most galaxies outside of clusters are spirals. The nature of Coma’s X-ray emission is still being investigated. Credit: Russ Carroll, Robert Gendler, & Bob Franke; Dan Zowada Memorial Observatory.

Naturally we are talking about very long-term solutions to a far-distant problem when we discuss moving a civilization to the most useful galactic cluster. The question comes down to whether it would be possible to travel, say, a hundred million light years within the age of the universe. To do this, Loeb says, it would be necessary to exceed one percent of the speed of light. At these speeds, no relativistic time dilation can shorten the journey for its participants. These would be civilization-spanning journeys by cultures capable of surviving on geological timescales.

But let’s mimic Fritz Zwicky and let our imaginations loose. Zwicky proposed that a moving Sun could reach Alpha Centauri in approximately 50 human generations. Loeb ratchets up the challenge in a grand way, though leaving the method of travel up to future scientists. A bonus in going where the fuel is: We might expect to find other civilizations that have made the same decision, with whom we could share cultures and technologies. Like individual species, perhaps all life-forms capable of making the journey will want to congregate around watering holes like these, a far future echo of the history of life on a planet we may or may not be taking with us.

The paper is Loeb, “Securing Fuel for Our Frigid Cosmic Future” (preprint).


{ 55 comments… add one }
  • Antonio June 22, 2018, 13:18

    “This kind of star, the M-dwarf comprising up to 80 percent of all stars in the Milky Way, appears made to order for civilizations dependent on liquid water.”

    Do you really think that billions of years in the future we will still be dependent on liquid water, or even on planets? I don’t think so for even a thousand years in the future.

    • ljk June 22, 2018, 14:10

      So what will our descendants do for liquid refreshment?

      • Michael June 22, 2018, 15:53

        A fusion of Gin and Tonic :)

        • J. Jason Wentworth June 23, 2018, 16:43

          An elementary student’s actual answer (and no, it wasn’t mine) to a science test question was quite similar: “Water is composed of two gins, oxygin and hydrogin. Oxygin is pure gin, hydrogin is gin and water” (another answer on another test was, “When you breathe, you inspire. When you do not breathe, you expire” :-) ), but:

          Any current or future civilization that has the ability to move and “herd” stars would probably also be able to synthesize liquid water (and most other elements and compounds) in bulk quantities directly from other ones via various types of nucleosynthesis, so that they might have no favored locations. They might somewhat prefer longer-lived K and M dwarf stars purely because of their long periods of stable energy output, but shorter-lived blue giant stars would give them more energy to utilize at any given time, so both “types” might interest them; the blue giant stars might be their sites of intensive industry, and:

          We can already perform such literal alchemy now (creating trans-uranic elements [and transforming mercury into gold using cyclotrons, for example]), but only in relatively small amounts and (especially gold from mercury) at great cost. A civilization able to move stars would have access to plenty of stellar energy for transforming substances in enormous quantities.

      • hiro June 22, 2018, 16:18

        AI drink no water!

      • Antonio June 22, 2018, 16:33

        Why would they need liquid refreshment?

        Apart from that, did you know that technology that was developed recently? I think they call it “fire” or something like that. Did you know that it can liquify ice?

    • ljk June 25, 2018, 9:40

      Water makes a good coolant and in both liquid and frozen form is a good barrier to radiation. So even advanced Artilects may have a use for it whether or not they have dry throats to unparch.

      • Antonio June 26, 2018, 15:06

        You overstimate radiation danger. There isn’t that much radiation in space, unless you go to very particular places, like near Jupiter, or move at relativistic speeds. And anyway other sustances could do as well, like methane, etc.

    • Robert June 25, 2018, 14:18

      Weren’t you one who called the EMDrive B.S.? Yet now you’ve redesigned humans to not need water or planets! Astounding.

      • Brett Bellmore June 25, 2018, 18:12

        More likely redesigning Pentium chips to be able to run a civilization.

        I think it’s inevitable that, if mankind endures, and our civilization does not fall, at least some of us will radically redesign themselves, over and over, until the end result is able to live in space without aid.

        After all, almost all of space is not suitable for present day humanity, and it’s an obvious goal.

      • Antonio June 26, 2018, 15:17

        Huh?? What is so astounding?? EmDrive is total bulshit: it contradicts very basic physical laws, its theoretical derivation has elementary mistakes, it has no real experimental proof, the experiments done are self-contradictory, … OTOH, what I said is perfectly possible under the laws of physics. Not only that, I have now on my hands something that doesn’t need water nor a planet. I’m using it to write this. Also, there are terrestrial life that can live in zero-g. There are also some papers that describe a possible chemical basis for life on Titan, without water nor DNA, … Seriously, don’t compare that BS with this.

        • Robert June 27, 2018, 15:50

          Feel free then to turn yourself into a bacterium and roam the universe. I’d rather wait for an EMDrive, Mach Effect Device or some other electromagnetically based thruster. :)

          • Michael June 28, 2018, 10:23

            “Feel free then to turn yourself into a bacterium and roam the universe.”

            We are cosmic microbes, our comparison in size dictates it.

            • Robert June 28, 2018, 12:22

              We are made in the image of God. Size isn’t everything.

              • Mark Zambelli July 6, 2018, 12:16

                Robert, with respect, I do not believe that superstitious-drivel for one microsecond so please refrain from making dogma-of-choice statements like that.

                One of the many things that makes this site such a rare gem is the strict guidelines on posting, unlike so many sites out there… this is where people leave their religious, pseudoscientific etc baggage at the door and discuss the latest of Paul’s treatments as on topic as possible, please remember that and keep your own personal religious views to yourself, thankyou.


          • Antonio July 9, 2018, 16:39

            Don’t hold your breath for those undrives.

  • José Moutinho June 22, 2018, 15:52

    A civilization capable of moving stars will probably be able to produce the necessary energy on its planet, or solar system, or galaxy, without having to use engineering on that scale. Or they can do it as cosmic gardening. A star here and other there in that corner of the galaxy …

  • Andrew LePage June 22, 2018, 16:35

    Fritz Zwicky was definitely an interesting character in astronomy and spaceflight. Here is a piece about his project which launched metal pellets into solar orbit (the first man made objects sent into that realm) using shaped charges on a sounding rocket in October 1957:


  • Patient Observer June 22, 2018, 17:28

    The Andromeda galaxy is already heading our way. Coincidence? I think not.

    • ljk June 25, 2018, 8:59

      There was a science fiction story about how galaxies are sentient and can communicate with each other. Turns out all the electromagnetic buzzing of the tiny species throughout the Milky Way galaxy were giving it the equivalent of a headache and it had requested medical help from the Andromeda galaxy, who is on its way to resolve the situation….

  • Joe June 22, 2018, 20:08

    I was under the impression that the accelerating expansion of the universe was only occurring between galaxies, not within them. Therefore the distance between stars within a galaxy stays pretty constant over time. If so, it seems to me that the night sky will still be full of stars even billions of years from now when the galaxies are too far apart to be seen.

    • J. Jason Wentworth June 23, 2018, 0:34

      That is what I’ve read as well. The galaxies are flying away from each other at ever-greater velocities, but the galaxies themselves aren’t flying apart. Even galaxies that are orbiting around each other (or that have smaller satellite galaxies, like the Milky Way and M31) will stay together; so should galaxy clusters (at least the more tightly-grouped ones), and:

      While Zwicky’s and Loeb’s ideas about turning whole solar systems into (literal) starships appear to be possible in principle, I think the concept is most likely just an intellectual “philosophers’ toy,” less likely to be found than a Dyson sphere. But if an advanced civilization saw that its solar system was heading toward an interstellar dust cloud that could freeze its home planet by blocking their star’s light (or, less likely, that their star was on a collision course–or on a dangerously close approach path–toward another star [perhaps a neutron star or a black hole]), arranging to move its star and planetary system would be a way to avoid disaster.

      • Michael June 23, 2018, 12:13

        Moving a BH or neutron star would be easier than moving a normal star. They have very powerful gravitation fields and so can accelerate objects towards them at great velocities. Normally it would be a six and two three’s scenario but with the Oberth effect which for BH and neutron star can be quite efficient it could move the object for less energy. It could be as simple as a shaped nuclear charge detonated at the right time and in the right direction.

        • J. Jason Wentworth June 23, 2018, 17:33

          That is true as far as it goes, but one factor would cause a major problem. A solar system could “slingshot past” a neutron star or a black hole (rather like the Pioneers’ and Voyagers’ flybys of Jupiter), but unless the stellar flyby was a distant one, the orbits of the planets would be greatly perturbed (and the more distantly-orbiting planets might even be pulled out of their orbits around their star), and:

          This is because small, super-dense, collapsed objects such as neutron stars and black holes have much greater gradations of gravitational pull over very short distances, even just a few centimeters (the gradations are far greater than those encountered around planets and un-collapsed stars, even blue supergiant stars). Unless–although even this might not help (or help enough)–the star and its planets happened to all be lined up in a straight line pointing “past” the neutron star or black hole during the solar system’s fast flyby of the collapsed star, ones closer to the collapsed star would be pulled on by it much more strongly than the others. Also, if any of the planets (and/or their star) passed too close to the collapsed star, the tidal effects would tear them apart (just as Comet Shoemaker-Levy 9, a physically weak object, was torn into numerous fragments by Jupiter’s gravitational tidal forces).

      • Joe June 26, 2018, 7:09

        Another amateur theory of mine is that perhaps the dark energy accelerating the expansion of the universe is also responsible for keeping galaxies intact. What if dark energy is subject to Newton’s third law of motion? Suppose, that as dark energy pushes galaxies apart, there is an equal and opposite reaction that pushes the stars within those galaxies together? IMHO, this opposite reaction to the push of dark energy between galaxies could counteract the centrifugal force pulling galaxies apart, so there is no need to invoke “dark matter.”

    • Patient Observer June 23, 2018, 9:48

      I think that the concept is space that is expanding everywhere. However, the expansion is very slow meaning chemical bonds to gravity; where sufficiently strong to overcome the expansion. The galactic gravitational field is believed sufficiently strong to keep the galaxy essentially unaffected (not sure if the remotest globular cluster would continue to orbit the galaxy).

      There was an article (Scientific American?) that speculated cosmologists in the far far future, when only the Milky Way would be visible, would have very little clue that there is a far larger universe out there.

    • Brett Bellmore June 24, 2018, 9:35

      The theory, (And at this point I think we can say that it’s just “theorized”, not “known”.) has the expansion being increasing with time. At first it overcomes the gravitational attraction between galaxies, but eventually would tear galaxies apart, then clusters, then planetary systems, and eventually even atoms.


      Key point: “Evidence indicates w to be very close to −1 in our universe, which makes w the dominating term in the equation. The closer that w is to −1, the closer the denominator is to zero and the further the Big Rip is in the future. If w were exactly equal to −1, the Big Rip could not happen, regardless of the values of H0 or Ωm.

      According to the latest cosmological data available, the uncertainties are still too large to discriminate among the three cases w −1”

      So, just theorized at the moment.

  • jonW June 22, 2018, 21:41

    I’m trying to figure out why some people believe it will be impossible to ever leave the Local Group due to the fact that nothing else is gravitationally bound to us. See for example this video: https://www.youtube.com/watch?v=ZL4yYHdDSWs . I tried doing a little math to convince myself: the Virgo cluster is moving away from us at 1200 km/s (0.004 ly/ yr). Let’s say we manage to get a generation-ship built that will travel at 0.01c. Without acceleration of cosmic expansion, this would be equivalent to travelling to a fixed point at 0.006c. The distance is 5 MLy, so it would take ~0.8 billion years to get there.

    Add a little extra for the increase in the expansion rate between now and then, but it seems unlikely that this voyage can be categorically ruled out as impossible. Are these calculations approximately plausible?

    • Antonio June 23, 2018, 15:47

      That “not gravitationally bound to us” stuff is total non-sense.

      • jonW June 23, 2018, 19:54

        Really? Care to elaborate? Because Avi Loeb and Freeman Dyson both also discuss things in these terms, meaning the gravitational attraction is sufficiently strong among members of the Local Group to overcome the expansion of space at all times in the future (even without the merger of members). But maybe you know something that they don’t…

        • Antonio June 26, 2018, 15:26

          Well, to begin with, what does “gravitationally bound” mean? It really only applies to objects in orbit around another object, but the Milky Way is not rotating around anything, nor the local group, nor the Virgo cluster, etc. At those scales, motion is more complex and everything is attracted to everything to some degree.

          • jonW June 26, 2018, 21:30

            Thanks for explaining your objection. I think you have a different concept of what “gravitationally bound” means. It does not apply to objects in orbit around one other as you suggest, but to objects whose mass, separation and relative velocity are enough to keep them from receding over each others’ cosmic horizons during the accelerated expansion of the universe that current models predict. To say that two objects (like the Local Group and the Virgo cluster) are not gravitationally bound to each other does not mean, as your reply would have it, that they are not attracted to each other. Just that that attraction is not enough to keep them from being irrevocably separated so they no longer can communicate at some point in the future.

            • Michael June 28, 2018, 13:06

              There is a theory that the Universe will end in the Big rip where even atoms are torn apart. Could this acceleration of the Universe be merely a series of ripples from the massive big bang. I find it hard that GW’s can expand the light waves in the direction they are travelling as they are meant to travel at the speed of light as well. But if Space Time is a surface which it appears to be then a transverse GW will have the effect of extending a light wave as it passes over it giving the impression of an accelerating universe. Think of matter travelling over the surface of a pond and a ripple or up welling under the surface expanding everything apart on that surface, including light waves.

    • J. Jason Wentworth June 23, 2018, 17:08

      I don’t remember, but it may be that the rate of acceleration–and thus of the increasing distance between galaxies–is also increasing over time, rather like how a rocket’s rate of acceleration increases as it becomes lighter and lighter as its propellant burns off (many have to throttle back to avoid structure-over-stressing rates of acceleration).

    • Marshall Eubanks June 25, 2018, 10:45

      This is mixing up concepts. The Earth is and should remain bound to the Sun for billions of years, but that doesn’t mean a spaceship can’t leave the solar system. Likewise, the binding of the local group doesn’t mean a spaceship couldn’t go to the Virgo Cluster, given enough patience. The presence of the cosmological constant would slow such a trip, but not render it impossible.

  • David June 22, 2018, 22:23

    Physics World has a good summary article on dark energy. Loeb is really jumping ahead on the cosmolgical constant. All we know for sure is we have an acceleration. Even the discoverers will not commit to anything more at this point. That said this is a lot of fun. Why not drag the Earth along if we can move stars.

  • Michael June 23, 2018, 11:53

    Why bother moving the Sun, if we wait for the planetary nebula stage it will eject enough hydrogen to form 4 to 6 low mass M-type stars and we will have the central white dwarf. Good times ahead if we are prepared to wait an eon or five.

  • Geoffrey Hillend June 23, 2018, 15:49

    Although it’s good to know star moving is possible, I like the idea that we figure out interstellar travel in a million years so humanity can leave the Earth before it becomes like Venus in three hundred million years due to the Sun becoming brighter.

    • Brett Bellmore June 24, 2018, 13:26

      Or just move the Earth out further from the Sun, if we’re sentimental about it.

      • ljk June 25, 2018, 9:54

        Or turn the whole Sol system into a Dyson Shell and keep our star on the Main Sequence far longer than natural, quoting from this article next:


        Reeves (1985) suggested the intervention of the inhabitants that depend on these stars for light and heat. According to Reeves, these inhabitants could have found a way of keeping the stellar cores well-mixed with hydrogen, thus delaying the Main Sequence turn-off and the ultimately destructive, red giant phase.

        Beech (1990) made a more detailed analysis of Reeves’ hypothesis and suggested an interesting list of mechanisms for mixing envelope material into the core of the star. Some of them are as follows:

        * Creating a “hot spot” between the stellar core and surface through the detonation of a series of hydrogen bombs. This process may alternately be achieved by aiming “a powerful, extremely concentrated laser beam” at the stellar surface.

        * Enhanced stellar rotation and/or enhanced magnetic fields. Abt (1985) suggested from his studies of blue stragglers that meridional mixing in rapidly rotating stars may enhance their Main Sequence lifetime.

        If some of these processes can be achieved, the Main Sequence lifetime may be greatly extended by factors of ten or more. It is far too early to establish, however, whether all the blue stragglers are the result of astroengineering activities.

  • Spaceman June 23, 2018, 20:41

    Is it true that there could be at least as many habitable planets in elliptical galaxies as in spiral galaxies?

    • J. Jason Wentworth June 24, 2018, 13:20

      Your question about habitable planets in elliptical galaxies (and also in irregulars and other types of galaxies [ordinary spirals versus barred spirals, like our own?]) would be an interesting (and helpful) matter that one or more of the SETI researchers and/or SETI-interested astronomers who read Centauri Dreams might examine in an article here. SETI usually refers to such searches only in the Milky Way (and more often than not, only in our little “county” [or “borough,” as we Alaskans have] of the Galaxy, within just a few hundred–or maybe a few thousand–light-years from Earth), and:

      While this implicit self-limitation is somewhat based on the inverse square law weakening of (humanly) conceivably-producible artificial radio or laser signals with increasing transmission distance, another factor in it is psychological. We could not hope to engage in two-way communication with a culture that is, say, even “just” a quarter of the way across the Galaxy, and sending Bracewell probes there (or finding such probes from them here), while not impossible, would take essentially forever (to us, if we tried sending probes), and the odds of a probe of theirs being here are not promising. While self-replicating von Neumann probes from just *one* civilization could reach every star in the Galaxy in only a few million years, we don’t know it such machines are possible, or if they are, if–especially if intelligent extraterrestrial life is rare–that they would think of building them, but:

      Some astronomers, especially Soviet ones (after Kardashev’s civilization types scale was published), suggested that making SETI observations of other galaxies might be worthwhile, partly because a radio telescope (or a “light bucket” optical SETI telescope) could view an entire galaxy at once, so that any sufficiently-powerful signals from Type II or III civilizations therein could be detected. This would–as far as we’re concerned–be “signal archaeology,” because the information transfer would be only one-way, as from ancient Egypt to Champollion in the early 1800s (when he deciphered the hieroglyphic inscriptions), and because intergalactic travel by probe (in either direction) would involve hopelessly long travel *and* signal time delays for us and for them (unless they were immortal, although they would have to be mighty patient folk). As well:

      “Declaration of presence” (“we’re here!”), attention-attracting beacons (made by “labeling” a star with artificial elements, as Arthur C. Clarke suggested) would be detectable over intergalactic distances in some cases. If elliptical and irregular galaxies have significant numbers of habitable planets, then the Milky Way’s and M31’s two (each–four in total [although our small Magellanic Cloud satellite galaxy may be split into two dwarf irregulars, one behind the other from our viewpoint, giving us three satellite galaxies]) satellite galaxies would also be worthwhile SETI observation targets. Such midget galaxies are more common than the large spirals and giant ellipticals (which are the largest of all galaxies). Ellipticals of all sizes are also nearly dust-free, making optical SETI/CETI within them–and to other galaxies–less difficult.

    • kzb June 25, 2018, 12:49

      The rate of star formation in ellipticals is much lower. That means there are far fewer bright stars because they burn out quickly. So if you start with the assumption that life needs sun-like stars, it is not true. If you believe habitable planets can exist around red dwarfs, then no problem.

  • Steve Bowers June 24, 2018, 7:02

    We’ve considered this possibility in Orion’s Arm many times. If it is possible then one might expect civilisations in other parts of the Galaxy, or in other galaxies, to have attempted it. The excess energy emitted by a self-propelling star should be identifiable as such; we should have better luck looking for instances of stellar propulsion in other galaxies than looking for Dyson Spheres in our own, simply because you can’t hide the exhaust of a rapidly-moving star.

  • J. Jason Wentworth June 24, 2018, 13:33

    Such self-propelled stars might also betray their (artificially-modified) nature by their anomalous proper motion. A star moving too slowly or too rapidly than its distance from the galactic center would indicate (via Keplerian motion, even for a quite eccentric orbit), and/or an “anomalously straight” path, or a strangely-inclined one, might call attention to its odd motion.

  • Geoffrey Hillend June 25, 2018, 1:19

    I think it depends on the size of elliptical galaxy. The larger ones have more stars, so therefore more planets, but they are also considered to be older galaxies that have used up more gas. Maybe there are a lot of red giants, white dwarfs and dead solar systems there.

  • Law June 25, 2018, 8:43

    Hmmm… with regards to superclusters, drawing civilizations like moths to a flame sounds like a recipe for endless, free-for-all genocidal war.

    • ljk June 25, 2018, 15:52

      Or making all sorts of new friends!

      • Mark Zambelli July 6, 2018, 12:24

        That’s the spirit! :D

  • Geoffrey Hillend June 25, 2018, 15:33

    I doubt it. Carl Sagan said that interstellar ET civilizations might have an ethic of non interference with emerging civilizations. If they have survived this long, they have learned to live with themselves or get along with other ET’s. We might be attributing our own backwardness to them. From the book Cosmos. I don’t have the page number handy. Also with interstellar travel, a ET civilization can travel anywhere or go to any planet that is not inhabited by humanoid life.

    • J. Jason Wentworth June 26, 2018, 3:09

      Here ya go–“Encyclopedia Galactica” in Cosmos (see: http://www.youtube.com/watch?v=m-NIwzBFJ_Y ). The simulated ET prime-number beacon signal reception at Arecibo (at about 42:00) is hauntingly awe-inspiring (the music helps to evoke that feeling), and the vine-like exploration paths –intertwining but not conflicting–of two different hypothetical ET civilizations are shown at about 49:00, but:

      That notion, that older and established ET civilizations wouldn’t communicate with emerging civilizations (including ours) due to an ethic of non-interference–is only an assumption, which might or might not be correct. If intelligent technological societies are rare, another one might be moved to communicate with us (“For so long, we feared that we were the only ones anywhere–at last we have someone else to talk with!”). If other civilizations are more common, it would seem unlikely that all of them would think and act in the same ways.

      • ljk June 26, 2018, 9:15

        In his 1985 SF novel Contact, Sagan said that emerging species with a penchant for conquest and destruction would destroy themselves before they could achieve interstellar flight and thus not be a threat to the rest of the galaxy.

        I have a level of ambivalence about that idea similar to the one I have about altruistic ETI beaming their equivalent of the Encyclopaedia Galactica across the Milky Way to enlighten and uplift younger species – also an idea by Sagan.

        Both ideas are certainly not impossible, but as we have seen with our own species, some of the more dangerous and conquering societies have also been among the most technologically advanced. This technology has often come about because of the domineering ambitions of such cultures.

  • ljk June 27, 2018, 9:56

    “A bird is safe in its nest — but that is not what its wings are made for.” – Amit Ray

  • ljk June 27, 2018, 12:57

    Thanks to a Gravitational Lens, Astronomers Can See an Individual Star 9 Billion Light-Years Away


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