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).