Dyson spheres are an outrageous concept. The idea of looking for alien megastructures in space appeared at the end of the 1950s and was widely discussed in speculations about searching for extraterrestrial civilisations. It also inspired some great science fiction novels. Today the idea seemed to have faded into obscurity, but recently as returned to the forefront of astronomy. Let us look again at these staggering hypothetical artifacts.
In the 1950s some specialists in the then new-found science of radio astronomy began to consider using their freshly built dishes to scan the skies for a very different kind of celestial radio source. In a galaxy of hundreds of billions of stars there could be billions of planets (today we know that to be a serious underestimate). Life appeared to have arisen easily on Earth and evolved into thinking beings. Perhaps this had happened many times on other worlds across the Milky Way. If these hypothetical civilisations existed, some of them at least might be trying to talk to us over the light years by radio. Some radio astronomers suggested a search for extra-terrestrial intelligence by using their radio telescopes to listen for artificial signals. Sixty years later no unambiguously unnatural radio signals have been received.
What if all these astronomers planning on looking for radio signals from extraterrestrial civilisations were barking up the wrong tree? What if most advanced alien beings are just staying at home and not bothering to broadcast to the rest of the universe? How would we find them? The very respectable British physicist Freeman J. Dyson (b1923) considered this and came up with a staggering idea which he published in his paper Search for Artificial Stellar Sources of Infrared Radiation in the journal Science in 1959 .
Dyson’s argument goes like this. Assume most extra-terrestrial societies need to use increasing amounts of energy as they develop, Dyson believed that energy consumption by technological societies would increase exponentially. A really advanced culture would need a lot of power to keep its replicators and holodecks running, in fact availability of cheap energy is essential to maintain civilisation. Eventually any fuel available on a terrestrial-type planet occupied by a long-lived technological civilisation will be exhausted, first the fossil fuel reserves will go, then the all the fissionable isotopes in the planet’s crust and even eventually all the isotopes of hydrogen and helium in the oceans will be expended in fusion reactors. Assuming no really exotic new source of power (say harnessing the zero-point energy of the vacuum) is possible a civilisation at this point will stop expanding in the near term and collapse in the longer term.
However, Dyson pointed out, a main sequence star will probably radiate heat and light for billions of years, millions of times as long as any civilisation in Earth’s history has endured. Using solar power -strictly speaking aliens from another star would be using “stellar power” – to keep a civilisation running sounds like a great idea but very little of a star’s light falls on its planets as their areas are so tiny (less than a billionth of the Sun’s output is intercepted by the Earth), the rest is wasted. Solar power collectors on planets can only supply so much.
Dyson asked what if technologically advanced aliens decided to collect every last joule of sunlight their sun emitted by completely surrounding it in a shell, a “swarm” as he described it, of orbiting solar collectors? If the star was identical to our Sun, the alien civilisation would have about 4×1026 watts on tap, in contrast our puny civilisation runs on a mere 12×1012 watts. That is to say, Dyson’s hypothetical extra-terrestrials would have about a hundred trillion times as much power on hand as we do! Dyson proposed that being able to completely tap a star’s energy was an inevitable stage technological civilisations would have to eventually attain- he suggested we would do it ourselves one day. In passing, Dyson has acknowledged that similar ideas had been discussed earlier in the pages of science fiction literature, notably Olaf Stapledon‘s Star Maker (1937). Dyson never gave a name to his hypothetical megastructure but this theoretical construct has become known as a Dyson Sphere in his honour and it is basically a star in a box!
Note that a Dyson Sphere is not a solid shell, in Dyson’s own words
A solid shell or ring surrounding a star is mechanically impossible. The form of ‘biosphere’ which I envisaged consists of a loose collection or swarm of objects traveling on independent orbits around the star.
Returning to Dyson’s original point about SETI, he argued that not all, possibly only a vanishingly small minority of, civilisations would make an effort to broadcast radio signals to the Universe. So few, possibly, that searching for them is pointless. However, Dyson spheres ought to be common (if intelligent life is abundant) and would be observable across interstellar distances.
Imagine that a Dyson sphere exists somewhere around a sun-like star. About 2 or 3 AU from the star is a great shell of essentially huge solar panels not unlike the Earth-orbiting Solar Power Satellite concept discussed since the 1960s. Each of these collectors is a light-weight structure (a few tens of tonnes at most), but possibly many kilometres across slowly orbiting the star on multiple inclined planes. These collectors may capture sunlight, convert it to electricity and beam it as microwaves to receivers on the inner planets, including the sphere builders’ homeworld. Among the solar collectors there may be millions of space habitats, artificial miniworlds, and the sphere itself could sustain a population of broadly human-like beings in the trillions at least. To wrap up a star takes vast quantities of raw materials, so the sphere is made of material extracted from demolished outer planets (think about that). Compared to this monumental structure even our mightiest engineering projects, undersea tunnels, dams, space stations, particle colliders are like the nests of ants or termites compared to a human city.
To outsiders like ourselves the star would be invisible as all its light is being captured, although the heat has to go somewhere and would leak out as infrared radiation (the central star’s neutrino and gravitational radiation would also escape) . Inside the Sphere its builders on their home planet enjoy abundant power but cannot see the stars. There are no dark skies at night either as sunlight reflecting from the sphere appears as a uniform milky glare across the firmament. Even the daytime sky would be abnormally bright to our eyes.
Madness! You may indeed be thinking that, but note that nothing about this concept is physically impossible (unlike, say faster than light travel which is disallowed by the laws of physics as understood today). Creating a Dyson sphere is actually more feasible than relativistic space travel. It is just the astonishing scale of the engineering that seems so shocking. Dyson revisited the idea a few years after his original paper to consider how a Jupiter-style planet could be dismantled into building material. In The Search for Extraterrestrial Technology (1966) he described how metallic grids would be applied to the planet, above it a series of enormous solar-powered satellites would apply powerful magnetic fields to the grid gradually increasing the planet’s rate of rotation. After a long time, tens of thousands of years, the centrifugal acceleration at the planet’s equator would exceed the gravitational acceleration and the planet would fly apart, some of the debris being intercepted for processing into solar collectors. Other researchers have proposed alternate mechanisms and scenarios, most recently Oxford University physicist Stuart Armstrong’s suggestion that humanity could convert the planet Mercury into a Dyson sphere over 40 years– starting in the next 30-50 years! Armstrong’s scenario has drawn criticism for its short timescale and general feasibility.
Essentially Dyson proposed in his paper that, rather than listen for radio signals, SETI researchers ought to search the skies for the waste heat from Dyson Spheres built by any supercivilisations out there. Real Dyson spheres would show up as fuzzy infrared sources. Astronomers looking for intelligence should seek peculiar cool (say 200-300 K), dark and fuzzy stars, emitting only infrared radiation, with spectra indicating the presence of heavy elements. There have indeed been occasional searches for them, usually based on the survey by the IRAS (Infra Red Astronomy Satellite) in the 1980s, but more recently data from the WISE and Kepler missions is being examined for oddities (such as Dyson spheres under construction). Richard A. Carrigan has suggested that other galaxies should be examined for unexplained apparently starless voids; these may be regions where a Dyson sphere-building culture is expanding into other star systems.
We now know that there have been planetary systems around some stars since 11 billion years ago, plenty of time you would think for intelligence to arise and reengineer its local planets into Dyson spheres in numerous systems across the Galaxy. Yet no searches for this “astroengineering” have succeeded despite looking at a 100 thousand galaxies (each containing hundreds of billions of stars), and this is probably one of the reasons why the idea of Dyson spheres has receded in popularity. Dyson spheres have joined starships and interstellar transmitters on the list of things that are physically feasible yet no one in the observable Universe seems to be actually building; the reasons for this are beyond our current understanding (but see the update at the end of this article).
By the 1970s the concept was widely described in books on searching for extraterrestrial life or futurology, but seems less frequently discussed today. The concept has been a gift to science fiction writers, appearing in many classic stories of the 1970s and ‘80. In fact, there were perhaps too many SF novels about space explorers wandering through enigmatic and deserted Dyson spheres (and related structures such as Ringworld and the rather smaller Halos from the famous video game series) ; eventually they became disparagingly referred to as “BDO stories” where BDO stands for “Big Dumb Object“.
In SF Dyson spheres are usually presented as solid shells with landscaped interiors but as Dyson himself said this is a physical impossibility. There are no real or hypothetical substances strong enough to create a solid structure so large. Even if it could be constructed, a solid structure around a star would be dynamically unstable, requiring its exterior to be studded with vast numbers of thrusters fed by literally astronomical quantities of propellent to prevent it inevitably colliding with its star. Also the surface gravity on the inside surface of a shell is zero so retaining an atmosphere or even walking around on the solid sphere’s internal surface are somewhat tricky.
If we did find a Dyson sphere somewhere in the Universe I imagine public reactions would be mixed. Some people would be elated to see proof that technological civilisations can survive and thrive beyond our current situation. But the implication that out there there are godlike beings capable of making-over whole solar systems out there would probably freak out more than a few others.
To my utter astonishment, since this article was written, astronomers have discovered Boyajian’s Star (also known as Tabby’s Star and KIC 8462852). About 1500 light years away from the Sun in the constellation of Cygnus, this F-type star is distinguished by its unusual lightcurves suggesting that it is orbited by several very large and irregularly-shaped objects. Some of these mysterious eclipsing bodies are so vast as to block more than 22% of the star’s light. The star also is claimed to have dimmed measurably over the past century. This peculiar behaviour could be signs of unusual natural phenomena, such as huge swarm of comets, or just possibly an indication of the presence of a Dyson Sphere-type structure. Obviously studies to learn more about this mystifying star are being enthusiastically pursued. I am amazed and fascinated to see that so obscure a concept as the Dyson Sphere is now back in the astronomical spotlight.
(Article by Colin Johnston, Science Education Director)