If advanced alien civilisations are building vast satellite swarms designed to harvest a star鈥檚 energy, we should be able to see them 鈥� so why haven鈥檛 we? One answer might be that these structures, known as Dyson spheres, will likely destroy themselves before we can spot one, according to new calculations.
The idea of structures that can siphon off most of the energy from their stars was first proposed by physicist Freeman Dyson in the 1960s. Astronomers have theorised that these structures, if they exist, should give off a telltale signature of light 鈥� a technosignature 鈥� that we could see with telescopes, but searches so far have been at best inconclusive.
Now, at the Breakthrough Listen initiative in Oxford, UK, says there may be a good reason for this. He has calculated that Dyson spheres, in the form of swarms of satellites, would likely be plagued by an avalanche-like cascade of collisions, eventually destroying themselves.
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鈥淲hen you have a whole bunch of things moving together like that in a swarm, a natural question is going to be, do they sometimes bump into each other?鈥� says Lacki. 鈥淎t their orbital speeds, that can be disastrous.鈥�
The risks of satellite collisions around Earth are already well known, with fears that out-of-control space junk could destroy vast swathes of satellites through chain reactions, a phenomenon known as Kessler syndrome. As a baseline, Lacki calculated that a swarm of satellites orbiting a star at random would destroy itself in hours or days in a Kessler-like scenario, but even a swarm made of fewer, larger satellites, or those placed in thin orbital shells to avoid satellites crossing paths, couldn鈥檛 last much longer than a few million years before descending into chaos.
The only way to avoid total destruction is for alien civilisations to employ some sort of active management system, says Lacki, like remote satellite tracking and control, or using the radiation pressure from the star to guide satellites equipped with solar sails, like a boat navigating trade winds.
鈥淚t鈥檚 not obvious that it would necessarily work out to be a big problem, but [Lacki] has done the math, and it鈥檚 definitely a big problem,鈥� says at the Pennsylvania State University. 鈥淵ou just can鈥檛 have that much material around a star without it colliding with itself and eventually shredding itself to pieces, unless each of those swarm components is careful about managing its orbit.鈥�
Actively managing a swarm of satellites to avoid collisions isn鈥檛 an unreasonable thing to expect from an advanced alien civilisation, says Wright, and is similar to technology we use for geosynchronous satellites on Earth.
One consequence of Lacki鈥檚 work, however, is that it means we鈥檙e unlikely to come across megastructure relics in the form of Dyson spheres once the alien civilisation has gone extinct, says Wright. 鈥淥ne argument that was sometimes made about Dyson spheres is that maybe a species would build it and then long after the species was gone, it would persist, but what Lacki is showing is that, like a lot of technosignatures, they鈥檙e only visible as long as they鈥檙e being used.鈥�
arXiv
