In addition to being a staple of science fiction, the concept of megastructures has long been the subject of serious scientific study. As famed physicist Freeman Dyson originally proposed in 1960, “Malthusian pressures will ultimately drive an intelligent species” to “occupy an artificial biosphere completely surrounding its host star.” In short, he theorized that advanced civilizations would dismantle their planet (or planets) to create a structure (now called a “Dyson sphere”) that would harness all of their star’s energy and provide vast habitats.
Over time, scientists have proposed many variations of this structure, collectively known as “Dyson structures.” However, extensive research has contradicted these suggestions, arguing that such megastructures would be unstable. In a new study, famed engineer Colin R. McInnes shows how two specific megastructures – Dyson Bubbles and Stellar Engines – could be built to be passively stable over time. These findings could aid the search for extraterrestrial intelligence (SETI) by limiting the technosignatures these structures could produce.
Colin R. McInnes is Professor of Engineering at the University of Glasgow and Chair of the James Watt School of Engineering. His results are presented in an article that appeared in the Monthly Notices of the Royal Astronomical Society. Although the concept is several decades old, megastructures have received renewed attention thanks to the discovery of Boyajian’s star and other cases where stars exhibited periodic dimming, were low in luminosity, or were “missing.”
In addition to being a leading figure in the field of solar sails, reflectors and satellites, McInnes has also written an article on the stability of megastructures. As he summarized in this latest study, megastructures have been proposed for a range of endeavors, including asteroid orbit modification, climate engineering (e.g. solar shields), terraforming (following Ken Roy’s Shell World concept), and planetary orbit modification (moving them into the star’s habitable zone).
On larger scales, scientists have considered how giant swarms of reflectors could envelop a star, known as a Dyson swarm, bubble or Matrioshka brain, or how they could be used to alter a star’s orbit, known as a Stellar Engine or Shkadov Thruster. In the former case, the reflective surface ensures that radiation pressure keeps the swarm (which could support habitats) hovering above the star. In the latter, a flat reflecting disk remains bound to a star through gravitational coupling, causing the star to move.
Similar to what Dyson suggested in his original paper, these studies assume that advanced civilizations will experience exponential growth and increasing energy demands as they age. “Freeman Dyson envisioned a swarm of energy-harvesting elements enveloping a central star as the endpoint for a civilization with ever-increasing energy needs,” McInnes told Universe Today via email. “It is obviously difficult to infer motivations. However, because of the universality of the laws of physics, we can at least speculate about how such structures might be constructed.”
While the idea is popular among scientists, extensive research by physicists and civil engineers has cast doubt on the existence of megastructures. In short, they argued that such structures would be inherently gravitationally unstable. But as McInnes explained, it is possible that megastructures could be built in a way that would ensure long-term passive stability:
Many concepts, such as a rigid Dyson sphere or a ring world, are not in orbit, and therefore a small displacement can cause the structure to drift and collide with the host star. They therefore need active control measures to stabilize them. However, my interest lies in understanding ways in which ultra-large structures could be engineered to be passively stable. We can imagine that engineers, terrestrial or otherwise, would prefer passive stability to more complex active control measures.
The simplest design (he notes) for a Stellar Engine would probably be a flat reflective disk. Starting with an ultra-large disk, he calculated the stability of the structure according to first principles using a simplified model of a perfectly reflective rigid disk. He then used the functional forms of gravitational and radiation pressure forces to study the stability of a radial engine and orbiting reflectors (forming a Dyson bubble) in various configurations. McInnes said:
Stability analysis involves adding a small displacement to the equations of motion that describe such structures and then determining whether the displacement increases with time. By then considering ways to design the structure’s properties, such as its geometry or mass distribution, we can determine whether it can be stabilized so that small displacements do not increase and are limited. There is no set process as such; It’s about looking at the equations of motion and considering how the acting forces could be changed, for example through changes in the geometry or mass distribution of the structure.
Ultimately, his analysis showed that although an ideal radial engine consisting of a uniform, reflective, rigid disk is unstable, a reflective disk with mass concentrated at its edge can (in principle) be passively stable. By balancing the gravitational and radiation pressure forces, such a design would also maximize the star engine’s propulsion. Meanwhile, a self-stabilizing Dyson bubble or swarm would avoid (or minimize) collisions between the cloud’s elements and maintain equilibrium, provided the proper configuration and design considerations were taken into account.
These structures would also create telltale technosignatures that SETI researchers could look for in the future. While a Stellar Engine would scatter the light reflected from its star, a Dyson bubble would appear like a dense cloud surrounding a star, changing its spectral properties. In a static cloud, no flickering would be noticeable to observers, in contrast to a swarm of orbiting reflectors passing in front of the star’s disk. And as Dyson first predicted, any solid Dyson sphere would be detectable by the infrared excess produced by the radiant heat.
However, as McInnes added, this study is not the final word on megastructures and their potential stability. “The analysis in the paper is simplified and makes a number of assumptions,” he said. “However, it is a starting point for understanding how ultra-large structures can be engineered to be passively stable. For example, a dense Dyon bubble can apparently stabilize itself because light pressure falls faster than gravity as we move through the cloud of elements. Perhaps by understanding how such structures can be engineered to be passively stable, we can better predict the technosignatures associated with them.”
Further reading: MNRAS
