Universe Today
In addition to being a staple of science fiction, the concept of megastructures has long been the subject of serious scientific studies. As famed physicist Freeman Dyson originally proposed in 1960, "Malthusian pressures will ultimately drive an intelligent species" to occupy an "artificial biosphere which completely surrounds its parent star." In short, he theorized that advanced civilizations would disassemble their planet (or planets) to create a structure (which has since come to be called a "Dyson Sphere" that would harness all the energy from their star and provide immense living space.
Over time, scientists have proposed many variations on this structure, collectively known as "Dyson Structures." However, considerable research has countered these proposals, arguing that such megastructures would be unstable. In a new study, famed engineer Colin R. McInnes demonstrates how two specific megastructures - Dyson Bubbles and Stellar Engines - could be built in such a way that they would be passively stable over time. These findings could aid the Search for Extraterrestrial Intelligence (SETI) by constraining the technosignatures these structures could produce.
Colin R. McInnes is a Professor of Engineering Science at the University of Glasgow and the chair of the James Watt School of Engineering. His findings are presented in a paper that appeared in the Monthly Notices of the Royal Astronomical Society. While 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 previously authored a paper on the subject of megastructure stability. As he summarized in this latest study, megastructures have been proposed for a range of ventures, including asteroid orbit modification, climate engineering (i.e., solar shields), terraforming (a la Ken Roy's Shell World concept), and planetary orbit modification (moving them into the star's habitable zone).
At larger scales, scientists have considered how massive swarms of reflectors could enshroud a star, known as a Dyson Swarm, Bubble, or Matrioshka Brain, or be used to alter a star's orbit, known as a Stellar Engine or Shkadov Thruster. In the case of the former, the reflective surface ensures that radiation pressure will levitate the swarm (which could support habitats) above the star. In the latter, a flat reflective disk remains bound to a star through gravitational coupling, causing the star to move.
Much like Dyson proposed in his original paper, these studies assume that advanced civilizations will experience exponential growth and rising energy demands as they age. "Freeman Dyson imagined a swarm of energy-collecting elements enveloping a central star as an endpoint for a civilisation with continuously growing energy demands," McInnes told Universe Today via email. "It’s clearly difficult to infer motivations. However, the universality of the laws of physics means that we can at least speculate on how such structures could be engineered."
While a popular idea among scientists, considerable research by physicists and structural engineers has cast doubt on the existence of megastructures. In short, they have argued that such structures would be, by their very nature, 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 Ringworld, are not in orbit, and so a small displacement can cause the structure to drift and collide with the central star. They would therefore need active control measures to stabilise them. However, my interest is in understanding ways in which ultra-large structures could be engineered so that they are 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 likely be a flat reflective disk. Using an ultra-large disk as a starting point, he calculated the structure's stability from first principles using a simplified model of a perfectly reflecting rigid disc. He then employed the functional forms of gravitational and radiation-pressure forces to investigate the stability of a stellar engine and of orbiting reflectors (making up a Dyson' Bubble) in different configurations. Said McInnes:
Stability analysis involves adding a small displacement to the equations of motion describing such structures and then determining if the displacement grows with time. Then, by considering ways to engineer the structure’s properties, for example, its geometry or mass distribution, we can determine if it can be stabilized such that small displacements do not grow and are bounded. There isn't a set process as such; it's a case of looking at the equations of motion and considering how the forces acting could be modified, for example, through changes in the geometry or mass distribution of the structure.
In the end, his analysis showed that while an ideal stellar engine comprising a uniform, reflective, rigid disc is unstable, a reflective disc whose mass is 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 stellar engine's propulsion. Meanwhile, a self-stabilizing Dyson Bubble or Swarm would avoid (or minimize) collisions among the cloud's elements and maintain equilibrium, provided the right configuration and design considerations were taken into account.
These structures would also produce telltale technosignatures that SETI researchers could look for in the future. While a Stellar Engine would scatter light reflected from its star, a Dyson Bubble would appear as a dense cloud enclosing a star, thus modifying its spectral characteristics. For a static cloud, there would be no flickering apparent to observers, unlike a swarm of orbiting reflectors, which would pass in front of the stellar disc. And as Dyson first predicted, e a solid Dyson sphere would be discernible from the infrared excess produced by radiated 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’s a starting point to begin to understand how ultra-large structures could be engineered to be passively stable. For example, a dense Dyon bubble can apparently be self-stabilising due to light pressure falling faster than gravity as we move out 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
