Suppose you’re a space-faring alien society. You’ve established colonies on a few planets and moons in your solar system, but your population is growing and you’re running out of space. What should you do? Your brightest engineers might suggest a radical idea: they could disassemble a Jupiter-size planet and rearrange its mass into a cloud of orbiting platforms that encircles your sun. Your population would have ample living area on or inside the platforms; meanwhile, through solar power, you’d be able to capture every joule of energy radiating from your star.
The laws of physics suggest no reason why this plan wouldn’t work; they merely require that all the energy collected be radiated out again as heat, lest the whole construction melt. This, in turn, means that your cloud of platforms should softly glow. A distant observer training a telescope on your solar system might see something like a hot, opaque screen encircling a dimmed star—a spherical entity, curiously bright at certain wavelengths.
The theoretical physicist Freeman Dyson first speculated about the existence of such structures in 1960. In the decades since, astronomers on Earth have looked repeatedly for so-called Dyson spheres, and nobody has seen one. There are different ways of interpreting this result. Jason Wright, an astrophysicist at Pennsylvania State University, told me that Dyson wrote his original paper while contemplating an abstract idea—that “the fundamental limit to an energy supply that a species could have is all of the starlight in their system.” The fact that Dyson spheres haven’t been found, Wright said, doesn’t prove that aliens don’t exist. It might just mean that astronomers should start looking for evidence of less ambitious alien projects.