Finding life is, perhaps, the main and most desired goal of astronomy, preferably intelligent, somewhere outside the Earth. Given the ease with which life spreads and multiplies on our home planet, and the ingredients for life throughout the universe, it is difficult to conclude that we are alone in the universe. There are about 400 billion stars in the Milky Way galaxy alone, each with its own unique history and chances for life. Despite how technologically advanced people have become, searches for extraterrestrial civilizations are unsuccessful, perhaps because technologically advanced civilizations do not communicate in the way we are used to. But an advanced civilization could build a sphere around its sun - the Dyson sphere - to absorb 100% of its energy. Incrediblebut we have the technology to detect them. If, of course, they exist.
Roy Dyson is seen as a step towards the Dyson sphere, when light is blocked by a series of spacecraft flying in front of the star.
On Earth, the amount of energy available to us is determined by the amount of sunlight striking the surface of our planet. At Earth's distance from the Sun, this is roughly equivalent to 1300 watts per square meter, but drops to 1000 if light is forced to pass through the atmosphere. If we covered the space above the Earth's atmosphere with solar panels, we would collect 166 million gigawatts of energy, all the time, all over the Earth. This is a colossal amount of energy: even a second of such a flow could provide earthlings with energy for a whole year. But only part of this energy is produced by the Sun. There are other ways as well.
The concept of a space solar power plant has existed for a long time, but no one dared even think about an array of billions of kilometers. A sphere or swarm of Dyson would go even further, encircling or paneling the Sun itself.
For example, we could build a swarm in space to collect even more energy from the sun. Imagine a large fleet of spaceships moving in a ring or a series of rings with a large collection area. This energy could be used for any purpose: it could be directed to the Earth in a beam, it could be used in situ to create a network throughout the solar system, or for interplanetary or interstellar communications. This is where the idea of alien megastructures was born - which has been proposed as one of the explanations for the darkening phenomenon of Tabby's star.
The most ambitious megastructure, however, will be the so-called Dyson sphere: an envelope around a star that absorbs all of its energy. We could do this by devouring a small planet like Mercury, breaking it down into iron and oxygen, and creating a reflective surface of hematite. If an alien civilization did the same, the shell would completely hide the star, making it virtually undetectable.
The Dyson sphere will completely cover the star, absorbing all of its ultraviolet and visible radiation. Only infrared radiation and long waves will pass through.
In any case, undetectable for telescopes operating in the visible spectrum of light, because such a sphere would completely block the star's light. But even a highly reflective surface must absorb some of the energy. And if energy is absorbed over time, it needs to be redirected somewhere in order to maintain a stable temperature. Therefore, the energy must go out into the Universe, even if there is no visible light. As the Earth radiates infrared energy at night, so the Dyson sphere will be.
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At night, the Earth emits electromagnetic signals, but the vast majority are in the infrared range, as sunlight and heat absorbed during the day are sent into space.
The European Space Agency recently released a massive dataset from the most powerful satellite to have ever charted and explored the stars of the Milky Way: Gaia. He managed to collect information on 1.7 billion stars in our galaxy, allowing us to create the most complex 3D map of the stars of the Milky Way. These are far from all stars, but an order of magnitude more than was recorded before.
One of the great things Gaia was able to measure was the color and magnitude of many stars, from faint red dwarfs (and even brown dwarfs) to stellar remnants like white dwarfs, main sequence stars, giants and supergiants that glow brightest. But Gaia observed not only in the visible, but also in the near infrared spectra, which means he saw objects that are hidden from the eyes of people. Among them are supercold stars, both giants and dwarfs. And Dyson spheres, if they exist and have specific temperature / luminosity profiles.
The big, bold line that crosses the diagram from the bottom left to the top right is the main sequence that contains the stars fusing hydrogen into helium. At the top right are stars in a giant or supergiant phase: they burn heavier elements and expand to much larger sizes. Even though they glow brighter, their temperature is lower because energy is scattered over a large area emitting energy.
The Dyson Sphere does much the same thing, but with a normal or low-mass star. You create a large surface area from which the star's energy will escape, and it radiates at a lower temperature, while giving out the same total energy. The infrared signature, in theory, should give us a similar sphere, but the Gaia satellite suggested another option, discovered by Eric Zakrisson: the discrepancy between the distance based on luminosity and the parallax distance.
The parallax method, used since the 1800s, involves observing the change in position of a star next to a more distant, background star. If the parallax and luminosity distances of the star do not coincide, this can explain the alien megastructure … or that the star is in a binary system.
When you infer a distance based on the light you observe, and then measure it in a completely different manner (using geometry), the two numbers should match. The fact that Gaia saw multiple discrepancies could indicate different things, including the structures of the aliens. Human nature is such that we immediately seek the most fantastic explanation. But a more mundane and reasonable reason would be that stars have double companions: this is a fairly common phenomenon in the Universe. The lack of excess infrared radiation required for structures such as the Dyson sphere leads us away from the hypothesis of aliens and their structures.
A number of observatories, including the Gaia spacecraft, have technologies that, in principle, can detect Dyson spheres that are several thousand light-years from Earth, if we assume that they are at the same distance from a star like the Sun as the Earth is from our star. The red dwarf star should be visible in Gaia's eyes with a small Dyson sphere up to a hundred light-years away, but a giant or supergiant star would be visible from virtually anywhere in the galaxy. Among the 1.7 billion objects assembled by Gaia, one could find Dyson spheres under construction. And by comparing data from infrared observatories, one could find ready-made Dyson spheres that emit enough energy. At the time of this publication, however, no Dyson sphere has been found in the Milky Way.
But this does not mean that they do not exist; this means that, if they are, we have not yet seen them. Dyson's spheres may be farther away than Gaia sees, positioned near smaller stars. Infrared observatories such as WISE define the boundaries of the search, and next-generation observatories could potentially detect a signature of heat removal from such an object.
Given the full array of observatories that have surveyed the sky, it is relatively safe to say that we have not yet found any Dyson spheres at this time. Perhaps, somewhere, there are intelligent aliens, using all the energy of their stars entirely and creating huge transplanetary empires, but there is zero evidence of this. There is only one reasonable conclusion: our galaxy, as far as we can judge, does not have these giant alien structures.
Ilya Khel