But that's not really what it means. What the study (which started in 2012) really found was that there were no infrared emissions that have the signature of a Dyson sphere.
Dyson spheres have appeared in many science fiction novels and shows, including Star Trek. Larry Niven's Ringworld was a variation on a Dyson sphere, a ring of superstrong metal that circled a star.
However, a truly advanced society would never find itself needing to build such a huge structure to house unchecked population growth. Long before they had the technological capability to dismantle entire planets and build structures hundreds of millions of miles across, they would have to learn to control their reproductive urges. If they didn't, they would wind up warring over limited resources, bringing their civilization to ashes time and time again until they finally killed everyone off, long before they were capable of building a Dyson sphere.
Furthermore, if you're really advanced, a Dyson sphere isn't that great an idea. First, it makes you completely dependent on a single star. That's a single point of failure, which is always a bad idea.
Then, the sphere itself is also a single point of failure. It's a really big and really heavy target with a massive star at its core. Every piece of junk in the star system and its environs is eventually going to be yanked into a collision with the sphere, in much the same way that Jupiter and the other gas giants keep pulling comets out of the Oort cloud toward the sun.
And this Dyson sphere would be huge, like two hundred million miles across. Every sizable piece of junk in its way as it orbits the core of its galaxy is going to put a big hole in it. An advanced engineer would make the design failsafe, which means all its defenses would have to be completely passive. A planet can take a big hit from an asteroid because its gravity will retain its atmosphere no matter how big a hole is punched in it (within limits, of course). Holes in a sphere will make it leak and eventually come apart. A Dyson sphere would require constant repairs, and a wise designer wouldn't assume that future inhabitants would always have the technology and knowledge to make such repairs.
Then, stars are messy. They have massive flares, spewing out lots of radiation and a constant flux of solar wind. All those waste products would have to be dealt with in a Dyson sphere. Earth's magnetic field and atmosphere shield us from that stuff. Most of the bad stuff just slips around earth into space. In a Dyson sphere you'd have to collect it all and deal with it, converting it to useful energy or ejecting it.
Over time stars grow, explode, shrink and ultimately die. That takes billions of years. But a civilization that builds on such a scale would be used to thinking in such terms. Building a structure that's supposed to be the eternal home for your race around a star with a finite lifetime is counterproductive.
Practically speaking, where's everyone going to live while you're dismantling all the planets in your star system and constructing a giant sphere that occupies the same space where your planet is orbiting? It just doesn't seem possible unless you build it around another star and then move everyone there. But if you can do that, why build Dyson spheres at all? There are billions of planets that could be made habitable with far less effort than building a Dyson sphere.
Finally, a civilization that wants to ensure its survival doesn't put all its eggs in one ... giant egg. It makes much more sense to build a lot of little planets that are all independent instead of a single gigantic system that's stuck around a single star.
Since the 1960s, when Dyson made his proposal, human technology has advanced. We've discovered a lot of ways to do things more efficiently. In some cases, much more efficiently. A truly advanced civilization will ultimately figure out an efficient means to produce energy via nuclear fusion, or using other principles that we've only speculated on -- antimatter or zero-point energy, for example.
The more advanced a civilization, the more efficient it would be, making it less likely that we would be able to detect its waste products. For example, as time has gone on, our communications technologies have become much more focused and lower powered. For our fastest data transmissions we use fiber optic cables. Our TV, radio and mobile phone broadcasts have been digitized and drastically lowered in power. Our powerful microwave transmitters are aimed in tight beams at receivers instead of broadcast like old-style AM radio.
We have concentrated on increasing the sensitivity of our receivers, decreasing the size of our electronics and reducing the power of our transmitters. This saves on battery life. Our cars and industrial processes are constantly doing more with less fuel and raw materials.
In other words, an advanced civilization won't need the entire energy output of its star. They would be smarter than that.
In the 1960s when Dyson came up with his idea, we were obsessed with bigness. We were building rockets to the moon. We were building nuclear aircraft carriers the size of cities. We still do big stuff, but now we know bigger isn't always better.
Dyson's underlying assumptions were that everything will grow bigger and bigger, and population will grow without check, resulting in energy consumption without check. These assumptions are without historical precedent: one thing history has shown us is that resources and technology have limits. Civilizations that recognize and work within -- or increase -- those limits survive.
Those that ignored these limits died.
