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At least some advanced civilizations might be producing tons of waste heat by now. And researchers are looking for them.

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Okay, I'm just gonna come out and ask what we're all thinking; where are all the aliens? Based on what we know about life, the universe and everything, there should already be advanced technological civilization sprawling across entire galaxies. Like, if you had a fleet of robots traveling between stars at just a small fraction of the speed of light, it would take them less that 5 million years to spread to every star in our entire galaxy.

Since the universe has been around for more than 13.7 billion years, you'd think there'd be plenty of time for a civilization to develop and spread around the galaxy, but we haven't seen or heard from, or found evidence of anyone, and that's the Fermi paradox. It's named for physicist Enrico Fermi, who, along with some of his colleagues, figured back in the 1940s that there must be other civilizations out there, and some of those, must have rocket technology at least as advanced as ours, and some of those advanced civilizations have had plenty of time to make themselves known.

But here we are, still making books and movies about finding alien civilizations because, I guess that's the next best thing. But scientists are still looking.

One of the best ways to do that is by checking for indirect signs of advanced life. Because we're pretty sure that advanced civilizations would be using a ton of energy, for example, and that should be traceable, which is where a Russian astrophysicist named Nikolai Kardashev comes in. According to Kardashev, once a civilization develops sufficiently, even all the energy and resources of an entire planet can't keep it going. So instead, they have to harness the energy output of entire stars.

But creating, capturing and using all that energy is inefficient, so it would produce lots of heat, usually, as infrared light, which is exactly what researchers might look for in their search for advanced civilizations.

Back in 1983, for example, astronomers used the infrared astronomical satellite, or IRAS to map nearly the entire sky. They ended up with the first large scale catalog of bright sources of infrared light, and while they were at it, they recorded spectra, or object's individual energy fingerprints. Later, other scientists combed through all the data looking for signs of advanced technology by comparing the energy and spectra of all of the bright infrared sources within about a thousand light-years. but they had to be careful because some naturally occurring systems could look a lot like waste heat coming from some advanced technology.

Some of the signals, for instance, could've come from a proto star, where a thick cloud of gas and dust completely envelops a baby star and turns its starlight into heat.

That's why the spectra were so important. Protostar dust contains molecules like silicon carbide, which absorb light at specific energies, changing its spectrum. And after looking through that whole huge catalog, they didn't find any signs of advanced civilizations, it was all just coming from naturally dusty objects. But new missions kept the search going, in 2011 NASA's WISE telescope did a new survey at a much higher resolution. It cataloged thousands of distant galaxies and a new team of astronomers began a project called glimpsing heat from alien technology, which was exactly what it sounds like.

The researchers analyzed WISE's survey of galaxies to see if they were super bright at infrared wavelengths, because that would be a sign that an alien civilization had spread through a galaxy, sucking up energy and producing waste heat.

But even in this huge sample of a hundred thousand galaxies, none showed signs of extra infrared radiation that couldn't be explained by natural processes.

These infrared surveys led to whole bunch of other amazing discoveries, though, like improving our understanding of how stars form and finding some of the tiniest brown dwarf stars ever.

And there are more sky mapping missions in the works, which will mean plenty of new data for future teams to analyze. A proposed mission would install a 300 megapixel camera on a telescope that's basically a clone of Hubble. One of the mission's goals would be to image the entire sky in the infrared, at even better resolution than WISE did. 

The map would give astrophysicists a lot of new information on how stars and galaxies develop, but it'll also be a useful place to continue our search for signs of alien life.

But meanwhile, our answer to "Where are the aliens?" is basically... we don't know. It's possible that we haven't spotted anyone because there are hurdles to building these types of technologies that we don't understand or we're not looking for the right signs. For now, we'll just have to keep looking.

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