In our search for alien lifeforms we scan for primitive biosignatures, and wait and hope for their errant artificial signals to happen by the Earth.

But these may not be the best way.

Any energy-hungry civilization more advanced than our own may leave an indisputable technological mark on the galaxy.

And yes, we’re very actively searching for those too.

Time to update you on the hunt for galactic empires.

Humanity is an energy-hungry species, and that hunger grows exponentially.

Fortunately we have a near-inexhaustible power source raining down from above.

We could satisfy humanity’s current energy needs by covering a tiny fraction of Earth’s surface with solar cells.

And we should probably do that by the way.

But we can expand that area indefinitely.

Not by covering the entire Earth, but by building solar collectors in space.

That’s a lot of room to expand if we were to cover the entire sphere surrounding the Sun with the radius of Earth’s orbit, we’d collect all of the Sun’s light - a billion times more than what we’d get just covering the planet in solar cells.

The idea of wrapping the sun in solar collectors is not new.

It was first proposed by British author Olaf Stapledon in his 1937 story Star Maker.

It was also named the Stapledon sphere - but only by the guy who was trying to avoid having his own name attached to the idea, without success.

In 1960, Freeman Dyson was inspired by Stapleton’s fiction to propose that a highly advanced civilization might expand to occupy an entire sphere surrounding its home star at the radius of its planet of origin, for the purpose of both habitation and energy collection.

To Dyson’s regret, the notion became called the Dyson sphere.

Naturally we’ve talked about the plausibility of Dyson spheres in the past - long story short, not plausible as a solid shell, but pretty reasonable actually as a swarm of independent craft, either in orbit around the star or kiting in place on the solar wind.

But Freeman Dyson’s original paper was not about how to build one of these things - rather how to find them.

If the Dyson sphere - or swarm or whatever - is a natural stage in the progression of many civilizations, might these be the best way to look for signs of intelligent life?

Let’s find out what our search has told us.

Earth’s biosphere is already a pretty good planet-scale solar collector.

If you were to observe Earth from a distant solar system, you might notice that it looks strangely dark.

It doesn’t reflect nearly as much of the Sun’s light as you’d expect for a globe of water and rock - it's as though that energy is being sucked up.

But if you shifted your gaze from visible light to infrared, you’d find all of the missing energy.

See, the Sun produces a thermal spectrum - light generated by its 6000K surface is distributed at all wavelengths, but it peaks in the visible part of the spectrum.

And Earth’s biosphere - mostly its forests - absorb a fraction of that light and use it to power biological processes.

And in the process they warm up, shedding all of that energy as a new thermal spectrum, now at 300 or so Kelvin, with its peak at infrared wavelengths.

To spot this spectral shift from another star you’d have to have an incredibly sensitive telescope to pick the Earth’s faint glow from the Sun’s overwhelming brightness.

But if a civilization has expanded to reprocess a significant fraction of its home star’s light, then not only could we detect that shift with our current telescopes, it would be difficult to miss.

And that was also true even back in 1960 when Dyson proposed that we look for these things.

So how do you go about searching for a Dyson sphere?

Dyson’s original notion was simply to search for points of light with temperatures of a few hundred Kelvin, but emitting the power of an entire star.

Anything capable of producing that much power at that low a temperature must be huge - the size of a planetary orbit.

In a 1966 paper, Carl Sagan and Russell Walker added some details to the calculations, figuring out how close a Dyson sphere would need to be in order to be detected with the technology of the time.

Short story, it's would have to have been pretty close.

But our infrared telescopes advanced quickly, and crucially they found their way into space.

Surveys started to find many, many objects that fit the description.

Unfortunately, there are some obvious natural explanations for a planetary-orbit-sized object at a few hundred Kelvin temperature.

For example we have protostars - the clouds of gas in the process of collapsing into a new star, circumstellar disks - the luke-warm disk of gas surrounding a new-born star that will eventually form planetary systems.

It turns out it’s near impossible to identify a full Dyson sphere from its thermal emission alone.

Sagan and Walker suggested that we consider these infrared sources as candidate Dyson spheres, to be followed up by scanning for narrow-band radio emission, a much more telling signature of technology.

So yeah, we could go flicking through the frequency channels of gas clouds looking for alien TV shows.

Or we could think about this a bit more.

Dyson and Sagan etc’s calculations were for a full Dyson sphere - one that reprocesses all of their star’s light.

But many civilizations may find it totally adequate to only partially harvest their star’s light.

Partial Dyson spheres could take the form of equatorial rings of collectors, or swarms that don’t fill the entire spherical surface.

Or really any so-called megastructure that intercepts a decent fraction of the star’s light.

It turns out that a partial Dyson sphere might be even more detectable than a full one.

To understand how to spot such a thing, let’s learn a thing or two about stars.

Stars are surprisingly simple beasts, for the most part ruled by laws of physics that we’ve understood for centuries.

A star in the prime of its life settles into an equilibrium state in which the outward flow of fusion-generated energy balances the inward crush of its own gravity.

If you know the mass of such a star, you can predict its size, its temperature, its brightness, its lifespan, and so on.

If a star deviates from the tight relationships between these properties then we quickly know that something is up.

For example, it might be surrounded by a Dyson sphere.

Let’s think a bit about what that will look like.

It would be an unnatural combination of a large, hot object - the central star, and a gigantic cool object - the partial sphere.

Each would produce its own thermal spectrum at different temperatures.

But from our great distance the light from the star and the sphere would be blended into a point.

Two pure thermal spectra would be stitched into one weird spectrum with too little light at visible wavelengths and too much at infrared wavelengths.

If we carefully broke up the star’s light with spectrographs spanning a huge wavelength range, we might be able to see two distinct thermal spectra.

But that’s a lot of work even for a single star, and we probably need to search a huge number of stars to hope to find even a single Dyson sphere.

Fortunately, even a crude observation of a star’s color and brightness can tell us that something is up.

In astronomy, color refers to the ratio of brightnesses at two different wavelengths.

For something with a pure thermal spectrum, we can exactly identify its temperature from, say, the ratio of red light to blue light.

I mentioned that most stars show a tight relationship between certain properties.

For example, the higher the surface temperature of a star, the higher its total power output - it’s luminosity.

If we graph temperature against luminosity we get something called a Hertzsprung-Russell diagram.

For temperature we can just use color - because that’s easier to measure.

All stars that are in the primes of their lives - those powered by fusing hydrogen into helium in their cores - lie on this tight band called the main sequence.

Stars drive off the main sequence when they expand into giants at the ends of their lives, or, presumably, when surrounded by giant alien astroengineering projects.

Let’s see what a Dyson sphere would actually do to a star on the HR diagram.

At visible wavelengths a star’s color might not change much - it’ll just look dimmer.

So a star might appear too faint for its apparent temperature - dropping below the main sequence.

But if we measure its colour using an infrared wavelength along with our visible light, we’d find too much of that IR.

The star would look cooler - lower temperature than suggested by its visible color alone.

Jun Jugaku and Shiro Nishimura, Japanese astronomers who’ve been searching for Dyson spheres for years, calculate that a partial sphere that intercepts only 1% of its star's light could shift the visible-to-infrared color by a factor of more than two.

Over the decades, several teams have used successively more advanced instruments to look for stars that were both too dim and too infrared, time and again they came up short.

But the search has expanded massively.

One of the latest efforts led by Swedish astronomer Erik Zackrisson looked at over 200,000 of the stars in the gigantic survey conducted by the Gaia satellite.

The team started by looking for stars that were a little too faint for their visible-light color - below the main sequence.

They found a couple of promising candidates.

Then they checked whether these stars had an excess of light in the infrared.

And … they did not.

Not enough at any rate.

Apparently there is non-Dyson-sphere-related weirdness going on, which may be interesting, but still isn’t aliens.

One other approach to finding partial Dyson spheres is to look for strange variations in brightness and colour over time, as might be expected if a giant orbiting structure partially eclipses the star in an irregular way.

And that’s exactly what happened with the famed Tabby's Star.

This weird star discovered by Tabetha Boyajian, exhibited complex dips in brightness, causing rumours of alien megastructures to abound.

But in 2019 it became somewhat clear this dimming was probably due to the debris of a tidally disrupted moon-size body.

Yup, still no Dyson spheres.

At this point, our surveys have found no evidence for star-spanning megastructures.

And with new surveys that are being planned, within a decade or so we’ll either discover one or rule out anything interesting in the local part of the Milky Way.

But that doesn’t mean Dyson spheres aren’t out there.

It would be incredibly difficult to detect such an object around a low mass star if it’s on the other side of the galaxy.

What we do know is that these structures are not very common through the Milky Way.

But that may not be true of other galaxies.

There are hundreds of billions of galaxies in the observable universe.

Unfortunately, we can't identify a single Dyson sphere's effects around individual stars at distant galaxies.

But what if a civilization in one of those has occupied its entire home galaxy and built a huge number of Dyson spheres, or similarly energy-hungry megastructures?

With this possibility in mind, a team led by Penn State astronomer Jason Write used the WISE survey to look for galaxies that had too much infrared light compared to their visible light.

Now this is a bit tougher, because galaxies aren’t as simple as stars.

There can be a huge variation in the amount of IR that a galaxy produces.

But even accounting for this, the team found no evidence for galaxy-spanning civilizations in the 100,000 galaxies that they analysed.

No very energy-hungry ones at any rate.

The search for galactic empires is expanding.

In the past couple of years researchers have been thinking about Dyson spheres built around the supermassive black holes in the centers of galaxies.

And about the effect on the enclosed star due to a partial Dyson sphere.

These are all long shots, and honestly might seem a bit hopelessly science-fictiony.

But some very serious scientists - starting with Freeman Dyson and Carl Sagan - take the question very seriously.

For good reason too.

The discovery of a single alien megastructure of any sort would massively change the way we think about our place in the universe.

It would also point to a possible future for humanity - something we might want to do when we grow up.

It would show us that we might even have a future - one that could even leave our own astroengineering mark on the galaxy, perhaps to be noticed by younger species when they emerge in distant, future parts of space time.