a century of Earth's radio transmissions has now washed over thousands of other star systems carrying with it some of our greatest broadcast masterpieces as well as our worst reality TV what are the chances that alien civilizations have detected and even decoded these signals we are a young technological civilization an advanced alien race monitoring our planet even from the nearest neighboring star would have seen nothing radio quietness until only around a century ago then beginning with the faint sporadic buzz of the first experiments with wireless transmission the radio brightness of this small planet would have bloomed into a continuous planet wide cacophony of TV and radio broadcasts satellite relays and radar pulses this bubble of chatter is as I speak spreading out into the galaxy of the speed of lights the unmistakable signature of an emerging technological power the outer edge of this bubble is now over 100 light-years away and it carries with it the first transatlantic radio transmission of Marconi himself that edge may be hopelessly diffused but it's followed closely by the much brighter 80 Lightyear shell on which rides the footage of the 1936 Berlin Olympics episodes of The Lone Ranger and Orson Welles ill-fated radio adaptation of War of the Worlds that shell has washed over several thousand star systems based on Kepler space telescope observations that also means thousands of potentially habitable planets lots of chances for hypothetical civilizations to have learned of our existence so the question is who could have seen us what manner of civilization would be powerful enough to sense this bubble what tech level would be needed to actually decode the signal it carriers if aliens arrive on our doorstep tomorrow would they have been alerted to our presence by Nikola Tesla's first experiments broadcast footage of the moon landing the Phantom Menace to answer the question of who can see us it's helpful to first ask who could we see after all humanity remains the one example of a technological civilization for now our best astronomers have been engaged in the search for extraterrestrial intelligence SETI in earnest since the 60s when Frank Drake first peeked at the Stars Tau Ceti and Epsilon Eridani Drake peered into the so-called waterhole a narrow frequency range in the radio spectrum between a pair of H and O H emission spikes which itself is within the broader window from 1 to 10 gigahertz there the natural universe is especially quiet surely this is the window where aliens would choose to broadcast their messages welcoming us into the galactic community and of course Drake saw nothing decades of SETI programs followed utilizing several of the world's great radio telescopes like Arecibo in Puerto Rico and the Parkes radio telescope in Australia these searchers often focused on the water hole or the surrounding quiet frequency window but with the exception of some one-off oddities the search was not successful the 1977 WoW signal is the most compelling a narrow frequency radio blast detected in the water hole that still has no broadly accepted natural explanation but no one ever saw it again apparently no one is trying particularly hard to get our attention and it's important to understand that these searchers could only have been successful if there were deliberate powerful beacons emitted by an alien civilization the radio leakage produced by their own into broadcasts would be much harder to detect our best targeted search so far the SETI Institute's project Phoenix scanned eight hundred stars within two hundred light years at that range it could have picked up a gigawatt beacon beam directly at us but it would only have picked up leakage from a civilizations internal broadcasts from within a couple of light-years if those broadcasts were the same strength as our own in that range there is exactly one star house an alien civilization at exactly our current tech level would never have seen our TV bubble we have sent out more powerful beacons like the stream of 10,000 tweets and celebrity videos that Arecibo blasted in the direction of the WoW signal but that's a long shot and perhaps for the best I'm not sure we want our first contact to be a typical slice of the Twitterverse one of the reasons it's hard to spot unintentional radio leakage is that a distant civilizations radio bubble is likely to overlap in frequency with our own transmissions this is another advantage for searching for alien signals in and around the waterhole at around 1,500 megahertz it's in the ultra high-frequency UHF band most TV broadcasts are our apt that tens to hundreds of megahertz of the merely very high frequency VHF band VHF works better for over air broadcast because it carries further and is less blocked by buildings and such so we often search the UHF to avoid the thick soup of our own VHF chatter but surely aliens are working with the same physics that we are perhaps we miss each other because we can't easily peer through our own local VHF soups but there is a way to pierce straight through our own radio noise as though it wasn't there the answer is interferometry two radio telescopes separated by a large enough distance can filter out local transmissions a signal that appears in one but not the other must be an earthly transmission now that's not the main purpose of interferometry that is to increase the spatial resolution of your observations you get the resolution of a telescope as large as the separation of the component telescopes also super useful interferometry has been around for a while but we're only now building an interferometer facility with the sensitivity to potentially see an alien TV bubble too many light-years and that is large enough to filter out local noise it's the Square Kilometre Array or ska the ska is actually many many telescopes arrays of thousands of radio dishes will be built in Africa and hundreds of thousands of antenna installed in Australia when connected up it will effectively form a giant radio telescope with over a square kilometer surface area but as an interferometer it has the resolution of a telescope thousands of kilometres across its sensitivity and pinpoint resolving power will be vastly greater than any telescope of any type that we have ever built the SKA is not being built to search for aliens one of its primary purposes would be to catch the radio emission from hydrogen gas in the extremely early universe that emission produced at 21 centimeters wavelength or 1420 megahertz frequency is by definition one of the boundaries of the waterhole but if such radio waves travel to us from the earliest of times then they become stretched out as they travel through an expanding universe that redshifted early hydrogen emission is now found slap in the middle of the noisy lower frequency part of our own TV broadcast spectrum to spot these radio photons we need a truly gigantic interferometer both for extreme sensitivity and to eliminate our own radio buzz that's the sk such a machine also just happens to be perfect for spotting alien television bubbles according to the calculations of Harvard University's Harvey lobe and Mateus alder Yaga the SKA should be sensitive enough to spot our own TV bubble from a hundred or more light-years away so will we soon be picking up extraterrestrial sitcoms not quite loeben Zelda Yaga's numbers assume pointing ska at a target star system for an entire month and adding up all of the radio emission over that time for an artificial radio source that would look like a mission over a narrow frequency range that doesn't correspond to any natural process these emission spikes may also shift back and forth in frequency due to Doppler shift as the distant technologically advanced planet orbited star so that's enough to identify a technological source but not to actually decode the signal an alien civilization with their own ska certainly couldn't watch earth TV to do that they would need to achieve sk a's one-month sensitivity in a tiny fraction of a second that means compensating for the difference in exposure or integration time with sheer size of the telescope let's figure it out what would be needed to say tune in to the first season of the original Star Trek series that signal is now 50 light-years away assume a bit rate of 10 million per second for a miserable 30 frames per second at 200 by 200 resolution and only 8-bit color forget audio which is a shame because the original series dialogue is awesome the aliens would need a radio telescope trillions of times the SKA surface area and equivalent to a dish around three times the radius of the moon's orbit this is perhaps conceivable for a type two civilization although it's exceedingly unlikely that there's one of those within our radio we may even be a little optimistic in thinking that the SKA will detect alien TV as the setting institute's John Billingham along with James Bedford point out that one month integration time requires a very consistent narrow frequency signal which may not be consistent with typical broadcasts but remember Humanity is young as a radio noisy civilization simple probability says that any other civilization is likely to be ahead of us on the curve it's not how to imagine them building a much larger ska without going full type to our ska will cost a few billion dollars by the time it's done in 2030 that's a significant investment but only a tiny fraction of the global GDP for a slightly more advanced civilization that's several Moore's law doublings ahead of us in various technologies and accounting for moderate GDP growth over a century or two a super ska with several hundred square kilometres of collecting area would constitute the same degree of investment as our own one Square Kilometre Array in that case Earth's TV bubble would be spotted within hours or even minutes of aliens pointing the facility to our solar system even if they weren't looking for us surely any advanced civilization is curious about its own origins that redshifted 21-centimeter hydrogen emission really is one of the most important keys to understanding the very early universe it's pretty natural to want to scan the heavens at exactly the frequencies where we Earthlings are the noisiest if there's a slightly more technologically advanced civilization within our radio bubble they've probably seen us however our steady broadcasts have only washed over a few hundred solar-type star systems and only a few thousand stars total technological civilizations would need to be extremely common throughout our galaxy and I mean tens of millions of them across the keyway for there to be any chance that there's another one so close to us but we just don't know if anyone else inhabits this very local region of the galaxy then they may have been aware of us for some time now any civilization within 40 to 50 light years could have sent a return signal that will reach us any day their fleets of welcome ships will be a little further behind but no doubt carry only the best wishes for their noisiest neighbors in nearby space-time hey