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They’re ghosting us Maybe aliens have a policy not to interfere with less advanced cultures. A bit like the “prime directive”, on Star Trek. They don’t live long enough to get in touch This idea, known as the Great Filter, says that no advanced civilisation survives long enough to still be around when its neighbours are thriving. For us, threats like nuclear war, climate change or pandemics might spell our doom. They’re already here From Roswell to The Lubbock Lights, there have been plenty of UFO sightings on Earth. Is the government hiding the evidence? Or is it fake news? If there are any aliens watching, why not say hi in the comments below? Better yet, click like and subscribe. You might learn a lot about human behaviour from our videos. And, both Aliens and earthlings can get 20 per cent off a New Scientist subscription by using the code SAM20. The silence from other civilisations is not for want of looking on our part. In 1960, Drake pointed a radio telescope towards two nearby stars, and waited for that hotline bling. Instead of intelligent messages he got a load of static, and interference from a secret military experiment. Nevertheless, the Search for Extra Terrestrial Intelligence, or SETI, was born. Then, on 15 August, 1977, another SETI program recorded a brief radio signal coming from the direction of Sagittarius. Jerry Ehman, the astronomer analysing the data at ‘Big Ear’ telescope that day, was so excited that he wrote “Wow!” in the margin of the print-out. So far, that ‘Wow!’ signal stands as our most hopeful sign of alien communication yet. But listening for ET to phone home is a bit like standing by the payphone, waiting for the phone to ring. Maybe we have to make the first move. Drake tried this too, sending a radio message towards globular star cluster M13. Known as the Arecibo message, it showed potential aliens our DNA structure, solar system and some of the biochemicals of earthly life. We also tried sending aliens a greeting onboard space probes Pioneer 10 and 11, which included images of humans and directions to our planet. Then, in 1977, we sent a more ambitious message, full of audio and images, on board the Voyager probes. We even included a mixtape of some of our favourite tunes, including Bach, Beethoven, Louis Armstrong and Chuck Berry, to impress any friendly lifeforms – or at least the ones with ears. And if that didn’t entice them to get in touch, maybe this illustration of humans eating, licking and drinking might?! By now aliens might know quite a lot about us – including what we need to survive and how to reach us. And you might be wondering whether it’s such a good idea to be broadcasting our location all over the galaxy, when we don’t know who’s listening. “It’s like shouting in a forest before you know if there are tigers, lions, and bears”, says Dan Werthimer, a SETI researcher. Worse still, these bears might have planet-destroying interstellar missiles. Putting aside the question of whether we should try to contact aliens, are we even doing it right? These alien societies might be millions of years old, so to them our radio messages might look as outdated as flip phones, pagers, and, well, radio. Plenty of other ways to look for life have been tried or suggested. Here are a few options: Look for biosignatures Over time, the biochemistry of billions of creatures can transform a world in distinctive ways. By analysing the wavelengths of light that come through the planet’s atmosphere, we can look for gases that suggest the presence of life. Look for technosignatures if we do see signs of life on another planet, that won’t tell us whether it is inhabited by mindless green slime or sentient city-builders. A more reliable approach to look for kindred spirits is to look for chemicals that only intelligent civilisations can produce. City lights Aliens with night vision as limited as ours might festoon their built environment with artificial lighting. So maybe we could spot