There isn't much of a plan for what we'd do if we heard from aliens, according to the man probably most likely to make contact with them.
Andrew Siemion, scientific director of the Breakthrough Listen project, has said that he spends just a few minutes each day thinking about how he would announce if the project actually found alien life. Breakthrough Listen is a huge plan funded by Russian billionaire Yuri Milner, which is picking through potential signals sent to Earth to find one that could be a message from aliens.
“Maybe for five or 10 minutes right before I go to bed, I think about what we might do if we actually found a signal,” he said in an interview with PRI. “It’s very difficult to predict how the world would react to that. But personally, I think it would be a very positive reaction. I think it would be a very unifying moment.”
He also said that he hasn't decided whether or not humanity would send a message back, if it heard one. Stephen Hawking, who has been involved in Mr Milner's projects, has actively warned that responding to any message would probably lead to the annihilation of life on Earth.
“Personally, I think... that’s a question that should involve the entire planet,” he said.
That is in line with most guidance – and there is only guidance, since no rules for announcing alien contact or replying to it have ever been written into international law. The International Academy of Astronautics (IAA) has a set of protocols for those involved in the search for extraterrestrial life – which stipulate that anyone who finds a signal should make the world aware, and shouldn't respond to it – but those are only protocols, and nobody could be prosecuted for breaking them.
As such, once a signal is found, it is unlikely that scientists would be able to stop someone with the right equipment from broadcasting a message. That is despite continuing warnings from people like Professor Hawking, and raging debate among scientists and others about whether we should ever actually speak to aliens.
But even if someone did make contact, it's unlikely that anyone alive would actually get to experience any kind of conversation – or see the wiping out of humanity that Professor Hawking warned would come.
The nearest potentially habitable world outside our solar system is Proxima b, which is a mere 4.2 lightyears away but probably doesn't actually contain any life. Even if it did, each message would take 4.2 years to send and the same time to get back – limiting any conversation – and without unimagined technologies, it would take many years for any alien life to actually arrive on Earth.
Some scientists claim to have intercepted messages which could have come from aliens, and Breakthrough Listen called those results "promising" early this year. The project published its initial results last week – but said that it hadn't yet found much.
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