Arecibo message: What happened when people claimed aliens contacted them – and why we might never want to

When the Arecibo message was sent into space in 1974 – blasting the most powerful signal ever broadcast deep into the universe – it was a pioneering attempt to reach out to aliens, wherever they might be.

When a response came back in 2001, it was a hoax that showed just how much some people hope we can actually communicate with extraterrestrials. But that fake reply might actually be the message we ever get back from aliens.

That's because some of the greatest minds on Earth fear rather than hope for contact with extraterrestrial life. Because the real reply to the Arecibo message might actually be something far more terrifying.

The hoax "Arecibo answer" or "reply" was a crop circle that was found in a field in the UK in 2001. Pressed into a field in Hampshire, it seemed to mirror the look of the original Arecibo message, which gave a host of information about life on Earth that the originators hoped would be decodable to people anywhere in the universe.

Where the original Arecibo message had shown blocks of information including depictions of important chemicals like carbon, an illustration of DNA and a picture of a human being. The hoax reply was probably too perfect: carbon was swapped for silicon, the DNA was altered and the human being was replaced with a big-headed alien of the kind seen in science fiction.

It was of course a hoax: among other questions, sceptics wondered why aliens wouldn't simply have sent a radio message, as we did, rather than dropping down to Earth to press a picture of themselves into crops and then failing to actually alert anyone. But it was a demonstration of the way the original Arecibo message and the search for alien life that it grew out of continues to occupy the minds of people around the world.

Scientists don't ever expect to actually receive a reply to the Arecibo message: it was powerful but specific, pointed at a galaxy cluster that will actually have moved out of the way by the time it travels the 20,000 light years to its destination. It was probably most useful as a demonstration of the technology and a thought experiment that led people to think about what such a message might actually need to say.

Similar efforts have been launched that could have more positive results, however. A new initiative called Breakthrough Message – part of a broader group supported by a Russian billionaire and Stephen Hawking, which aims to find life elsewhere in the universe – has invited people to consider how best to write and send a message out into space, though has cautioned that for the moment it will not send out those messages.

But some fear that a response wouldn't come in the form of a message at all. Instead, it would be something altogether more dangerous – and that's why they advise that we shouldn't send further signals like the Arecibo message, for fear that they would only put us in peril.

Stephen Hawking, for instance, famously said that we should be wary of sending out messages into the universe for fear that they could be answered by someone looking to wipe us out – or, perhaps worse, someone who might wipe us out purely by accident. "‘If aliens visit us, the outcome would be much as when Columbus landed in America, which didn’t turn out well for the Native Americans," he said in a 2010 documentary.

And such criticism was even in place when the Arecibo message was first sent. Just days after the dispatch was shot into space, the Royal Astronomer of England, Martin Ryle, launched an intense blast of criticism at the idea, arguing that it might only alert aliens who are "malevolent or hungry", and asking the International Astronomical Union to ban any future messages.