Even before Stephen Spielberg’s latest film, Disclosure Day, began unspooling at local multiplexes, the internet was debating whether we would ever experience a real-life disclosure day – when the US government admits that it’s aware of aliens here on Earth, a secret it has supposedly kept since the 1940s.

That would be dramatic news. But don’t hold your breath.

For decades, surveys have shown that a majority of Americans believe that extraterrestrials inhabit the cosmos. That’s a view shared by many scientists (including me). The reasoning is straightforward: either aliens exist, or there’s something extraordinarily remarkable about the existence of life on our planet – an argument that sounds like special pleading.

Many among the citizenry also suspect that the federal government is hiding the fact that aliens are present here on Earth. The authorities supposedly keep this under wraps to forestall widespread panic.

Does this make sense? Could the extraterrestrials really be here, sequestered in Nevada’s notorious Area 51 or some other clandestine locale?

Well, of course it’s possible. You can’t rule out this cinematic premise with an appeal to science. After all, it doesn’t violate physics to rocket from one star system to another, even if only to conduct breeding experiments on the locals. However, science does tell you that it’s improbable. Interstellar travel is enormously difficult and immensely costly (in terms of energy), given........

© The Guardian