It has been called the Fermi paradox, after the physicist Enrico Fermi, and it asks why – if it is so probable that life exists elsewhere in the universe – haven't we found any?
The common suggestion in response to this paradox is to explain that the universe is very, very big – and there are a lot of places we could potentially find some life. Looking everywhere takes some time.
Now, a trio of astrophysicists have taken another look at the "needle in a haystack" analogy and have created a model to assess how much work has actually been done to search for aliens.
In the paper written by the scientists from the Pennsylvania State University, the trio question how much work projects such as SETI (Search for Extra-terrestrial Intelligence) have actually done.
They compared this work against the hypothetical amount of searching that would need to be done before scientists can believe it is probable that there is no extra-terrestrial life.
In 2010, Dr Jill Tarter famously compares the search for alien life to looking for fish in a glass of water while standing beside the unexplored ocean.
The new paper by Jason Wright, Shubham Kanodia and Emily Lubar, examines how much of the universe SETI can be estimated to have analysed, and in what detail.
The researchers note Dr Tarter's suggestion, stating that she "and others have argued strongly to the contrary" about the lack of signals from space which have been detected.
"Bright and obvious radio beacons might be quite common in the sky, but we would not know it yet because our search completeness to date is so low," they argue.
In response they have developed what they describe as "the metaphor of the multidimensional 'Cosmic Haystack' through which SETI hunts for alien 'needles' into something scientifically measurable".
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With this model, they compare the volume of space that has been searched as closer to a large hot tub or small swimming pool set against the world's oceans – but suggest that there are different kinds of needles which could be located.
The paper describes different qualities of the radio signal "needles" that might be detected, and notes that you only need to search the whole haystack to prove that there are zero needles – otherwise, just finding one will be enough evidence.