He made the remarks shortly after hosting the national science academy’s first conference on the possibility of alien life.
“They could be staring us in the face and we just don’t recognise them. The problem is that we’re looking for something very much like us, assuming that they at least have something like the same mathematics and technology,” he said.
“I suspect there could be life and intelligence out there in forms we can’t conceive. Just as a chimpanzee can’t understand quantum theory, it could be there as aspects of reality that are beyond the capacity of our brains.”
Lord Rees used the conference in January, entitled “The Detection of Extraterrestrial Life and the Consequences for Science and Society”, to ask whether the discovery of aliens would cause terror or delight on earth.
He told Prospect magazine that improved telescopes made the chance of finding extra-terrestrial life “better than ever”.
But Dr Frank Drake, the world’s leading “ET hunter”, told the conference that satellite TV and the “digital revolution” was making humanity invisible to aliens by cutting the transmission of TV and radio signals into space.
At present, the Earth is surrounded by a 50 light year-wide “shell” of radiation from analogue TV, radio and radar transmissions. But although the signals have spread far enough to reach many nearby star systems, they are rapidly vanishing in the wake of digital technology, according to Dr Drake.
The scientist, who founded the Search for Extra-terrestrial Intelligence organisation in the United States, said digital TV signals would look like noise to a race of observing aliens.