Attempts to probe the Universe for advanced civilizations may be unscientific, says Philip Ball, but that shouldn't stop us.
Enrico Fermi's famous question about intelligent extraterrestrials — "Where is everybody?" — still taunts us today.
In New Mexico in the summer of 1950, Fermi was walking to lunch at Los Alamos National Laboratory with Edward Teller, Emil Konopinski and Herbert York1, discussing a recent spate of UFO reports. When they sat down to eat, Fermi suddenly piped up with his question, then did a back-of-the-envelope calculation to show that we should have been visited by aliens long ago.
As this hasn't happened, Fermi reasoned, it must mean either that interstellar travel is impossible or that technological civilizations don't last long enough to develop it.
Fermi's thinking was fleshed out in the 1960s by astronomer Frank Drake, then at Cornell University in Ithaca, New York. Drake's celebrated equation estimates the probability of extraterrestrial technological civilizations in our Galaxy by breaking it down into the product of the various factors involved, such as the fraction of habitable planets and the number of them on which life appears.
Meanwhile, researchers began to wonder whether we might see signs of technological civilizations from afar, for example through radio broadcasts of the type that are currently sought by the Search for Extra-Terrestrial Intelligence (SETI) project, based at the SETI Institute in Mountain View, California.
Anybody out there?
Now, another approach to hunting for extraterrestrial life has been described by physicist Richard Carrigan of (appropriately enough) the Fermi National Accelerator Laboratory in Batavia, Illinois.
Carrigan has combed through the data from 250,000 astronomical sources — found by the IRAS infrared satellite during its 1983 mission — to look for the signature of solar systems that have been technologically manipulated in the manner proposed by physicist Freeman Dyson in the 1960s2.
Dyson suggested that a sufficiently advanced civilization would baulk at the prospect of its star's energy radiating uselessly into space. They could capture it, he said, by breaking up other planets in their solar system into rubble that would form a spherical shell — known as a Dyson sphere — around the star, creating a surface on which the solar energy could be harvested3.
A Dyson sphere would be warm, re-radiating some of the star's energy at a much lower temperature, perhaps around 300 K. This would show up as a far-infrared object unlike any other currently known. If Dyson spheres exist in our Galaxy, said Dyson, we should be able to see them — and he proposed that we should look.
That's what Carrigan has now done. In 2004 he reported a preliminary search4, but the new data set is sufficient to spot any Dyson spheres around Sun-like bodies out to a distance of 300 parsecs (approximately 1,000 light years).
It will surprise few that Carrigan finds no compelling candidates. After weeding out false positives, for example from old stars surrounded by thick dust clouds, just 17 candidates remained, of which most — indeed perhaps all — can be given more conventional interpretations.
Dyson feels that Carrigan may have been rather too stringent in whittling down the list of candidates. Carrigan, for instance, excludes any source that doesn't radiate energy like a so-called 'black body'.
Yet the obvious question hanging over all of this is: who says advanced extraterrestrials will want to make Dyson spheres anyway?
History shows that we find it hard enough to predict where technology will take us in just 100 years' time. Although it might be fun to speculate about civilizations more technologically advanced than ours, one might say that this strays beyond science.
But in a sense, Dyson was leaning on the Copernican principle, which assumes that the human situation — and by extension, human ingenuity — is representative rather than extraordinary. It has recently been proposed5,6 that this principle may be put to the test in a different context, to examine whether our cosmic neighbourhood is unusual — whether we are, say, at the centre of a large void, which might provide a 'local' explanation for the apparent cosmic acceleration that motivates the idea of dark energy.
It is hard to see that a survey like Carrigan's could ever 'prove', or even provide compelling evidence for, the existence of a Dyson sphere and the hypothesis that the paper probes might therefore be called 'unscientific'.
But how sad it would be to declare that speculations like Dyson's, or experimental searches like Carrigan's, are out of scientific bounds. As long as we see them for what they are, efforts to gain a foothold on metaphysical questions are surely a valid part of the playful creativity of the sciences.
Jones, E. M. Manuscript LA-10311-MS. Published by Los Alamos National Laboratory (1985).
Carrigan, R. http://arxiv.org/abs/0811.2376 (2008).
Dyson, F. J. Science 131, 1667–1668 (1960).
Carrigan, R. IAC-04-IAA-1.1.1.06, 55th International Astronautical Congress, Vancouver (2004).
Caldwell, R. R. and Stebbins, A. Phys. Rev. Lett. 100, 191302 (2008).
Clifton, T., Ferreira, P. G. and Land, K. Phys. Rev. Lett. 101, 131302 (2008).
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Ball, P. Is the hunt for extraterrestrial life misguided?. Nature (2008). https://doi.org/10.1038/news.2008.1287