Far from being deterred, Loeb doubled down. In another equation-packed paper, the pair argued that it was fantastical to imagine solid hydrogen floating around outer space. In it, he recounts the oft-told story of how Galileo was charged with heresy for asserting that Earth circled the sun.
And the only way the object could be propelled by solar radiation is if it were extremely thin—no thicker than a millimetre—with a very low density and a comparatively large surface area. Such an object would function as a sail—one powered by light, rather than by wind. The first planet to be found circling a sunlike star was spotted in by a pair of Swiss astronomers, Michel Mayor and Didier Queloz.
Its host star, 51 Pegasi, was in the constellation Pegasus, and so the planet was formally dubbed 51 Pegasi b. By a different naming convention, it became known as Dimidium. For their work, Mayor and Queloz were eventually awarded a Nobel Prize. The planet turned out to be very large, with a mass about a hundred and fifty times that of Earth. It was whipping around its star once every four days, which meant that it had to be relatively close to it and was probably very hot, with a surface temperature of as much as eighteen hundred degrees.
Mayor and Queloz had detected Dimidium by measuring its gravitational tug on 51 Pegasi. In , NASA launched the Kepler space telescope, which was designed to search for exoplanets using a different method.
During the last transit of Venus, in , viewers on Earth could watch a small black dot creep across the sun. Kepler measured variations in the brightness of more than a hundred and fifty thousand stars in the vicinity of the constellations Cygnus and Lyra. By , it had revealed the existence of a thousand exoplanets. By the time it stopped operating, in , it had revealed sixteen hundred more. This is the average number of rocky, roughly Earth-size planets that can be found orbiting an average sunlike star at a distance that might, conceivably, render them habitable.
Since there are at least four billion sunlike stars in the Milky Way, this means that somewhere between 1. Kershenbaum argues that the key to understanding cosmic zoology is natural selection. From this premise, Kershenbaum says, it follows that life on other planets will have evolved, if not along the same lines as life on this planet, then at least along lines that are generally recognizable.
Additionally, it would need to maintain a dedicated receiver for each target star to be certain not to miss a return message if and when it arrives. The cost of this strategy to ET in time, energy and materials would be immeasurable. Further, by announcing its presence to so many stars, it invites disaster should any civilization prove aggressive. Added to this is the problem of communicating with a target civilization of which it would know nothing.
Perhaps the transmitting civilization communicates in color oscillations like a cuttlefish, while the recipient only understands bee-like waggles. Building on the work of others, I have hypothesized that aliens would be better served by sending robotic probes. Relatively simple flyby probes might intermittently surveil nascent solar systems, for example, at million-year intervals. Star systems with biogenic planets might be surveilled more often. Highly capable probes might be placed permanently in the vicinity of planets that have achieved multicellularity as indicated by their oxygen-rich atmospheres or other biosignatures.
Once a permanently placed probe had detected artificial electromagnetic leakage, indicating that one multicellular species had become technologically intelligent, it would attempt to decode the species.
After many decades of work by E. Wilson and others, we now know a little something about ant communication but are still far from a complete decoding. How very much more difficult would it be for ET to decode humans? Even if it has been watching episodes of I Love Lucy that have been leaking out into space since that show was first broadcast, it may still not understand them.
If the probe began transmitting data to its home in after its detection of early television signals, and if that home base were located at the modest distance of light-years, then the earliest year in which the probe might receive instructions to make contact with Earth would be However, when we do finally hear from a local probe, after it has decoded us, its transmissions may be in a terrestrial language. The ensuing dialogue will take place in near real time, as opposed to the painfully slow dialogue between ourselves and an alien civilization transmitting from a star at hundreds or thousands of light years distance.
An alien probe need not reveal the location of its home base, obviating any danger to the progenitor civilization. A fully autonomous probe would be able to communicate with us even if its progenitor civilization is long extinct. What should we expect to detect if these searches succeed? My suspicion is that it is very unlikely to be little green men — something I speculated about at a talk at a Breakthrough Listen a Seti project conference.
If it lagged significantly behind, then that planet would plainly reveal no evidence of extraterrestrial life to our radio telescopes. But around a star older than the Sun, life could have had a head start of a billion years or more.
Human technological civilisation only dates back millennia at most — and it may be only one or two more centuries before humans, made up of organic materials such as carbon, are overtaken or transcended by inorganic intelligence, such as AI. Computer processing power is already increasing exponentially, meaning AI in the future may be able to use vastly more data than it does today.
It seems to follow that it could then get exponentially smarter, surpassing human general intelligence. Perhaps a starting point would be to enhance ourselves with genetic modification in combination with technology — creating cyborgs with partly organic and partly inorganic parts. This could be a transition to fully artificial intelligences. AI may even be able to evolve, creating better and better versions of itself on a faster-than-Darwinian timescale for billions of years.
If we were to detect extraterrestrial life, it would be far more likely to be electronic than flesh and blood — and it may not even reside on planets. We must therefore reinterpret the Drake equation, which was established in to estimate the number of civilisations in the Milky Way with which we could potentially communicate.
The equation includes various assumptions, such as how many planets there are, but also how long a civilisation is able to release signals into space, estimated to be between 1, and million years. But the lifetime of an organic civilisation may be millennia at most, while its electronic diaspora could continue for billions of years.
If we include this in the equation, it seems there may be more civilisations out there than we thought, but that the majority of them would be artificial. In contrast, extraterrestrials might be a single integrated intelligence. If Seti succeeded, it would therefore be unlikely to record decodable messages.
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