The science fiction convention for first contact between humans and aliens is a face-to-face meeting in meatspace. This has the advantage of immediacy: No speed-of-light time lags to contend with! In-person meetings do have the disadvantage of proximity, should it turn out that humans and aliens cannot (or choose not to) peacefully co-exist. Therefore, while distant detection does not offer immediate gratification, it is not nearly as dangerous as in-person first contact. Or at least, so it would appear…
What brought this to mind was an arXiv paper with the catchy title “What no one has seen before: gravitational waveforms from warp drive collapse.” The question raised was straightforward: Assuming for the sake of argument that warp drives were possible1, just how far away would they be detectable by our civilization? The authors consider the extreme (and from the warp drive user’s perspective, undesirable) case of a warp bubble collapse. Their conclusion? With the right equipment, a warp bubble collapse could be spotted as far away as a megaparsec (somewhat farther than the Andromeda Galaxy).
While overhearing the gravitational scream of a dying starship is not as information-rich as a conversation, detecting a warp bubble collapse would still provide valuable information. We’d know that warp drives were possible and that at least one other technologically sophisticated species existed2, at least at the time of the mishap3.
There are other signals that might hint at the presence of other intelligent beings in this universe. Let me count the ways.
The presence of certain atmospheric pollutants could reveal (or at least hint at) the presence of intelligent-tool-users. Take lead, for example. Roman silver smelting produced lead vapor as an unintended by-product. The Romans put enough lead into the atmosphere that the evidence was preserved in Greenland ice, over four thousand kilometers away. Extraterrestrials noting the rise and fall of lead in the Earth’s atmosphere might deduce what they were seeing was the product of intelligent activity (though so incidentally unintelligent as to dump massive amounts of lead into the atmosphere).
The catch would be determining if the unusual pollutants were being produced by industrial activity by intelligent beings, or if some unfamiliar but purely natural phenomena were at work. Aliens intermittently monitoring Earth 66 million years ago might note a sudden spike in various pollutants (including iridium) combined with a dramatic drop in species diversity… all of which were purely natural in origin.
Less challenging and certainly longer range would be inadvertent electromagnetic signals. While the radio and television noise humans generate drops below detectability at inconveniently short distances, which would presumably be as true for aliens as it is for us, certain activities might be detectable across many light-years. For example, military radar could be detectable beyond 10,000 light-years, while Thomas Womack once calculated4 that exo-atmospheric nuclear explosions were in theory detectable by 10-metre telescopes anywhere in the Milky Way.
Yeah, deducing that there used to be an alien civilization a few thousand light-years away because we picked up decades of military radar emissions followed by several thousand x-ray flashes over a few hours, then silence, would be a bit depressing. But at least we would surely learn an important lesson!5 And if signals began again after a few years, that too would be a valuable lesson.
Artifacts provide a special case of durable, potentially long-range signal. Aliens in the vicinity of Ross 248 might deduce from the passing Voyager probes that humans existed or at least existed thousands of years earlier, when the Voyagers were first launched. Scientist Nick Oberg concluded that the Golden Records on the Voyagers could remain at least partly legible for as long as five billion years.
The obvious implication of all of the above is that if unintentional signals can betray our existence or that of our alien counterparts to observers thousands of light-years away, then deliberate signals should be detectable at comparable distances. In fact, since the signals could be optimized for detectability, greater distances should be attainable6. There does not seem to be any inherent reason a civilization could not announce itself to the whole galaxy.
The fact that we see no evidence that anyone is trying to alert the galaxy to their existence is a bit concerning. There are many explanations as to why this is so, only some of which should keep you awake at night, wide-eyed with terror. The least alarming might be that we are alone (although then one has to wonder why), followed closely by the possibility that there is a superior means of communication we primitive humans have yet to discover.
Have I overlooked something obvious? Feel free to enlighten me in the comments below.
