This week's column by Ctein
Recently there have been some interesting scientific developments in the Search for Extraterrestrial Intelligence (a.k.a. SETI) that I picked up at September's "100 Year Starship" Conference in Houston.
No, we haven't found anybody yet. If we had, you wouldn't first be reading about it here! But, the astronomical community has constrained one of the possibilities and thought up a new way to search for others.
This is not a speculative column on the nature of extraterrestrial intelligence and why we have or haven't found evidence for it. I hit that topic two years ago in "Where Are They?" If you're inclined towards comments and questions in that direction, you ought to review that column and the very thoughtful comments from readers that follow it; your concerns may already have been addressed.
First, one specific and negative result. Dyson spheres are either extremely uncommon or entirely nonexistent.
Most of you are asking yourselves, "What the heck is a Dyson sphere?"
Freeman Dyson suggested, half a century ago, that a sufficiently advanced solar-system-spanning civilization (such as we might plausibly become in a few hundred years) might eventually decide to surround its star with a densely packed network of solar collectors to capture most of that sun's output as useful power. There is no reason why that can't be done; it's one of the less fanciful extrapolations of current engineering.
A Dyson sphere is a highly recognizable object with today's telescopes. No natural object has its luminosity characteristics. It would be a compact source with a total brightness equivalent to a star, but with very strong, even dominant emissions in the 10–20 µ infrared. That's simply because the waste heat from all that power consumption still has to get radiated away from the roughly 1 astronomical unit radius sphere.
(Just to be clear, the sphere isn't necessarily a solid structure; in fact that's the least likely form. It's really a very closely spaced array of solar collectors. Doesn't much matter in this discussion.)
We've now done a substantial amount of sky surveying in the mid-infrared. We don't see anything like this. If there are any Dyson spheres out there, they are far fewer than one in a million.
Now on to some more hopeful news.
The trickiest part of SETI has been figuring out what to look for. The Drake equation is a lovely conceptual tool for trying to estimate the number of civilizations that might be out there, but the further you get into the terms of the equation, the more uncertain the numbers are. At this point we have some handle on the number of stars, the number of planets per star, and the percentage of those planets that are nominally habitable (for a loose definition of habitable). But the last terms in the equation have nothing to do with basic science as we know it. They're essentially sociology on unknown creatures: What fraction of civilizations do something that we would detect as communication and how long do they do it for?
The answer, of course, is that we don't have the foggiest idea. We look for what we can look for: radio and optical signals. Does any technology use that for very long? Who knows? We can only look for what we know how to look for, but we have little confidence that we're looking for the right things.
Now there's a new search proposal which gets around that problem entirely. It doesn't look for specific technology or uses of technology, it just looks for civilizations that are large enough to be using a lot of power. And by "a lot" I don't mean much more than we're using today. Maybe a factor of two.
The power consumption of all of humanity, in all forms, is about 1/2000th the power the Earth gets from the sun. Which doesn't sound like a lot, but it's close to an excess that we can detect with current telescopes. Almost all of that power ends up as waste heat that gets radiated away from the planet. The distribution pattern of that radiation, both geographically and spectrally, doesn't look the same as what the planet radiates naturally.
What is elegant about this search aproach is that it doesn't depend on any assumptions about the technology of the civilization, where they get their energy from, how they're using it, how they communicate—none of that. All they have to do is be big enough to be consuming a little more power than we do, and we'd be able to see them.
But how far out, you ask? That's where Colossus comes in. This proposed 75-meter Earth-based telescope uses a novel design that reduces its weight by a factor of five. The cost of a telescope scales pretty well with its weight. This puts Colossus within the reach of the "big astronomy" budgets—that is, under $1 billion.
There's lots of truly nifty astronomy that Colossus can do, but what interests this column is the SETI search. Colossus will be capable of seeing human-scale civilizations out to about 20 parsecs. There are a lot of stars within 20 parsecs of Earth. In fact, based on the Kepler data, it's estimated that there are several hundred Earthlike planets within that sphere.
Will Colossus find evidence of ETI? Oh, probably not. If it were that common, we'd likely see other signs of it.
Or so we imagine. As my earlier column explained, we really don't have a clue how aliens might think or what they might do.
And that's the whole reason for the survey. We don't have much in the way of clues. If Colossus turns up zip, then that puts another upper limit on the number of such civilizations in our neck of the woods, much as the lack of Dyson spheres put a limit on those kinds of civilizations. It'll tell us a lot more about just how alone we are (or aren't?) in the Universe.
©2013 by Ctein, all rights reserved
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Featured Comments from:
Richard: "It seems to me that we should still be Searching for Terrestrial Intelligence. Once that is found, then perhaps we might disperse with spending trillions of dollars on war and use it instead to start exploring. There is Life out there; one only needs to look up at the stars on a dark night and know you are only seeing a fraction of what's there. Will we be intelligent enough to find it and will we know what to do if we do find it? Great article."