In the previous segment, I discussed parameterizing alien civilizations by their energy consumption. This time, I'd like to further attempt to shoehorn the concepts of life and intelligence into a physicist's set of parameters.
One problem with looking for alien species is that you don't know what you are looking for. Ostensibly, you are looking for evidence of "unnatural" applications of energy like radio signals that demonstrate intelligence (prime numbers, factors of pi, etc.) or evidence of mega-engineering. But science demands natural explanations for everything it observes, and theorists are clever, and can come up with possible explanations for just about anything.
For instance, we have a good unified model for active galactic nuclei, but is it simpler to just hypothesize that they are power plants for extragalactic civilizations? Or, given that AGN are less common now then they were in the distant past, perhaps most alien civilizations harness their galaxy's central engine for their own purposes and dump the waste heat into the dark sector or out as photons at 3K, here we can't see them? Pulsars and quasars were both initially hypothesized (however facetiously) to be aliens before their physical nature was understood.
So you have to avoid the problem of the "aliens of the gaps": invoking aliens to explain any phenomenon that you don't understand... yet. The good news is that if you do find something, it means it's either aliens or a new astrophysical phenomenon, which is interesting either way.
So, if an alien civilization could easily look like some sort of natural but not-yet-thought-of phenomenon, how can we focus our search? One way is by following the outline of the previous sections of this thread, and argue that intelligent, spacefaring life will sometimes (or, perhaps, inevitably) become free-energy limited.
To recap, but to put things into physicist's terms, we argue that life, by definition, has the capacity for exponential population growth, consumes resources, and uses energy. Malthus warned that eventually exponential population growth would hit some resource limitation, causing catastrophe, but humans so far have managed to increase the resources we can use. We have done this through technology, and can do this because we're smart.
I have further argued that it is not land or space that ultimately limits our growth, but energy. We can pack each other in pretty tightly on this planet, and we only use a tiny fraction of its surface for our own purposes, but we already consume power equal to 0.01% of the light we get from the Sun. We are good at applying this energy to solve resource limitation problems. Our current solutions might be expensive, but sunlight is cheap.
Not enough fresh water? Desalination is just the application of energy to seawater. Not enough arable land? Hydroponics requires little more than water and energy. We will run out of energy before we run out of anything else.
I think this ability of ours makes a good "physicist's definition" of intelligence: the application of energy to overcome an external resource limitation.
Mold reaches the edge of the petri dish and stops: it cannot solve the problem of how to get more agar. A barnacle laps up food that happens to drift by, but does not have the intelligence (or ability, for that matter) to get more if the supply stops. Trees take what sunlight they can get. There is no evidence of intelligence here, just the natural tendency to take what's nearby and spread through reproduction.
Now, my definition runs into some ambiguities: it can be hard to distinguish between spreading due to purely instinctive behavior and deliberate behavior, and reproduction can look like an organism going out to get more resources if you look at the species as a whole instead of just the static individual (those intelligent trees are coming over the hill to get more water! Verrrrrryyyyy ssssssllllloooowwwwwllllyyyy....) Also, it means that playing chess is not evidence of intelligence, since it overcomes no external resource limitation (unless you use the prize winnings to buy food).
But applied to the problem of alien waste heat, I think it works. A space-faring intelligent species will be able to use its intelligence to travel to other stars to get more energy to produce more resources needed for itself. This helps define exactly what we mean by "aliens": von Neuman machines would count, even if they were just leftover automata filling the galaxy with copies of themselves long after their creators and programmers have gone extinct.
So if intelligence means that a population can grow until it runs out of energy (because intelligence allows energy to overcome all other limitations) then intelligence means that a species can become free-energy limited. If it is possible, then given the incredible age of the Universe, it is hard to see how a species could avoid it forever.
Phrased this way, a hunt for intelligent life becomes a rigorous physics experiment. The question "Is there intelligent life in the Universe?" contains an answerable sub-question: what is the abundance space-faring, free-energy limited species in the local universe?
I think the answer to this, or a tight upper limit on it, is knowable, but only because of recent mid-infrared surveys. If we find none, then either there are no species that meet this definition of intelligence to the degree of being free-energy limited, or there are physics we don't have access to that allow them to hide their waste heat. Either possibility sharpen's Fermi's Paradox.
We are hiring a postdoc for this project!We have received a grant from the New Frontiers program funded by the John Templeton Foundation.Some press coverage of our award:An article in The Atlantic online"Astronomers assume aliens are more open to solar... Read More