Petigura, Howard, and Marcy published a paper in PNAS attempting to count the extraterrestrial civilizations:
Fermi therefore asked the question "Where are they?" and the question may look even more pressing today than it was decades ago. In "Billions of Worlds", Sean Carroll proposes an explanation. We don't observe any visible civilizations at the galactic scale because they get bored and stop any growth before they reach this level of influence.
This explanation is known as EBH which stands for "Environmentalism Busts Happiness". We are speculating about the psychology of different creatures – or humans/creatures on Earth in a very distant future – but we may still speculate. Is EBH a viable theory? Well, I don't think so.
Some readers on Carroll's blog raised important objections to the EBH way of thinking. Joe Bloggs wrote:
While fun to think about, the EBH seems impossible as defined! What definition of boredom results in inaction? Whether we’re talking about restlessness or existential ennui, boredom is a problem to be solved. It is the unique trait that _triggers_ creativity and exploration.Indeed, it is very problematic to identify "boredom" with "inaction". People are doing – and have been doing – many new and unconventional things exactly because they were bored by everything else. So the sentiment needed for EBH to work isn't really "boredom" but some kind of self-effacement or material modesty, something that actually curbs expansionism and supports inaction.
Fine, so do these sentiments win as the civilizations get more advanced? Are more advanced intelligent beings inevitably closer to monks? Maybe, you could think if you had a look at modern monks such as your humble correspondent. However, Greg has pointed out an important oversimplification in this line of reasoning:
Suppose most members of most intelligent species eventually get bored of life. Those members will die out, or at least fail to conquer the galaxy, leaving only those individuals who aren’t bored of life. So there is natural selection in favor of not getting bored, which makes the EBH unlikely in my view.In other words, it doesn't really matter how bored or modest or passive the average intelligent beings become. Despite Sean Carroll's obsession with mediocrity as the golden standard for everything, they are not those who will dictate the overall character of expansionism of their civilization. It's enough if some intelligent beings want to expand, want to compete, want to be richer and more powerful than ever before. Those – and perhaps their disciples or offspring – will determine what the visibility of their mother planet etc. will look like.
This observation is nothing else than Darwin's natural selection and there's no good reason why it should break down at a particular scale – even though we're trying to extrapolate to ridiculously distant moments in the future. One reason why it seems implausible that the "desire to grow and expand" will completely disappear is that it could have happened a long time ago but it didn't.
MSIE 11: Off-topic: fans of Microsoft Internet Explorer should download MSIE 11 (64-bit or other languages) which is faster and supports more modern standards (and is personified by an Aryan Japanese clumsy anime girl). In a few weeks, it will be automatically pushed through the Windows update service. There is a tool that forbids the updates, too.Soon after the bacterial life was born, they could occupy much of the planet and they could think that the "mission was accomplished". But they didn't. They kept on evolving. OK, let me skip many similar steps and jump straight to humans who actually use their brains to think. Well, some of them and occasionally. Centuries ago, humans were able to feed themselves and they could become bored and stop any progress. They didn't. They found important theories of physics – Isaac Newton's theories, physics at the end of the 19th century – that made them believe that they almost had the theory of everything. That could lead them to stop expanding and terminate the progress. It didn't. And so on. In fact, the ability to do and understand certain things invites many other people to apply them. Key breakthroughs in science may turn their finders to modern monks but they have the opposite effect on the rest of the society. The society is looking how to benefit from anything and everything.
We may actually be facing some threats resembling EBH today. The Luddites – most of whom have largely adopted a new trademark, the "environmentalists" (and the "global warmists" represent the most aggressive branch of this movement so far) – are trying to curb the economic growth and especially the "extensive expansion" of the mankind and its influence. Does it mean that they will make the progress stop? Is this cancer of "environmentalism" what stops almost every extraterrestrial civilization and prevents it from becoming visible from the Earth?
I don't think so. The environmentalists and similar -ists are annoying and may look like a threat for the human progress. The society seems to tolerate them. But it's only because they haven't really made any \(O(1)\) impact on the life on the Earth. They're just a minor gang of parasites and demagogues. If they actually started to threaten 50% of people's wealth and income, ruin 50% of their dreams and 50% of their freedom, I am confident that a violent backlash would erupt. A sufficient number of the environmentalists would be physically eliminated so that the movement would disappear.
Soon or later, such liberation is bound to happen in regimes that restrict the human freedom. The bad news for you is that the process may be too slow and most of your life may be affected. The good news is that dreams about 1,000-year-long totalitarian, stagnating empires can't work. Even if such socialist or environmentalist empires manage to defeat their external enemies, they're not protected against internal developments. The smaller the external threats and foes become, the more the people focus on internal struggles. The stronger bodyguards you hire to protect yourself from external freedom warriors, the more you risk that you will be assassinated and stopped by your own bodyguard. This ultimately leads to the restoration of the diversity and the revival of the natural selection and progress, too.
No doubts about it, environmentalism is a major example of a disease that may cripple whole societies and stop their progress. But it can't do the same thing with all the life on Earth. The societies suffering from similar diseases may slowly die or become irrelevant but others continue to expand and make further progress. It can't be otherwise.
The most likely interpretation of the absence of extraterrestrial signals is still the obvious thing: There are simply not too many extraterrestrial civilizations if there are any at all.
The number of nice enough planets in the habitable zones may be very large and life may start to spread all over the planet soon after the seed of life is created there or gets there (this speed is what our geological records indicate) or but it's not necessarily the case that the "seed of life" appears on all habitable planets.
In recent years (which is relatively recently), my belief was increasing that the early life was actually born outside Earth. See Amino acids born on an artificial comet and especially Panspermia follows from Moore's law. In the latter article, I already coined the concept of "increasing concentration of sophisticated genetic or industrial capital" as an explanation of many patterns and hierarchies here.
Let me add a few words.
Protons and neutrons are "pretty much everywhere" in the Universe. More complicated atoms are "almost everywhere". Simple molecules appear at some special places, simple organic molecules in an even smaller fraction of the Universe. Now, life-creating minimal organic molecules may only be present on comets in various clouds of hope, in a small percentage of the galaxies. 4.7 billion years ago, one such cloud could have surrounded the newborn Earth and the early primitive organisms may have found the Earth to be a great place to expand and evolve.
Only the most fertile places saw the growth of the human civilizations 6,000 years ago. A small portion of the world today is covered by prosperous economies. I could continue with this ever increasing concentration. At the end, we could mention that the top string theorists only occupy a few hundred cubic meters somewhere in the physics departments at Princeton, NJ and in Cambridge, MA. Apologies to the Holy Father and others who may have expected that I would identify a different tip of the pyramid of intelligent life. ;-)
Note that this increasing concentration actually does solve the Fermi paradox. It is not enough for a planet to be Earth-like and to belong to a hospitable zone for the planet to host life. Why? The sequence above offers us many analogies. One analogy is: It is not enough for a physics department to have a staircase similar to Harvard's staircases if you want the department to contain the world's best string theorists. ;-)
The very early, extraterrestrial life didn't need conditions that were too luxurious; it didn't really need the Earth. (I hope that this claim may be supported by independent arguments based on the analysis of how this life worked.) The surface of comets – with a negligible gravity etc. – was enough for these creatures that were satisfied with the surface tension instead of gravity. Such an early life could have ignored the separation between planets and zones. It depended on lots of surface of solids and cosmic dusts or comets may have dominated in this discipline (even though they have a smaller volume or mass than the planets – most of the mass of planets is "inside the solid" and therefore not helpful for life).
The dust that was containing seeds of life wasn't covering the whole Universe 4.7 billion years ago. It only occupied some clouds and stellar neighborhoods. The stars with such seeds in their vicinity could have been as special as the string theory departments are special among the academic buildings today. But this could have been just the initial condition in the evolution towards even higher concentration.
My main points are that the increasingly sophisticated structures and life are increasingly rare and increasingly concentrated to special places; and that the first forms of life on Earth weren't really at the very beginning of the evolution of life so they were already somewhat concentrated to some places in the Universe. Such a general big picture may also be seen to imply that the planets with life are rare and extraordinarily powerful civilizations are even rarer.