Lets face it, we can do fusion, but we can’t seem to control it except as a bomb. Maybe it can only be fully controlled in the conditions existing in stellar cores. That means no fusion power, no fusion drives for spacecraft, perhaps not even any near relativistic ship speeds….We haven’t really come up with any real new physics since relativity and quantum. Our biggest successes lately have been in computing and biochemistry.

As for the writing, I didn’t like it, it didn’t flow smoothly and effortlessly like an essay should. It seemed stiff and blocky, the rhythm was all wrong. The facts are OK, but the sentences and paragraphs are choppy and abrupt. I chose not to publish it, but I did submit the third draft to

https://www.centauri-dreams.org/

as a post.

Your criticism was spot on. I knew it lacked a unifying principle, but the obvious one you suggest never occurred to me. Or maybe I just got lazy and got tired of working on it.

You cover very well all the difficult (and well discussed) basic requirements for intelligent life to exist on other worlds, and then present your own possibility toward the end for an apparent empty universe. My only real suggestion would be maybe have a short opening sentence, or paragraph, indicating that you are examining Fermi’s Paradox before diving right into it. Maybe that is in your title?

Aside from any constructive criticism of your excellent well written article, here’s my view regarding your thoughts that there is a limit to engineering, technology, discovery, POSSIBILITY. We live in a physical universe so indeed there must be some limits to it at some point. But scientific history has shown continuous and spectacular advancements, especially in modern times. There seems to be if anything a tremendous acceleration of discovery in many fields rather than a slowing down. Paradigm revolutions about the nature of the universe are by no means done with either. Odds are we can expect more of the same in the coming centuries. Humans have gone from hunters and gathers to an early space-faring civilization in a few thousand years, but most of that advancement has been in the last couple hundred years. I personally expect we are much farther away from the possible limits of engineering and technology than where we have advanced to so far. The answer to Fermi’s Paradox might simple be that we are in a lonely and quite spot in the universe, and the real answer might await those further advances (FTL; new types of communication, etc?) we will possibly someday achieve. Indeed, what if the universe was awash right now in a form of communication we have yet to discover? Possibility is almost endless it seems.

Everything we know about ourselves serves as the template on which we can base speculations on potential extrasolar societies. We know it happened here so its possible it happened somewhere else. We also realize that it is possible that it didn’t happen anywhere, or that it happened differently, or that something totally different is happening ‘out there’. But we can always rely on using human history to justify our conjectures as reasonable. We have an example we can point to.

However, we also realize we are talking about aliens and the very definition of the word tells us they will be different, they will be strangers to us, they will look different, think different, live different, their environment will be unlike ours and their behavior may be incomprehensible. We can qualify these doubts by adhering to some very reasonable assumptions such as they will be the result of a biological evolution whose effect on ourselves we have studied extensively. They will be affected by physical laws we know intimately and which we have every reason to believe are the same for them as they are for us. Their home star and planet may have had a different history than ours, but it couldn’t be too different. Even if they turn out to be machine intelligences created by now-extinct silicon creatures that breathed ammonia; F=ma in their home world works just like it does on ours.

There are some things we can say with some confidence: ETI will have arisen on a world where conditions were stable enough for long enough that they were able to evolve and adapt just like we did. We know Earth’s environment has changed dramatically since Life first appeared here but it did not change so drastically, or so quickly, that Life was not able to adjust. Of course, we are pretty sure a species can escape devastating cosmic catastrophes once it has achieved space travel or some other extraordinary technology, but until that happens, it requires relative stability in its natural environment.

The evolution of biological life requires a long-lived star and planet that does not change too abruptly. The home star cannot go supernova or undergo any other rapid evolution, even a perfectly suitable home planet must escape radical orbital changes, destructive collisions, close encounters with other large objects in its system. A binary companion to the system must also be far enough or stable enough to not threaten Life on the home world during its own evolution. (Remember, MOST stars are members of binary systems and most binaries are composed of members with highly different masses and subsequent histories.) We have already identified system characteristics that MAY be essential to Life, or intelligent Life, such as the presence of a nearby massive moon, plate tectonics, a strong magnetic field, oceans, I can go on. Some truly cosmic catastrophes may even make substantial portions of entire galaxies uninhabitable. No doubt there are many other potential obstacles to Life we haven’t even guessed at yet. But the Universe is so old and so big we can confidently hope that Life has arisen in other places. After all, we made it. Why couldn’t they have gotten lucky too?

The SETI community is aware of these pitfalls and opportunities and is currently engaged in healthy debate as to which of them may be significant. There are biological bottlenecks, too; is the development of multicellular life inevitable or is it a lucky accident? We rely on the geologists and biologists to answer those questions but there are other assumptions which seem to have escaped criticism altogether.

The arising of highly complex life forms makes the appearance of intelligence possible but does that mean it is inevitable? Its late appearance and explosive development suggests it might be a fluke, a lucky accident. Does the appearance of intelligent and social creatures make science inevitable? Does the appearance of technology always lead to the type of engineering capable of interstellar travel and communication? What DOES an alternative form of engineering even look like? We may have only gotten lucky because of a fortunate but unlikely combination of biological, psychological, social and physical factors. Do savanna dwellers evolved from arboreal ancestors have a leg up on everybody else? We have no way of knowing any of this is possible, or even likely, for all intelligent reasoning organisms and societies. And finally, how do we know all civilizations who develop complex cultures choose to immediately found colonies on other worlds and start systematically exploring the Galaxy? I suggest even the spacefaring races that do exist will cool off their ambitions once they have ensured their security by populating multiple, separate worlds, or perhaps engage in relations (hostile or benevolent) with other species they encounter.

I continue hearing from SETI enthusiasts how ongoing and unstoppable interstellar diaspora is inevitable for any species that once managed to chip a projectile point from the local stone. This has never been persuasively demonstrated to me. Even if other civilizations have shown they are as aggressively expansive as we believe we are (remember, the SETI community selects for space and exploration enthusiasts), how many are there? What fraction? Even advanced terrestrial maritime societies (Portuguese, Spanish, English, Chinese, Polynesian, Viking) have sometimes suddenly decided to stop voyaging (for a variety of reasons). Even the USA, with its impressive nautical history, has pretty much given up building ships, except for men-of-war. Our excuse: “it is no longer economically feasible”.

All of these conjectures involve speculations on the motivations of extraterrestrials, or observations of our own history, both of which involve assumptions we can’t always justify. But there are further obstacles to the inevitability, or even potential, of a society to achieve a spacefaring technology, especially one that persists long enough to make the existence of such societies common. The question SETI asks is, “What is the likelihood we will ever encounter another intelligent species?” They may very well be out there, but they may be so far apart in space and time that we will never meet or communicate with them. Conversely, they should be so common that the fact we have not found any suggests we are alone. The universe is very big, and very old, but is it big enough AND old enough? And how long must these communities exist, on the average, for there to be a fighting chance for them to find one another?

We may assume that the technological development of our own civilization has been so rapid in the last few hundred years that it is reasonable to believe that this pattern must also be the case with many others. After all, there is no particular reason to think we are unique in this respect. But does this period of rapid growth continue indefinitely, or does it tend to quickly damp out as soon as all the ‘easy’ discoveries are made. The only reason the entire Galaxy is not teeming with multiple, expanding extrasolar cultures might be because there is some general process, characteristic, reality (a ‘filter’) that simply shuts down or eliminates cultures after some relatively short period of time. This could be some sort of recurring occasional cosmological catastrophe, or perhaps an inherent risk that advanced technologies destroy themselves through pollution, war, decadence, social collapse, etc.

But I suggest there may be another reason. Maybe interstellar travel and communication is just too hard; no technology exists, or can exist, to make it possible, or even likely. Perhaps the rapid advance in technology we have exhibited for the last few centuries is a phase many cultures experience, but natural limits eventually intervene for all societies that stop their scientific advance. We have assumed that that which is not forbidden must be mandatory, but that may simply not be the case.

We suspect that the speed of light is the maximum velocity possible in nature, but there might be other natural limits which cannot be overcome. Materials can be developed with higher melting points, but maybe there is a natural limit to the highest temperature solid matter can tolerate. Perhaps the energy sources available to us are limited and we will never be able to command enough to do everything we like. Direct matter-to-energy may exist in nature but perhaps we just can’t harness that with mere gadgets. We may already be approaching the actual limits of progress; we have gathered all the low-hanging fruit and we can’t reach the higher ones because nature will not allow it. Is it possible that we are within just a few decades of achieving the limits of engineering? We have long assumed that there is no limit to our future progress; we’ve imagined an entire pantheon of Kardashevs and Matrioskas and other secular deities which may be just fantasies because nature will simply not allow them. For all we know, when (and if!) we reach our pinnacle of technological development, the universe may be populated with species that are more or less as scientifically capable as we are. The intelligent universe may be analogous to the Earth of a thousand years ago, many civilizations, some of them even aware of one another, but none with steam or electricity.

This is not an attractive prospect to those involved in SETI. We tend to be oriented to scientific progress and ever-increasing knowledge and capability but we cannot demonstrate that that future is achieved anywhere. Maybe Fermi was right and perhaps the reason they’re not here is not because they don’t exist, but because there is no way we can reach out to one another. A first contact may still be possible but it can only be with a contemporary who is not too far away, and it may not be for a long, long time.