OTHER | THE GREAT SILENCE BECOMES DEAFENING

 *DEAFENING SILENCE*

I expect my audience to be already familiar with the concept of the Great Silence, but for those who need a refreshment - it is the current situation that we find ourselves in. That despite all odds being apparently stacked in favor of Life being abundant in the universe, and by consequence intelligence as well - we see no obvious technosignatures in the sky, mainly radio signals, hence the name Great Silence.

My stance regarding possible solutions to the Fermi Paradox is to think we are, if not the earliest, among the ealiest civilizations in this side of the galaxy. And my reasoning for this is the following:

Earth has been technologically detectable for at least 64 years (circa 1958), this means that if our signal had reached any technological civilization with intent of immediate response we would have gotten that response in now in early 2020s (30 years to send, and 30 years to reply), we can be sure to a considerable degree that our interstellar neighborhood all belongs to humanity for a minimum radius of 30 light-years from the Sun. In the same train of thought, we can be sure that we are not within 60 light-years from any other nearby technological civilizations which happen to be on a similar or slightly superior technological level, else we would have already received unambiguous radio signals from space.

Moving further away from Earth, and time dilation kicks harder, had another civilization 100ly away started broadcasting in 1900s, we would be receiving their signals in the early 2000s, and the same for one 200ly away starting in the 1800s, and so on. The further we start looking into space for technosignatures, the older these civilizations would have to be in order to justify the Great Silence, and that is just a lower bound which assumes we would be receiving their signals any time soon. Now, if you consider that the age and epoch of first broadcast differ enough from one another, then those civilizations would have become detectable from Earth before we had ever finished building the first radiotelescope array.

For example, a technological civilization that goes on air circa 3000 BCE from 4000ly away would have become detectable on Earth in the year 1000 CE, only to be finally discovered post 1950. But that assumes that they had been on air for nearly 1000 years, this becomes difficult if civilizations become more silent over time due improvements in technology, security concerns, or setbacks due internal conflicts or disasters. Yet a civilization doesn't simply become invisible after switching SETI policy or technology, there is no way to stop a signal once sent, and so a predictable pattern would emerge in that civilizations appear very bright in the dawn of their development, only to minimize their radio footprint over time.

This strongly indicates, given the distances we can look inside the Milky Way, that no technosignatures younger than 100 thousand years and farther than 100 kly exist in our field of view. Note that we cannot see the side of the galaxy opposite to us, since it is blocked by the galactic bulge. If a civilization is located 50 kly from the Sun, but then hasn't started broadcasting since the last ice age, we won't hear about them for a good while, however we won't know about them either if they are way older than some 50 thousand years as well, because we woul already find ourselves way into their security-era radio bubble.

Since radio travels at the speed of light, it is particularly easy to graph a radio-horizon as an | y(x) | = x line, and any civilization which coordinates in time and space are contained below that line will be considered undetectable. Fun thing that this drawing is actually just as slice of a Light-cone graph. Based on this, we can be pretty sure there are no civilizations in the local group of galaxies which are near or really Kardashev 3, since not only it would require millions of years for such civilization to emerge, but also that those galaxies are millions of light-years away as well. We can't be sure that Andromeda isn't now inhabited by a K-2.5 civilization, but we can be sure that it wasn't 2.5 million years ago because of our light-cone, and the same is valid for other galaxies.

Hence, we come back to the start of this post, if we aren't the first, we are amongst the first technological civilizations in the local space. And I dare propose a scary solution to the Great Silence along those lines: Life across galaxies emerge in bursts, only to die out shortly after.

GALACTIC GRAVEYARD?

Let's picture the following, the Milky Way galaxy isn't exactly a galaxy in its prime of stellar formation, for its size and metallicity it used to be much more active in the deep past, and a deep past marked by merger events with other galaxies. The interaction between stars passing near each other and through gas clouds during a merger is one way star formation can be increased for a period of time, and thus the rate at which potentially habitable systems emerge. We also know that some regions of the galaxy have a higher average metallicity than others, it isn't like the galaxy uniformly increases its metallicity over time, the galactic thin disk sees way more action than the thick disk or halo, and so some regions could very been host to the prime conditions for planetary systems and habitable planets aeons before the Sun was born, thounsands if not millions of worlds would have a headstart of a few billion years, yet we are stuck the Great Silence, leading to the Fermi Paradox. Now, if we find out that a planet hosting Life far away from Earth, orbits a star just as old as the Sun, say like another yellow dwarf or orange dwarf, then we can start to suspect that all Life that currently inhabits the galaxy was born in the same epoch, a merger epoch approximately 5 to 8 billion years ago, and we would see clumps of similarly aged planets across the galaxy.

And so the solution to the Fermi Paradox would be that the right conditions for the emergence of Life are only common during short geologic periods across time, and that it becomes rare outside of these burst events, thus the Great Silence we currently experience is a result of nearly all planets inhabited by intelligence having more or less the same age, differing by a few hundred millions of years. Yet, for some reason there must exist a Great Filter that doesn't allow older civilizations to interact with younger ones, that is, if Life had several eons of headstart, why we don't see elder K2.5 or K3 civilizations? Perhaps interstellar travel becomes increasingly difficult from the point of view of a biological civilization, and not really necessary for artificial civilizations (or perhaps those operate on timespans of billions of years instead, overarching biological Life). Thus an interstellar empire could still have thousands if not millions of years, and only occupy a volume of a few hundred light years in radius from its homeworlds, due the abundance of local resources.

Let's say you want to mine a large asteroid 12km wide, at an average rocky density this would only amount to half a billionth of the Earth's mass, or about 3.2 trillion tons of resources. And there are rocks way larger than that across the whole of the solar system, most of which can be readily acessed in moons and the asteroid belt. A quick search shows that in 2019, humans have mined over 3.2 billion tones of metal of the Earth, 94% of which is iron ore - our little12 km-wide rock at 1% metal could supply almost 10x our current yearly needs. And we are talking about an asteroid that's 99% rock, your typical metallic asteroid is nearly 90-95% metals by mass, a single rock could supply the Earth with metals for the next 1000 years, or hundreds of times more infraestructure built per year if our consumption increases several fold. And so, mining your own solar system would provide nearly all the metal, rock, and volatiles you may ever need for thousands of years, and even millions of years if you reach out for other stars. You will only ever run out of stuff if you run out of personel or machinery to do so. So civilizations would be "forced" to stay at their local space, because there is nothing outside of it that they cannot obtain in their collection of worlds.

- M.O. Valent, 09/10/2022

- M.O. Valent, published 09/10/2022