My first thought is of the sheer quantity of extraterrestrial organisms which must exist in the incomprehensible vastness our universe.
Maybe the laws of physics and mortality will prevent us from meeting them, let proving their existence, but in such an ultimate vastness where we know that life does exist, life must exist.
I don’t get Fermi though. If you take the inconceivably large problem space that is “life finding other life in the universe” and cross it with the infinitesimally small amount of space-time we’ve occupied actively looking for other life, how is not having found other life yet a paradox?
Then, to quote Fermi, where are they? If even one of those species had become intelligent enough to build self-replicating Von Neumann probes, then we would see evidence of it somewhere in our solar system. Unless... we are the Von Neumann probes.
The farther we look, the earlier back in time we see whatever we look at. There is good reason to think that the rise in complexity of organisms is a global feature of the universe, so other intelligent life may simply not have developed early enough that we've had time for the light from them to reach us yet.
Life on Earth is about 3.5 billion years old. If some other species in the Milky Way had beaten us to intelligence by 1%, even assuming they didn't start out another billion or two earlier, that would give them 35 million years to spread exponentially through our 100,000 light-year-wide galaxy.
It is possible we are first. The Copernican principle, to my eyes, is taking a bit of a beating lately anyhow. It was a fine heuristic for a while, but many people seem to have forgotten it is only a heuristic that seemed to work for a while and not an actual law of the universe. So maybe we are that unique. But it is worth pointing out that would itself be a significant sea change in the current philosophy dominant in science.
It's possible that whilst the universe is full of life, intelligent life is quite scarce. It could be we are the first in this Galaxy.
It could be that others have come and gone but never quite made it to space. Cave paintings go back 40,000 years. I'd say that displays a level of intelligence. Yet there was a significant period of time until we managed to develop agriculture (30,000 years), or writing (37,000 years).
At lot can happen during that time. In fact during these years humans could have died out many times, succumbed to the changing climate or disease.
But considering all this, you have to take into account that the universe consists of upwards of 2 trillion galaxies, at least as far as we can see. That's a lot of potential. But if we can never make contact, does it even matter if its there or not?
I highly recommend Stephen Webb's book Where Is Everybody? on the pros and cons of these arguments about the Fermi paradox. For example, your point is reasonable regarding the overall scale of the universe and life in other galaxies. Yet if life naturally arose on our planet in a relatively short habitable window (<100MM), considering the number of other potential habitable planets in our own galaxy alone, many scientists find it unreasonable to think that we would be the first or only life (and/or intelligent life, depending on assumptions) in our galaxy, within however many billions of years we are into a habitable universal time window. There are fairly conservative calculations that argue we should at least have seen evidence or been visited by at least one civilization trying to escape threats to its own planet within our own galaxy.
The simulation hypothesis: intelligent species are destined to invent computing, and therefore also invent recreational simulations (video games). Eventually the generation and detail of these simulations becomes reasonably sustainable and sufficient that the species retreats into absolutely controllable sims and stops interacting with the outside world for lack of interest.
Also broadcasting is inefficient. Surely there are better use of energy than wasting so much of it that it becomes detectable from another solar system.
An advanced species would mostly broadcast on purpose only, which indeed, raise other concerns.
There was a similar theory (well more a science magazine one) than Fermi paradox to explain there is no other planet in the universe. Until we started to know what to look for and suddenly they appeared everywhere.
Fermi paradox is at the same time clever at the same time you wonder if you are actually doing something akin "There is 0 chance you can throw 1000 dice and produce to produce a positive integer number", just because you look at a specific sequence and indeed that exact sequence is statistically impossible to exist randomly.
The Fermi paradox may not be a paradox, depending on the values you assign to various probabilities. It's more of a observational heuristic for guessing the probabilities... if intelligent life was common, we should see some signs of it.
Bacteria, plants, insects, and even some animals can all live in a human city and not realise their environment is artificial. The city is conceptually invisible to them, and interactions with it will appear random, or at least extremely unpredictable.
Recognisable interspecies intelligence may be a question of relative cognitive ability, not of physical presence.
I don't think that's a fair comparison. Even though humans are limited in our senses, we've built machines to see and monitor every frequency of light. And we're also really good at finding patterns. If there was an active intelligence that had colonized most of the galaxy, we'd know it. (Unless the intelligence was actively trying to hide from us?)
Why would we? Our own EM emissions drop below the noise floor at only a few light years out, and AFAIK our total emissions have been going down as we transition away from pure broadcast to cell and wifi.
Even assuming that there was a large civilization out there that communicated via EM across interstellar distances, they'd almost certainly use point to point lasers to keep the SNR usable.
To elaborate, independent probabilities multiply, resulting in exponentially smaller probabilities, whereas as you look out into space, the number of possible places for life increases only at somewhere around O(n^2)ish as distance increases. (It is not O(n^3) as you might naively think as matter is far from uniformly distributed. I don't recall what it is exactly, it's not O(n^2.000), but it's somewhere in there.)
It doesn't take hypothesizing all that many low probability events for the creation of life, or intelligent life, before you get to numbers like "less than one per galaxy", and it isn't all that far from a logarithmic perspective to get to "less than one per universe".
So it is indeed the case that our ignorance is such that we can't simply look up at the universe and go "it has to be teeming with life!", because we do not yet know that the probability of life is not in fact orders of magnitude smaller than the observable universe. We don't know that it isn't teeming with life either, but that's where you get into the Fermi paradox and the Great Filter arguments.
There are some science fiction authors that postulate a galactic civilization putting the equivalent of traffic cones around the solar system telling others to not disturb the developing life there.
If you believe it, then it makes you wonder at what point in our development are we going to be able to see the cones for ourselves and then wonder about them? We now have two probes that have gone extra-solar and perhaps soon we'll have rocketry to send something toward Alpha Centauri.
I love this school of thought (if you want to call it that)! It seems like a very reasonable and simple answer (considering what advanced technologies other lifeforms could have) to why we haven't seen any concrete evidence anywhere yet.
My personal favorite theory for your question of 'when?' is that it actually has nothing to do with reaching a physical point in our universe. Instead, we make some advance that either A) allows us to perceive another dimension/energy that we couldn't before, or B) raises our consciousness to a level to match what should be the tons of aliens in space.
I think it comes down to the fact that it appears in 10,000 - 1,000,000 years (very soon relative to earth's birth) humans will be able to colonize nearly the whole galaxy using von Neumann probes[0] It would not require technology much more advanced that we have now.
If life evolved on a planet a billion years before earth, why haven't they colonized the galaxy?
Kurzgesagt recently published a video covering the idea of the "Great Filter".[1] The idea is that there must be something (some "filter") preventing life from colonizing their galaxy.
I don't think one can be certain that a "von Neuman probe" is not "much advanced than we have now."
Maybe the challenge of producing a thing capable of going interstellar distances and foraging the resources it needs to create copies of itself is only short distance (in time) away from our present capabilities but maybe it is vastly far away or impossible. Given we definitely don't know how to do this now, we basically can't say which it is.
> We constrain the planet mass fraction to be larger than 0.0001 of the halo mass, which is equivalent to 2,000 objects ranging from Moon to Jupiter mass per main sequence star.
So the paper is saying there there probably aren't more than 2000 planets per star? That seems like a pretty large upper-bound.
Maybe the laws of physics and mortality will prevent us from meeting them, let proving their existence, but in such an ultimate vastness where we know that life does exist, life must exist.
I don’t get Fermi though. If you take the inconceivably large problem space that is “life finding other life in the universe” and cross it with the infinitesimally small amount of space-time we’ve occupied actively looking for other life, how is not having found other life yet a paradox?