Left over artifacts from their civilizations are just as exciting a find.
Personally, I feel the thought that we are alone in this universe is a wee bit arrogant.
The universe is so big and existed for so long, we have to assume that sentient life existed in the past, perhaps exists now and will definitely exist in the future.
Hopefully, we find at least artifacts that prove alien life, just because the minds of humans expand a bit more, because imagine how shocking it is to religions around the world if existence of aliens is confirmed.
the most convincing meta-analysis i've seen, which considers the probability distributions implied by the error bars in our current best estimates of the relevant parameters (https://arxiv.org/pdf/1806.02404.pdf), suggests a 39–85% chance we are alone in the observable universe. i'm less impressed by those particular numbers than by the attempt to rigorously quantify our ignorance on the issue, where some component values span fifty orders of magnitude. while the universe may indeed be big, i'm not sure it's broader than the sheer span of our ignorance.
Basically, the universe is so ludicrously huge and so ludicrously old that it is as close to statistically impossible as one can imagine that human beings are the only things ever to evolve in such circumstances as to be sapient.
Note that the paper you have linked uses the term "detectable civilization", which is significant limitation due to our current level of technology, the age of the universe, and the propagation speed of light. It wasn't that long ago that we couldn't even detect extra-solar planets.
There is of course an argument to be made for us being "alone" in the universe in the sense that sapience is rare enough that pretty much all species, including ourselves, will die out before ever meeting another sapience, but the idea that in all the unimaginable vastness of the universe we are the only ones?
Maybe even out of 1,000,000,000 of similar universes only 1 would actually happen to have life. It's similar with planets. If you only know of 1 planet within 1B to have life, how can you tell the odds are not low enough to have 1 universe out of 1B to have life?
This could be an awesome example of survivorship bias to assume because there's life here, there must be life elsewhere in the universe too. We only have experience with a planet where there's life, so we are innately biased to think that it might be more frequent occurrence than it really is.
In other words, the fact that it has happened at least once very strongly suggests it has happened more than once.
No, it doesn't at all.
Maybe just the opposite.
The universe is mind-bogglingly large, but it's also really, really empty, and really, rally sparse. The numbers games starts to dwindle very quickly in terms of all the ingredients necessary.
And the Fermi paradox helps provide crudely indirect evidence.
If there was any validity at all to the 'common life theory' - then it would have started long ago and probably spread everywhere to the point wherein there would probably be ample evidence.
In the 'common life' scenario, it should be rare/impossible to find life that's disconnected from other life scenarios.
There's a decent chance we are alone.
The common argument in favor of more life is that the probability of life evolving is some very small number, such that there must be a very large number of opportunities for life to evolve in order for life to actually evolve. At the same time, there are a large number of opportunites given the size of the universe. Given that there is presumably no relation between the "small" probability for life to evolve and the "large" number of opportunities in the universe, we don't expect these two numbers to conspire to be near the same size, which is what would have to happen for just one life to evolve. Rather, we expect there should be lots of life that evolved in different places or that the probability that any life evolved at all is extremely (vanishingly) small. So if this argument is correct and we know of one life form, we should expect many more in the universe. (Not to imply we would necessarily be able to find them.)
There is one potential problem with this argument. It assumes there is just one universe. If there are many universes then it is possible that life did not evolve in almost all of them. In universes where life did evolve, odds are it happened just once. And of course, that life (us, for example) would see that univese and the life that evolved there, however improbably that would be.
I read a saying "You know an argument is wrong if it invokes quantum mechanics." But I'll do it anyway. Some educated people believe quantum mechanics works this way - the univese effectively branches into parts that are essentially distinct (referred to as different universes too). It is not as outlandish as it may seem. The whole reason quantum computers are supposed to be powerful is that different things happen in parallel.
You don't have to buy the quantum mechanics part. If there are multiple universe for whatever reason then the probability argument above is not valid.
I do want to add one thing. The number of "opportunities" for life in our universe grows linearly with each dimension in the universe. (Lets just say to the fourth power of the size of the universe.) On the other hand, the size of probability grows/shrinks exponentially. So it is not hard to imagine between the two of these numbers, the exponential one wins.
The non-existence proof of ETI also means laws of physics are cruel and cannot be violated. There is almost 100% chance that advanced life forms exist in other galaxies, may be even billion year old. But no one has figured out how to travel faster than light.
If anything I'd hazard our ignorance is more on the side of things we don't know we haven't observed than things we observed incorrectly.
I think compared to how long it took for somewhat intelligent life to evolve on Earth, from the point of first life. Assuming* a comparable evolutionary cadence, the question then becomes - how often does life begin?
*This I assume because the cadence of living things on Earth is at the very least tied to the day/night cycles, winter/summer seasons, and lunar tides, none of which are especially unique.
If anything like us existed in the milky way it's almost certainly dead now. If there's something alive it will be almost incomprehensibly different than us.
I think this may be making some weird assumptions about motivation. Let’s say, for the sake of argument, that travel at 5% of the speed of light is practical. Would you be signing up to go colonise a planet around Alpha Centauri? What would be the motivation opportunity such colonisation? Who would pay for it?
If interstellar travel turns out to be permanently slow, difficult, and expensive, then that may cast doubt on the idea that anyone who theoretically can spread out all over the place given a hundred million years actually will.
But I think both of these opinions come from human-centric reasoning. A species that could hibernate may be able to control hibernation through technology, so a ten year journey wouldn’t be as complicated for them. Other species on earth simply don’t have the curiosity to explore the way humans do.
Speculation about alien intelligence is interesting to me, but I imagine it’s a lot weirder than we think. Especially around topics adjacent to intelligence, like curiosity, motivation, and cultural values.
Earth gives us a wide range of organisms with different lifespans and metabolic rates. There are not that many species that live longer than us, and extremely few that live significantly longer than us. Even if something lived 100 times longer than us, only a very small portion of the galaxy would be traversible within a lifetime, and there is still no reason to go more than a few lightyears.
An insult repeated to the point of cliche in this context which I suspect isn't normally personally meant, but nevertheless seems to deny the counter-arguer the abilities of imagination and wonder, and also presumes ("arrogantly"?) they haven't considered the known numbers first before coming to their conclusion that either there is little hope of communication or we don't know enough to decide.
For most I think that we are hear seems to indicate some inevitably but I like that we are an anomaly, we shouldn't exist, the we die.
Getting to complex cells took 2 billion years. And getting to where we are now another 2 billion years.
All that time conditions were fairly stable.
Given that life might need some elements created in supernova explosions mixed into other solar systems, and that we had just 13 billion years since the beginning, when you add some randomness, schedule starts to look tight for me.
We might be one of the first civilisations in our neighborhood.
I'm not saying that I believe that's the case but that's entirely possible.
Perhaps the evolving spectroscopic signature of our atmosphere will provide a durable signpost. The earth terraformed itself well before intelligent beings ever evolved, and maybe an astute and patient observer might have been able to guess that we were on our way towards intelligent life at some point.