At 2 billion years older than Earth, I can't begin to imagine what kinds of things could possibly be living on this planet. Given, of course, that it's got the right ingredients for life to arise. What if it's suffered huge extinctions recently? What if there's a domineering species out to conquer the planet just like humans? What if there's a society there more sophisticated than us? It's a very exciting discovery.
It's jarring to think that if intelligent life were on Kepler 452-b and observing Earth, we're so far away that they would be seeing our "civilizations" in 615 AD: shrinking global population as the old Roman and Byzantine empires are overrun by barbarian hordes and Arab conquest, with some pockets of human progress in China. They would have no way of knowing that the descendants of those squabbling warlords just cooperated to fling a space probe at 36000 MPH past a tiny ball of rock at the edge of our solar system.
Actually no! Estimates I found are of around 10^23 stars in the universe. There are over 10^38 IPv6 addresses, so even if there were tens of inhabitable worlds per star and billions or even trillions of addresses per world, we'd have enough! (Although it is possible that stars number is a significant underestimate.)
I was beginning to think we collectively lost our sense of humor.
There is hope. Now quick, someone post a cat pic and a relevant gif.
Edit: Not complaining about the downvotes, but guys, this is the internet, not work. Loosen up. Smile for once. It's good for you, and confuses people.
My favorite thought is that they've sent probes millions of years ago and we could send a request to watch some HD surveillance of dinosaurs roaming the Earth. It'll just take ~3k years to receive the requested footage.
You're right about the idea, but a few hours is way too short, because of how the Human body tolerance to strong accelerations.
Assuming an acceleration of 10 m/s^2 (similar to Earth's gravity), it will takes roughly one year to accelerate to a speed close to the speed of light were relativistic effects occurs, and you will need one more year to decelerate.
> It's jarring to think that if intelligent life were on Kepler 452-b and observing Earth, we're so far away that they would be seeing our "civilizations" in 615 AD: shrinking global population as the old Roman and Byzantine empires are overrun by barbarian hordes and Arab conquest, with some pockets of human progress in China.
Is there something uncivilized about the Arab conquest of Byzantine lands? Had the Kepler 452-b-ians been observing Earthians but a few hundred years prior, they'd have seen the Romans doing much the same to expand their own empire. And the Arabs did quite a good job to not only preserve the Roman intellectual and scientific state of the art, but to contribute their own advancements of those fields.
Not a dig at anyone, especially not the Arabs who basically preserved philosophy and mathematics while Europe entered the Dark Ages. My point was simply that Kepler 452b-ians observing Earth would be seeing lots of war and destruction even though we're currently peacefully collaborating among mankind to discover and explore planets.
> My point was simply that Kepler 452b-ians observing Earth would be seeing lots of war and destruction even though we're currently peacefully collaborating among mankind to discover and explore planets.
There's a quite a bit more human energy going into war and destruction right now than into "peacefully collaborating among mankind to discover and explore planets."
> observing Earth would be seeing lots of war and destruction even though we're currently peacefully collaborating among mankind to discover and explore planets.
If they were observing us today, they'd still see plenty of war and destruction.
It just so happens that a fraction of the worlds population lives under "peaceful" conditions. That's not universally true.
It is more proper to refer to them as Muslims, not Arabs. The whole point of Islam was to unite everyone under the same ideology. That's why it's called the Islamic civilization.
Was anybody but Arabs involved in the conquest of the early 7th century? Ie I'm sure other people took on Islam afterwards but he was talking about a specific point in history.
We don't call the Spanish conquest of the Americas a Christian one even though they were Christians who ultimately converted everybody to Christianity.
Quite a few of the Prophet's companions were not Arabs, but yes the majority were. Nonetheless, it is more correct to identify as a Muslim before an Arab.
What irritates me greatly is when historians refer to the Muslim scientists of the Islamic Golden Age as "Arabs", when most of them were not Arabs at all. It's such a simpleminded and sweeping generalisation that clearly shows the thoroughness of the so-called orientalists.
I think it may be the preferred nomenclature in the Muslim world since all the people who were conquered are now Muslim - so they should have an easier time accepting the fact of being Islamicized as that's their culture now (whereas bringing up the fact that they were conquered by a foreign people might stir up tensions).
I agree with you about the second point. I guess it's due to the fact that they wrote in Arabic and many of the people they actually belonged to no longer exist (for example al-Khwārizm was from a Persian family but born in what is today Uzbekistan - I actually visited his hometown :)
>What irritates me greatly is when historians refer to the Muslim scientists of the Islamic Golden Age as "Arabs"
To me it's strange to ascribe such areas of study to the religion. What does Islam really have to do with optics besides Ibn Al-Haytham being a Muslim? No one ever ascribes enlightenment science as 'Christian science'.
I'm sure some people track these sort of tribal affiliations between inhabitants of the British isles over the course of a millenium, too, but again I'd consider it a silly thing to get riled up about.
If they have the ability to telescope us before we do. They can probably guess that technology accelerates.
In fact I feel more pessimistic of the human's future as we found more and more such older planets -- If their million or billion years older civilization cannot reach us, then we might not reach out that far as well in the next billion year, or ever.
>we're so far away that they would be seeing our "civilizations" in 615 AD: shrinking global population as the old Roman and Byzantine empires are overrun by barbarian hordes and Arab conquest, with some pockets of human progress in China.
Err, the Byzantine empire was still going strong for 6 centuries after that (and still existed, but not in a good shape, for 8).
It never got overrun by "barbarian hordes", but it did have a run in with thieving "supposedely christian horders" on their way to the Crusades in the 12th century, and was finally overrun by the muslim Ottoman empire in the 14th century.
One can perhaps take comfort in the fact that a civilization advanced enough to be aware of us would likely be aware of the fact that we were more advanced than what they could see at any given moment (or they had found a way to narrow the perception lag).
They would likely have extrapolated the savagery they saw and realized that we may pose some problems for the galactic community. RKKVs are probably on their way towards Earth as we speak.
I propose it's unlikely that any technological civilizations more advanced than our own have existed on this planet in the past few billion years, otherwise some subgroup of their species would likely have seeded the local universe with life (it's only ~1400 lightyears away) and they would be here instead of us. Unless of course the panspermia hypothesis is correct and that is indeed where life on Earth came from.
Apparently it has a diameter 60% larger than that of the Earth, and so if we assume it has the same density as the Earth it would have about 16m/s^2 gravity on the surface, and an orbital velocity of around 12km/s @ ~1000km (in comparison to the Earth, where a 1000km circular orbit is closer to 7km/s).
This means that merely achieving orbit would be approximately as difficult as launching a probe on a Hohmann transfer orbit to Mars is. (Except that such a launch vehicle would need nearly twice the thrust/weight, cutting down the mass ratio even further.) It is not unreasonable to think that a species on such a planet would have never seriously developed space travel.
To colonize or seed life you don't need to launch huge space vehicles with enough life support to maintain a breeding population, though. You can just launch some kind of self-replicating microbes capable of directing themselves into the forms of life you want to encourage over time. The precise ability to do this would depend on your current state of technology, but we're pretty close to developing full blown synthetic biology and genetic engineering ourselves, so it's hard to imagine they'd need to be more than a few hundred years more advanced than us at most. Microbes can be engineered to be immune to most dangers from space travel as well, similar to tardigrades.
To head off the common rebuttal of "maybe they wouldn't have any interest in it," with more advanced technology smaller and smaller groups of individuals would be capable of instituting such a project, to the point where any high school kid with a synthetic biology kit and some toy rocketry kits could colonize the entire universe (owing to relativistic effects). You'd have to assume their society underwent some kind of massive convergence of social norms away from any individuality such that no few members of their species ever had both the capability and desire.
By the same logic, any depressed high school kid could also engineer a pathogen to wipe out their entire civilization before that happened. (I think this is a plausible great filter candidate)
You seem to be assuming a notion of continual human progress. In a few hundred years, we'll probably be in another Dark Age, judging by current trends. We're like the Roman Empire in 200 AD.
Progress is the exception, not the rule, so far in human history.
This might work on a baron world, but any place with a developed ecosystem would likely see these things out competed very quickly do to the massive overhead of needing to survive in space for a long time.
Or they would be engineered with huge advantages against an ecosystem which had never adapted defenses to them (typical of successful invasive species on Earth), and work on timescales much faster than typical evolutionary ones. These would basically be microscopic von Neumann probes, presumably with some kind of self-organizing emergent AI to direct their progress, not just spraying random bacteria everywhere and hoping for the best.
Only a tiny fraction of species end up as invasive, because the local ecosystem tends to be better adapted. As to ‘grey goo’ there is fairly good evidence that’s far less possible than you might think.
If you don’t stick with Hydrogen Carbon Nitrogen Oxygen you’re going to have a much harder time finding what you need to grow on earth. For extra solar stuff even those might be hard to find.
If you do stick with HCNO it's hard to get a leg up on nature without simply making a few minor improvements which has limited value outside specific situations. There are also a lot of tradeoffs with binding energy etc.
Or the "zoo hypothesis"[1] is true and we're quarantined off on our little planet out here. I figure we're like the people on North Sentinel Island[2]. We haven't even invented fire, and we kill anyone who tries to shows up on our island. We're so remote that the local authority in the area has declared our Island a protected zone, just as India has with North Sentinel Island. We occasionally see helicopter and try and shoot arrows at them.
As a counter point, it's always possible that a civilization that developed past where we are now would have been masterful at space travel, but not advanced enough to completely figure out a 1400 light year long journey. We're still having hard times figuring how to get squishy meat sacks to Mars without being irradiated to death or creating a launch vehicle made of lead. Besides the engineering problems there's the social cost of a venture like that. Unless their societies would be structured in a wholly different way (which is likely) then there is significant political and social momentum that would need to go behind a project like a mission where the minimum acknowledgement time of success is 2800 years away. There's plenty of time to go extinct or make yourself die off.
You don't have to figure out a 1400-ly trip to be here. You just ("just") need to be able to move from one star system to the next.
If they took 100 years to get to the next star system 5-ly away and then 500 years to populate that system enough to start a new colonization, that's just ("just") 168,000 years to be here. Given a 2 billion year head start, that's easy.
Even if it only took them 100 years to get to a star system 5 ly away, I would be amazed and impressed by a society that would be okay with investing in and working on at minimum a 105 year mission. Perhaps they live much much longer than we do :)
Cathedral building in the middle-ages was a multi-generational 80+ year affair and some of the Pyramids took even longer IIRC. So societies were/are definitely capable of working on ~100 year missions :)
Or the Sagrada Familia in Barcelona for a contemporary and in-progress example.
Maybe a generation ship needs to have some sort of 'cathedral-in-the-sky' religion connotation in order for us to commit to building it. Sounds like an interesting idea for a sci-fi plot!
If they lived much longer, comparisons on earth would lead one to expect they'd be much slower to get anything done. Small burrowing rodents that live a couple of years hit the ground running, build extensive dens, and reproduce before they're a year old. OTOH giant tortoises never actually do anything except walk around slowly.
> We're still having hard times figuring how to get squishy meat sacks to Mars without being irradiated to death or creating a launch vehicle made of lead.
But we've only been working on that for a few decades. 2 billion years is a lot longer. If there is a civilization there at all, it's extremely improbable that it hasn't figured out how to do exploration on a scale of thousands of light-years.
> I suspect that's unlikely, even if there were advanced life there for 2 billion years.
Why do you think so? "Continuity" doesn't have to be having the same government or the same political structure, or even the same physical form (the article itself talks about uploads and nanomachines). It just means continuity of technological development. It doesn't have to mean steady progress either; our civilization hasn't had that, yet in ten millennia or so we have progressed from basic agriculture to space travel. For this planet, we're talking about two million millennia, or 200,000 times the time we've had. All the variables we've observed in our civilization's history become rounding error on that timescale.
A mass extinction event would mean there wouldn't be a civilization on the planet at all. That's certainly possible, but for this particular subthread I am taking as a premise that there is a civilization there.
Yes, I mean continuity of technological development. I suspect that the prior millennium’s dark ages are but a pale inkling of how far a civilization can collapse, especially given geological time scales. I also suspect that technological development can plateau for sociological reasons.
Even during the dark ages here on Earth, there was technological advancement. Also, the dark ages weren't worldwide. I would be skeptical of a plateau in technological development for any significant length of time for the same reason: even if one part of the planet was in one, the rest of it wouldn't be.
It's fun to think about all of the things that could prevent a far more advanced civilization relatively nearby in the cosmos from making contact; it gives us a better perspective on how lucky or fragile our own development is and where we might be focusing on the wrong things.
Perhaps no civilization of any sort has ever developed there. Maybe conditions on the planet are inhospitable, perhaps evolution simply never went that way. I think people tend to assume that evolution naturally leads towards more intelligent creatures, but that's not true. Humans are a very very recent development on our planet, in geologic time scales, and our early existence was pretty improbable.
Perhaps civilizations have existed there, but never achieved technological advancement. There are still tribal civilizations on Earth that are using 50kya technology; there's no guarantee that a civilization must develop science and technology.
Perhaps it has had civilizations with similar technological advancements to our own, but they have been destroyed or set back by any of a number of things: extinction-level impact events (could easily happen to us), disease, warfare, or resource shortages. Maybe, for some reason, they never developed easy access to a high density fuel; imagine if humanity had tried to go directly from steam engines to nuclear or solar power because oil didn't exist or wasn't easy enough to find or its significance was never understood.
Perhaps they reached a level of technological advancement far, far superior to our own, but they never made an effort to explore the cosmos. The vast majority of people on Earth right now don't see any point to even exploring other worlds in our own solar system; that mindset could easily dominate for the rest of mankind's existence. Maybe we assume that there is some kind of shortcut around the speed of light and there isn't and, no matter advanced a civilization gets, there simply aren't enough volunteers for a 10,000-year journey to another tiny little planet that may or may not harbor life.
Perhaps all of these are wrong, but they simply visited Earth a million years ago and found it to be entirely uninteresting and haven't been back since. Maybe their civilization has even ceased to exist, and our civilizations will never ever cross paths because we missed eachother by a few minutes in geologic time.
I'm generally optimistic about the chances of advanced civilizations elsewhere in our galaxy, even relatively close by, and entirely pessimistic about our odds of ever being able to interact with those civilizations in any meaningful way.
Here's another scenario: they could have colonized other worlds, but had no interest in setting foot on every last planet. Perhaps their culture is insular, and they are either apathetic to (or forbidden from) contacting us. It's not hard to imagine that we would be unable to detect them, either.
It could also just be their resources were insufficient and they plateaued or fizzled out before escaping their own planet was possible.
Or that the distances and difficulty of transporting anything very large or complex between star systems turns out to be just too great of an engineering problem for any life form. The energy required to move a life-sustaining amount of mass between stars is mind-boggling huge and beyond our own technological capabilities for the foreseeable future ( we only anticipate interstellar life is a possibility by assuming our technological growth curve will continue unimpeded and then guessing there's life out there further along tha us, which may not be accurate or possible).
Or that they are indeed spreading but doing it in a way undetectable to our technologies we're looking with and we just didn't come up lucky enough to be directly contacted by them in their spread (yet?).
Or maybe they developed a Matrix-like utopia, a self-maintaining and long lasting support system and stopped outward appearing progress?
Or going in another direction, perhaps we really are inside a simulation and $life_bearing_planet is set to 1.
The hypotheticals are nearly endless and not necessarily due to overtly negative outcomes.
Maybe they tried and failed? Maybe they arrived on a sterile Earth 3.6 billion years ago with supplies and huge tomes of knowledge, promptly died off within a couple hundred years of arrival, tectonic movement subsumed their landing ship and all other physical records of their existence here and we're all just descendants of their gut flora?
I am surprised more people don't believe all this is just a trick because we base everything on our knowledge what if something out there is tricking us? We see the universe through a telescope and other sensors. But if something puts something in front of it, you will see different reality. It is like if you use binoculars and I put a fake image in front of you, you could start believing there is something else. It is also like magician.
So we could be in ZOO experiment. Other civilizations want to see if we are capable of evolving and joining them so they make sure we are not disturbed by other life out there because if we discover intelligent life, we will freak out. We might start fighting for our survival and it could affect our evolution to higher species.
We could also be a part of some computer simulation (with enough power you can create rules with particles and simulate the universe). Maybe somebody is trying to answer this question: If you create a big enough simulation, will be the simulation able to figure out it, it is being simulated?
Or maybe everything that we see is true but it is just impossible to overcome some physical limitations. We assume there are no limits and everything is possible and one day we will master it. But what if communicating and traveling through universe is nearly impossible? Maybe there are limits that we don't know about yet.
Maybe every civilization reaches point where they know the answer to everything and they don't see a reason to live anymore or they leave the universe and move somewhere else.
My point is that we assume something that will eventually turn out false and everyone will be surprised by the truth.
Totally agree with you. There are so many possibilities of evolution besides building high tech spaceships and conquering planets and stuff.
Imagine how much more wisdom (besides technological advancement) the average humans will posses in just 200 years (provided things evolve like they have been evolving during the last century minus the wars ).
What wisdom and knowledge will we have in 1000, 2000, 50000 years (again, provided we are still alive) ?
Maybe there is no need to physically travel through light years of space, maybe the 'truth' is much closer - right inside our own minds.
In just a couple of decades we'll be able to build simulations which simulate all our senses and are just as real as our 'current' Universe - enter there and take a pill which makes you 'forget' about this reality and you can basically live another life without knowing about your current one.
What if our current life is the result of such a 'game' which we entered and forgot about the 'outer' life. What if we're level 64 deep in such a simulation within simulation ? :)
So many possibilities, but hopefully we'll somehow figure them out ..
True but all of our planet has an atmosphere conducive to supporting life.
Don't get me wrong, I'm not saying that Earth-like planets generally won't support life. I have no idea how many will. It just seems unlikely that the first one found happens to tick all the relevant boxes.
Having said that, the fact that they've found one so quickly, relatively speaking, seems like a good indicator that there are an awful lot of good candidates for life-supporting planets. The more there are, one would assume, the more chance that some will have everything in place.
But then again, we could just get lucky and hit it first time. Which would be stellar (sorry).
There is good evidence that life apeared on earth more or less as soon as it cooled down, so there is just as much reason to think that this new place may have life as not.
However, by the same reasoning it seems to me that the life may be bacteria like!
I find myself thinking that the existance of Dark Energy/Dark Matter is the solution to the fermi paradox. there is some kind of energy creating system which creates dark matter. This matter is no good for normal matter so it is transported far away from stars/galaxies and dumped. This would probably be true if we could create a physics model that can explain like 1% of Dark Matter occurring naturaly. Then we just assume that other species have discovered how to create a ton of energy with Dark Matter as the by-product. We haven't been able to observe them for the same reasons our telescopes can't see the New Horizons spacecraft out by Pluto, just too small. And as far as observing communications. It's likely that all true interstellar communication on frequencies we could understand are sent with a targeted focus, like a laser, rather than a blast. Think of all our deep space equipment which has their uplink bits pointed toward earth.
Or maybe there is no Dark Matter and we can't see a huge portion of stars because all of the light is being captured like a Dyson sphere. Unfortunately both of those hypotheses are rather improbable, we can observe very very distant galaxies and they all seem consistent with respect to Dark Matter AFAIK.
If there was an intelligent life form, like humans, it would have expanded into the cosmos and colonized the entire galaxy within only a million years of developing technology. Since we don't see traces of them everywhere, we must presume they don't exist.
Perhaps government funding for their space exploration programs dried up, industry regulations interfered with commercial space exploration programs, and they all just gave up and shifted technology resources to making games for their mobile phones.
It's like, nobody has considered that advanced life might not even feel a need to live on planets at all. It's expensive to get resources from, and there's a near limitless supply of easy to get smaller debris.
Additionally, the difference a 2 billion year head start could make is hard to imagine. One thing that's not hard to imagine is that they don't live in meat bodies for most of that time. They could easily be machines or virtual, and then would look and act nothing like anything we're looking for.
We have sensitive telescopes. If there was life out there, we would have noticed. You would have astronomy conferences filled with people trying to figure out what all these weird spectra objects are, or trying to decipher all these structured radio observations.
What? No, absolutely no we would not just see them with sensitive telescopes. Perhaps, if you're thinking of some sort of interstellar space travel, maybe we'd see the engines? But no, even that seems unlikely.
Any space habitat of any plausible size will be completely undetectable, or at least indistinguishable from a planet. And I would think they'd be much smaller than that.
And structured radio? Lots of possible reasons not to see it. One, possibly not using radio. Two, encryption. Communication will likely look like noise. But even so, the distances involved means long range transmissions will need to be very directed and narrow beam. And even then, it's only a few light years before the signal is undetectable. I would suspect any really long distance communication would be bounced through relays. Also, why? You have to wait years for messages to go around. All of this means no, we're not likely to easily find stray radio signals.
I think you are completely misunderstanding the point. He have to vent heat somehow in the most efficient ways of doing so are not natural in their spectra. An encrypted radio transmission is still perfectly detectable -- you may not know what it says but you know that it is there. Artificial signals are blindingly obvious.
And this is without getting into Dyson spheres and other efficient but detectable technology.
Yes, of course. And the heat of a star will make that heat completely invisible in comparison.
The "spectra" that heat would be vented at is infrared, which is infrared no matter the source.
Say you had some gargantuan fusion power harnessing space station in deep space. lots of assumptions here about alien cultures, but this is likely the easiest to detect. They will do whatever they do with their energy, and then vent the waste heat. That heat will be infrared. What does a dim infrared source look like (if even visible at all)? It looks like a brown dwarf. But this is assuming a lot. It assumes some alien would even build brown dwarf sized space stations. We don't know if or even why they would do that.
Encrypted signals are not blindingly obvious. An encrypted radio transmission will appear as pure random noise and no different from all the rest of the noise that we pick up.
If an astronomer picks up a radio signal it is extremely obvious whether it is an artificial source or not. Natural sources have very inefficient spectra of very specific types. Artificial sources have very clear frequency cutoffs, having come from an antenna. Even frequency-hopping sources may jump around in a way that makes eavesdropping difficult, but if you stick around long enough on a single channel (as radio astronomers do) you'll get little blips now and then as the signal hops through your channel and is gone again.
How do we know this? Because astronomers see these every single night and have developed sophisticated filters to remove them. But by checking how the source is moving it is quite easy to tell if source is local or not, and sure enough all these artificial sources come from satellites, the troposphere, or an unshielded microwave.
It is actually remarkably difficult and a challenging problem in itself to create a communication mechanism which looks like a natural source. It defies imagination to think that every single intelligent species out there has purposefully masked their existence in such a way, including giving up useful technology like ground control radar or mobile communications.
The reason they have to filter out artificial sources, and the only reason they're a problem, is we are literally right on top of the source. These aren't even as far away as, say, Mars. New Horizons, for example, can't communicate with Earth unless it points its transmitter / receiver directly at us. And that's in our own solar system! Only a few light hours away.
Just think about it. The signals are going to be so weak by the time the photons get here they will be indistinguishable from the CMB.
> We have sensitive telescopes. If there was life out there, we would have noticed.
I'm sorry, but that's way off.
Our current telescopes can't even see the majority of the exoplanets in our galaxy; we are made aware of their existence by observing the behavior of their host star (wobbling/dimming). Unless something is extremely close, unfathomably enormous, or emitting/reflecting large amounts of electromagnetic radiation (light, radio, etc.) that isn't bent/filtered/reduced to noise, then we are completely unaware of it.
That assumes that civilizations derive power by encasing their host star in a Dyson sphere, which is not a foregone conclusion; that prediction was based on our current understanding if technology. Think of Huygens who imagined that Venusians sailed the seas on ships held together with hempen rope.
An "amorphous blackbody" source, from anything reasonably distant, just looks like a smaller star. There are a bunch of if's and unknowns involved, but it could be maybe possible to detect a Dyson sphere. This[1] article talks about it. The upshot is that it is nowhere near blindingly obvious, and we don't currently have the ability to differentiate between a natural or artificial infrared source.
We just found the first planet that is somewhat similar 1400 light years away... and you think our 'sensitive telescopes' would have definitely discovered all possible life in the universe if it existed???
"SETI estimates, for instance, that with a radio telescope as sensitive as the Arecibo Observatory, Earth's television and radio broadcasts would only be detectable at distances up to 0.3 light years, less than 1/10 the distance to the nearest star."
Or maybe the next step in the evolution of a civilization is the realization that 'colonizing' is much more fun if the colonists are just dropped off without any technology and let to develop into a civilization and discover who they are and do the same thing over again.
You know, just to keep the game going. All that's needed for that is already encoded in their DNA.
Maybe there are other channels of exploring spacetime than just the physical reality. Maybe that's where all the advanced civilizations reside.. Like the place we are in before we are born or after we die...
1 million years of mind evolution should not only bring advanced tech, but also advanced wisdom and understanding of who we are and what we are doing here. I doubt that expansion to all possible planets is the ultimate goal, the mystery must go much deeper than that ;).
It's like an anthill in the middle of the Amazon saying "If there were life on Earth that is more intelligent than us, why can't we see their enormous anthills? Why haven't they tried to contact us yet?"
I sometimes wonder if "like humans" is the key. What if past some level of technical advancement, organisms cease inhabiting the same scale as us. Physically and temporaly; they become microscopic and perceive+act on a time scale faster than us by orders of magnitude. Why inhabit the days/hours/seconds timescale when you can have the same experiences in micro/nanoseconds? So we would be mostly not interesting to them and expensive to interact with at our timescale. Not sure if we would notice them right under our noses.
Based on what evidence? I would posit that it there was an intelligent life-form, it would have descended into war and eventually extinction. Self-interest isn't exclusive to Earth.
Edit: I've said that self-interest isn't exclusive to Earth, without providing evidence and I don't have any; I should have said that self-interest isn't exclusive to humans, the simplest forms of life (bacteria, as an example) make war also.
Your big unstated assumption is that any intelligence will always consume the maximum resources it can and cause the maximally possible transformation of any habitat. Normally, when physicists look for these kinds of large-structure signs, the implicit assumption is that it's sufficient for one civilization to behave this way, but you seem to assert they all must be like that.
Any of the hypotheses we discuss in this thread (whether they are over-confidently stated as facts or marked as speculative) are inevitably deficient due to data derived from a sample size of one.
If I were writing a science fiction story there would be primitive villages on higher inland areas, and partially submerged technologically advanced cities on the coastlines :-)
But what I find most amusing is that when I was a kid "science" was saying "but we don't know if any other star even has a planet!" and to me as a kid it seemed preposterous that other stars wouldn't have planets. And now of course we know that pretty much ever star has at least some planets. The unusual case is no planets.
Similarly with life, it seems ridiculous to think that somehow this planet at this time was the only place anywhere that sentience emerged. So it really makes me wonder, why the heck aren't we already talking with them?
What is it that keeps intelligent life that originated on other planets from communicating with the intelligent life living on this planet? Why won't they talk to us?
