Type II, III and above. The problem with interstellar travel and energy requirements are not really a problem for gravitational constraints. Remember these civilizations can harness the power of individual stars (type II) and many stars (type III). Generally speaking higher order civilization types are likely to have access to abundant energy and likely the technology that deals with also energy requirements as related to 'super earths'. Type I civilizations no doubt deal with gravity but really some of the bigger problems to 'space program' is their thicker atmospheres. One this translate to much frictional energy that represent constraints to travel, secondly biology often deals with constraints to the amount of accelerated force that would be dealt with. For Earth's type I civilization traditional chemical rockets have served purpose for graduating energy payload expense, dealing with atmospheric friction, and secondly dealing with the problem of accelerated forces that would certainly harm flights containing biology. The downside to chemical rockets is that its pretty rudimentary (combustion) related stuff to get stuff flying...couldn't there be better ways to lift payloads up into space, or has been quipped in the movie Interstellar, there really isn't so much of a type I solution to evacuate an entire planet (for its biology) for the amount of chemical combustion (energy requirements) up into orbit, save petri dishes and genomes.
One theory...
Transporting biology across interstellar space is a wasteful for energy and spatial requirements. It seems cool but really, its a waste for type I civilizations. As to orbital lift solutions, maybe there is some physics solution (discovered) in the future that simultaneously handles high friction burns with shock waves and cavitation more adequately than exists now and simultaneously deals with acceleration/deceleration problems (then one shot energy burst lift assists could be a possibility), but otherwise, conventional means are a likely given today and well into the future as it relates to biology and transport. The same applies in where atmospheres are more rarefied, except mostly its the accelerated forces problem...maybe the use of cryogenics to some advantage can ramp up accelerated motion for biological flight problems, but this is approaching a type II civilization's solution? So Type I civilizations and other types may not really bother so much with living biology for interstellar travel. It seems far less cool, but hey, its the reality of Earth's type I civilization. Cheaper to send machines and control these.
Theory Two...alien civilizations exist and are quite active here...we aren't looking for them in the right places. They aren't so interested in convincing the world with a massively idiotic mother ship hovering in the sky that creates massive panic about the world and mobilizes any to think that aliens are allergic to water and bad show tunes.
They could be embedded in a much larger microbial population (quite small) and integrated with respect to any attributes that would make for genetic dissimilarity. In other words, they don't look like alien biology at all relative to our own. They may be assimilation oriented as well, assimilating the features of their own genetics with that Earth life making for ease in transitioning to Earth's biology without being invasive or destructive. Adding to this theory, why is intelligent life always restricted to corporeal biology? Or given to likely type I constraints? If you could be an intelligence that were nearly without mass, it seems travel becomes easier, doesn't it?
Theory three...when radio telescopes are used to look for intelligent life, really type I civilizations are going to be found more likely...given that type II, and above aren't as interested in type I technology or communicating via this way. Are there faster than light means, for instance? Really my type I existence 150,000 years ago, isn't so meaningful to you now if I am long since gone! And secondly, we had to wait 300,000 years just to say 'Hi'? Local communication is better up to a few decades, but really going beyond this, seems to get more cumbersome in the exchanges. Other than relaying the message 'Hi' (and given the energy requirements for transmitters), greater utility for radio based communications probably warrants justifications. At times type I civs politic the notion of radio telescope probing the universe on the basis of 'we're likely alone if we see no evidence to suggest otherwise', and the ramifications for type I civs even desiring to spend the monies either transmitting or looking for transmissions diminishes.
Theory four...civilization types tend to be interested in one another more likely when there is type parity...thus type II bonds well with type II, type I with type I, and so forth...
type IV studies type III but isn't so much interested with a lot of interaction...'Hey look we are type IV would you at type I like to be type IV?' When a cargo cult is made of beings? I mean the ethics of this sort of thing gets studied right? Aliens of different type orders seem more omnipotent and all powerful and well...hmm...maybe its just as well to go incognito and call it a day! They could be interested, they may not be interested. If they've been here chances are they've been elsewhere, and anyways, as in the scale of time in the cosmos, what are several thousands years of recorded history? The Fermi paradox fails as a longitudinal argument...existentially here and now, yes, a possible issue...but we see starlight from millions of years ago coming to Earth and if ever any civilization existed there, its probably not there anymore.
The paradox in reality provides for complex answers while the mistaken assumption is the simple reasoning of obvious presence. For instance, you think, the answer could mean more likely, 'Why is the alien friend not living down the street and waiving at you as you come to greet?' If it Is an assumed bias that intelligent civilizations seek to inhabit (by greater populations) more places and potentially consume more resources part of the argument 'why aren't they here by now'? If the goal of higher order civilization type is neither obvious presence which infers the condition of colonization and consumption of resources, then it could be inferred that such civilization isn't obeying either by principle of population scale, settlement/colonization, and consumption of resources. Even if colonization is also a logical prospect what is an easier means to colonization? Is it transformation of a given environment or integration and assimilation of existing biology into an environment. We know that CRISPR gene editing allows for the insertion of genetic code, so if it is possible with a type I civilization, how to identify 'alien' DNA versus Earth DNA? The other part of the colonization answer, is that it may not be so simple to transfer components of a biome into another real estate without transferring more completely many different parts of that biome. Transferring an 'alien' biome may require unique climate conditions and all other parameters that need be met. This is organisms from the microscopically small to larger organisms that work in such environment to make any particular component sustainable. It seems another likely ethical and legal conundrum (likely present in many such type I and above civilizations) is what responsibility is born existentially in eradicating indigenous planetary life. As it turns out, many of such civilizations will likely have complex legal systems and likely a basis of ethics presiding with respect to interactions. Can you imagine a chaotic and lawless group of aliens in mass being organized well enough to produce the technology to travel interstellar space...as in the case of number of monkeys that randomly manage to reproduce the works of Shakespeare? Well maybe it is very very very remotely possible, but for the vast majority of intelligent civilizations this is true?
The 'why are they not here' may also means that even if existential proof is given the validity of evidence by continual contact. Surely the voyage across the Atlantic is by far shorter, and abundantly trade is globalized on our planet, relative to the scale influence of civilizations past, but even 12,000 light years across is but a fractional distance of our own galaxy. That distance by the way describes the limiting time frame for luminal velocities. That distance also describes much of recorded history on our planet (not pre history or natural history but recorded history). Thus, in such a distance of travel, one could imagine enormous changes having happened in a type I civilization and what about trade and contact clearly? For a relative modest scale distance of travel. The people of 12,000 years ago may share some physical and emotional similarities but the cultures could be quite distinct! And who would future aliens come to meet relative to a visitation 12,000 years ago if they had such meeting? My argument then is this: the concept of continual contact and trade for type I civilizations over extremely modest and short scale distances becomes economically cumbersome and faces any number of difficulties. Of course, even for aliens traveling when relativity is quite noticeable for near luminal velocities, the journey is one into the future. It is in knowing that the information of ancestors will likely be history upon arriving at any destination. That is, almost universally the truth for most stars likely hosting intelligent civilizations throughout our galaxy. Then what are the economics and what is the sociology of continual visitation and contact to look like? Many wars fought, the rise and fall of any number of civilizations happened in such time frame. If that impression of us, a first civilization so long ago reached in ever the faintest of reflecting photons from our planet to such receiving host alien world and that such could be extrapolated from the darkest of images to much greater luminosity, the light of today would reveal how much has changed. The 'why are they not here' is also in keeping to the likelihood that most travel is sub luminal and near luminal at best, but almost unlikely superluminal. While we hold out hope that wormholes could miraculously transport us from one world to another, the likelihood of travel by this means is enormously expensive if it were possible. Thus even for the observable universe, the concept of scale cannot be fully realized until interstellar missions have set about conveying the problems related to communication and contact for most. The question 'why are they not here' is not so easily satisfied. A handful of settlers don't land on a planet to survive with existing indigenous biology neither likely providing adequate real estate, and even where hosting biology shares enough genetic similarity, look to history. All the perils of science in such question is contained in the 'we are doomed by what we only know', and how do ancestors record the incidental meeting of one alien group (a mere handful of such individuals) in history, and how is such history to survive? How is the evidence of their existence ever to survive? There have been lone voyages by Irish monks alleged to have sailed all the way to Iceland in sojourn for a hermit like existence. Likely a place that afforded solitude as much as the alleged travels of another Irish monk supposedly even as far reaching to the America, but in some ways maybe this history is almost recalled in the way that mythology is regarded. There is probably little to any evidence ever to be found of a lone monk that ever would have reached a place like New York long before European colonist ever settled. Without scale and numbers, unless aliens were using machines in fabricating ever so much grafitti with a sense in conveying a permanence of their existence in some incontrovertible way, it seems like the fate of human voyagers in the past, there are some relegated to the now unwritten chapters of history. That is to say, that there is also a probability that such contact and communication simply vanished into narrow time window of civilizations past if any receiving civilization was there to record it even. If a tree fell in the forest but no one was there to hear it, did the tree fall? Certainly it did, but not to the broad consensus of the scientific community.
As an analogy consider the symbolic language used at Yucca mountain. I think this perfectly illustrates the problem of scale of communication over time. For instance, scientist reasoned, likely rightly, that America as we know it may not exist in the next tens of thousands of years, and with this, there could be a possibility that our own language hadn't existed, and so to communicate with future inhabitants the dangers of the Yucca mountain site, they instead relied upon using universal like symbols that would convey danger of the site, or likely be interpreted as a warning. While this isn't to say that aliens might not be doing the same thing somewhere else in some nearly eternally ruined (or just a long long time to go back before its safe) spot in the cosmos, but it is to convey the difficulties of communication over the scale of time.. Just whose language would people of the star system 12,000 light years away would such intelligent speak? Consider the possibility that alien visitors came to Earth in prehistory, the problem of contact and communication follows more along the lines how to discern visitors of millions of years ago, or whether if a civilization existed millions of years ago, the Silurian hypothesis forms some idea of figuring out a civilizations existence. Even for 12,000 years of communication return and response (more like 24,000 years cycle), two intelligent civilizations are mindful in communicating with some key (as in the movie 'Contact') making use of logic and natural structures (constants) in the universe, for instance, that would form the basis of such language. Given the likely mutability otherwise of existing language over any span of time. Who will exactly manning the transmitter on the next cycle of communication?
Theory five....the scale distance relationship of the America's to Europe is vastly different from the scale relationship of Earth to Mars, Mars to Alpha Centauri, Alpha Centauri, and so forth. While technological increase in type I civilizations potentially increases the rate of travel, there are (beyond the theoretical) fundamental limits to the speed of travel, and these aren't likely fundamental limits that can be broken. While holding out hope in cosmological oddities like wormholes, travel in the cosmos in an intergalactic sense is beyond being a turtle, its much slower if it is at the speed of light...maybe from the perspective of travelers this is fine given relativity, but its generally a one way ticket into the future. Type I civilizations are really doomed when it comes to deep exploration (beyond the one way ticket). Sure one can look in one's own backyard but mostly for type I civilizations, the civilizations are luck to have vessels designed to house people for hundred's of light years. For technological growth on Earth, when would you estimate a first interstellar journey? Scale distances (as indicated above) relate another problem which is that signal communication obeys inverse square laws in terms of signal amplitude...the farther things are away changes the necessary requirements for sending a signal that isn't lost in the chaotic background of cosmic noise. Certainly stars could send signals but look how massive these structures are! Communicating between Earth and Alpha Centauri mentions the transmitter's aperture requirements for our nearest star neighbor...and that is several light years away. As scale distance increases so to the size of transmitter and cost with it, so even if a type I civilization couldn't afford literally sending something there, even communicating could be costly with all the risk of sending signals to a place that hosted no intelligent life.
Theory six...type II and higher civilizations progressively get rarer in terms of existence. Even if each type I civilization has managed to survive its nuclear childhood and adolescence. Is it likely social evolutionary destiny that such civilization aims to harness the power of its own stellar neighborhood, and subsequently the evolution of higher civilization types is to maintain abundant population growth with power and resource control on galactic scales? Type II and higher may not be as interested in growing population and inhabiting more space and subsequently consuming more resources (energy or otherwise). Biological drives and existential purposes could be distinct relative to those found on Earth. If exploration is driven by population pressures (type I civilizations), imperatives even on Earth for different groups of peoples based upon geography have been shown true, which relates to desire for exploration, communication and technological adoption. It maybe that some higher type order civilizations aren't so interested in spending time and resources to exploring the cosmos even given all the factors that hadn't prevented such civilization from doing so. The same could be true for communication. Thus a rarer civilization type also holds the possibility that social evolution makes even rarer the possibility for exploration and communication in general. Thus the one's that are able to are less likely to do the communicating or traveling leaving those that are (type I s) less likely to do so beyond a quite local basis ( 100ly < ). Extending to this idea, higher order civilizations aren't as myopic as type I civilizations and thus consider things well beyond the scope of a few years or decades for planning exploration and communicating.
Theory seven...Earth is known and avoided (for some reason). It implies something about our civilization type indeed, but it could be possible that we have offended or are considered uniquely hostile relative other intelligent civilization types found in our local galactic neighborhood. Consider all the wars, genocide and destructive capacity that we've exhibited up till now...higher civilization types probably didn't get to be where they are at by continuing to waste resources and energy warring with one another, or possibly blowing themselves up to smithereens with nuclear weapons. Higher order civilization types may have little to do with war, are far kinder to their own, and generally dislike much of Earth in terms of social climate, and thus Earth is left communicating more likely in a universe of isolation.
Theory eight... skepticism requires much proof. Is a monolith found buried in some location on Earth sufficient evidence for alien intelligence? We urge others to practice 'leave no trace' in consideration to ecology, yet the graffiti of travelers passing through tell us irrevocably about those that have been. Time relates another problem, even rocks can erode in time (millions of years). Artifacts can buried under sedimentary layers of Earth. Even for the scale of time on the order of millennia, or far less, old settlements may be found in time. However, given much the evidence pointing where no evidence exists for travelers having passed through millions of years ago. How would an otherwise anomalous artifact (say object composed of iron) but found millions of years before any human civilization be treated without the lack of collective evidence even it were sufficiently dated? Would it be treated by the scientific community as purely 'anomalous' or likely fake? Of course, being 'iron' doesn't fit the extraordinary claim that it is also the product of an 'alien civilization', does it? Inherent biases would likely have that any such artifact or 'evidence' of any alien civilization requires something that is a technology that we couldn't reproduce and likely gaining acceptance requires scale of evidence. The whole point in stating this is that 'we only know what we know' is an inherent assumption also biasing the study of the observable universe.
We don't absolutely know what happened on this planet back hundreds of millions of years ago. We have many theories and evidence about the natural history of our planet what has likely happened, and that collective ignorance of conservative judgement puts much greater limiting factor on probabilities of life elsewhere. As to discerning history of dating artifacts beyond recorded history, that is another matter because we have biased against this consideration. A few decades ago, a similar skepticism would range about the possibility of exo planets beyond our solar system, yet the thinking was that our solar system was uniquely privileged in harboring the necessary ingredients for forming an harboring planets? What is remarkable about the vast cosmic web of the universe is that much of it looks homogeneous even if galaxies themselves bear distinctions in terms of stars and the elements that they are likely to have greater abundances or lesser amounts. That there is little distinction for the sun relative to other points in our galaxy, or own galaxy in the cosmic web plays to some central notion that if the conditions are neither so exceptional here, why not elsewhere? And if in time (the longitudinal argument) makes that the formation of life is far less exceptional given the conditions and ingredients than we believed (in laboratory and simulation testing), why should life be exceptional on Earth? Earth was not exceptionally the center of the universe, or the solar system...nor the sun similarly. So why not life? And why not intelligent life?
Other reasons you can think of?
The whole point of this is to say the universe is also so large in scale for intelligent life not to exist, but this will likely become more true once examples of exo biology are found. This will happen in the next ten years.
What are own thinking reveals to us, is that for the 15,000 years past having seen the birth and death of civilizations over is a very minuscule window in the picture of cosmic time, and that even our own anthropocentric biases have repeatedly been undone.
One theory...
Transporting biology across interstellar space is a wasteful for energy and spatial requirements. It seems cool but really, its a waste for type I civilizations. As to orbital lift solutions, maybe there is some physics solution (discovered) in the future that simultaneously handles high friction burns with shock waves and cavitation more adequately than exists now and simultaneously deals with acceleration/deceleration problems (then one shot energy burst lift assists could be a possibility), but otherwise, conventional means are a likely given today and well into the future as it relates to biology and transport. The same applies in where atmospheres are more rarefied, except mostly its the accelerated forces problem...maybe the use of cryogenics to some advantage can ramp up accelerated motion for biological flight problems, but this is approaching a type II civilization's solution? So Type I civilizations and other types may not really bother so much with living biology for interstellar travel. It seems far less cool, but hey, its the reality of Earth's type I civilization. Cheaper to send machines and control these.
Theory Two...alien civilizations exist and are quite active here...we aren't looking for them in the right places. They aren't so interested in convincing the world with a massively idiotic mother ship hovering in the sky that creates massive panic about the world and mobilizes any to think that aliens are allergic to water and bad show tunes.
They could be embedded in a much larger microbial population (quite small) and integrated with respect to any attributes that would make for genetic dissimilarity. In other words, they don't look like alien biology at all relative to our own. They may be assimilation oriented as well, assimilating the features of their own genetics with that Earth life making for ease in transitioning to Earth's biology without being invasive or destructive. Adding to this theory, why is intelligent life always restricted to corporeal biology? Or given to likely type I constraints? If you could be an intelligence that were nearly without mass, it seems travel becomes easier, doesn't it?
Theory three...when radio telescopes are used to look for intelligent life, really type I civilizations are going to be found more likely...given that type II, and above aren't as interested in type I technology or communicating via this way. Are there faster than light means, for instance? Really my type I existence 150,000 years ago, isn't so meaningful to you now if I am long since gone! And secondly, we had to wait 300,000 years just to say 'Hi'? Local communication is better up to a few decades, but really going beyond this, seems to get more cumbersome in the exchanges. Other than relaying the message 'Hi' (and given the energy requirements for transmitters), greater utility for radio based communications probably warrants justifications. At times type I civs politic the notion of radio telescope probing the universe on the basis of 'we're likely alone if we see no evidence to suggest otherwise', and the ramifications for type I civs even desiring to spend the monies either transmitting or looking for transmissions diminishes.
Theory four...civilization types tend to be interested in one another more likely when there is type parity...thus type II bonds well with type II, type I with type I, and so forth...
type IV studies type III but isn't so much interested with a lot of interaction...'Hey look we are type IV would you at type I like to be type IV?' When a cargo cult is made of beings? I mean the ethics of this sort of thing gets studied right? Aliens of different type orders seem more omnipotent and all powerful and well...hmm...maybe its just as well to go incognito and call it a day! They could be interested, they may not be interested. If they've been here chances are they've been elsewhere, and anyways, as in the scale of time in the cosmos, what are several thousands years of recorded history? The Fermi paradox fails as a longitudinal argument...existentially here and now, yes, a possible issue...but we see starlight from millions of years ago coming to Earth and if ever any civilization existed there, its probably not there anymore.
The paradox in reality provides for complex answers while the mistaken assumption is the simple reasoning of obvious presence. For instance, you think, the answer could mean more likely, 'Why is the alien friend not living down the street and waiving at you as you come to greet?' If it Is an assumed bias that intelligent civilizations seek to inhabit (by greater populations) more places and potentially consume more resources part of the argument 'why aren't they here by now'? If the goal of higher order civilization type is neither obvious presence which infers the condition of colonization and consumption of resources, then it could be inferred that such civilization isn't obeying either by principle of population scale, settlement/colonization, and consumption of resources. Even if colonization is also a logical prospect what is an easier means to colonization? Is it transformation of a given environment or integration and assimilation of existing biology into an environment. We know that CRISPR gene editing allows for the insertion of genetic code, so if it is possible with a type I civilization, how to identify 'alien' DNA versus Earth DNA? The other part of the colonization answer, is that it may not be so simple to transfer components of a biome into another real estate without transferring more completely many different parts of that biome. Transferring an 'alien' biome may require unique climate conditions and all other parameters that need be met. This is organisms from the microscopically small to larger organisms that work in such environment to make any particular component sustainable. It seems another likely ethical and legal conundrum (likely present in many such type I and above civilizations) is what responsibility is born existentially in eradicating indigenous planetary life. As it turns out, many of such civilizations will likely have complex legal systems and likely a basis of ethics presiding with respect to interactions. Can you imagine a chaotic and lawless group of aliens in mass being organized well enough to produce the technology to travel interstellar space...as in the case of number of monkeys that randomly manage to reproduce the works of Shakespeare? Well maybe it is very very very remotely possible, but for the vast majority of intelligent civilizations this is true?
The 'why are they not here' may also means that even if existential proof is given the validity of evidence by continual contact. Surely the voyage across the Atlantic is by far shorter, and abundantly trade is globalized on our planet, relative to the scale influence of civilizations past, but even 12,000 light years across is but a fractional distance of our own galaxy. That distance by the way describes the limiting time frame for luminal velocities. That distance also describes much of recorded history on our planet (not pre history or natural history but recorded history). Thus, in such a distance of travel, one could imagine enormous changes having happened in a type I civilization and what about trade and contact clearly? For a relative modest scale distance of travel. The people of 12,000 years ago may share some physical and emotional similarities but the cultures could be quite distinct! And who would future aliens come to meet relative to a visitation 12,000 years ago if they had such meeting? My argument then is this: the concept of continual contact and trade for type I civilizations over extremely modest and short scale distances becomes economically cumbersome and faces any number of difficulties. Of course, even for aliens traveling when relativity is quite noticeable for near luminal velocities, the journey is one into the future. It is in knowing that the information of ancestors will likely be history upon arriving at any destination. That is, almost universally the truth for most stars likely hosting intelligent civilizations throughout our galaxy. Then what are the economics and what is the sociology of continual visitation and contact to look like? Many wars fought, the rise and fall of any number of civilizations happened in such time frame. If that impression of us, a first civilization so long ago reached in ever the faintest of reflecting photons from our planet to such receiving host alien world and that such could be extrapolated from the darkest of images to much greater luminosity, the light of today would reveal how much has changed. The 'why are they not here' is also in keeping to the likelihood that most travel is sub luminal and near luminal at best, but almost unlikely superluminal. While we hold out hope that wormholes could miraculously transport us from one world to another, the likelihood of travel by this means is enormously expensive if it were possible. Thus even for the observable universe, the concept of scale cannot be fully realized until interstellar missions have set about conveying the problems related to communication and contact for most. The question 'why are they not here' is not so easily satisfied. A handful of settlers don't land on a planet to survive with existing indigenous biology neither likely providing adequate real estate, and even where hosting biology shares enough genetic similarity, look to history. All the perils of science in such question is contained in the 'we are doomed by what we only know', and how do ancestors record the incidental meeting of one alien group (a mere handful of such individuals) in history, and how is such history to survive? How is the evidence of their existence ever to survive? There have been lone voyages by Irish monks alleged to have sailed all the way to Iceland in sojourn for a hermit like existence. Likely a place that afforded solitude as much as the alleged travels of another Irish monk supposedly even as far reaching to the America, but in some ways maybe this history is almost recalled in the way that mythology is regarded. There is probably little to any evidence ever to be found of a lone monk that ever would have reached a place like New York long before European colonist ever settled. Without scale and numbers, unless aliens were using machines in fabricating ever so much grafitti with a sense in conveying a permanence of their existence in some incontrovertible way, it seems like the fate of human voyagers in the past, there are some relegated to the now unwritten chapters of history. That is to say, that there is also a probability that such contact and communication simply vanished into narrow time window of civilizations past if any receiving civilization was there to record it even. If a tree fell in the forest but no one was there to hear it, did the tree fall? Certainly it did, but not to the broad consensus of the scientific community.
As an analogy consider the symbolic language used at Yucca mountain. I think this perfectly illustrates the problem of scale of communication over time. For instance, scientist reasoned, likely rightly, that America as we know it may not exist in the next tens of thousands of years, and with this, there could be a possibility that our own language hadn't existed, and so to communicate with future inhabitants the dangers of the Yucca mountain site, they instead relied upon using universal like symbols that would convey danger of the site, or likely be interpreted as a warning. While this isn't to say that aliens might not be doing the same thing somewhere else in some nearly eternally ruined (or just a long long time to go back before its safe) spot in the cosmos, but it is to convey the difficulties of communication over the scale of time.. Just whose language would people of the star system 12,000 light years away would such intelligent speak? Consider the possibility that alien visitors came to Earth in prehistory, the problem of contact and communication follows more along the lines how to discern visitors of millions of years ago, or whether if a civilization existed millions of years ago, the Silurian hypothesis forms some idea of figuring out a civilizations existence. Even for 12,000 years of communication return and response (more like 24,000 years cycle), two intelligent civilizations are mindful in communicating with some key (as in the movie 'Contact') making use of logic and natural structures (constants) in the universe, for instance, that would form the basis of such language. Given the likely mutability otherwise of existing language over any span of time. Who will exactly manning the transmitter on the next cycle of communication?
Theory five....the scale distance relationship of the America's to Europe is vastly different from the scale relationship of Earth to Mars, Mars to Alpha Centauri, Alpha Centauri, and so forth. While technological increase in type I civilizations potentially increases the rate of travel, there are (beyond the theoretical) fundamental limits to the speed of travel, and these aren't likely fundamental limits that can be broken. While holding out hope in cosmological oddities like wormholes, travel in the cosmos in an intergalactic sense is beyond being a turtle, its much slower if it is at the speed of light...maybe from the perspective of travelers this is fine given relativity, but its generally a one way ticket into the future. Type I civilizations are really doomed when it comes to deep exploration (beyond the one way ticket). Sure one can look in one's own backyard but mostly for type I civilizations, the civilizations are luck to have vessels designed to house people for hundred's of light years. For technological growth on Earth, when would you estimate a first interstellar journey? Scale distances (as indicated above) relate another problem which is that signal communication obeys inverse square laws in terms of signal amplitude...the farther things are away changes the necessary requirements for sending a signal that isn't lost in the chaotic background of cosmic noise. Certainly stars could send signals but look how massive these structures are! Communicating between Earth and Alpha Centauri mentions the transmitter's aperture requirements for our nearest star neighbor...and that is several light years away. As scale distance increases so to the size of transmitter and cost with it, so even if a type I civilization couldn't afford literally sending something there, even communicating could be costly with all the risk of sending signals to a place that hosted no intelligent life.
Theory six...type II and higher civilizations progressively get rarer in terms of existence. Even if each type I civilization has managed to survive its nuclear childhood and adolescence. Is it likely social evolutionary destiny that such civilization aims to harness the power of its own stellar neighborhood, and subsequently the evolution of higher civilization types is to maintain abundant population growth with power and resource control on galactic scales? Type II and higher may not be as interested in growing population and inhabiting more space and subsequently consuming more resources (energy or otherwise). Biological drives and existential purposes could be distinct relative to those found on Earth. If exploration is driven by population pressures (type I civilizations), imperatives even on Earth for different groups of peoples based upon geography have been shown true, which relates to desire for exploration, communication and technological adoption. It maybe that some higher type order civilizations aren't so interested in spending time and resources to exploring the cosmos even given all the factors that hadn't prevented such civilization from doing so. The same could be true for communication. Thus a rarer civilization type also holds the possibility that social evolution makes even rarer the possibility for exploration and communication in general. Thus the one's that are able to are less likely to do the communicating or traveling leaving those that are (type I s) less likely to do so beyond a quite local basis ( 100ly < ). Extending to this idea, higher order civilizations aren't as myopic as type I civilizations and thus consider things well beyond the scope of a few years or decades for planning exploration and communicating.
Theory seven...Earth is known and avoided (for some reason). It implies something about our civilization type indeed, but it could be possible that we have offended or are considered uniquely hostile relative other intelligent civilization types found in our local galactic neighborhood. Consider all the wars, genocide and destructive capacity that we've exhibited up till now...higher civilization types probably didn't get to be where they are at by continuing to waste resources and energy warring with one another, or possibly blowing themselves up to smithereens with nuclear weapons. Higher order civilization types may have little to do with war, are far kinder to their own, and generally dislike much of Earth in terms of social climate, and thus Earth is left communicating more likely in a universe of isolation.
Theory eight... skepticism requires much proof. Is a monolith found buried in some location on Earth sufficient evidence for alien intelligence? We urge others to practice 'leave no trace' in consideration to ecology, yet the graffiti of travelers passing through tell us irrevocably about those that have been. Time relates another problem, even rocks can erode in time (millions of years). Artifacts can buried under sedimentary layers of Earth. Even for the scale of time on the order of millennia, or far less, old settlements may be found in time. However, given much the evidence pointing where no evidence exists for travelers having passed through millions of years ago. How would an otherwise anomalous artifact (say object composed of iron) but found millions of years before any human civilization be treated without the lack of collective evidence even it were sufficiently dated? Would it be treated by the scientific community as purely 'anomalous' or likely fake? Of course, being 'iron' doesn't fit the extraordinary claim that it is also the product of an 'alien civilization', does it? Inherent biases would likely have that any such artifact or 'evidence' of any alien civilization requires something that is a technology that we couldn't reproduce and likely gaining acceptance requires scale of evidence. The whole point in stating this is that 'we only know what we know' is an inherent assumption also biasing the study of the observable universe.
We don't absolutely know what happened on this planet back hundreds of millions of years ago. We have many theories and evidence about the natural history of our planet what has likely happened, and that collective ignorance of conservative judgement puts much greater limiting factor on probabilities of life elsewhere. As to discerning history of dating artifacts beyond recorded history, that is another matter because we have biased against this consideration. A few decades ago, a similar skepticism would range about the possibility of exo planets beyond our solar system, yet the thinking was that our solar system was uniquely privileged in harboring the necessary ingredients for forming an harboring planets? What is remarkable about the vast cosmic web of the universe is that much of it looks homogeneous even if galaxies themselves bear distinctions in terms of stars and the elements that they are likely to have greater abundances or lesser amounts. That there is little distinction for the sun relative to other points in our galaxy, or own galaxy in the cosmic web plays to some central notion that if the conditions are neither so exceptional here, why not elsewhere? And if in time (the longitudinal argument) makes that the formation of life is far less exceptional given the conditions and ingredients than we believed (in laboratory and simulation testing), why should life be exceptional on Earth? Earth was not exceptionally the center of the universe, or the solar system...nor the sun similarly. So why not life? And why not intelligent life?
Other reasons you can think of?
The whole point of this is to say the universe is also so large in scale for intelligent life not to exist, but this will likely become more true once examples of exo biology are found. This will happen in the next ten years.
What are own thinking reveals to us, is that for the 15,000 years past having seen the birth and death of civilizations over is a very minuscule window in the picture of cosmic time, and that even our own anthropocentric biases have repeatedly been undone.
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