If Aliens Exist, Where Are They?

[Foxaèr]

Life existing elsewhere in the universe is more credible, to me at least, than the existence of a supernatural deity.

[Joods]

All the books and all the movies that humans could ever create most likely don't hold a candle to what is really out there. We can wonder all we want but until we actually come face to face with life from another planet - we're probably imagining it all wrong.

[Anomalocaris]

1. All civilizations, societies of sentient beings, or large individual sentient beings (like a planet-wide network of neurons or computers) tend to develop.
This fact is a similar statement to the principle that living things evolve via natural selection, except that sentient beings, being natural apex predators and highly intelligent, are freed from much of the pressure that natural selection imposes on most living creatures. This is coupled with the fact that sentient beings develop/advance via the acquisition of knowledge, whereas normal organisms tend to advance via evolution, and it makes the analogy a bit poorer. Nonetheless, both evolution and civilization development ultimately result in increases to the organisms fitness and (probably) complexity.

Disagree on 2 counts.   1, regarding slackening of natural selection pressure,  Unless apex predators specifically go out of its way to ensure all heritable traits existent or arising in its population attain exactly the Same probability of Perpetuating, regardless of how they each individually or in combination affect metabolism, cognition, desire, outlook, or other aspects of behavior, nature selection would continue to operate and impose the same pressure as before, even if the pressure is not superficially in the same form of "red in tooth and claw" that we've associate with natural selection on the African savanna.
2, there is no compelling reason clear to me to believe civilization will necessarily tend to increase in complexity.  In fact, I suspect complexity in aspects of civilization brings with it its own problems, such as the more complex the organization, the more likely it is for the whole organization to respond non-linearly in unanticipated ways, and more likely would the civilization fall apart under stress and bring a stop the increase in organizational complexity it had hitherot nurtured.

2. As civilizations or organisms increase in complexity, their desire to communicate or "bother with" beings of less complexity/importance decreases.
This should go without saying: Humans are not concerned whatsoever with the opinion of an ant, and nor does anyone of sane mind try communicating with them.

You are projecting. It only goes without saying because it appears self evident based on our innate prejudice, and we are willing to overlook the need to support it with a rigorous case.   The prejudice here is our specie's non-rational behavioral tendencies in this regard must somehow be common to all sentient species, and no other imperative arising in the course of further development of civilizations would seriously hamper the expression of this tendency.  I think no rigorous case can be made for this view.

3. Simultaneously, as civilizations/sentient beings advance, they become increasingly concerned with the waste of energy they collectively generate.
The United States, for example, in the late 2000s led a campaign of regulations, subsidies, and other actions to increase the efficiency of consumer and business appliances. As a result, the energy usage of machines continues decreasing to this day, which makes way for more technology and new power-hungry inventions to consume the void in energy usage. So, from this we can be certain to a high degree that societies will seek to limit their energy usage in order to allow 'more... More... MORE!', and in addition, any sufficiently advanced civilization will recognize that if the Universe continues on it's current path and is not stopped somehow, energy will inevitably become more and more valuable over time.

Disagree.  Waste depends on perception.  In a fundamental way, We as a civilization never wasted energy.  All energy we ever expended as a civilization were expended for specific purposes, and were likely the result of intuitive trade off assessments under them current situation.   We simply had exolving perception of which purposes may not be worth the energy expended to achieve it.    For example, burning forests may seem like a waste of energy to us now.  But we didn't do it to waste energy.   At certain point of evolution of agriculture, burning down forests seems like a necessary and efficient step to reach the next level.    There is no reason to believe any highly advanced civilization would not be confronted with a succession of needs or an imperatives that could only be achieved in a timely manner by expanding vast quantities of energy in a manner that allows a great deal of the energy to leak away to be detectable to us.  In other words, from certain point of view, they waste energy.

4. Therefore, given all of the above, the civilization in its final stages of development will not bother expending energy on communicating or attempting to communicate with other beings which may likely not have anything to offer for them, but rather, will focus all efforts on preserving itself and achieving maximal efficiency. This civilization would not be detectable, at least not with any realistic level of ease; all electromagnetic waves would be absorbed on their planet's surface to be transformed into energy for their use. Their planet would be a perfect black body.

See the above.
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[vorlon13]

Amazing how many more ways a planet can "go wrong" regarding the development of life. Yeah, were finding many more planets, but it's bewildering how many more ways they can turn out unpleasant than may have been appreciated in the early 80s when Sagan (and others) were discussing odds and probabilities of all this.

[Gawdzilla Sama]

Amazing how many more ways a planet can "go wrong" regarding the development of life.  Yeah, were finding many more planets, but it's bewildering how many more ways they can turn out unpleasant than may have been appreciated in the early 80s when Sagan (and others) were discussing odds and probabilities of all this.

Well, it's just happenstance that we're in the Goldilocks Zone and all the rest. I doubt we'll find a "typical" planet like Earth out there. The variables are, after all, astronomical.

[ignoramus]

Guys, here's a question. How do we know how much variation another planet's conditions can vary from our early earth and still bare successful life?
In other words, no-one really knows the acceptable boundaries of the so called goldilock zones. Maybe they are prohibitively narrow?

Maybe the odds compound exponentially if we take into account the necessary(?) climatic changes over the millions of years of early earth evolution and the appearance of oxygen creating simple life as a necessary middle stage for complex life to form?

The only thing in its favour is that we are made of common and abundant elements of the universe so it makes sense that other planets also would as well. But I'm still not convinced that even simple life can spawn as a matter of chemicals and energy in the right proportions with the right temp given enough time (goldilocks specs).
But with a proven sample of 1 of 1, maybe I'm being just as illogical as a theist.

[Anomalocaris]

Amazing how many more ways a planet can "go wrong" regarding the development of life.  Yeah, were finding many more planets, but it's bewildering how many more ways they can turn out unpleasant than may have been appreciated in the early 80s when Sagan (and others) were discussing odds and probabilities of all this.

When Sagan was discussing odds and probability of life, he probably seriously overestimate how typical our solat system is amongst all planetary systems. But be probably also seriously underestimated how common planetary systems were amongst all stars. So it may not all that far off in the end.

At the last count, we've found around 4,000 exoplanets. If on average only 1 in 100,000 planets in the milky way is hospitable to our mode of life, meaning the chances of us having found such a planet amongst the 4,000 we discovered so far is very slim indeed, there would still be on the order 40,000,000 planets hospitable to,our mode of life in the Milky Way.

At the same time, we also found life fundamentally related to us don't need to be in the Goldilocks zone to survive, nor maybe even to arise. This implies planets like ours may be few and far in between, but life may not be.

[Mister Agenda]

Our big moon had a lot to do with our evolution, probably sped up the transition from sea life to land life by tens of millions of years. There are a lot of earth-like planets out there, but how many with oceans and big moons in the Goldilocks Zone? Maybe we're the elder race of this galaxy, thanks to our strong tides. I think an aquatic technological species would be handicapped in developing electronics and space travel.

[Anomalocaris]

I think the solution to Drake's paradox is simply that the power, reach and capability granted by science and technology to civilization raises so exponentially as a function of time that each civilization spends only a very brief amount of time, compared to the age of the galaxy or indeed the total life span of a civilization or technic specie, in the era when what it does and how it interact with the universe is detectable and recognizable to us.

If 1,000,000 technological civilizations had arisen throughout the history of the Milky Way at random times since ~9 billion years ago, when the first terrestrial planets have reached 2-3 billion years of age, and on average each civilization spends 100,000 years in the era when it interact with the universe in a way we can both detect and recognize, then on average, at any given time, there is only 10 civilization across the entire Milky Way that at a stage where it would be interacting with the universe in a manner we can both detect and recognize, despite the presence of many more of the total of 1,000,000 may have survived, prospered, and advanced to a capability, mode of interacting with the universe in modes we do not even have the basis to speculate.

[bennyboy]

If life isn't everywhere, then if there is other life, it may be very far away, indeed. If life is ubiquitous, or at least common, then we are not the special snowflakes we think we are, and might therefore not merit much attention.

The only reason I'm confident there is life out there is that the Universe is fucking huuuuuuuuuge. That's a lot of rolls of the evolutionary dice, and if you aren't necessarily sure about the Big Bang, there is no limit to the number of galaxies that may be out there beyond the lightspeed horizon in an expanding Universe.