There's an entire body of knowledge that we cannot chart yet because we have no data to formulate it from. It seems likely and intuitive that if there are life forms on planets other than Earth, they are subject to evolution; but we cannot hypothesize much further than that.

Under what conditions does sentient meta-cognizant life evolve? How does it develop culture, religion, mathematics, art, philosophy, language? (has the Church-Turing thesis been formulated identically somewhere else than on Earth?) Which of those are constants amongst such species, and for the ones who aren't, what leads to their creation? How intertwined are they to biology? (e.g., death and religion) How frequently does the notion of "gender" happen in complex living structures? Do principles of cultural development such as the ones presented in "Guns, germs and steel" hold for non-Earth civilizations?

And if we find advanced sentient life that is much older than we are, that will be even more interesting. Are there events that this civilization went through/constructs that it built that are similar to ours? (various notions of democracy/freedom/government; mechanical transportation, "computers", electricity, digital communications, artificially expanding life (i.e. healthcare), etc.)

"Xenopology" is going to be one fascinating subject[1]. And on a related note, if any one is aware of serious academic work in that field (it seems pretty much impossible right now, because we just don't have the data; but you never know), I'd love to hear recommendations :)

[1] With fascinating consequences when it arises, too. We'll have to rewrite a lot of the more "meta" Wikipedia articles, for starters (Wikipedia is terribly anthropo-centered). And then, what will happen with our body of knowledge? We will merge human mathematics with xenomathematics? (if it's even possible) If they have a communications construct comparable to the internet, do we keep the two separate? Do we want them to be able to communicate at all on an individual level, much like any human on Earth can email any other human on Earth freely right now? Those are all of course very hypothetic considerations, but they're probable and fascinating to think about.

It could well be that all advanced civilizations decide that it's easier to answer these questions with simulations than observation. More interestingly if that is true and you also believe the odds of a singularity in our future is high, then the odds of us being in a simulation now are quite high.

It is more specific than it needs to be - it uses the hypothesis that we will want to run simulations of our past history. But of course such simulations would not be constrained to that.

Philip K. Dick wrote a short story about this, where little spheres with artificial worlds becomes a massive fad, but then someone realises the people on those world might actually be alive and intelligent. I can't remember the title.

[1] Pretty sure it was Seth Lloyd's "Programming the Universe": http://en.wikipedia.org/wiki/Programming_the_Universe

At first brush (and to the layperson who is not steeped either in digital physics or simulation theories) this seems like a good contender in the legion of theories of this sort that seek to explain the nature of our observable universe (as it appears to the human consciousness)

I looked up the reviews of Amazon for the book, "Programming the Universe" and some reviews seems to suggest that the author largely treads on thin ice in several areas.

For example:

I've always admired the notion (first promulgated by Voltaire?) that the true measure of intelligence is the ability to simultaneously comprehend two mutually contradictory ideas. So I tend to take a mellow approach to ideas that I disagree with. However, this book angered me, not because of its ideas, but because of its serious flaws.

The first serious flaw is that the author cannot keep his ego from seeping into the text. He regales us with triumphant tales of how he confounded his students with deep questions and then nobly revealed the true answers. Sheesh, man, why use the dialog approach using weaklings as your interlocutors? Pit yourself against somebody who can do more than behave as your straight man. Argue with yourself, if you have to! But presenting yourself as the all-knowing professor rubbed my fur the wrong way.

The problem of author vanity permeates the entire book. At no point does the author admit to uncertainty, or present two sides of a case, or even admit that anything he writes is controversial. One gets the strong impression that everything is crystal clear to this author. That impression raises my hackles.

The overwhelming self-assurance of the author explodes in his face when he gets it wrong. In the section "Exorcising Maxwell's Demon" in Chapter 4, he writes:

"The full exorcism of the demon was not accomplished until recently. (I played some part in this ceremony myself.)"

Perhaps Mr. Lloyd is older than I imagine. The exorcism of Maxwell's Demon was accomplished by Leon Brilloun, the physicist who patented the atomic bomb, in a paper published in 1951. Mr. Brilloun does not mention any contribution by Mr. Lloyd. It would appear that Mr. Lloyd is unaware of Mr. Brilloun's paper. Worse, his explanation of the exorcism of Maxwell's Demon is a turgid mess that makes no sense at all. Between claiming credit for another man's achievement and botching the explanation of Maxwell's Demon, I reached the limit of my tolerance. I literally threw the book away from me at that point.

Perhaps the material after Chapter 4 redeems the book; I do not know, because I did not read it.

The other serious flaw in the book is its smarmy vagueness. In attempting to avoid the intimidating reliance on mathematical and technical definitions, Mr. Lloyd resorts to poetic phrasings. These would be acceptable if they weren't so damned cute -- and if they made sense. For example, in attempting to make clear the difference between energy and information he writes, "Energy makes physical systems do things. Information tells them what to do." At first glance, that seems a pithy observation. But go back and read it again; what is the author really saying? Does energy give molecules speed, and information give them direction? Does the energy in an A-bomb make the bang and information tells it what to destroy?

I was disconcerted by the author's fuzziness regarding information. He never defined it -- which is not necessarily a fatal flaw, given that a book for the educated public should not burden its readers with undue technical detail. But he used the term in such a myriad of ways that I started to think that he was using it to refer to any magically powerful force. Information, in this book, seems capable of performing wondrous feats.

Physics is finally coming to terms with the concept of information as a physical concept. The change began after World War II and has been edging forward for fifty years; in the last ten years, progress has accelerated. A clearer concept of information and its relationship to the physical universe is emerging. Mr. Lloyd misses one of the most important factors in this process: that information itself is inextricably bound with the concept of time, in something like the way that mass and energy are bound together, only more complex. It is not information that is the fundamental quantity; it is information flow, or bandwidth.

Source: http://www.amazon.com/review/R1HXWI1JRL1M75/ref=cm_cr_dp_tit...

Is there a more authoritative piece of writing that tackles the key arguments against simulated reality, more comprehensively and perhaps more convincingly?

Especially the arguments of

a) Computationlism b) Computability of physics c) Nested simulations

http://en.wikipedia.org/wiki/Simulated_reality#Arguments

Probably no, the reason is any one who is smart enough to simulate us will also be smart enough to stop us messing up with the simulation. And it ends there.

The scary situation is simulation theory perfectly accommodates things like religion, god and miracles. And probably even things like heaven, hell and the judgement day.

Nick Bostrom's rationale against this objection:

(1) the human species is very likely to go extinct before reaching a “posthuman” stage;

(2) any posthuman civilization is extremely unlikely to run a significant number of simulations of their evolutionary history (or variations thereof);

(3) we are almost certainly living in a computer simulation. It follows that the belief that there is a significant chance that we will one day become posthumans who run ancestor-simulations is false, unless we are currently living in a simulation.

If we are in a simulation, doesn’t that undermine the reasoning in the simulation argument?

The simulation argument relies on an assumption about the technological capabilities of a mature civilization. Our evidence for this assumption is empirical: it is based on our best theories about the physical limits of computation and the kinds of constructions that could be built with advanced molecular manufacturing techniques etc., and our confidence in these theories rests ultimately on observations of the world we see around us. But if we are in a simulation, then how could we trust these observations? Might they not inform us only about the simulated reality and not about any hypothesized underlying of reality in which the simulation is being run? And if so, does this not undermine the simulation argument by casting doubt on its empirical assumptions?

If we reflect more carefully on this objection, however, we see that it fails. The claim that we cannot have any information about the underlying reality if we are in a simulation is false. In a simulation, we can certainly know the following two conditional claims:

A. If we are in a simulation, then the underlying reality is such as to permit simulations, it contains at least one such simulation, and (3) is true.

B. If we are not in a simulation, then the empirical evidence noted in the simulation argument is veridical taken at face value, suggesting that a technologically mature civilization would have the ability to create vast number of simulations; and consequently, by the simulation argument, there is a very high probability at least one of the disjuncts in (1)-(3) is true.

Since we either are, or are not, in a simulation, we can conclude, even if we are in a simulation, that at least one of the disjuncts (1)-(3) is true.

Source: http://www.simulation-argument.com/faq.html

If we don't receive any such signals, then finding life elsewhere is going to be a long, hard slog. It seems we don't yet have the energy/technology levels required to get a probe to another star within a reasonable amount of time (100 years travel time, would perhaps be reasonable? Could any of our equipment last that long?) Self-replicating space probes that could exponentially explore the entire galaxy seem to be right out of the question, at present.

Assuming it is possible to make such probes for a relatively young civilisation, and most civilisations that reach that stage does send out even just one probe that starts relicating, if life is abundant around us we should expect to have swarms of them everywhere.

Of course it could be we just can't detect them yet, or that life is abundant but that life old enough to be advanced enough to do this is rare for some reason.

Then if we find a planet with an atmosphere that is far from the thermodynamical equilibrium, like our 21% oxygen is because of photosynthesis, then that might be a planet with planetary scale life.

I find this particularly fascinating, because it could lead to a corollary to the Drake equation. This equation would quantify the (possibly insurmountable) barriers that a species must overcome to become a space-faring civilization: strength of the planet's gravity well, ease of access to and total planetary quantity of rare elements required to build space hardware, etc.

Basically, Guns, Germs, and Steel -- but on a planetary scale.

Note some "fun" consequences of this argument:

-- quote --

With no evidence of intelligent life other than ourselves, it appears that the process of starting with a star and ending with "advanced explosive lasting life" must be unlikely. This implies that at least one step in this process must be improbable. Hanson's list, while incomplete, describes the following nine steps in an "evolutionary path" that results in the colonization of the observable universe:

    1. The right star system (including organics and potentially habitable planets)
    2. Reproductive molecules (e.g., RNA)
    3. Simple (prokaryotic) single-cell life
    4. Complex (archaeatic and eukaryotic) single-cell life
    5. Sexual reproduction
    6. Multi-cell life
    7. Tool-using animals with big brains
    8. Where we are now
    9. Colonization explosion.

-- quote ends --

(emphasis mine)

In other words, finding life on Mars would suggest that the Great Filter event may be ahead of us, so we're probably screwed.

    1.        70%
    2.       1–3%
    3.       1–3%
    4.        15%
    5. and 6. 10%
    7.         4%
    8.      0.03%

The ones advanced enough to colonize space(whose stars formed way before) are likely very far away from us.

Then it took ca. 500 million years to Homo, and 5 million years for our species to develop technology.

So I'd say the road from microbial life to cavemen was 1000x larger barrier, then the road from cavemen to technology. Also the barrier from microbial life to first animals (let's assume some kind of worms) was 10x larger than the barrier from worms to technology.

In summary, getting early microbial life to get organized into some kind of macroscopic animals, this is the hard part. When that is done, vertebrates, dinosaurs, hominids and internet will follow at 5–10x faster pace.

http://en.wikipedia.org/wiki/Evolutionary_history_of_life

The thought that 1 out of 7 suns has earth-like planets is still amazing though. Wouldn't it be rather likely that an advanced intelligence is watching us at that point?

A factor in the development of life, especially sentient life, is metallicity [2]. Almost all of the first 100 elements naturally occur on Earth. This is pretty amazing. Nuclear fusion in stars naturally produces elements up to iron [3]. S-capture is thought to produce elements up to Bismuth (atomic number 83). Elements higher than this are thought to be produced only in supernovae or other cataclysmic events.

It's amazing to think that enough time has gone by in the Universe to take the abundant hydrogen and helium, form stars, have those stars run out of fuel and enough of them explode to scatter their elements to form heavy planets around later stars.

While heavy elements may not be required for life it seems less likely that starfaring sentience can form without them. Semiconductors for example require an odd mix of heavy elements.

It seems amazing that even we exist given that you need metals, a long-lived and stable star, likely billions of years of evolution, a sufficiently stable (and probably geologically active) planet with an atmosphere that has water in liquid form (a pretty narrow temperature band even taking into account different atmospheric pressures) and no cataclysmic events to happen in all that time (it seems to have been ~65 million years since the last big one).

It may just be that sentient life is fleeting. We've had "civilization" for X0,000 years, which is the blink of an eye. It's entirely possible we'll wipe ourselves out due to war or simply lack of (sufficiently cheap) resources in another blink. The problem isn't lack of resources. They are plentiful in space. The problem is the cost of getting them if energy is anything other than free.

Additionally interstellar distances seem almost unconquerably large that one has to wonder if we'll ever meet another sentient life form face to face (assuming they have faces).

Even if we do, one wonder what evolved elsewhere for facilitate communication. Will they see? Will the wavelengths of light they see cross over ours at all? These are questions we can only really speculate about.

It's also been suggested (by Hawking among others) that we really don't want to meet a much more advanced species as it will probably be bad for us, perhaps even an Outside Context Problem [4] and it seems unlikely given the rate of change we're experiencing that another civilization will be at the same point as us.

We could of course be "first". I mean someone has to be right? It does seem unlikely however. But I suspect there were many "firsts" (in the sense that they evolved independently and without any contact or evidence of other species).

The seeming lack of self-replicating robots, something we'll almost certainly be able to build and start filling the galaxy with within a thousand years I'm sure, is also somewhat puzzling.

Personally I just hope we find evidence of extraterrestial life sometime soon as it will settle [5] a whole bunch of issues about us being "special" (in the typically religious sense).

[1]: http://en.wikipedia.org/wiki/Drake_equation

[2]: https://en.wikipedia.org/wiki/Metallicity

[3]: http://hyperphysics.phy-astr.gsu.edu/hbase/astro/nucsyn.html...

[4]: http://en.wikipedia.org/wiki/Excession#Outside_Context_Probl...

[5]: http://xkcd.com/1235/

I always assumed that the estimates there were purposefully low to show "Hey, even with all these low estimates, we should still find something, right?"

"In other words, roughly 15 percent of all suns could, in principle, be hosting a place suitable for life as we know it. "

If planets are abundant, that puts the kibosh on the "rare Earth" theory, as shaky as that was to begin with. The other explanations are less optimistic: Maybe they all nuke themselves or turn to grey goo before they can muster the technology and energy to build a starship. Or they are roasted by random blasts of radiation before they become space-faring.

It's either something universal about civilizations, or something is wrong with the Drake equation or the underlying cosmology.

We do know that microbial life did appear quite soon after the formation of the Earth, so we have some evidence that life can bootstrap itself readily given a favorable environment. However, we also know that life then spent the next three billion years quite content at being unicellular, until the Cambrian Explosion made lots of things happen very quickly (at least on the astronomical timescale). We don't really have much ideas about why the CE happened, and how inevitable it was to happen at some point at least by sheer probability. I believe the shift from unicellular to multicellular life was the most fine-toothed of the Great Filters we've already passed - let us hope there won't be even stricter ones in our future. Given the number of ways it appears a technological civilization could snuff itself out of existence in an eyeblink, I'm not very optimistic.

Look at the graph here:

http://en.wikipedia.org/wiki/Oxygen_catastrophe

It took 1 Ga from the invention of photosynthesis to grow enough biomass so that photosynthesis started to be a notable process in geological scale.

Then it took 0.5 Ga of photosynthesis, when the produced oxygen would dissolve into the oceans, until the oceans were saturated. Atmospheric oxygen remained at 2–4%.

Then it tool 1.5 Ga of hard work, when the land surface minerals would be oxidized, until they were all oxidized and finally oxygen started to accumulate in the atmosphere.

Then around 600 Ma ago the atmosphere hit above 10% oxygen, and this pretty much coincides with the Cambrian evolutionary explosion.

http://en.wikipedia.org/wiki/Cambrian_explosion#Possible_cau...

It's been a decade since I read the book but IIRC very little of it had to do with whether lots of planets exist. In fact they assume microbial life is abundant but argue against complex multi-cellular life being very common.

It's a fascinating book ("Rare Earth") but even without their arguments, I'm curious: why would one assume that planets equal civilizations?

There are hundreds of plausible reasons why, if there is life outside, we haven't been contacted. Maybe they know about us but we're not interesting, maybe they don't know about us, maybe they tried to contact us but we haven't been able to detect them, maybe we just don't have the right technology, or we're not looking in the right place, maybe we're all limited by these pesky laws of physics and there is no chance that we could ever meet or even know about each other, maybe they are not looking in the right place and missed us, maybe they are not yet aware of our existence since we've been emitting radio for such a short time, etc

The thing about the scale of the universe is that the odds are always cosmically against "That just hasn't happened yet" being the right answer.

But there are many other constraints: It may have happened many times, but they may not currently be at a level where we'd be able to communicate. We have only been emitting radio for about hundred years, and only tried to detect others for a few decades. Maybe we evolve to a point where we lose interest in being known and stop "leaking" radio signals all over the place within the next hundred.

Then we're not just talking about whether or not it "has happened yet", but our interest in finding others - perhaps that'll only be a hundred year timespan - must intersect with the couple of centuries that such civilizations broadcasts hit earth, and they'd need to be in the regions of sky we've tried to detect signals from.

Suddenly there can be vast number of current or past civilisations out there that'd we'd never notice.

Here's how far our out going call has gone:

http://zidbits.com/wp-content/uploads/2011/07/how-far-in-spa...

Maybe for the same reason we're not "calling" that much anymore? For the last several decades we've been investing tons of money and engineer-hours to make radio waves stop going up, as we mostly need them on the ground, and we don't have cable or cellular subscribers in space.

A) The universe is very large. The nearest star is 4 and some change lightyears (ly) away. That means any message take over 8 years to get, process, and transmit to the nearest star. All other stars are further than that. Since we have been weakly radio broadcasting for about 100 years, the number of stars we have been able to speak with is at best 1600 (http://wiki.answers.com/Q/How_many_stars_within_50_light_yea...).

B)Messages get lost in noise. We assume that we are heard out in the universe. But it is very very difficult to hear a mosquito at a rock concert 100 miles from you. The reverse is true for us. Also, you have to know to look. The concert has been going on for 100 years but the venue, so to speak, is 4 billion years old. Also, you might be on the other side of the galaxy, or in another one entirely.

C)Messages aren't understood as such. Do you know the protocol for USB to serial conversion? (If so I have many questions for you) We may think an actual message is just noise. Or, that noise is a message. We cannot possibly tell. Yes hopefully the other side will start out easy on us but define 'easy' to an alien.

D)Lets assume that they have spotted us and are pre-sending signals. In what medium? We haven't even figured out the basic laws of the universe yet. Heck Dark matter is ~25% of the mass-energy of the universe. The other ~75% is some crazy stuff we just labeled Dark Energy. We have no clue what is going on there. They may already be blasting us with data, but we just haven't got the interstellar radio invented yet.

E) They simply don't give a shit. What do we have that they want? Lets assume they are about 10,000 years older in tech than we are. Geologically speaking, thats nada, thats the last ice age. People were around then. And look now we have radios and iPads and ISS's and stuff. 10,000 years is a lot of time to think and make stuff. What could we do in that much more time? Well, make a computer that'll calculate just about anything, probably. And if your civilization is all entertained, why bother looking at the stars? Why bother with a,likely, giant telescope array that beams out gibberish for a thousand years. What stuff on Earth over 1000 years old? Not all that much but piles of rocks and some windows. The engineering of a giant thingy is very hard. And all for what? Some algae that you hope turns into a little curious squid or something?

F) They dont give a shit. Not because they are hedonistic but because they live under an ice-sheet the planet wide. Or they are at the bottom of a giant cloud where pressure is just right. Or they are frozen periodically in ammonia ice for 700 years. Or they have no idea of math and science because they don't have nerves that work like that. Or because they don't have cells at all. Or because they have no photoreceptors and can't even begin to imagine light. Or because they think that thing we call matter is just a rounding error. Or because they already migrated into dark energy beings and are just waiting to see us.

G) We are just plain jane alone.

H) They do give a shit, but they are busy hiding. Consider this: If there's only one civilisation out there sufficiently advanced to be able to accelerate large, heavy objects towards others at sufficient speed, and who has decided that clobbering everyone else is their best bet at long term survival, everyone else better hide unless/until able to throw rocks first. A sufficiently large object constantly accelerated over a distance of light years would be near impossible to protect against.

The solution might very well be that anyone chatty gets hit by a planet.

I) Something natural periodically wipes life out. E.g. consider if gamma-ray bursts occurs frequently enough, and are powerful enough, to cut down everyone that don't reach an extremely high technological level in a very short timespan.

J) We're in a simulation and they're not interested in simulating life on other planets (or the purpose is to simulate a universe with life only on earth).

Hiding has few benefits though. We can see them, and they can see us. It just takes a big telescope. The time lags are the big problems still. Say they can send a school bus at us at 0.9c. That'll still take a few hundred years or so to get to us. We have plenty of time to skedaddle or send our own school bus at their's. And we'll always see it coming too. In fact, just as in any hard problem you want more minds on the task. A different view point, or math method, or formulation of chemistry or art, will be a great benefit to you. Considering that you will never actually "shake hands" talking is the only thing you can do.

Hmm, actually, I wonder if the interstellar ham radio network is more like the internet than a semaphore network, aka full of trolls. Guess we have to find out.

I am pretty in favor of the periodic angel of death argument. The conditions for life on earth are incredibly rare as is. So the conditions to not have a GRB near us are also even more rare. The odd asteroid, charismatic lunatic with nukes and a grudge, disease, suicide cult, volcano etc amplify the fragility of a continued intelligence.

I don't believe that a simulation is a good thing to bet on. It would be nice to think my thoughts are archived in a giant blobbery computer somewhere in the 4th dimension, but all evidence points to us actually being here. This gets to swirl the great toilet hole of existentialism really quick, so i prefer not to look at it a whole lot.

They probably have, or are trying to. But even if they want to, a start system say 2000 light years away looking at us is seeing the world as it was during the time of Jesus. And we are looking at them when they were equally ancient.

And not to mention, if they tried to communicate with us we won't know until the next 2000 years. Basically you get the point. For a civilization to communicate to us, they should've started the process of sending signals out x years, based on for x light years far they are.

And that's just the first problem. The universe is so big, each galaxy has billions of stars and there are some billions of galaxies in the universe. Given the vastness and size of universe, its next to impossible to search such a large space with already existing limitations of distance and time.

Thirdly, we've hardly been listening for a few decades, we might have missed a signal if it was ever sent out or haven't received one.

Let's say someone was beaming a very strong signal at us, would we have detected it by now with all our SETI programs? Nope, not very likely we would have. We've only scratched a tiny bit of the search space with SETI, spatially and in wavelength.

Similarly, what if an Earth-twin existed in a stellar system within 20 light-years of us, would we have detected it by now? Again, also no. It would take rather a lot of luck for us to have detected it by now, our methods of detecting planets are very limited. It will likely take many more decades before we can confidently have a nearly complete survey of exoplanets near Earth, let alone at greater distances.

As far as the Fermi paradox, I never bought it as a paradox. All it says is that we probably don't understand the nature of space-faring technological civilizations, which we clearly don't, nothing more.

See http://science.psu.edu/news-and-events/2013-news/Luhman3-201... for proof that we found stars THIS YEAR within 6.5 light years of us. What else is out there? We don't know.