The Big Alien Theory(thebigalientheory.com) |
The Big Alien Theory(thebigalientheory.com) |
How did this ridiculous tinfoil-hat article even make it into Hacker News!?
Even if the arguments it makes are wrong I'm not under the impression that they are particularly kooky.
e: Maybe you are implicitly assuming that the workflow for "determining" who you are born as goes: 1. pick a random species 2. pick a random individual in that species. But it's more like: 1. pick a random individual.
Huh? I see no justification for this.
I'm firmly in the "We know nothing about aliens and won't until we get some hard data" camp.
Alien life will, by definition, be alien. We have no basis for assuming it's even going to be recognisable as life.
Life essentially seems to be a persistent self-reproducing dissipative structure that responds to evolutionary pressure. There is nothing in the manual that requires liquid water, gravity, a planetary surface, carbon, or any of the other ingredients that define life on Earth.
Have you heard of the German Tank Problem? https://en.wikipedia.org/wiki/German_tank_problem
You can make estimates from a sample size of one. That's literally the point of the entire article. If you want their justification, it's here:
============
A similar connection between area and population is seen among countries on Earth. Those with a larger population also tend to have a larger area. This effect is not quite as strong as you might think, due to the way countries are formed. A tiny region of land is less likely to declare its independence if only a handful of people were living in that region to begin with. The smallest countries therefore tend to have slightly higher population densities. Overall though the trend is clear, larger countries do hold significantly larger populations. Most countries are smaller than sixty thousand square kilometres, yet most individuals live in a country of over one million square kilometres.
If we are to estimate the size of an ordinary alien planet - one that hosts intelligent organisms - we first need to make two decisions. The first is the connection between population size and planet size. The simplest approach is to suppose that, on average, the population density will not change with planet size. For small changes in planetary radius this ought to be a good approximation. For planets much larger than Earth one could imagine that a larger proportion of the planet’s surface - for example near the poles and the equator - tend to become uninhabitable. A more detailed study of planetary atmospheres, and the prevalence of water, is needed to give a better answer here. For now we shall stick with the simple model where the average population of a civilisation increases with the planet’s surface area.
> For now we shall stick with the simple model where the average population of a civilisation increases with the planet’s surface area.
This is exactly what keeps it as an amusing thought experiment a la the doomsday argument—it's an unintuitive result from treating our population growth as simple as possible and extrapolating it to a general law.
Of note, if fusion get's cheap enough planets far from stars may be more useful as they get to radiate more energy into space.
On carbon, this is from [a recent Astrobiology textbook](http://books.google.com/books?id=x83omgI5pGQC&q=%22there%20m...) which probably does count as a manual : )
"There are, after all, only a finite number of elements in the periodic table, and many of these are very poorly suited to support life for any of a fair list of reasons. Consequently, many of the 90-odd naturally occurring elements can be ruled out. So many, in fact, that in the end there may very well be only a single element--carbon, the basis of all life on earth--that is able to support the complex chemistry presumably required to create any self-replicating chemical system. The easiest way to appreciate the special, perhaps even unique, qualities of carbon is to compare it with silicon, its closest cousin.
Many of the properties that suit carbon so well to its central role in Terrestrial life are shared or even exceeded by silicon. For example, silicon, like carbon, is tetravalent--that is each atom forms four bonds, allowing for the formation of a rich array of complex molecular structures. And, while silicon-silicon bond is weaker than a carbon bond, the discrepancy is only about 25%. Consistent with this, both silicon and carbon can form long molecular chains, For example, compounds of silicon and hydrogen, called silanes, with up to 28 consecutive silicon-silicon bonds have been reported in the scientific literature. Likewise, while carbon is the fourth most common element in the Solar System as a while, silicon is many orders of magnitude more common on the surface of Earth. Indeed, silicon is second only to Oxygen in terms of its abundance in the Earth's crust. Nevertheless, silicon simply cannot support the same rich chemistry as its "upstairs" neighbor in the periodic table. The problem lies in both the thermodynamics (equilibrium stabilities) of silicon's interactions with other atoms and the kinetics (rates) of these reactions...
So carbon wins over silicon. But what of the 90 or so other naturally occurring elements? They fare even worse than silicon."
This assumes the fine structure constant has the same value throughout the entire Universe for all time. Tentative results from recent observations suggest it could increase in one direction and decrease in the other along one of the spatial dimensions of the Universe.
Like, only elements we know about exist, and no matter what conditions might be carbon is an absolute must for life.
Hortas disagree with you.
That's because we've only discovered or figured out how to make a finite number of them. Is there a reason that other (alien) elements can't exist that we've never been exposed to?
What about the fact that Hydrogen, Oxygen and Carbon are the basic building blocks for all types of life that we have observed so far on Earth, and they are also 3 of the 4 most prevalent elements (H, He, O, C) in the observable universe in general?
Sure, people have theorized that there could be Silicon- or Germanium-based biochemistries out there that work similar to Carbon-based life, but it's probably more likely we're going to find something closer to a Carbon-based biochemistry than not, just given the distribution of elements out there, right?
I think we can still talk about statistical probabilities without having actual observable data on a non-Earth life form, can't we?
Yes I want to believe this but, I'm afraid there is no science behind it. Based on # of galaxies and stars it does seem logical however, what unknown variables also equate into determining this? Needless to say I stopped reading after this assumption in the first paragraph.
If you don't bother to actually observe planets similar to your own, then of course you're not going to observe any alien life similar to your own.
So if you are a human - then it is most probable that you live in a country that is more populous than the typical country. (OK)
Now they say - ok - so now instead of choosing humans let's do the same thing with sentient beings. If you are a sentient being it is more probable that you live on a planet where there are many other sentient beings rather than on a planet that there are few of them. But if you go to some random planet with sentient life - then the expected number of sentient beings there would be average.
Then it goes on that "Physically larger species will on average have lower population densities." - so most probably the random alien planet will have fewer and larger sentient beings than us.
I don't know if I buy that whole argument - but I am too lazy to write the bayesian equations to nail it down.
What if our imaginary aliens are plant-like, squid-like, or something we can't even begin to fathom, like symbiotic unicellulars? Sentient, self-replicating machine clusters the size of planets? Take a minute to think about this.
I think his conclusion should then have been worded like: the random alien planet will have fewer and larger VERTEBRATES than us. And that's a probably very meaningless conclusion.
I'm not saying his intuition is wrong, though. In fact I quite like it. But in this scenario, I don't think "whatever remains, however unlikely" lifeforms can even begin to be imagined by our tiny little human brains.
If you're confused by the examples given [for the approximate size], that's understandable, but the examples don't change the analysis. That's like saying "All of the countries in the example are in the northern hemisphere, so the analysis is flawed."
Is it individuals or civilizations?
An average civilization will be average sized. An average individual will belong to a larger-than average civilization.
It's also a bit like the problem that in average, your friends have more friends than you do. (That's easy to understand. It's because they are not a really random sample of all people. People with more connections are over-represented in your friends.)
If we assume that observation doesn't depend on civilization size, then we're sampling civilizations, and on average would find average sized civilizations.
If we assume that we observe individuals and not civilizations, then we're sampling individuals and are likely to see individuals of a big civilization.
Now, if I look at myself, if I'm a random sample from all individuals in the galaxy, it's likely that I'm part of a large civilization. That would mean other civilizations would on average be smaller than mine.
If I look at my civilization, and assume it's a random sample from all the civilizations in the galaxy, it's likely that it's an average sized civilization. A random other individual in the galaxy would be likely from a larger civilization.
I don't think either way of thinking is really justified.
You can extend this to a doomsday argument by the way. Since I am alive now, it's most likely that most people are alive now. Hence in the past and in the future, there will be less people alive.
Can you expand on the "waking amnesiac problem"? Google is coming up with nothing.
Consider "the size of alien species". Okay... so we are extrapolating about the size of beings we know nothing about based on those beings that have come to existence in our particular situation? Assuming that the distribution of weight across animals on Earth is the same as the distribution of weight across beings in the universe is dubious.
This is a wonderful example of Brandolini's law.
For example, if a human did this thought experiment 2000 years ago - a blink of the eye in the scale we are talking about - we would have perhaps 500,000,000 humans on the planet, or something along those lines. We have 14x as many humans now.
Yet we have not shrunk in size as the species has grown in population, and if anything, have grown larger.
If we look at the total biomass on earth, we are a fairly small portion of it. So shouldn't we assume, as we are assuming our situation is average, that intelligent aliens are also a fairly small portion of their planet's biomass? And if so, wouldn't the size of the aliens themselves be something that has very little to do with the total energy reaching the planet surface?
I get that it's just statistical probability and math, and it's fun, but this particular thing stuck out for me.
It was a fun read regardless, so thank you for the break from work!
> "What if people who lived several centuries ago did a calculation on how many births there would be?"
> This appears to be one of the most widespread misconceptions on the topic. Many scientists have fallen into this trap, such as Lee Smolin's article from 2004 . In science there is never absolute certainty, only varying degrees of confidence. We should never be 100% sure of anything. When stating the degree of confidence in a result, typically 95%, it should be in full knowledge that one time out of twenty, we will be wrong. 5% of the time we will be misled by statistical chance.
> Now if someone who lived tens of thousands of years ago estimates the total number of human births, based on how many there had already been, they will underestimate the truth. Because we now know there has been many more. But those first 5% of people who ever lived represent the 5% of the time we expect to be wrong. This is a basic premise of how science functions, how it uses statistics. We must be wrong some of the time. In reality, we are wrong much more frequently than statistical chance suggests, because of human error or misunderstanding.
> tens of thousands of years ago
This appears to be blatantly moving the goal posts.
> those first 5% of people who ever lived represent the 5% of the time we expect to be wrong
This implies that for only 5% of human history the calculation would be wrong, and the reset of the time it would be 100% correct.
I'm not sure that is how statistics work.
In summary: "We’re rare, we’re first, or we’re fucked."
The article seems to assume that there are lots of populations just because there are lots of planets. But the time that those planets have been around matters too.
We, as a single species, found to the question "is there alien life besides us?". I'm no individual independent from the culture of our species. I don't come up with this question randomly, you pointed me to this today.
The other way around: I have to expect to be in the large group only, if the large group makes it more likely that someone in it has questions about his group (more members -> more random thoughts -> greater total of thoughts about which group one is in). This is true for blood types (unless people with weird blood types commonly get in to issues making them wonder about their blood type...). But for aliens, probably either more or less all wonder collectively through cultural exchange, or it wasn't part of a public debate.
Hm, you get the knot in my brain? can you solve it?
I would have thought that a planet's life form, shape and variety would be determined by:
1- the energy output of nearest star
2- the planet's gravity
He barely mentions gravity which is surprising. Earthlings probably wouldn't be as tall with 1.3x more gravity. Maybe life wouldn't even have made it out of water, or much more slowly.
Evolution would mean "heavier" eggs would be harder to carry. The entire evolution process hangs around reproduction so what would that mean?
Same for less gravity - except it would _probably_ be on a smaller planet. Gravity correlates with planet size in the solar system. Would <0.8G be enough to retain water, atmosphere, etc?
Somehow I'm not surprised to find out one day that an intelligent alien life would look a lot like us, on a planet that looks a lot like Earth.
Oh yeah? :) http://prokopetz.tumblr.com/post/57702943181/mikhailvladimir...
However, I do think that we can scientifically study some aliens today, or more precisely what we can see from them, through the UFO phenomenon. Yes, that thing! For me the only paradox in the Fermi paradox is that the UFO phenomenon is boycotted as a manifestation of Aliens on earth.
The following three articles are a product of such study. They present a new electromagnetic propulsion system called PEMP inspired by the data of UFO observations. It also present a totally innovative method to produce the intense EM fields required by this propulsion system. These are currently only theories waiting for an experimental validation. The author is a physic theorist, not an experimentalist.
"Pulsed EM Propulsion of Unconventional Flying Objects":http://www.meessen.net/AMeessen/Propulsion.pdf
"Evidence of Very Strong Low Frequency Magnetic Fields": http://www.meessen.net/AMeessen/Evidence.pdf
"Production of EM Surface Waves by Superconducting Spheres: A New Type of Harmonic Oscillators": http://www.meessen.net/AMeessen/Production.pdf
So there is no need to speculate. Just open your eyes and look at the data we already have for so many years.
Note that this is the product of an inductive research process. The initial working hypothesis was that UFO are real and witnesses report real data on them. Now see if we can derive a valid propulsion system matching the described artifacts using only conventional physics law.
It was initially a test, an experiment on a pure theoretical ground. The test is apparently conclusive. We now have a theory we can test in our lab and we could validate a disruptive discovery.
Objectively, we still don't know if UFOs are real and they are aliens visiting earth. But we now have an opportunity to indirectly test that possibility with pure solid ground science and engineering. Thanks to these theories.
- I am an ordinary sentient being (for the sake of argument... just nod)
- I am a member of an ordinary species.
according this theory one of these statements is wrong.
Hypothetically, let's imagine there were 10 normal-sized planets with 10 people each, and 1 big planet with 1 million. In this scenario everyone is either on a strange (big) planet, or they are on a normal planet but they are not typical individuals, because over 99% live on the big planet.
- I am an ordinary human
- I live in an ordinary country
One of these is wrong, and it's the second one.
So what about:
- Humans are an ordinary species
- Humans live in an ordinary galaxy
Does that change our relative height?
btw fergussimpson your posts are showing up dead but then are getting undeaded after a certain period of time I guess. Your first post here was dead for a while and now it isn't. Your reply to this parent is dead as of writing this post. I guess it's because you have a new account - I wasn't aware we were auto-hellbanning people now, though. How annoying.
If it's #1, those are just aliens. If #2, I don't think you can discount the possibility entirely, but there's no evidence for their existence, but it's entirely possible. But spending any energy on the question seems a bit pointless, since no members of the Q Continuum have made their presence known to us yet. If #3, the problem there is there's no good evidence for their existence, only ancient stories passed down from oral tradition, and the old "telephone game" shows how reliable that is, plus the well-known phenomenon of hallucination, which can happen to people when they eat certain tainted foods.
The question of aliens is worth considering seriously because we do know that life is possible (look in the mirror), we know under what conditions if can form (look outside; we have a planet to study that formed life), and now we know that lots of other planets are out there, and some of them may very well be similar to our own. If we can evolve here, it's quite possible some other beings evolved elsewhere under similar conditions. And with many billions of stars out there (just in our galaxy and nearby ones), the probability of other planets existing with conditions similar to ours is high. Furthermore, as our ability to detect exoplanets improves, it's quite possible we may detect signs of alien life: radio signals, industrial emissions in their atmosphere, weird starlight patterns indicating a possible Dyson swarm, etc. There's no way to detect any kind of god (whether it's one from some old book or the Q).
I want to be generous here and assume I'm misunderstanding, but it does seem a bit like the argument begs the question a bit.
The intended conclusion is that we should consider non-earth-like (i.e. non-earth-sized) planets as just as likely to be inhabited as earth-like planets. Which is to say that we shouldn't expect that population density is strongly correlated with planet size.
And this is shown starting from a model where "mean population density is invariant to planet size". Hmm...
>However if there is any hope of finding life on other planets, there must be a huge number of planets with life in the Universe. Therefore, for the case we're interested in...
So during the entire analysis, we were limiting ourselves only to those universes in which we do make alien contact, regardless of how likely that event is.
Unbridled Physics-ism (as in [2]) and Bayesianism definitely don't mix well.
The normal evolution of a civilisation of smart species is to arrive to a point where they experience an Intelligence Explosion [0]. (or how Elon Musk puts it: Chances are we're the biological boot loader for digital superintelligence) [1].
In a cosmic scale, given the fact that the timespan to go from industrial/high technology civilisation to a SuperAI is like a drop in the ocean (1000 or 2000 years), it'd be impossible to establish contact unless they co-exist at the same point of evolution, in the same (or near) star system, and in the same period of time.
Something far from probable...
I think that this is the most plausible solution for the Fermi Paradox, we haven't been contacted by Aliens for the same reason that we haven't "contacted" with ant colonies or microbes. We simply can't, we're in a different state of consciousness.
We are probably living among Aliens, but they're too advanced for our reasoning and live in a different dimension/cosmic state.
To quote:
> Given the prevalence of the four different blood groups in the cartoon above, the most profitable strategy here would be to bet on "A", as that gives you the greatest chance of winning. Another way of looking at it, is that if everyone adopted that strategy, the bookmaker would lose the most money.
It is true though, that if you spin a black and white patterned disk, you will see colors.
How did this tinfoil-hat ridiculous article even make it into Hacker news!?
As a human you're far more likely to live in the XX or XXI century than in any other.
Is that all you have as "argument" ?
Supposing you referred to my comment on the big alien theory, as a biologist, I would say that this pure statistical analysis ignores the possible existence of yet unknown factors that could modulate the probability of existence of alien civilization of different size, planet size or intelligence.
This is why I concluded that this work is based on a pure speculation that these unknown factors don't exist. I didn't say this work is false or bad. I said it won't move us reliably forward on this research topic.
Regardless, if there is a gradient, any meaningful change in the fine structure constant beyond the range at which we could observe anything in the universe, so it still makes sense to only assume carbon-based life.
But if you're looking for life, it seems very likely that most life is made out of carbon; possibly nearly all of it. You might as well start looking there. We have limited resources, after all.
The interesting thing with elements is that they describe almost all conventional matter in a very predictable pattern and we can pretty much enumerate them. Some scientists are even trying to make the next elements (Ununennium for instance), even though there are incredibly unlikely to occur in nature.
Granted, elements only describe a fraction of all matter, but we do not really expect anything complex to last long as plasma (stars), and we do not know much of dark matter anyway.
> no matter what conditions might be carbon is an absolute must for life
GP was answering this exact point. In short, it's just that the other elements do not look as promising as carbon for building complex molecules (whether we know them or not).
You make a statement with some degree of confidence (often 95%) and fully expect to be wrong 5% the time.
It doesn't actually matter. All we care about are values by %. So we can say that someone in the first 5% of humans could have done this same calculation and been wrong. And that's okay.
If it turns out that we are in the first 5% of humans, then we too are wrong. That's a risk we're willing to take, and statistically, it doesn't matter. It allows us to make informed decisions right now.
If most species looked at themselves and said "my species is ordinary" the majority of them would be right; however, it doesn't give you any statistical reason to think that humans are ordinary.
The counter-intuitiveness of it all is really intriguing. We really are evolved to think in small numbers.
I accept the doomsday argument, but I had heard that it is controversial, so I expected more debate here.
[1] https://en.wikipedia.org/wiki/Self-indication_assumption [2] https://en.wikipedia.org/wiki/Self-sampling_assumption [3] https://en.wikipedia.org/wiki/Self-Indication_Assumption_Doo...
Or another way, you at least need to argue AGAINST rational rationing of existing resources to make calculating potential populations over all time quite difficult and run up against heat death calculations.
That said, I love both these thought experiments because they highlight how hard it is to figure out population growth with (theoretically) rational populations.
Hth.
You should rapidly become considered a normal user, but I'm told emailing HN is what you do if you continue to be auto marked dead.
You need a karma level of 30 for the flag / vouch links to appear. You need to have showdead turned on to see the comments that have been flagged.
Here are two comments from dang about it: https://news.ycombinator.com/item?id=10638699
If most species are large, and from small planets, they will have much stricter resource constraints than we did.
As a result, they are unlikely to have industrialized the way we did, as it would have depleted their resources too quickly. As a result, they are unlikely to be a silicon-age species.
Drawing from the author's biological/size argument, they're likely to have longer lifespans than we do, and considerably fewer births.
Given their resource constraints, and longer lifespans, they are likely to have strong communal systems of resource allocation, and are unlikely to have moved towards Capitalism.
Given their longer lifespans, they are probably more risk averse than our own species -- as willingness for an animal to die is strongly related to size and lifespan. This suggests they may be less war-faring than ourselves, and thus less likely to have nations.
Given their closer relationship with nature, due to their resource constraints, they're likely to have developed a Pagan system of thought, rather than a monotheistic theology.
As a result, they are unlikely to be motivated towards large infrastructural, imperial or technological projects as a result of: profit motive, national motive, or religious motive.
I'd argue then, that it is unlikely they have produced a civilization of significance.
--
Long story short; local alien species are likely to be tribal, communist, non-technological, and environmentalist.
If through some strange force a species of this kind managed to become technologically advanced, they would likely look down on a species such as our own as a sort of vermin, because of our numbers, hunger and short lives.
But Carl Sagan would probably refute that - "It is perfectly possible to imagine civilizations of poets or (perhaps) Bronze Age warriors who never stumble on James Clerk Maxwell’s equations and radio receivers. But they are removed by natural selection. The Earth is surrounded by a population of asteroids and comets, such that occasionally the planet is struck by one large enough to do substantial damage." http://gencodesignal.info/the-abundance-of-life-bearing-plan...
I think the Drake Equation solution that Sagan proposes puts values that are orders of magnitude too large for most variables. In this case fi (where he confuses intelligent life with civilization) and fc.
As far as discovering species on Earth: we have many data points there so it's much easier to make meaningful predictions.
So far we don't even know for certain that there is other life in our galaxy. For all we know life may occur at a rate of, say, once per galaxy per billion years, and then only survive long enough to evolve to any significant level of complexity 10-20% of the time. Personally I doubt it's that low, but we have no evidence for either side of the argument. It amounts to just intuition at this point.
This gets into combinatorial search stuff that is hard to explain concisely, but here's a programming analogy:
Ask two programmers to develop the same huge project. Tell one to do it in JavaScript, and another to do it in C# on Windows. You will get two radically different architectures due to how these languages affect the way the programmer traverses combinatorial search space. The JS programmer might give you a bunch of Dockerized micro services that runs against a NoSQL database, while the C# programmer might give you an OOP-based monolith built around an ORM. Those two languages "want" to become those things-- it's built into their structure.
So a different biology might, for example, be biased toward less or more biodiversity, or different levels of evolutionary variation, or a different trade-off between stability and adaptability, or different levels of radiation tolerance, or different average life spans due to more or less chemical stability, etc. That in turn might yield radically different biospheres with radically different distributions of species. If it gets complex it might yield a radically different form of intelligence, like a hive mind or a distributed system or maybe something we can't even imagine. Or... some initial starting positions might never yield intelligence at all. For all we know most initial setups have a low probability of ratcheting up to this level of complexity.
We only have one data point, so we have no idea. It's like using one example of a hurricane to generalize about the behavior of all possible cyclonic storms in all possible atmospheres in the entire universe.
We know what a country is. And there's very (very, very, trust me I'm an engineer) little chance we'll find new countries with characteristics unknown to us...
Thus, the statistics are purely for the discussion of "what should other sentient life look like?"
Finally, it's ridiculous to complain in a discussion about aliens "that we can't begin to imagine what type of life form constitutes an alien." You have to make some fundamental assumptions. For instance, we should be able to assume that they obey the physical laws of the universe.
Anything else puts you in a totally unscientific world of discussion [your claims are no longer falsifiable], and that's not one I care to participate in.
I don't think anyone is assuming otherwise... all I see is statistics and sampling being argued. You'll have to endure my wall of text, though, sorry XD.
Let's take the planets again. We don't know the size of every single planet in the galaxy (i.e. the planet size histogram), so we need to make a guess. So we've been using Kepler's exoplanet observations, knowledge of planetary geology, etc. to fit a model, and it's well developed (http://exoplanetsdigest.com/2014/07/25/exoplanet-statistics-...). The size distribution is quite well understood, its bounds and modes are well defined. It's unlikely that our best guess of what the average size of exoplanets is will change drastically the more we know about planets. That is why the article's first conclusion is legitimate.
Now back to (sentient) species, and my original point. The definition of life in the SETI context has to be VERY wide. It has to encompass any scenario for a species that we might consider "intelligent" -- and not just little green men. It might have (some would say, inevitably) evolved beyond the biological, and still be considered sentient. After all, life on Earth has only existed for ~4By, compared to ~13By for the Milky Way. As a base for extrapolation, just think how different to humans extreme life on Earth is. (http://www.livescience.com/13377-extremophiles-world-weirdes...) In any case, there are bound to be some weird-as-shit species out there, whose composition still obeys the laws of physics. In other words, there is almost certainly many statistical modes of life out there that are, or can become, sentient.
With that in mind, there's a very good chance that the overall size distribution of (sentient) species does not match the one the author used (that of vertebrates only). In statistics-ese: if the distribution is multimodal, the average of our unimodal sample is not a good guess as to what the true average really is.
Maybe we can agree upon that?
But, when considering life, if applying the Big Alien logic, I should expect to be a single bacteria. And if not that, then, say, some insect.
But, instead, I'm a human. Or a mammal. Or a vertebrate. Any of these very fundamental descriptions of myself already put me in the SMALL group not the LARGE group when classifying the population of all life on this planet.
It feels like confirmation bias to select "vertebrate" or "mammal" to show that we are already indeed in a large group.
And indeed you can argue scientists are already applying this logic by looking for signs of simple life, say, or Mars. Or Europa. Because, that's the most likely to be found.
Maybe we should call it the Bacterial Alien Theory. Of course, it wouldn't be very controversial.
But when we restrict our search to sentient life, here is what the statistics suggest [and as you correctly point out, that restriction is what makes this an interesting discussion].
The signal detection thing is just dumb. Signals quickly fall in strength to background noise levels, so you need to broadcast a huge amount of power in a tight beam to overcome that over any significant distance. So basically, you're arguing that just because no ETs have bothered to pour a lot of resources into 1) finding us and learning of our existence, and 2) building a gigantic, power-consuming radio transmitter and directing it at us, that they must not exist.
The probes thing assumes that the ETs actually want to talk to us. That's a pretty huge assumption. We've had civilizations here on Earth which had no desire for outside contact (namely the Chinese during some of their dynasties). What makes you think the ETs are so intent on pouring resources into making artificially-intelligent probes to establish communications?
By that logic, WE don't exist, because we haven't bothered to do these things either.
"Some"? I would bet on 99.99999999%.
Popular assumption that all evolution everywhere would inevitably progress towards intelligence strikes me as very biased.
Even on Earth evolution had to be "restarted" several times (great extinction events; the one that killed dinosaurs wasn't the biggest one) before humans came about.
Hell, the jump from unicellulars to multicellulars by itself took 3 billion years. It's nothing short of amazing to have such a long period of good/relatively stable "weather" on the planet.
Then, having a complex nervous system is very expensive and evolution often backs off from it: http://www.bbc.com/earth/story/20150424-animals-that-lost-th...
Note that again, even on Earth, intelligent life made it by the skin of its teeth. Neanderthals didn't survive, homo sapiens were very close to following them (according to some theories, eg. https://en.wikipedia.org/wiki/Toba_catastrophe_theory - "between 50,000 and 100,000 years ago, human populations sharply decreased to 3,000–10,000 surviving individual [...]It is supported by genetic evidence suggesting that today's humans are descended from a very small population of between 1,000 and 10,000 breeding pairs that existed about 70,000 years ago"
In addition to the stability argument, there are also energy requirements. It turns out fusion (stars) only gives energy up to iron, then it requires energy to make bigger atoms. In other words, you get energy back out when you split big atoms (I.e. nuclear reactors). In nature, all elements past iron are created only in the spectacular energies of supernovas, which occur in less than a second.
Basically, we have a really good grasp on the elements that can exist. We're only missing the details on a few things that occur on nanosecond and less timescales.
That's actually a common misunderstanding. If you fuse hydrogen (or even lithium) with iron, you can get a higher numbered element and some excess energy. See the chart on this page:
https://en.wikipedia.org/wiki/Nuclear_binding_energy
While iron is at the top of the curve, that just means you won't see iron-iron fusion. There really is no reason the lighter (common) elements can't fuse with the heavier elements.
This is also the idea the LENR (low energy nuclear reaction, formerly known as cold-fusion) guys are considering. If you fuse Hydrogen with Nickel62 to produce Coppper63 you could get some energy out. Notice that Nickel is already heavier than Iron. Some claim to have seen this copper production in hydrogen-nickel cells. The claims are not really relevant - the math supports it as a possibility. Weather it can happen on earth or in a star is open for debate. One key question is how the excess energy would get out as heat, and there are ideas about that.
I for one find it amusing that people don't think something like this is where all the naturally existing heavy elements came from.
Furthermore, the formation of elements in the Universe is also a fairly well understood process. For elements lighter than Fe it generally occurs through nuclear fusion in the center of stars. For elements larger than Fe it generally occurs through the r-process and s-process. With these we can model nucleosynthesis extremely well and it gives us a very good idea of the elemental composition of the Universe. That being said, there could be some crazy unknown element out there but it would contradict almost everything know about atomic physics.
[1] https://upload.wikimedia.org/wikipedia/commons/8/80/Isotopes...
Yes, because you form new chemical elements by adding protons (and stabilizing neutrons) to the nucleus, and humans have found or synthesized the first 118 of these. The ones that don't occur in naturally on earth are short-lived an unstable.
Even if such elements are out there in the universe in tiny quantities, we'd run into a billion carbon-based biospheres before we found any.
(This only works if the element is common enough and hot enough to emit light that is seen from Earth.)
Of course, this is today's understanding. We may be wrong. :-)
You're assuming that there aren't other spacefaring civilizations out there who are opposed to these probes, and that don't actively seek them out and destroy them.
You're assuming that the galaxy and nearby vicinity is old enough for one of these civilizations to become this advanced and to build these probes and for one of them to reach us. Who knows, maybe there just aren't that many really old civilizations around yet. Just look at how long it took us to evolve to this point.
For anything more than 10^6 km away and less than 10 metres in size, we just wouldn't see it. We have only just noticed some enormous lumps on Ceres, and it seems a whole planet has evaded detection until now.
I don't think it's particularly likely we will find a vN probe. But to consider their non-detection to be of statistical significance is, I think, an overestimation of our technological capabilities.
