Planets in the habitable zone, the Drake Equation, and the Great Filter

post by JoshuaZ · 2011-10-01T02:44:05.968Z · score: 11 (12 votes) · LW · GW · Legacy · 64 comments

One serious issue for evaluating existential risk is working out whether most of the Great Filter is behind us or in front of us. This relates to the Drake Equation and similar attempts to estimate the frequency of life in an obvious way.

Over the last few years, it has become increasingly apparent that extrasolar planets are common. However, what fraction of these planets lie in their stars habitable zone has still been an open question, primarily because most of our current methods for planet finding easily find planets that are either very large or are very close to their star (ideally both). 

A new study, using the data from the Kepler spacecraft, estimates that about a third of all stars similar to the sun have at least one planet in the habitable zone. There are some issues with this estimate, and Phil Plait discusses them at his blog.  The estimate has a large amount of variance. The paper actually estimates 34% +/- 14% and the issues that Phil brings up increases the uncertainty in both directions but it seems safe at this point to consider this not being very far off.

One obvious issue from a Fermi perspective is that some systems will likely have multiple planets in this zone. Also, having planets in the habitable zone is clearly not sufficient for life. By the standard estimates for habitable zones, Venus and Mars are both in the habitable zone of the sun. And there may very well be ways for life to arise outside the habitable zone. Moons like Europa and Titan seem to be excellent candidates, and we can't rule out more exotic forms of life in other habitats although that seems not too likely right now. 

However, one thing this makes clear: The part of the Great Filter that is behind us that is due to planets not  lying inside the habitable zone is small. So the question is, what does this mean for our estimates of how much of the Filter is behind us and how much is in front of us? 

64 comments

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comment by DuncanS · 2011-10-01T10:47:51.641Z · score: 14 (14 votes) · LW(p) · GW(p)

Perhaps the good news is that having a planet in the habitable zone is only a part of the problem. There are plenty of other constraints.

Firstly, the star has to be the right size. A bit larger than our sun, and the star evolves off the main sequence more and more rapidly. It's not sufficiently stable to last the several billions of years needed to get to our stage of life. It's reckoned by some people that our Earth only has a billion years or so left as a home for multicellular life before our sun overheats it. (minus the human factor.)

This also shrinks the effective size of the habitable zone enormously - a suitable planet ought to be towards the outside of the zone at first so it doesn't end up too close to the star later on when the evolving star heats up.

A smaller star is more stable, but gives a different problem - since the habitable zone is closer in to the star, the star exerts larger tides in the planet, which brake its rotation to a stop. This has already happened to Venus, for example.

Then the star has to have the right mix of elements. These elements can only come from the explosions of other stars - so the universe has to live a bit before planets are possible. Equally, once the planets have formed, you want the process of stellar formation not to carry on next to you - all these criteria essentially require the star to be formed in a spiral arm of a decent-sized galaxy like ours - not in the core, and not in the first round of stellar formation.

Earth contains elements from several kinds of stellar explosion, and needs them. This implies you might get planets with differing proportions of the chemical elements, which might not be so conducive to life.

The planet itself has to be the right size - too small, and it won't retain an atmosphere, and also won't retain geological activity over the needed timeframe. Too large, and it will retain hydrogen and/or helium, which results in a huge atmosphere and no life at all.

The planet needs a large moon - otherwise the axis of rotation of the planet tends to tumble chaotically, as Mars does, and Venus apparently did when it rotated faster. The moon stabilises the axial tilt, which could be stabilised at practically any tilt, not just Earth's slight tilt which gives rise to reasonable seasons. The moon itself is a bit of a constraint in its own right - it would need to form in the late stage of planetary formation - computer simulations suggest this happens about 1 time in 12. Too small a moon might be driven out of orbit by the underlying planets rotation (our own moon's been driven out a loong way as it is), and will cease to be any help at stabilising rotation. Too large a moon will again brake the planet's rotation to a much lower level, resulting in long days and inhospitable temperatures. Apparently our moon will cease to protect the Earth from axial wobble in around another billion years. (if not for the human factor)

The orbit shouldn't be too elliptical.

Earth is a bit special, it seems.

comment by occlude · 2011-10-01T19:57:44.937Z · score: 10 (10 votes) · LW(p) · GW(p)

There are plenty of other constraints.

Many of these factors seem, with high probability, like genuine constraints (e.g. a second generation star). But I wonder whether others might be examples of anthropic generalizing from one example (e.g. availability of a specific ratio of elements). Presumably alien life would adapt to whatever conditions actually exist in their world.

comment by JoshuaZ · 2011-10-01T21:45:16.267Z · score: 9 (9 votes) · LW(p) · GW(p)

Your post got me thinking on a completely different tangent: How much of the filter might be at a high tech level for most species but we managed to escape it based on what resources we actually lacked?

The most obvious example is the amount of U-235. If humans had arose 2 billion years ago there would be about six times as much U-235 on the planet (since U-235 has a half-like of around 700 million years), making it much easier to develop nuclear weapons. That could have a substantial negative impact on a species chance of not wiping themselves out.

I'm not sure how much this would matter in that fission weapons are a lot easier already to produce than fusion weapons. Moreover, this would also make it easier to use nuclear power for productive purposes. Even a Fermi style pile would be much, much easier to construct (Fermi's original pile did not use enriched uranium). So the ability to use nuclear power in this way would help for simple electricity generation a lot as well as nuclear rockets (which would directly help beating the Great Filter). So overall this seems like a wash without a lot more data.

Is there some other isotope that could have a similar impact? Possibly something that now is very rare so we aren't paying much attention to it? The other obvious candidates don't seem to work. For example, tritium has a very short half life but natural processes produce more of it so that shouldn't matter. Similarly, plutonium 239 has much too short a half-life so that any species that arose even after the first billion years wouldn't see any substantial amounts of it. Maybe Plutonium-244? It is primordial, has a half-life around 80 million years, so would be around in larger quantities on a young planet. But I don't know of any obvious fission chain for it, and the quantities produced would be very little, since it is not easily produced in supernovae.

comment by Desrtopa · 2011-10-02T18:18:39.048Z · score: 2 (2 votes) · LW(p) · GW(p)

Most accessible nuclear power on our planet isn't stored as U-235 anyway. We can get a lot more by converting uranium into plutonium, which is heavily restricted because of how much easier it is to make weapons out of, or out of thorium, which is safer and more abundant but requires a different procedure to extract energy which has never been developed for commercial applications.

comment by amcknight · 2011-10-02T23:27:03.718Z · score: 7 (7 votes) · LW(p) · GW(p)

Each of the constraints you name, though plausible, don't seem to be strong enough to act as a full filter. They appear to filter on the level of full percentage points rather than billionths of percentage points. Without finding dozens more filters of this kind, there would still be human-level life all over the place and hence (assuming no life in the universe) the Great Filter would be ahead of us.

A bunch of tiny filters is an unlikely scenario for explaining "The Great Silence".

comment by JoshuaZ · 2011-10-02T23:39:10.013Z · score: 1 (1 votes) · LW(p) · GW(p)

Duncan seems to be just talking about the Earth related constraints. It is possible to have those constraints as well as other constraints. I don't see Duncan as directly claiming that the Earth filtration issues are the entire filter. His point seems to be more that being in the habitable zone is not the only filtration issue for a planet.

comment by DuncanS · 2011-10-03T20:00:26.230Z · score: 2 (2 votes) · LW(p) · GW(p)

This is true. However, I also think the planetary filters are perhaps easier to evaluate than some of the others. How would I know whether the origin of life is something that's really unlikely to happen on Earth in half a billion years, or something that will happen in trillions of places all over the planet in under half an hour, given the right conditions? I'm more tempted by the latter thought, but without knowing what the process actually is, or having any suitable examples other than Earth, it's tough to know what the answer might be. We have no reason for thinking it's difficult other than that we haven't replicated it yet, though. That and the great filter question itself - if making intelligent life is easy, why isn't it everywhere?

comment by jhuffman · 2011-10-03T21:41:00.575Z · score: 0 (0 votes) · LW(p) · GW(p)

It is pretty easy to imagine lots of smallish filters as responsible. If I am doing my math correctly, then 9 filters that each weed out only 9 out of 10 solar systems would reduce the expected population of intelligent life in the Milky Way to just .1 systems. Since we know so much more about star formation I am a lot more comfortable saying "habital planet" will weed out 90% of systems, that is not nearly a big enough filter to account for what we see by itself but as you stack them filters of this size are quite substantial.

edit: er no that's wrong, off by a few orders of magnitude. So yeah, gosh where is everyone?

comment by amcknight · 2011-10-02T20:17:44.703Z · score: 7 (7 votes) · LW(p) · GW(p)

I think this provides a tiny update in the direction of great-filter-ahead because it removes one more possibility from behind us. I still think great-filter-behind is much more plausible though.

comment by see · 2011-10-01T07:24:41.364Z · score: 5 (7 votes) · LW(p) · GW(p)

I'm still comfortable with the hypothesis of it being behind us, having previously considered habitable-zone planets around Sun-like stars highly probable.

The two big filters I tend to favor are development/survival of multicellular life and evolution of tool-using intelligence. Multicellular life is such a recent froth on Earth and such a small part of the biosphere that I'm satisfied with it being highly improbable. And intelligence? High animal intelligence is a resource hog that makes the species much more vulnerable to environmental disruptions. I think it's unlikely to simultaneously get paired with tool-using capability and wind up a feature that gets sexually selected far beyond its advantages to survival all the way to true sapience.

comment by MichaelHoward · 2011-10-01T10:26:04.943Z · score: 7 (7 votes) · LW(p) · GW(p)

High animal intelligence is a resource hog that makes the species much more vulnerable to environmental disruptions.

cough

...a feature that gets sexually selected far beyond its advantages to survival...

This sort of runaway selection pressure happens a lot in nature (eg. peacocks).

comment by see · 2011-10-01T11:34:55.104Z · score: 7 (7 votes) · LW(p) · GW(p)

First, yes, the unit of selection is of course the individual. But a species-wide feature that causes each individual's chance of survival and propagation to go down does, in fact, imperil the survival of the species when it is selected against in every individual's case. When every individual dies, so does the species; similarly, if merely enough individuals die that there's a genetic bottleneck, the species also risks extinction because of inbreeding-related unfitness of individuals. Next time try actually thinking through what is being said, instead of partial-pattern-matching in an effort to score points.

And yes, runaway selection pressure in general happens a lot. Now, how often does it happen specifically on the characteristic of intelligence, instead of all the other possible features it could happen on? Looking at the only paleontological record we have available, it seems to be awfully rare.

comment by MichaelHoward · 2011-10-01T12:58:33.459Z · score: 0 (0 votes) · LW(p) · GW(p)

First, yes, the unit of selection is of course the individual.

I'd argue it's the gene.

But a species-wide feature that causes each individual's chance of survival and propagation to go down does, in fact, imperil the survival of the species when it is selected against in every individual's case. When every individual dies, so does the species; similarly, if merely enough individuals die that there's a genetic bottleneck, the species also risks extinction because of inbreeding-related unfitness of individuals.

To the extent that this works against runaway selection pressure for intelligence, it also works against runaway selection pressure in general, which as you say, still happens a lot.

Next time try actually thinking through what is being said, instead of partial-pattern-matching in an effort to score points.

Please tell me why you think I was doing this, if you've good reason for not assuming good faith then I'll try to avoid giving this impression in future.

And yes, runaway selection pressure in general happens a lot. Now, how often does it happen specifically on the characteristic of intelligence, instead of all the other possible features it could happen on? Looking at the only paleontological record we have available, it seems to be awfully rare.

Fair point. I'm not sure if they'd count as runaway selection pressure, but aren't there lots of examples for animal-level equivalents of social intelligence? If so, then that counts as it looks like that's the main reason we got smart.

comment by see · 2011-10-01T14:07:08.005Z · score: 1 (1 votes) · LW(p) · GW(p)

I'd argue it's the gene.

Fair enough. The point is, nothing I said depended on the premise that the species as the unit of selection, it was merely stating the effect that selection (on whatever unit you like) had on the species. These are distinct concepts. Nothing I said was dependent on the erroneous idea of the species as the unit of selection.

The most charitable interpretation of your "cough" link was that you had read my statement, saw I wasn't making the mistake addressed in your link, but thought I should have made a big verbose show of avoiding the mistake, and thought the appropriate way to say that was the word "cough". This did not strike me as very probable.

A more probable (in my view) interpretation was that you saw something that looked like the error in the link, and without analyzing whether the error was actually being made, decided to point out I did something that looked like an error. That's got several possible interpretations itself, of course. I likely shouldn't have assumed that you were trying to score points; it is, on reflection more likely you were trying to alert me to a possible error that you simply didn't spend the time to analyze.

I apologize for my hasty imputation of bad faith, but suggest a more verbose message than "cough" would have

To the extent that this works against runaway selection pressure for intelligence, it also works against runaway selection pressure in general

Ah, I wasn't addressing runaway selection pressure there, I was addressing the survival value of high animal intelligence. There is a tendency among people (for example, the participants in the 1961 Green Bank meeting where the Drake Equation was first introduced) to evaluate higher intelligence as automatically translating to higher chances of survival, and thus set the fi term in the Drake Equation to 1, on the assumption all life evolves toward intelligence. But high animal (sub-sapient) intelligence, like any other adaptation, is not inherently pro-survival, and so does not monotonically increase. That's fairly basic evolutionary biology, of course, but between the popular notion of the chain of being and the fact that Drake and his colleagues made the error, I've developed a reflex of addressing the mistake in discussions of the Fermi Paradox.

Hmm. Looking back, I was very brief and dense in what I wrote, which makes it difficult to comprehend. I know I'm supposed to say what I'm going to say, say it, than say what I said to maximize comprehension, but I find in the event that I am annoyed at the time it takes and bored by the repetition involved. So I skimped on the effort to communicate effectively, and then when I wasn't understood, I got angry instead of being sensible.

So I again apologize for being short with you. It was my error.

I'm not sure if they'd count as runaway selection pressure, but aren't there lots of examples for animal-level equivalents of social intelligence? If so, then that counts as it looks like that's the main reason we got smart.

I tend to evaluate the lots of examples as evidence that it doesn't likely lead to runaway selection for intelligence, since it so rarely seems to. It could, of course, instead be understood as there being lots of opportunities for social behavior to cause a runaway selection for intelligence, and so it is almost inevitable that social behavior will eventually do so.

Hmm. I really want some other planets to look at, this one isn't a large enough sample size.

comment by MichaelHoward · 2011-10-01T15:10:49.669Z · score: 2 (2 votes) · LW(p) · GW(p)

...your "cough" link...

Your point wasn't dependent on it so I didn't want to go verbose & waste the reader's time when they've heard those arguments before. My first thought was Eliezer's ERROR: Postulation of group selection detected but I thought a subtle cough would be kinder. Oh well.

There is a tendency...to evaluate higher intelligence as automatically translating to higher chances of survival...assumption all life evolves toward intelligence. But high animal (sub-sapient) intelligence...is not inherently pro-survival.

Ah. Yeah, that wasn't where I was arguing from. I agree with you about this. Do many people (who should know better) still believe this nowadays?

I tend to evaluate the lots of examples as evidence that it doesn't likely lead to runaway selection for intelligence, since it so rarely seems to. It could, of course, instead be understood as there being lots of opportunities for social behavior to cause a runaway selection for intelligence, and so it is almost inevitable that social behavior will eventually do so.

Maybe not inevitable, but I really doubt it's hard enough to be a Great Filter.

comment by MichaelHoward · 2011-10-01T16:03:50.071Z · score: 6 (6 votes) · LW(p) · GW(p)

development/survival of multicellular life

Hasn't Multicellularity evolved independently many times on Earth?

Multicellular life is such a recent froth on Earth and such a small part of the biosphere that I'm satisfied with it being highly improbable.

It, or something leading up to it.

comment by see · 2011-10-02T05:29:36.527Z · score: 13 (13 votes) · LW(p) · GW(p)

Hasn't Multicellularity evolved independently many times on Earth?

Well, I started to cache-dump on colonies versus multicellular organisms, but when I got to the bit on the origin of mutlicellular plants/animals/fungi, I noticed I had a lot of what sure looks like special pleading. And then I noticed myself "rationalizing" the arguments instead of engaging them, because of my emotional dislike of the idea that a Great Filter might be in the future instead of safely in the past.

I am currently raising the probability of a strong filter still in our future on the basis of having one of my two strong past filters seriously weakened. I still have the desire to argue that plant/animal/fungi multicellularity is single ancestral cause (either a single evolution with branches that went back to unicellular life, or "parallel" evolution from a single origin instead of "independent" evolution), but I'm finding things like, for example, "oxygen levels finally got high enough to efficiently support multicellular life" coming in with higher probability.

comment by jhuffman · 2011-10-14T20:48:51.143Z · score: 3 (3 votes) · LW(p) · GW(p)

Recently as I learned more about the social context for development of intelligence I updated further in favor of a future filter. I couldn't understand why it would be useful enough to get a little more intelligent than being able to wear an animal skin for warmth and club a seal with a rock for food. But the reproductive benefits of going up the pecking order of the clan by way of clever plots seems pretty well supported and intuitively likely once I'd heard of it.

comment by Jack · 2011-10-01T11:11:48.201Z · score: 5 (7 votes) · LW(p) · GW(p)

Tool use is... quite common.

comment by see · 2011-10-01T11:38:34.725Z · score: 1 (3 votes) · LW(p) · GW(p)

I would suggest, against the number of species that actually exist, tool use is actually fairly rare.

comment by Jack · 2011-10-01T11:46:59.879Z · score: 12 (12 votes) · LW(p) · GW(p)

Rare, sure. Rare enough to be a plausible candidate for a great filter? Not at all. Anything that independently evolved multiple times on Earth is likely to evolve in any similarly populated planet.

comment by see · 2011-10-01T13:17:52.127Z · score: 5 (5 votes) · LW(p) · GW(p)

Yes, but it's the conjunction with intelligence that I proposed as the filter, not the tool use on its own.

Even if you get a runaway sexual selection for intelligence, the species has to have some tool-manipulation ability for it to result in technology. A primate that gets human intelligence through runaway sexual selection is in good position to develop a technological civilization that can communicate or colonize. A corvid that gets human intelligence might manage something, or it might be stuck in a stone age forever because of inferior manipulators and insufficient body strength. A human-intelligence baleen whale, I don't see how they can possibly get to technology from where they are.

comment by [deleted] · 2011-10-01T06:33:02.890Z · score: 4 (6 votes) · LW(p) · GW(p)

One serious issue for evaluating existential risk is working out whether most of the Great Filter is behind us or in front of us.

My favorite hypothesis is that there is no great filter - someone's got to be first in our light cone, and we're it.

comment by CarlShulman · 2011-10-01T07:05:43.028Z · score: 9 (11 votes) · LW(p) · GW(p)

We observe not just an empty galaxy, but one which apparently had hundreds of millions or billions of years before us for intelligence to develop. If there were no Great Filter this would be surprising.

comment by Vladimir_Nesov · 2011-10-01T11:50:40.949Z · score: 3 (5 votes) · LW(p) · GW(p)

(An old hat response that your comment still seems to warrant.)

The fact that we are still here reduces the surprise significantly. If intelligence did develop, we probably won't be still here, and we know that we are. It's not just an empty galaxy, it's an empty galaxy where we live. It is not surprising in the same intuitive sense as observing the sequence 0101100100010 of coin flips is not surprising, once you take it as data you have (specification of which phenomenon you are dealing with) and not the property to be explained.

comment by byrnema · 2011-10-01T19:19:38.088Z · score: 7 (7 votes) · LW(p) · GW(p)

I agree.

My perpective was that it is surprising that the entire universe is only 13.7 billion years old (WMAP) while the Earth is already 4.5 billion years old and it took about that amount of time for intelligence to evolve.

Science has taught us that we're usually not special, but if our evolution already took a third of the time of the entire universe's existence, it seems we're exceptionally early. In comparison, a human lifetime is 60-100 years and I was born approximately 400-600 lifetimes into the cumulative period of human lifetimes which seems much more random than '3'.

Given the information that we are in the second or third time span required for evolution, the expected number of alien civilizations doesn't have to be zero to expect that we're the first. I don't know the relevant statistics, but, for example, it the probability of intelligent life developing in a 3 billion year period is 1/50, we might just need to wait a couple more billion years for the next group. Solar systems are still being born ... aren't we objectively early in that respect?

comment by [deleted] · 2011-10-02T00:56:22.323Z · score: 10 (10 votes) · LW(p) · GW(p)

Given the information that we are in the second or third time span required for evolution

There's a twist that actually reinforces your point. The first stars (Population III, all dead now) were metal-free. The next generation, Population II, had low metal content. Our Sun belongs to Population I, with higher metal content. So it looks like our Sun belongs to the first, maybe the second, generation of stars that could possibly support intelligent life.

comment by Nornagest · 2011-10-02T01:14:08.660Z · score: 8 (8 votes) · LW(p) · GW(p)

To further that point, it might be worth noting here that "metal" in astronomy parlance means anything heavier than helium. Complex life might conceivably evolve without, say, iron, but life without carbon is much less likely.

comment by [deleted] · 2011-10-02T07:47:48.100Z · score: 1 (1 votes) · LW(p) · GW(p)

Yes - did you see that I linked to the metallicity article? (Astronomers are funny.)

comment by Nornagest · 2011-10-02T18:54:17.598Z · score: 1 (1 votes) · LW(p) · GW(p)

Should have clicked through, in retrospect.

comment by amcknight · 2011-10-02T23:50:51.411Z · score: 3 (5 votes) · LW(p) · GW(p)

Science has taught us that we're usually not special

Maybe, but due to anthropic effects, this is one of the times in which we definitely cannot use the we're-not-special rule of thumb. Noting that we happen to have developed gives us absolutely no evidence about the rarity of observers that can notice that they've developed (except that it rules out theories that make it so rare that even 1 observer is unlikely).

if the probability of intelligent life developing in a 3 billion year period is 1/50 we might just need to wait a couple more billion years for the next group

Without more information about The Great Filter, most of the probability density does not reside in such perfectly balanced orders of magnitude (like 2% per 3Gyr-galaxy) to make us happen to be first. Though it's an open possibility that we're the first but not the only life that will develop, it's extremely unlikely that two huge numbers that could conceivably be orders of magnitude apart, happen to line up so closely.

comment by JoshuaZ · 2011-10-03T00:02:28.565Z · score: 3 (3 votes) · LW(p) · GW(p)

I'm not sure I follow your last paragraph. What are the two huge numbers in this context?

comment by amcknight · 2011-10-04T15:41:09.067Z · score: 0 (0 votes) · LW(p) · GW(p)

The numbers I have in mind are something like: total number of planets and probability of any given planet to allow life to flourish across the galaxy. These numbers are independent. You could start with 'region of space' or involve time, but the numbers will still be independent. (i guess I should have said a huge and tiny number balancing when multiplied)

comment by see · 2011-10-01T07:10:03.350Z · score: 3 (7 votes) · LW(p) · GW(p)

That really doesn't work.

If there is nothing making the development and survival of intelligent tool-using life intrinsically improbable, the odds are at least billions to one against Earth being first in our light cone. Any hypothetical Great Filter that has odds of mere millions-to-one against being true is a substantially superior hypothesis to "we just happened to be first".

comment by Nick_Roy · 2011-10-03T20:18:29.277Z · score: 1 (1 votes) · LW(p) · GW(p)

Wouldn't this mean that the Great Filter is behind us?

comment by [deleted] · 2011-10-04T05:41:46.041Z · score: 9 (9 votes) · LW(p) · GW(p)

Not necessarily. As Wikipedia says, "According to the Great Filter hypothesis at least one of these steps - if the list were complete - must be improbable." That is, if "Great Filter" means anything, it's that one or more of the steps to achieving a technological civilization that can expand throughout the galaxy is very difficult ("improbable").

What I'm talking about goes like this: suppose that none of the steps are very difficult. Of course, that doesn't mean they're instantaneous - each step takes time. You need elements other than hydrogen and helium for life, so you have to watch the supernova clock ticking until Population I stars form (maybe Pop II, but remember that this is hypothetical - let's assume that only Pop I stars have enough "metals"). Then you need planets - but we're seeing planets everywhere as the limits of our vision increase. Once the first replicator forms, you have to wait for evolution to grind its way up the complexity ladder (mandatory disclaimer: evolution doesn't "prefer" higher complexity, but there is a complexity lower bound - there is literally nowhere to go but up). In this scenario, where none of the steps are improbable, but they do take time, what would the first intelligent species in a given lightcone see?

They would arrive on the scene, and they would see a young-looking universe. Their star would be among the first stars capable of supporting life. Their planetary formation would have been almost immediately followed by the first replicator (and returning to reality for a moment, we see fossilized life as far back as 1 billion years after the Earth's formation). Their evolutionary history, while marked by giant impacts and mass extinctions, would appear relatively free of long reigns of nothing happening complexity-wise. And they would see an empty universe, and wonder where everyone else is.

Obviously, I don't know the answer to the Fermi Paradox. But if anyone exists, someone's got to be first. Maybe it's us, and maybe that's why the paradox is so baffling.

(Instead of "where are they", I think the LW way of phrasing the paradox is "why aren't we paperclips?".)

comment by Rob Bensinger (RobbBB) · 2013-05-07T12:53:30.457Z · score: 1 (1 votes) · LW(p) · GW(p)

That's a very attractive scenario. But I don't think 'Someone had to be the first' is sufficient to explain why we are the first. On your view, intelligent life takes some time to get going, but then is incredibly abundant for as long as stars and metals abound. On standard cosmological models, star formation will continue for some 100,000 billion years (or at minimum 1,000 billion years). Anthropically, our occurring only 14 billion years into our universe's lifetime is then profoundly surprising. If a lot of intelligent life precedes us (and/or intelligent life is generically rare and will never be very abundant), then our location remains surprising, but a lot less so.

Your theory predicts what we see given that we evolved very early in our universe's lifetime, but if we don't build in our temporal location then it actually strongly predicts the opposite -- that we'll open our eyes and see a galaxy teeming with life, many tens or hundreds of billions of years after the beginning of the universe.

comment by [deleted] · 2013-05-08T03:50:19.046Z · score: 0 (0 votes) · LW(p) · GW(p)

I don't claim it's sufficient, I just claim it's possible. And while it's true that most civilizations would see a galaxy teeming with life, somebody's gotta be first.

comment by Rob Bensinger (RobbBB) · 2013-05-09T07:39:37.637Z · score: 1 (1 votes) · LW(p) · GW(p)

Anything's possible. If your claim is interesting, it's because it's probable, at least relative to its explanatory rivals.

The problem with 'somebody's gotta be first' is that it's either a fully general explanation or no explanation at all. Suppose you and a hundred billion of your friends are each assigned a distinct number between 1 and a hundred billion. You're assigned the number 1. Should you dismiss this surprising outcome because 'someone had to get 1'? No. Not without a lot of evidence that it's a coincidence, anyway. The fact that someone had to get 1 only means that it was possible, on the terms on the game, for you to get 1. It doesn't make that possibility, when it occurs, any less surprising or confusing. The goal of explaining our observations isn't to show that they're possible; it's to show that they're a lot more likely than they would have been in the absence of the explanans.

comment by [deleted] · 2013-05-10T02:37:46.579Z · score: 1 (1 votes) · LW(p) · GW(p)

I would be surprised because that violates the pigeonhole principle.

(Yeah, I'm a programmer.)

comment by NancyLebovitz · 2013-05-10T05:25:51.545Z · score: 0 (0 votes) · LW(p) · GW(p)

Isn't "someone had to win" the rationalist explanation for most lottery winners?

comment by Rob Bensinger (RobbBB) · 2013-06-15T08:52:59.866Z · score: 1 (1 votes) · LW(p) · GW(p)

'Someone had to win' isn't a full explanation on its own, no. (If it were, it would be a fully general explanation schema; 'something had to happen', for example, can explain any event.) Rather, 'someone had to win' is an explanation for an unlikely victory when you posit a large sample space. In most cases, selection bias will also play a role in the explanation -- it will account for the prima facie salience or interestingness of the event.

If selection bias and the size of the sample space don't help make you any less confused about why some X happened to you, then revisiting 'someone had to X' shouldn't alleviate your confusion.

comment by Nick_Roy · 2011-10-04T18:28:07.218Z · score: 1 (1 votes) · LW(p) · GW(p)

I see. Good answer.

comment by jhuffman · 2011-10-14T20:36:39.340Z · score: 0 (0 votes) · LW(p) · GW(p)

Yes, someone has to be first and this is what they would see. Or actually, this is what the first dozens or hundreds or thousands would see as without great filters you'd probably have many hundreds of species reaching an information age at around the same time but there would be a good while before they produce enough evidence to detect each other with any likelihood.

This era of galactic development though before the first civilizations become detectable to each other - on the order of a few thousands of years - is unimaginably short in terms of the time-span of the cosmos, unimaginably short even in terms of the lifetime of our own sun. It seems like tens of millions of civilizations would develop in the life-span of our Sun and its peers and yet we have to be among this very small sample at the beginning. That would be surprising.

A great filter is I suppose, equally surprising even though the mechanisms of it would maybe not be surprising.

comment by Hyena · 2011-10-01T06:03:50.091Z · score: 3 (5 votes) · LW(p) · GW(p)

The Fermi paradox can be restated as the conclusion that the Great Filter is behind us since we are radio users now but have not seen another radio user; so whatever it is which prevents one coming about is behind us not ahead by virtue of the technology I'm using to make this post right now.

comment by roystgnr · 2011-10-01T15:03:29.003Z · score: 4 (4 votes) · LW(p) · GW(p)

Who said anything about radio? We don't seem to see anything that points to more advanced technology either, not even partial occlusions of stars. Those spectra all look suspiciously like untouched low entropy starlight going to waste, and nothing at all like high entropy space colony radiator output.

And that's after we've already pushed the goal posts back from "why aren't they here" to "why aren't they anywhere else, either".

comment by Hyena · 2011-10-02T11:44:34.619Z · score: 1 (1 votes) · LW(p) · GW(p)

Part of me tires of people phrasing their agreement or extension as an objection.

The original Fermi Paradox, "where are they", seems sufficiently explained by general relativity and its implications for target selection. Unless you think there is an alien civilization very close by, you'd have no reason to expect a visit since you're not just far away, but unlikely to be even the closest interesting object among the those which are considered "far away".

comment by JoshuaZ · 2011-10-02T15:04:17.546Z · score: 5 (5 votes) · LW(p) · GW(p)

That doesn't work. If a civilization reached slightly beyond our tech level around twenty million years ago (a blink in the eye as far as the universe is concerned) anywhere in our galaxy then we should expect to have seen some sign of them. Radio and visitation aren't the only issues. We don't see any evidence of Dyson spheres or ringworlds (although the second is something we've only recently been able to detect- Kepler should be able to detect ringworlds albeit I don't know if anyone is looking for them.) We see no evidence of any form of stellar engineering or large scale structures at all. And distance only matters when you look into the near past. Civilizations that have had millions of years to prosper should have spread much farther. Note that our galaxy is only about 10,000 light years across. (ETA: 100,000 not 10,000 see roystgnr remark below.)

comment by roystgnr · 2011-10-02T18:16:40.823Z · score: 4 (4 votes) · LW(p) · GW(p)

100,000 light years across; you may be thinking of 10,000 light years thick.

But in any case, small enough for an expansive civilization to colonize, slower than light using nothing more "speculative" than fission-based rockets, in a fraction of a percent of the age of the galaxy.

comment by JoshuaZ · 2011-10-02T18:54:32.027Z · score: 0 (0 votes) · LW(p) · GW(p)

Yes, thanks. That's what I was thinking of.

comment by Hyena · 2011-10-02T15:24:17.510Z · score: -1 (5 votes) · LW(p) · GW(p)

Why would we expect such things to begin with? That we don't detect the elements of Anglophone science fiction doesn't seem particularly worrying. Should we be updating our belief in extraterrestial civilizations based on their absence or should we be updating the credence lent to these speculated technologies?

comment by JoshuaZ · 2011-10-02T17:08:33.267Z · score: 5 (7 votes) · LW(p) · GW(p)

That we don't detect the elements of Anglophone science fiction doesn't seem particularly worrying. Should we be updating our belief in extraterrestial civilizations based on their absence or should we be updating the credence lent to these speculated technologies?

Presumably this should be an update in both directions, but the update should be much more towards an absence of civilizations since these are all technologies which seem to be both doable and desirable for a civilization. Moreover, it isn't just an absence of these technologies. We don't see anything that looks like a sign of stellar engineering. That isn't just the "Anglophone science fiction" but the fact that the universe looks essentially natural. It is possible (indeed likely) that there would be other technologies out there that we haven't thought about yet that would also involve large scale harnessing of energy and other resources. But we don't see anything anomalous.

comment by Hyena · 2011-10-03T00:25:10.606Z · score: 0 (2 votes) · LW(p) · GW(p)

The universe looks natural; it also looks homogeneous. This militates in favor of alien civilization and is settled. You're proposing that I should doubt either naturalness or homogeneity because a purely speculative technology is not observed.

comment by JoshuaZ · 2011-10-03T00:31:23.752Z · score: 5 (7 votes) · LW(p) · GW(p)

You're proposing that I should doubt either naturalness or homogeneity because a purely speculative technology is not observed.

Not just a speculative technology. Those technologies are just some examples. As noted, there's also a complete lack of radio sources. There's no sign of any sort of alien life at all. If an alien civilization arose and prospered a billion years ago in our galaxy, we'd expect to see some sign of it, such as colonizing planets. And the lack of specific speculative technologies isn't the problem, it is the lack of any sign of any sort of larescale use of the massive quantities of energy available to stars. It is all getting wasted. And that's not just true for our galaxy, every galaxy we look at looks completely natural as far as we can tell.

Of course if you are absolutely set on preserving a strong form of homogenity and the naturalness of the universe there's one obvious solution: There's a Great Filter in front of us. Things are homogenous, and species get wiped out by something, probably a technology they decide to play with. If you are convinced of homogenity and naturalness then the Great Filter should frighten you.

comment by DanielLC · 2014-12-06T08:01:24.783Z · score: -1 (1 votes) · LW(p) · GW(p)

We have a star. Those aren't exactly rare, but they have far more resources than it takes to reach them. Regardless of your goals, sending probes to every star would be a useful first step.

comment by MichaelHoward · 2011-10-01T10:35:52.501Z · score: 2 (2 votes) · LW(p) · GW(p)

Not necessarily.

comment by Jack · 2011-10-01T11:17:54.360Z · score: 9 (9 votes) · LW(p) · GW(p)

Obviously the aliens don't use photons to communicate. They use muonic neutrinos, much faster that way.

comment by Rob Bensinger (RobbBB) · 2013-05-07T13:41:06.646Z · score: 0 (0 votes) · LW(p) · GW(p)

One could also interpret the Great Silence as an extremely optimistic sign, rather than a pessimistic one: Intelligent species may be common, but as a rule they completely take over their galaxies (and anything else they can get their hands on), exploiting the resources so thoroughly that they crowd out any other species that might have evolved independently. So any intelligent species will tend to be the only one in its part of the universe, and the first to develop. If it hadn't been the first, its star, metals, etc. would have been converted to another species' ends and it would never have arisen.

comment by Nic_Smith · 2011-10-02T06:36:27.870Z · score: -2 (4 votes) · LW(p) · GW(p)

A thought I've wanted to bounce off of LW for a while -- could clothes be a great filter? They allow and encourage on-planet exploration as individuals are able to survive and thrive in climes they're not innately adapted to, and also allow for quick-switching of adaptation for different weather, which seems to aid long-distance communication and trade. While I think that Wikipedia exaggerates slightly when it says that "The wearing of clothing is exclusively a human characteristic" (hermit crabs?), it is nonetheless much rarer than even intelligence or tool-use (and like both of these traits, humans go to extremes). It might not occur to even a very intelligent species to wear something in order to go into space. The technology used just to move around the surface of their own planet might be really clunky and uncomfortable for several generations of development.

comment by wedrifid · 2011-10-02T07:59:09.926Z · score: 4 (4 votes) · LW(p) · GW(p)

A thought I've wanted to bounce off of LW for a while -- could clothes be a great filter?

I'm going with 'No'. How about "insy winsey filter" that is optional?

Or you could just subsume 'clothes' into 'technology' and leave it at that.

comment by Rob Bensinger (RobbBB) · 2013-05-07T15:21:51.643Z · score: 3 (3 votes) · LW(p) · GW(p)

A more likely Great Filter would be a lack of distinct climates, seasons, etc. altogether. The problem that would raise isn't that the species would be too stupid to come up with the idea of tools-to-put-on-your-body (and yet smart enough to otherwise be capable of reasoning and tool use?), but that a lack of variation over time and space would discourage the evolution of generalist or adaptive intelligence in the first place. Instead, all life-bearing planets would be dominated by highly niche-specific super-effective super-simple organisms with no real competition. This seems like a plausible explanation of the Great Silence to me, because very few planets have seasons. If inhabited planets also tend to be 'boring' (e.g., to have a fairly uniform temperature or terrain, or to be sheltered from major asteroid impacts), that could explain why generalist species, including adaptive reasoners, haven't evolved.

comment by NancyLebovitz · 2013-05-10T06:19:07.795Z · score: 1 (1 votes) · LW(p) · GW(p)

Could there be an inhabitable planet without distinct climates?

comment by Rob Bensinger (RobbBB) · 2013-06-15T08:45:27.269Z · score: 0 (0 votes) · LW(p) · GW(p)

Yes. Why would climatic homogeneity prevent any of the building blocks for abiogenesis? At first glance, I'd expect it to make things easier for life.

comment by NancyLebovitz · 2013-06-15T12:01:35.510Z · score: 1 (1 votes) · LW(p) · GW(p)

I was asking whether it's physically possible for a planet, especially one that's got land masses, to not have climates.