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There Have Probably Been Aliens

nytimes.com

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Re: There Have Probably Been Aliens

#151

Post by madaxe_again »

sevenless wrote: post #20
madaxe_again wrote: post #6

The most likely reason that nobody seems to be out there is the great filter. The author asks if our civilisation will last millenia or millions of years - but I think a more apt scale would be decades or centuries. It just seems pretty obvious that we have an ever increasing set of opportunities to extinct ourselves, and therefore the probability of a civilisation ending disaster only grows. All the aliens either bl…

The Doomsday Argument implies we've got centuries or less left. https://en.wikipedia.org/wiki/Doomsday_argument

Oh, I think we've decades or years rather than centuries. We always underestimate exponential growth. That's OK, at least it'll make for an interesting existence.

Re: There Have Probably Been Aliens

#152

Post by lisper »

ulmus wrote: post #146
lisper wrote: post #138

Earlier quoted context omitted.

Sorry, I wasn't very clear. > the authors essentially claim to know the probability of their own existence The "quantity of interest" is the probability of aliens, but not the probability of aliens in all possible worlds, but rather the probability of aliens in this world. In this world, the probability that the authors exist is 1.

I respectfully disagree. I claim the probability of human existence could have been less than 1, just as the probability of heads when tossing a fair coin is less than one, even when heads is observed as an outcome. We simply can't observe "tails" in this case.

You are confusing two (perhaps three) different things. The probability of a fair coin landing heads is 1/2 before the coin has been flipped. After the coin has been flipped, the probability is either 0 or 1. You may not know which it is, but it is definitely one or the other (assuming classical mechanics).

Now, if you are being a strict Bayesian then the probability of the coin being heads is never quite 0 or 1 because there is always the possibility that your eyes are deceiving you, or you are suffering from delusions or some equally unlikely contingency. But I assume that's not what you mean, and that you accept that humans do in fact exist in this universe, in which case the probability that humans exist in this universe is exactly 1. P(A|A)=1 by definition.

Re: There Have Probably Been Aliens

#154

Post by skissane »

chmike wrote: post #116
skissane wrote: post #47

The universe is so big, that I think it's very likely that somewhere out there are, there are many other intelligent civilisations, in distant galaxies–but so far away we'll never be able to visit them, never be able to communicate with them, we'll likely never even know of their existence (not in a million years–and I mean that literally.) In the long-run (of millions of years), the two most likely outcomes for huma…

Why do you believe it is surely extinct by now ? Why do you believe that colonisation of the galaxy means eliminating all other advanced civilisations and life form ? Are you russian ;) Your reasonning is based on many assumptions. Look at us. When did we discover how to fly ? When did we landed on the moon ? Human evolution in science and technology mastering is evolving exponentially. Things can happen much faster…

I'm not suggesting our species will go on a genocidal rampage through the galaxy. Rather, I'm suggesting that if we manage to spread through the galaxy, we won't encounter any other intelligent life, and our galactic colonisation will prevent any other intelligent species from evolving in this galaxy since we'll be fully occupying that ecological niche, just as being the dominant species on this planet makes it much less likely that other species (such as chimpanzees) will evolve to our intelligence level. (Galactic colonisation might plausibly result in speciation of homo sapiens, however.) It's extremely unlikely for two intelligent species to evolve within a single galaxy at the same time; if other intelligent life ever exists in this galaxy, it is almost certainly many millions of years older or younger than us. Any civilisation following our present trajectory is likely to colonise the galaxy within a few million years. Since we are here, no species before us has successfully colonised this galaxy, which implies either we are the first, or the others have already gone extinct before reaching that level.

I discount UFOs because I believe the alternative explanations–such as natural atmospheric phenomena, secret testing of military aircraft, confusion, hallucination and delusion–are collectively much more probable than the hypothesis that extraterrestrials have visited us.

Re: There Have Probably Been Aliens

#155

Post by EndlessSky »

unclebucknasty wrote: post #66
EndlessSky wrote: post #54

Earlier quoted context omitted.

I'm not sure you can draw the conclusion that since there is one intelligent (as defined above) species, out of many on earth, that intelligence is unlikely to develop. The main issue I see is that there were other species that were close to as intelligent us, but we out competed them and they went extinct. I wouldn't be surprised if there was a sort of "first one wins" effect where the first species to reach a certa…

And, I don't think it's even correct that we are the only intelligent species on Earth. Most intelligent, yes. But, certainly not the only intelligent.

The guy I was replying to defined as intelligence as attaining radio telecommunications. In that sense, yes we are.

Re: There Have Probably Been Aliens

#156

Post by ThePhysicist »

apenwarr wrote: post #119
ThePhysicist wrote: post #76

Earlier quoted context omitted.

Fascinating, thanks for pointing this out! Discovering a new kind of life on Earth that has coexisted with us without being noticed would be a huge event of course, and it would probably challenge what we regard as life in a general way. This would be good stuff for a science fiction story as well.

It does make a good sci fi story: https://www.amazon.com/dp/B00M64A8EG/

Wow, thanks for the link :)

Re: There Have Probably Been Aliens

#157

Post by throwaway_yy2Di »

gus_massa wrote: post #105
throwaway_yy2Di wrote: post #56

Earlier quoted context omitted.

I think what will really help us to get at a better estimate is to answer the question whether there exists life in our solar system that evolved independently of that on Earth. That's definitely the big question. But you left out the elephant in the room: is there life right here on Earth that's evolved independently? Abiogenesis doesn't simply halt after it's achieved its "goal". Perhaps we take the common-origin h…

I think it's a winner take all task. If you are the first almost live form, you have plenty of time, and can evolved in an environment that has plenty of food. Something like the brown goo of Titan, with lots of hydrocarbons and other small molecules that you can pick up for free. And nobody is trying to eat you. So the reproduction process can be slow and unreliable, perhaps a few reproductions per day/week?/month??…

    If you are the first almost live form, you have plenty of
    time, and can evolved in an environment that has plenty of
    food. Something like the brown goo of Titan, with lots of
    hydrocarbons and other small molecules that you can pick up
    for free.
But there's no food on Titan anywhere! There's no oxidizers for hydrocarbons to react with; hence no accessible chemical energy.

That's likely generic for pre-biotic environments. Planets don't naturally form redox gradients; if there's a large redox potential, that's reactive chemistry, which will react away to equilibrium over geologic time. It takes work to maintain such a gradient; for example, Earth's oxidizing atmosphere is actively maintained by life. Oxygen is geologically removed from atmospheres, reacting with exposed rock surfaces (oxygen weathering). It takes active expenditure of work (solar energy) to keep it existing.

There is some chemical energy in abiotic planets, but only small amounts, at very low power levels compared to Earth's aerobic biosphere (which eats something like 10^14 watts!). There's photochemistry caused by solar UV light, and there's atmospheric lightning (which is a major player in the Earth's N2 cycle). Most important is geochemical energy; i.e. primordial chemical gradients that aren't yet at equilibrium, because the two reactants are physically separated by rock layers. On Europa for example, a very slow process brings rocks into contact with the ocean, where slow serpentinization reactions can occur: oxidation of Fe++ to Fe+++ by water, with the production of H2. This would be the food source of Europa life, if it's there.

Re: There Have Probably Been Aliens

#159

Post by EndlessSky »

sktrdie wrote: post #65
EndlessSky wrote: post #54

Earlier quoted context omitted.

I'm not sure you can draw the conclusion that since there is one intelligent (as defined above) species, out of many on earth, that intelligence is unlikely to develop. The main issue I see is that there were other species that were close to as intelligent us, but we out competed them and they went extinct. I wouldn't be surprised if there was a sort of "first one wins" effect where the first species to reach a certa…

Even if we outcompeted the Neanderthals (or other very intelligent species), they were still very close to us in the tree of life (they were cousins). For a trait to be a high probability outcome of evolution it needs to happen independently in many different species - take binocular sight for instance which happens in all kinds of very far related species. Even if our close relatives had similar intelligence and we…

One species having binocular sight does not significantly effect the evolutionary advantage of binocular sight in other species.

And define what you mean by "high probability", I would agree that intelligence (as defined as building radio communications) does not have a high probability of evolving compared to binocular sight.

What I am getting it, is I would not be surprised if intelligence (building radio communications) was like a race. A race only has one winner. Just because a race may have a thousand contestants, does not mean that the probability of there being a winner if 1/1000. Rather the probability of there being a winner goes to 1 as the duration of the race increases.

Or its like concluding that because life only appeared once on this planet, its very unlikely that life could have appeared at all. Another hypothesis is that once life appears, it makes it very difficult for new life to appear by preventing the conditions that lead to abiogenesis. For example maybe chemical evolution can not occur, because in all places where it would the are living organisms that interrupt the process because those chemicals are useful to the organism.

Similarly, I Humans have an unprecedented degree of control over the evolutionary trajectories other organisms. Maybe other apes, if left alone for hundreds of millions of years would evolve into a species that develops radio communications. But organsim's need evolutionary pressure to evolve, and humans really, really like preserving species. If a species of apes start dying off due to some evolutionary pressure, rather than let nature take its course and let the apes which can survive that pressure out compete the others, humans will intervene to keep them around, and as a result they will remain stagnant evolutionary.

Re: There Have Probably Been Aliens

#160

Post by jessriedel »

tim333 wrote: post #107
apsec112 wrote: post #9

"In other words, given what we now know about the number and orbital positions of the galaxy’s planets, the degree of pessimism required to doubt the existence, at some point in time, of an advanced extraterrestrial civilization borders on the irrational." This is terribly silly. It reminds me of the Onion talks: "Step 1: Devise an idea to create a car that runs on compost. Step 2: Create the car. We’ve already compl…

An alternative is if you believe in the principle of terrestrial mediocrity, that we are on a fairly average planet in a fairly average bit of the universe where fairly average things happen, the fact that life popped up on our planet in less than a billion years implies it probably has done so in loads of other places. Quote Martin Amis's The Information: “The history of astronomy is the history of increasing humili…

The actual time taken to randomly clear a hurdle actually tells you very little about how unlikely it was to clear, so long as there are several hurdles and you condition on them all occurring (i.e., that you exist). In particular, the expected time to clear a hurdle asymptotes at a constant value even as the difficulty becomes arbitrarily hard. This is a very unintuitive but very important result for thinking about this topic.

http://mason.gmu.edu/~rhanson/greatfilter.html

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