The Odds of Life and Intelligence

at some point or another we have all probably wondered are we alone it's one of the most basic questions that science can ask one of the most human scientists and non-scientists alike have professed their beliefs on this matter for centuries and the consensus wanes with her fashions of the time but in all of that there's no data it's just guesswork opinion even faith well I've never been one much for faith I want an answer an objective solution conditioned upon data and corrected for bias and so today I'm gonna give you one just maybe not the one that you're expecting [Music] do you think that there is life outside of Earth intelligent life in the universe absolutely there is life elsewhere and almost certainly intelligent life it has to be though odds are just overwhelming as soon as the most frequently asked question are we low my best guess is that the universe is teeming with life surely other civilizations out there in the universe there are 350 billion galaxies in the universe just over a year ago we posted a video entitled why we might be alone in the universe it understandably generated a huge reaction and has become one of our most popular videos I hope that you'll check it out but the basic idea is that it is a commonplace fallacy even among scientists to proclaim that the huge number of habitable worlds out there means that therefore it is highly likely that life and even intelligence is abundant in the universe well why what's wrong with that logic let's write that the number of inhabited planets equals the number of habitable planets multiplied by the probability of life starting on those worlds let's call that FL and for intelligent life it's basically the same but now with another multiplication factor which is the probability of life developing intelligence let's call that F I okay so far so good but herein lies the problem the number of habitable worlds is surely huge yes I agree but we have no idea absolutely no idea what the probability of life or intelligence developing are they could be anything one in ten one in a billion or even one in septillion in which case we really shouldn't expect anyone else to be out there so these two probabilities FL and fi are pretty important so important that they feature in the famous Drake Equation this is the equation that Frank I wrote down in 1961 in one of the very first conferences about the search for intelligent life but as elegant and convenient as the Drake Equation is it's actually quite frustrating because we have no idea what values to plug in for FL fi and indeed some of the other terms not even a hint now you might say hang on a minute actually we do have a way of estimating this probability of life developing because when we look at the earth we see that life developed very soon after conditions on the earth became suitable for its existence therefore doesn't that mean that life is easy therefore doesn't that mean that FL should be close to one mom sadly it's not that simple this is where something which Brandon Carter calls the weak anthropic principle comes in because it may very well be that life had to start early else we simply wouldn't be here a selection effect so it may be that on most planets life takes billions even trillions of years to typically start but here on the earth it just so happened to start much faster than usual and critically if it hadn't had started quickly then we wouldn't be here to talk about it so you might well ask why would a quick start to life somehow be necessary to our existence today and the answer to that lies in our last video where we covered the history of the earth and its future a crucial point from that video is that the earth will become inhospitable to multicellular life relatively soon in geological timescales less than a billion years now remember that it took about four billion years for life to evolve from whatever it started out as to us intelligent life a full billion year gestation period let's say for the moment that this is typical that intelligence usually takes about four billion years to evolve so if the earth becomes inhospitable to us in less than a billion years then life has to stop pretty quick in order to have enough time for this gestation period to play out and evolve to us intelligent life and this is the source of a anthropic die lemma the fact that life started quickly on the earth may simply be a necessity to our existence rather than any kind of useful information about how easy it truly is and this is why in our why we might be alone video I explained that my answer to the question are we alone in the universe was that I don't know now sure it's possible that life starts quickly all of the time that is totally consistent without one data point but it's also possible that life usually takes a very long time to get going if at all and we're only here to observe ourselves because it happened quickly here on the earth that's also consistent without one data point so both possibilities appear equally valid like a 50/50 coin toss [Music] ever since I made that video perhaps like you I've been chewing on this quite a bit after all I grew up watching Star Trek where intelligent life seemed to be on every planet that they came across the idea of a lonely universe just seemed wrong counterintuitive maybe it was really just countercultural but I wasn't satisfied with this 50-50 I don't know situation any more than you and so for the last year I've been working on a new research project to try and finally shed some light on this damn question one that has frankly haunted humanity since antiquity but after everything that I've told you so far what new insights could possibly shed new light on this ancient problem how can we make progress barring say direct evidence for alien life which of course we don't have the story that I told you in the first chapter is really just a simplified description of a seminal paper by two colleagues of mine dr. Dave Spiegel and professor Eadie Turner despite Drake writing down his famous equation half a century ago and dozens of papers trying to tackle it since for me this is the most important paper written on the topic Spiegel and Turner tagged with the problem using mathematics specifically Bayesian statistics the key to Bayesian thinking is Bayes theorem which states that the probability of X being true given that we see Y happen is proportional to the probability of seeing Y happen when X is true multiplied by the prior probability of X being true that sounds complicated but it really just states that probability depends on both the data that we have and our prior assumptions ignore those prior assumptions at your peril as we've discussed the time scale for life to develop from its start to us as intelligent observers is pretty crucial when thinking about this problem as far as we can tell it took about four billion years here on earth but maybe that's not always true so Spiegel and Turner tried to different values a nominal 3.5 billion years and a more optimistic 1.4 billion years if we use 3.5 then life really has to start quick in order for there to be enough time to get to us in such a case we're in the I don't know scenario where anthropic bias dominates and life could either be common or rare we just can't say on the other hand if we use the one point four billion years value then life doesn't have to have started quickly it could have started two billion years after the earth cooled and still have had enough time to get to us in such a case then the quick start to life is actually quite telling here because now it really is telling us that life is a quick and easy process so this number the time scale for intelligence to evolve is clearly pretty crucial it has a huge impact on our results and so as wonderful as the paper of Spiegel internet is my criticism of their work would be that fixing this time scale - - somewhat arbitrary choices doesn't go far enough rather than assuming - fiducial values we can actually try to learn this number already we are trying to learn the time scale for life to develop so why not go a step further and simultaneously try to learn how long it takes for intelligence to evolve - and that really encapsulates one of two key improvements that I've used in my work and has been published this weak link down below in the description adding in an additional parameter to an already lost problem might seem overly ambitious I mean after all if we're struggling to measure one parameter how is it gonna help to add in a second but by letting us explore both the rate of life and the rate of intelligence together we can explore how they depend upon one another we can more honestly encode our ignorance about the problem and ultimately we can answer the question I'm not just life but also intelligences prevalence but this is just one of two improvements that I made in my paper so let's turn to second one of the most common complaints about Bayesian statistics which remember is the tool with both Spiegel internet use and I am using in this paper concerns the priors recorder in Bayesian statistics probabilities are a product of what the data tells us and our prior beliefs and if you ask different people well they might have different prior beliefs and thus arrive at different answers in other words probability is subjective and not universally agreed upon this is understandably frustrating but if you really think about it it's kind of inevitable after all let's say you and I have different problems about the reality of ghosts in the world well when we are presented with a hazy photo of a dude with a sheet on his head we are probably gonna arrive at different conclusions in fact I would argue that prize are not a weakness of Bayesian inference at all but actually one of its major strengths because they acknowledge this fact quite openly and transparently nevertheless for our problem it does pose somewhat of a dilemma how do we choose these priors spiegel and turner tackled this by trying a range of different pride distributions for the time scale for life to emerge a process known as abiogenesis the simplest assumption is that every value is just as likely as any other value so that would mean that a valley between 100 to 200 million years is just as likely as a value between 800 and 900 million years that's called a uniform or flat prior but uniform priors actually impose quite strong assumptions into the problem for example a fast time scale say somewhere between zero and point two million years is highly unlikely just because that's a very thin window against the overall range when spiegel and turn attack with this by trying different reasonable priors they and that each of them lead to markedly different answers is there anything better to be done here well the choice of prize doesn't have to be arbitrary that does exist a system of rules for figuring out how to choose a prior objectively known as objective Bayes nism the idea is to choose the least informative prior remember that the prior always influences the answer but objective Bazin ism looks at all of the possible priors and uses the one which at least informs the answer which is called an objective prior if we could find an objective prior for this problem then we wouldn't have to throw up our arms and just show possible answers we can actually figure out a single objective solution great but how do we do that well we don't have to reinvent the wheel here objective Bazin ism has been around for decades and we can lean on that there are two ways to think about this abiogenesis probability and i've been using them somewhat interchangeably thus far the first is to call it a time scale it typically takes X years for abiogenesis to happen and that's how Spiegel and Turner thought about the problem equivalently I show in my new paper that we can take that time scale and compare it to the age of the earth to arrive at a probability that abiogenesis succeeds at some point in its history that's really what Frank Drake was talking about with F L they're really just the same thing just different ways of looking at the problem but when we pose this as a time scale problem it becomes difficult to argue that any prior is any more objective than any other prior and that's really the position that Spiegel and Turner ended up in but when we think more like Drake and cast this as a probability of success over an interval of time it becomes more tractable to objective Bazin ism and why so ok this probability F L is just a number somewhere between 0 & 1 and the outcome of this process is binary we would technically call that a Bernoulli process it's either life appears or life does not yes or no if FL is close to zero then it's a lonely cosmos with very few inhabited worlds if FL is close to 1 then well it's a crowded party over there could it be that FL is close to 0.5 though that would mean that on a group of literally identical planets and conditions half of the planets succeed and half of the plants do not intuition Li that just feels a little contrived either the conditions and laws of physics are amenable to life or they aren't it's a bit odd if it ends up being split right down the middle like this an analogous example from chemistry will be taking a random chemical chemical X and pouring it into water in a series of experiments to see if it dissolves now we would expect that either it will dissolve every time F equals 1 or it won't dissolve at all F equals zero but we wouldn't really expect to dissolve half of the time yes and half of the time note so let's pour it in with my lab assistant here then see what happens let's see what happens no no no for knows epic was zero take your job over and so whatever prior we use for these types of canoeing variables we should expect it to have more way to the edges the extremes rather than in the middle this was the argument made by a statistician John Haldane and so became known as the Haldane prior one of the architects of objective of Bayesian ISM Harold Jeffries presented a systematic way of deriving objective Bayesian priors in the 1940s and he arrived at a similar but somewhat more rigorous answer to Haldane all right so here at the cosmic casino and definitely not my dining room table we're gonna visualize the prior probability space on the x-axis I'm going to f out that's the probability of abiogenesis and on the y-axis I'm gonna put F I the probability of intelligence evolving now if we use the uniform prior than any combination of FL and fi is just as likely as any other I could bet here anywhere and be just as likely to be right a-flat prior probability space but now let's modify this scheme to the objective Bayesian framework plotting the prior as a 3d function we initially have a flat prior but as we move to the objective Bayesian prior you can see had the prior curls up at the corners the objective Bayesian prior tells us that one of these four extreme corners of the Fi and F our parameter space is most likely to be the correct answer and so the objective Bayesian Gambas in the room would be placing their chips in the four spaces that we see here for example this corner corresponds to a lonely universe Inori where both life and intelligence are very rare whereas up here in the upper diagonal we have the exact opposite we have a situation here of a crowded universe where both life and intelligence flourish and so if we had to place a bet on just one of these we had to choose it would be very difficult because each of these four possibilities is just as likely as each other at 25% roll of the dice so your bet is just as good as mine thinking about the problem this way not only follows the objective Bayesian logic but it also simplifies the situation because we now only have four distinct hypotheses to compare rather than an infinite continuum the use of this objective a Bayesian ism is the second big change in my paper over that of Spiegel and Turner with the other one of course being the fact that we now let intelligence be another free parameter in our model so we're now finally ready to combine the two and see what we get hopefully by now you have an appreciation as to just how subtle and tricky this problem that truly is it's deceptively simple and yet requires very deep and careful thought but now we are ready for the results after all in any analysis tenth resource is the most exciting part but especially one where we're asking the question are we alone we've done the rounds on priors but we should mention what ad data is here it's really just two data points the first is that life must have emerged sometime before 4.1 billion years ago this is based on evidence for carbon 13 depleted zircon deposits found in Western Australia something we actually discussed in our last video now we actually don't know when life really appeared we only know that it had to have appeared by that time and that fact is fully accounted for in our model as is the anthropic bias that we would not be here had that not have happened I also want to highlight that this evidence is still under some debate and others prefer to defer to more conservative micro fossil evidence my paper explores both possibilities but we'll stick to the 4.1 billion year number in this video the second data point we use is that intelligent life appears to have only emerged very recently now we can argue about what constitutes intelligent life I mean is it when we start to build cities and technology is it when we start to speak language or maybe even it's not really directly us at all it's when the first hominid started to walk the planet it actually doesn't really matter because you're moving the clock here by at most a few million years in a multi billion year landscape the important number is that this event occurred quite close to the end of the habitable window which closes in 900 million years due to the evolution of the Sun and so I'm back at the cosmic casino train once more to place a bet on one of these four possibilities but now unlike my fellow gamblers in the room I have this additional information about what happened here on earth how does that extra information change where I'm gonna place this bet for now let's imagine that I have a gamblers hunch that the answer is going to be down here in rare intelligence part of parameter space so I have two options really I can either bet on life is rare and intelligence is rare or life is common but intelligence is rare but how to choose between these two working through the math my paper shows that if we assume that intelligence is indeed rare then this corner over here that life is common but intelligence is rare it actually nine times more likely than the life is rare scenario and so if I was forced to place a bet between these two I'd be nine times more likely to win if I put my chips right here but hold on a minute I use this gamblers hunch to assume that intelligence is rare but what if that is not true what if the probability of intelligence is just a little higher maybe 0.1 or maybe even all the way up here in the top corners is the abundant life bet still the smart money remarkably yes because it turns out that that ratio of nine represents the minimum no matter what value of FI we assume the ratio is always at least nine times better for the life is common scenario and so whatever we think about fi we have a rather profound conclusion here that life is most likely common now when I was doing analysis at this point I stumbled across something which truly blew my mind it really took me aback so far we have assumed this objective Bayesian prior but you know what if someone walks in the door and says hey I think that's bogus I don't want to use that prior well normally at this point we would have to just pick up all of the chips take our bets off the table and repeat the entire analysis but here's what really blew my mind because it turns out that it doesn't even matter what prior you use you can use any damn priori one objective subjective or whatever else you will always find that the probability of being in the column the life is common scenario is always at least nine times more likely than being down here in the life is rare scenario the explanation for this is that when we are comparing the four corners of parameter space like this technically known as Bayesian model selection the prize end up cancelling out and so even if you don't like the prior that we are using in this study it doesn't matter you would still end up with the same conclusion that life is at least nine times more likely than no life now before we jump up and down in excitement I want a caution that we are really just considering re running the tape on earth here if truly earth-like planets are rare by which I really mean everything the chemistry atmospheric properties climate and orbital stability then it doesn't really matter that FL is high because Earth twins are so rare that it's essentially still an empty universe but it is surely encouraging that we have arrived at this resort now finally intelligence now I've been dancing around this issue so far but what does this sophisticated base analysis have to say about earth I the probability of intelligent life evolving to get at this we have to do something called marginalization where we essentially integrate over all of the possible values of FL dissolving in away from the equation when we do this we find that the rare intelligent scenario versus the common intelligent scenario are pretty close to one another and using the objective Bayesian prior we find three to two betting odds in favor of rare intelligence because we marginalized the prior sadly doesn't cancel out for the case of intelligence but even so if we switch out to a different prior say a uniform prior that number barely changes so this looks pretty robust odds are then if we were forced to choose between these two scenarios the safest bet would be on the rare intelligent scenario what's really driving this result is the fact that it took intelligence four billion years to evolve if it were an easy process it could have happened far sooner than this but it seemed to depend upon a long series of evolutionary steps to get to this point the fact that we appear so close to the end of Earth's habitable window gently nudges us towards the hypothesis that we are not a typical outcome so there you have it my bet is that life is common but intelligent life may be rare not informed by good instincts religious beliefs or personal feelings they're just a cold hard insights of rigorous Bayesian inference and we haven't talked about Fermi's paradox in this video today but it's worth noting that this bet provides a natural explanation as to why we don't see anyone else out there but make no mistake this is only a probabilistic statement I can't offer you any guarantees or certainties here and once the abundant life bet looks comfortable at 9 to 1 the rare intelligence probability is just 3 to 2 definitely not the kind of odds you'd wanna bet your house on and yet we began our journey with even odds across the board every possibility seemed just as likely as any other but now by looking at Earth's chronology and a bit of Bayesian logic we've been able to shift the playing field and this is why I personally consider this to be the most profound paper I've written today if we choose to do so one day either us or our descendants will find the answer to these questions through a lot of hard work research funding and societal support it will change the way we think about who and what we are forever we have here some hints as to what lies ahead but answers answers await us if we choose to seek them so until the next video stay thoughtful and stay curious life is a comparative rarity you can survey dozens of worlds and find that an only one of them does life arise and evolve and persist but we continue to search for inhabitants we can't help it life looks for life [Music]

Channel: Cool Worlds

Views: 416,094

Rating: 4.8654742 out of 5

Keywords: are we alone, could we be alone, probability that we are alone, odds that we are alone, probability of alien life, chance of aliens, chance of alien life, is there life, are there aliens, astrobiology, life universe, statistics alien life, bayesian statistics aliens, bayesian statistics life

Id: iLbbpRYRW5Y

Length: 28min 13sec (1693 seconds)

Published: Mon May 18 2020

Reddit Comments

[u/undadatunda]

something i always found interesting is that at one point in time way back...the entire universe was room temperature

[u/SilentNightSnow]

The fact that this provides an explanation for the Fermi paradox is a lot. The Fermi paradox is the only way I can think of to cross reference the age of the Earth data. It all makes perfect sense.

And the implication is actually awesome. If no-one else is in our galaxy, the whole thing is ours. Humanity can go exploring the whole thing safely, and still find aliens. And the more we learn, the closer we can get to answering this guy's question.

Also puts a ton of pressure on us not to fuck up and kill ourselves. Possible we aren't super expendable.

[u/OneMustAdjust]

There doesn't seem to be any one aro-ound

[u/WorstRengarKR]

I love their channel, incredible production quality and the director is incredibly eloquent. Watch the video “journey to the end of time” on their channel, not as detailed as issacs content, but goddamn it plays with your heartstrings.

[u/synocrat]

Hello Science Daddy.... sploosh.

[u/goat-worshiper]

Thought this said Cool 3D World for a second. Very very different channel.

[u/tamerlanol]

Great channel, although it gets dark when they talk about the death of the Earth or deep space travel

[u/mrmonkeybat]

Another explanation for the early start of life on Earth is panspermia. Life could have started somewhere else in the solar system before raining down on Earth with all those planetary collisions. Bacteria can survive a long time trapped in rocks so it seems quite likely to me. That means when we check for life on Mars and Europa etc we have to remember to check whether it shares a common ancestor with Earthlife by checking for DNA. If it does then such a headline find says little about how common life is in the galaxy. Unless there is interstellar panspermia. If panspermic algae have spread across the galaxy with an R nought over 1 en that would make colonizing Earthlike worlds easier. Intergalactic panspermia seems unlikely though, baring some serious warp drives.