Into the Unknown, page 10
These spontaneous “brains” are referred to as “Boltzmann brains,” after the late physicist who helped to invent thermodynamics.2 Boltzmann brains could even believe themselves to be self-aware and think that they have memories—simply because the arrangement of particles would mimic that perception. Many a physicist has argued that they are certain they are not a Boltzmann brain, but the evidence for this claim typically involves stating that the idea of Boltzmann brains is ridiculous. It would be good to have slightly more logic to back this up than our own misgivings. The upshot is that in a very large (possibly infinite) universe with enough time on its hands, radically different forms of intelligence could arise via pathways far different from the chemical reactions of complex molecules that we hypothesize happened on Earth—whether in structures like Boltzmann brains, or something entirely different.
What of Life Elsewhere?
Boltzmann brains aside, for all these reasons discussed earlier, folks interested in possible life elsewhere in the galaxy (and universe in general) tend to focus their efforts on planetary systems around stars not terribly dissimilar from our Sun. Is that narrow-minded? Probably. Can we envision scenarios in which life might emerge around other kinds of stars? Absolutely. But when we have limited resources and an existing data sample of one planet known to have life, we have to start somewhere, and it makes sense to start where we think success is most likely.
There is one other catch that knocks out half of the candidate stellar systems: most stars are in what are called “binary” systems—meaning they have a partner star and the two stars orbit a common center of mass. This may sound innocuous, and it may be fun for the stars to have a friend to spend billions of years with, but it is not good news for any planets around. In such a binary system, it is virtually impossible for planets to have nice stable orbits. If planets don’t have nice stable orbits, the physical conditions on the planet will change radically on short timescales. Let’s say some microbial life emerged in a lovely warm tide pool and was adapting well to that environment. If the planet abruptly goes from warm and humid to scorching hot and dry, or frigid and frozen solid, that life doesn’t have much hope of adapting fast enough.
We’ve already seen that just in our solar system there are a couple good places to look for life, so you might be wondering how many more potentially habitable places there might be in the whole galaxy, or even in the entire universe. We have an actual equation for this, which is pretty famous as far as equations go, and named after the late scientist Frank Drake, who was one of the pioneers in the search for extraterrestrial life. The Drake equation works by multiplying likelihoods, and despite its length, it is straightforward.
Be warned: just because we have an equation doesn’t mean we have the “correct” answer to the equation. My students often get frustrated with this equation because it doesn’t produce a nice tidy confident result. However, the power of this equation is its ability to lay out the variables at play and help us see the impact of different assumptions we might make about them.
In the particular incarnation of the equation used here, we are really asking whether we are likely to hear from any advanced ET life in our galaxy. But we can think of this equation like a Swiss Army knife; we can also dial the equation back a bit and ask how much life—of any form—is likely to exist regardless of its intelligence and ability to have interstellar communication. All we have to do is drop the last three terms in the equation. Or we can ask about “intelligent” life, regardless of whether it is communicating, in which case we drop the last two terms. Or we can zoom out from our galaxy and ask what these estimates are for the observable universe, in which case we get to multiply by the number of galaxies we can see (which is a really big number, so this is fun to do). The point is, there is a lot of flexibility built in, depending on exactly how we formulate the question we want to ask.
In the way this equation is usually presented (as it is here), our confidence in the estimates for the variables gets increasingly lower as we go from left to right. Some of the variables we have a decent handle on (at least by astronomical standards). By the time we get to the end of the equation, the best we can do is speculate. To have any real confidence in the values we adopt for the last couple variables in this equation, I would argue we would need a solid grounding in astropsychology and astrosociology, neither of which exist yet (for the record, I am also excited for astrolinguistics to emerge as a field).
Let’s go through these variables one at a time, and I will give you my best range of estimates (and guesses) for these values, but I want you to feel empowered to substitute in your own estimates/guesses as well.
N—Total number of currently broadcasting civilizations. This is the holy grail of the equation. In the equation’s formulation in this case, N is our estimate for how many technologically advanced civilizations in the Milky Way are currently sending out signals (in other words, with whom we have some hope of communication).
R*—The rate at which Sun-like stars form in the Milky Way. If we assume that life can only form on planets that orbit stars, we need to know how many stars there are. There are a couple caveats, though. First, you will note that this number is a rate, which means how many stars are formed per unit of time. We need a rate because we need to consider time in this calculation—stars don’t live forever, and (probably?) neither do civilizations.
If we only care about Sun-like stars, on average, roughly three of these stars are formed each year in the Milky Way. There is a final caveat, though—at least half of the stars formed are in binary systems, and for reasons we discussed in the previous section, we think those stars are probably not a great stable home for life to grow up in. Taking these estimates, R* is in the range of one to two stars/year.
fp—Fraction of these suitable stars that have planets of any kind. Assuming that life needs a planet to form on (which might not be true if we are thinking out of the box), we need an estimate of what fraction of the stars (estimated for R*) have planets. When the Drake equation was originally rolled out, we had no data on what this value might be. In the intervening years we have had major technological advancements enabling us to go out and determine what an increasingly large number of other stellar systems are like. The bottom line is that fp seems to be 100 percent, or at least close enough to 100 percent to not matter for our purposes here. That makes this variable easy to deal with, because we just put in fp = 1.
ne—Fraction of these planets that have a habitable environment. As promised, moving toward the right in this equation, things start to get a tiny bit more uncertain, but they are not totally un-estimable yet. If our solar system is typical, this value is for sure at least one (since we are here), and may be even as high as three to four if we consider other planets and moons that have (or have had) water. However, when I consider the likely values that ne might have, I find I need to leave room for stellar systems that just really are not set up for life. If I’m feeling especially conservative, I think this value could potentially be as low as perhaps one-tenth of possible places suitable for life to emerge per planetary system, leaving us with a range of ne = 0.1 to 4.
fl—Fraction of habitable planets on which any kind of life emerges. Of all these planets that might be suitable for life to emerge, on what fraction does life actually emerge, even if only briefly? We are now getting out of our nice scientifically testable comfort zone. As of today, we know of one planet in a single stellar system on which life has emerged, and you are living on it. Having a single data point makes it dicey to extrapolate, and we are cornered into having to guess whether we are particularly special. For the moment, let’s focus on what we know about life on Earth: as a reminder, we find some type of life everywhere we find liquid water, and life emerged on Earth as soon as it possibly could have. Taken together, these two facts might suggest that life is likely to emerge relatively quickly (on astronomical timescales) on any of these habitable planets that have liquid water.
To be clear, there are a lot of “ifs” involved. And we should be wary of extrapolating too much from a sample of one, which you do not need a degree in statistics to appreciate. On one hand, the emergence of life on Earth could be entirely unique, but pretty much every time in history we’ve thought that we have some privileged place in the universe, we have turned out to be wrong. Because of this, most scientists are inclined to assume, until evidence suggests otherwise, that we are “typical.” In the absence of evidence, a default philosophical position that we are prone to adopting in science is the “principle of mediocrity,” which says there is not likely to be anything particularly special or unique about us or our universe. If you go out to a clover field and pick a clover at random, it is unlikely to have four leaves. That being said, four-leaf clovers do exist.
If the principle of mediocrity holds, then fl should more or less equal 1. This may seem odd, but fl could also be greater than 1—if life emerged independently multiple times,3 which is like buying more lottery tickets. However, if Earth is, in fact, a special four-leaf clover, then the question is how special? I’m not comfortable with values more pessimistic than about 1/100, but if you want to pick 1/1,000, or 1/1,000,000, go right ahead because I can’t refute you with any hard data.
fi—Fraction of life that evolves to become intelligent. Continuing into variables that we have no empirical way to constrain at the moment, what fraction of this life that emerges becomes “intelligent”? And how do we even define “intelligence” anyway? I mean, sometimes we humans are phenomenally stupid. In terms of evolution, this issue revolves around the extent to which intelligence has survival value. There are examples that both support this (like humans, maybe) and refute this (like cockroaches).
For the practical purposes of what we want this version of the Drake equation to do, the definition of “intelligence” in this context is the ability to create technology that could potentially broadcast signals across interstellar space. To be clear, defining intelligence in this way gives me a bit of heartburn because I think it is far too narrow of a definition, but in this version of the equation we need to know whether a species could communicate if they were so inclined, otherwise we are unlikely to hear from them.
Even determining what fi is on Earth is a little tricky. For sure fi is at least 1 on Earth, because here we are with our big radio antennas. But it isn’t like the evolution of species on Earth has stopped—if we wait around long enough, could chimps or dolphins make this leap in intelligence, too? On the other hand, had evolution taken a very slightly different route, we might still be closer to chimps. The principle of mediocrity suggests we adopt fi=1, but who knows? 1/100? 1/1,000? It really depends on how pessimistic you’re feeling today.
fc—Fraction of intelligent life that broadcasts technological signals. If you thought estimating fi was bad, hold on to your hat. Of the life that becomes “intelligent” (fi), what fraction goes on to emit signals into space? At least fi had to do with biology; fc brings us into the realm of alien psychology, sociology, and politics. In our anthropocentric view, it is very tempting to just assume that of course aliens would be curious, and of course they would want to know if anyone else was in the universe, and of course the alien science foundation would fund projects to do this. We may not quite be ready to make assumptions about what values and priorities ET civilizations might have. Even among cultures on Earth, the adoption and use of technology is deeply heterogeneous. Our good friend the principle of mediocrity suggests fc is 1 over the course of a civilization’s existence. Other than that, you can really pick any value you think is reasonable. Maybe 1/100 feeling appropriately pessimistic?
L—Length of time civilizations emit signals to space. We’ve made it to L, which I think is the most painful of these variables to consider. In a nutshell, after a civilization develops and uses technology that sends signals into the universe, how long will they be around and do so? This number really matters if we have any interest in communicating with any ET life while they are still alive.
Shall we take another quick look in the mirror? We have only had the technological ability to transmit signals to space for about 100 years. Let that sink in for a moment—over the long arc of human history, with our brains being “intelligent” for tens of thousands of years, 100 years is pretty much nothing. Yet during that 100 years we have already developed the ability to annihilate ourselves (and one could argue have come precariously close), which calls into question our working definition of “intelligence” entirely. We are currently in our “technological adolescence,” meaning we are “advanced” enough to kill ourselves off, but are we “intelligent” enough not to? Think of it as a civilization not yet having a fully developed prefrontal cortex to help us make good decisions. So, L could be as low as, say, 100 years, but I’m going to go out on a limb and express some cautious optimism that we will make some colossal mistakes, but we will learn our lessons and rebound, which is part of growing up (at least this is the parenting philosophy I’ve tried to take with my own teenagers). Given the timescale over which humans have been “intelligent,” I think 10,000 years is not a totally crazy number to adopt for L.
If “intelligent” civilizations can make it out of their adolescence, then the options open up dramatically. If a stellar system is like ours, their planet might be due for a catastrophic asteroid impact roughly every 10 million years, on average. But I tend to think that if a civilization were even a couple generations more advanced than we are, deflecting an asteroid might not be a big deal. The next key time frame is the evolution of the host star itself—once their star reaches the end of its life, the life on that planet needs to move on, develop some great new technology to harness other forms of energy, or die. This timescale is perhaps a few billion years or so, depending on the host star. If the civilization survives the death of the host star, we are in a whole new territory.
I feel impelled to confess that I am taking several shortcuts here, largely because I’m assuming that someone who picks up this book is not necessarily interested in all the painstaking (and sometimes deeply annoying) details involved. But as they say, the devil likes to take up residence in the details. Modern exoplanet scientists and astrobiologists have modified ways of thinking about the Drake equation, which you may come across upon deeper reading. For example, a new fashion is to refer to a variable called η ⨁ (pronounced “eta Earth”), which combines a few of the separate variables in the Drake equation and tells us the mean number of rocky planets that are roughly the size of the Earth and in a potential habitable zone of the host star. There is a whole army of researchers working to determine the value of η ⨁ at this very moment, so stay tuned for revisions.
Now that you have a sense of these seven variables and the values we might assign to them, we can put them all together to estimate the number of technologically advanced civilizations sending signals into space at this very moment. Heck, go ahead and put your preferred numbers into a calculator and see what you get. Depending on how optimistic or pessimistic I’m feeling with assumptions about these variables, I find that the chances of there currently being another advanced broadcasting civilization in the Milky Way are in the range of roughly 1 in 10,000 (so unlikely, but not impossible) to there being a billion or more in our galaxy (virtually unavoidable, and they should be everywhere). However, the only scenarios in which I find it is unlikely that there is another broadcasting civilization in our galaxy right now are the ones in which I assumed the civilization would survive less than one thousand years after becoming technologically advanced. Sleep well, dear reader.
Of course, we might be interested in any form of life, and not just limit ourselves to whether that life ever becomes intelligent and starts to communicate. In this case fi and fc don’t matter. What do we do about L, though? On one hand, non-“intelligent” life isn’t likely to destroy all life on the planet with nuclear warfare. However, it seems to me that nonintelligent life would probably have a hard time deflecting a catastrophic asteroid impact, and is probably even less likely to survive the death of the host star, which would bring us up to timescales of a few billion years or so. I’m also assuming that if primitive life starts to emerge, it will find a way to flourish. It could well be that if microscopic life emerges, it will get snuffed out right away by who knows what. But I also think that if the conditions are good for life to emerge, then it will just go on and keep emerging over and over until some form of life gets a foothold in the right place at the right time. For all we know, this is what might have happened on Earth.
If we don’t care about whether life becomes “intelligent” or decides to try to communicate, the numbers look a lot more promising, ranging from there “only” being 1,000 (or so) other planets with life in the galaxy to a billion (or so). The primary feasible ways these numbers could be significantly off is if we are either radically misestimating the fraction of habitable planets on which life appears or lots of the life that emerges just doesn’t survive for very long, which could be for a whole host of reasons, but probably not nuclear war, because they are not “intelligent.”
We do have one more option to consider with this equation. Up until now, we have only been considering the possibility of life in our own galaxy. There is a good reason for this: if we are interested in whether we have any chance of communicating with them, even if it takes generations between messages, this hope all but disappears if we start looking outside the Milky Way (barring, of course, some super-advanced technology that might allow cool things like wormholes that could act like subways connecting regions of the universe). But the universe is a lot bigger than our galaxy. If we replace “galaxy” with “observable universe” we get to add a new variable to the Drake equation—the number of galaxies in the visible universe. This is a really big number—around 2 trillion, which is 2,000,000,000,000 or 2 × 1012. By these estimates, the chances of there being some kind of life somewhere in the universe are, well, astronomically high.
