Jun 18 2020

Intelligent Life in the Galaxy

Published by under Astronomy
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The headlines (taken from the press release) read: “New light shed on intelligent life existing across the galaxy.” But here’s the thing – I don’t think the referenced study does that at all. So what are they talking about?

The study uses their own version of the Drake Equation, which is a way of calculating how many spacefaring civilizations there are likely to be in the universe. The equation itself is correct – you consider the number of stars, the subset of those with planets in the habitable zone, the number of those who develop life, then intelligent life, then technology and multiply all that by the average lifespan of such civilizations. The equation works, as far as it goes, it’s just not terribly useful. The reason is that we don’t know the values of any of the variables. We can guess some of them, those dealing with stuff we can see, like how many planets are out there, but we essentially have no idea about any of the variables dealing with life.

The reason we have no idea is basic scientific logic – because we have one data point, Earth. Remember when we encountered the first interstellar object? That one encounter left us with no practical way to calculate how common such objects were. It could have been a one-off extremely unlikely event. But as soon as we encountered a second interstellar object, we had a rough idea how common they were. We had something to calculate.

You just can’t extrapolate from one data point. We may be the only life in the entire universe, or the universe might be teeming with life – both ends of the spectrum are consistent with our one known data point. We have no idea how common life is, how common intelligent life is, or technological civilizations, or how long they survive on average. None – really. So any numbers we put in are just wild guesses, and the errors on those wild guesses multiply.

The Drake Equation, therefore, is not useful in terms of estimating what is actually out there. It is only useful as a thought experiment regarding what effect each variable has on the number of civilizations out there. And in that respect, it is quite obvious. Sure, the longer civilizations survive, the more of them there should be at any one time. At best the Drake Equation helps us grasp the immensity of space. Even if civilizations are relatively rare, there must be millions of them in the universe.

So what does the new study do?

“We investigate the possible number of CETI civilizations based on different scenarios. At one extreme is the Weak Astrobiological Copernican scenario—such that a planet forms intelligent life sometime after 5 Gyr, but not earlier. The other is the Strong Astrobiological Copernican scenario in which life must form between 4.5 and 5.5 Gyr, as on Earth. In the Strong scenario (under the strictest set of assumptions), we find there should be at least ${36}_{-32}^{+175}$ civilizations within our Galaxy: this is a lower limit, based on the assumption that the average lifetime, L, of a communicating civilization is 100 yr (since we know that our own civilization has had radio communications for this time).”

This is meaningless. They just assumed that our one data point, Earth, is typical. Intelligent life formed on Earth after 5 billion years, so let’s assume that all suitable planets form not only life but technological civilizations on the same timescale. They just assumed the answer to the big unkowns in the Drake Equation, and their assumptions don’t even make sense. Technological civilizations survive for 100 years on average, because we are at 100 years and counting? That makes no sense. I don’t think this sheds any light at all.

The only interesting part of such studies, in my opinion, are those that look at actual data – how many stars have planets, how many of those systems have rocky worlds in the habitable zone, and how many stars have properties suitable for sustaining life over billions of years? These are numbers we can potentially calculate by observing the universe. We are making some progress with finding exoplanets, but the methods we are currently using bias the data in certain ways. It is easier to find big planets closer to their stars, for example. But as we refine our methods, we are starting to get some statistical picture. It’s too early for firm conclusions, but it is fair to say there seems to be many potentially Earth-like worlds out there.

That is about the extent of the actual science. The rest is speculation, as interesting as it might be. We don’t know the limits of the kinds of worlds that can form life. We don’t know what different kinds of life are possible. We don’t know how typical vertebrate-mammalian-primate “intelligence” is, and how often a combination of traits amenable to the development of advanced technology will evolve.

It is possible that, even if life is extremely common, most worlds will be covered in moss or algae. Those with more complex life may mostly resemble invertebrates. We have no idea how common things like central nervous systems are. There are just so many different strategies for survival, the odds of one strategy including tool use is extremely low. We can, I think, look at life on Earth and see that humans are one tiny twig. Most life is not evolving in a direction that leads to technology – by tens of millions to one. We don’t even have a second example of a technological species on Earth, a planet teeming with life.

This is what we don’t know. Evolving technology seems to be rare, but there are lots of opportunities even on a single planet. How does that balance out? We have no idea. What percentage of planets with complex life develop a technological civilization? The authors of the current study assume the answer is 100%. But what if it is 10%, or 1%, or 0.0001%? All of these numbers are compatible with our one data point.

On the other hand, civilizations on average may survive for 1000 years, or a million. Or perhaps, once they get past a certain point (such as colonizing multiple star systems) perhaps they tend to persist indefinitely – for billions of years. These are orders of magnitude variations.

In any case, this is all fun to think about, but it is humbling to realize that we essentially have no idea how much life is out there, and how many civilizations there might be. This allows us to imagine many possible scenarios, but for now that is all that it is, imagination. But we should not pretend that plugging essentially random numbers into an equation “sheds light” on the actual answer. It doesn’t.



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