For Bbella: a mathematician's take (Introduction)

by David Turell , Monday, July 18, 2016, 14:44 (2833 days ago) @ David Turell

A response to the previous article:-http://www.npr.org/sections/13.7/2016/07/14/485992587/were-there-aliens-before-us-"Today, I would like, once again, to present our argument and dive a little deeper into its meaning and its limits. In particular, I want to address two excellent rebuttals written by Ross Andersen in The Atlantic and Ethan Siegel in Forbes. Neither Andersen or Siegel was buying some of my contentions and they both made good points. The thing about science (take note climate deniers) is that it's really a call and response. Both Andersen and Siegel are great writers. Their skepticism made me think even harder about the ideas in our paper and that was really helpful.-[I've skipped Drake equation discussion]-"By looking at the problem this way — and using the new exo-planet data and rearranging things — our results provide an empirical constraint to a very different question than the one Drake's equation usually focuses on. Here is our question:-"What would the bio-technical probability per planet have to be for us to be the only civilization that ever occurred in the entire history of the universe?-"Putting in our exo-planet date, we found the answer is 10^22 or one in 10 billion trillion. We call this number the "pessimism line," and you can think about its meaning in a bunch of ways.-***-"Also, while its true that we can't say anything explicitly data-driven past our derivation of the pessismism line, the history of debate about the Drake equation provides ample material to think more deeply about our result. While many have argued that exo-civilizations would be rare, the sense of what rare means is rarely specified explicitly. If you scratch below the surface, rare often means orders of magnitude above our 10^22 pessimism line.-"To see this point, let's take a particularly famous example. In 1983, the physicist Brandon Carter developed an absolutely ingenious argument against exo-civilizations based on the observation that the time for intelligence to arise on Earth was close the total age of the sun. Using this one fact, he further made the case that intelligence required evolution to pass through a series of "hard steps," each of which would be highly improbable.-"Imagining there were 10 evolutionary "hard steps," he did a calculation where he found the total probability for exo-civilizations to form to be 10^20. He then claimed, this value "is more than sufficient to ensure that our stage of development is unique in the visible universe."-"But it's not! The pessimism line we derived shows that Carter's 1983 calculation still allows 100 exo-civilizations. Carter intended his calculation to be hyper-pessimistic, but it turns out to be optimistic instead. It should also be noted that researchers now believe only five hard steps exist (if they exist at all). This, along the other values in Carter's original paper, imply a probability of 10^10 which, along with our pessimism line, implies a trillion exo-civilizations across cosmic history. (It's also noteworthy that authors like Mario Livio present arguments that undermine the basis for Carter's work).-"Of course, it's still possible to construct arguments leaving the probability far below our pessimism line, ensuring we're the only exo-civilization that ever formed. But it's here that, I believe, the most important implication of our result emerges.-***-"Thus skeptics are entirely right that without any more data one must remain formally agnostic about exo-civilizations. You can't assign a probability to an unknown process. But to stop there misses a key point about our moment in science and in history. Astrobiology, the study of life in the universe, has made tremendous strides through studies of our world, the other worlds in our solar system and, famously, the newly discovered exo-planets. The study Woody Sullivan and I carried out is firmly situated in the midst of these expanding astrobiological horizons. Taken together, I believe our results mean that most pessimists (on the question we asked) are actually optimists and the remaining hyper-pessimists — well, they really have some 'splaining to do.-"Finally note that our study said nothing about the existence of civilizations now. We were dealing with a kind of exo-civilization archeology. If that all important lifetime factor L is not long, then our neighborhood the Milky Way galaxy might be entirely empty (other than us) in the current cosmic epoch."-Comment: Huge article worth reading. Previous alien civilizations are slightly possible.