A new paper published May 18 in the journal Proceedings of the National Academy of Sciences offers an answer: Life likely spawns quickly and easily under Earth-like conditions. But intelligent life is probably rare and slow to emerge, suggesting it might not re-appear.
His paper leaves questions about other planets unanswered. He used a statistical method called Bayesian analysis to study the handful of data points available, landing on the conclusion that we're probably lucky to exist at all. Related: 7 wild theories on the origin of life. There are two main approaches to statistics, said Pauline Barmby, an astronomer at the University of Western Ontario who wasn't involved with Kipping's paper: frequentist and Bayesian. When news networks announce who just won a presidential election, meteorologists predict the weather, and public health officials estimate coronavirus infection rates from limited samples, they're usually using frequentist approaches.
In other words, they use the limited information they have to judge what the truth about the world most likely is. Bayesian analysis more closely resembles the way human beings actually think. For instance: How likely am I to make a free-throw this time, given that I've missed the last 20 times I tried? What about if I missed the last 50? The approach forces researchers to examine the assumptions involved in the questions they're asking and their confidence in those assumptions, Barmby said.
Kipping's paper took the handful of data points that have been collected on how long it took life and intelligence to emerge on Earth, as well as estimates of how long Earth will be habitable based on the sun's life cycle. He then used a Bayesian approach to figure out the odds on whether each event is a "rapid process" or a "slow and rare scenario. If life's emergence from inanimate stuff " abiogenesis " was fast, we'd expect that on an Earth rewound and rerun, life would probably happen at some point in our planet's billions of habitable years, Kipping wrote.
But if that emergence was slow, life might have been a lucky break. The same caveats apply to the emergence of intelligence. Still, that's not a lot of data, certainly not enough for frequentist analysis. We've only run one "Earth" experiment, and have no other similar planets to compare ourselves to yet. But as the journalism we do is costly, we invite readers for whom The Times of Israel has become important to help support our work by joining The Times of Israel Community.
Gendler via AP. Newsletter email address Get it By signing up, you agree to the terms. For example, the oldest large-scale stretch of ancient surface lies in the Negev Desert. Go back much further than the Quaternary, and everything has been turned over and crushed to dust. That means the question shifts to other species, which is why Gavin called the idea the Silurian hypothesis, after an old Doctor Who episode with intelligent reptiles. So could researchers find clear evidence that an ancient species built a relatively short-lived industrial civilization long before our own?
Perhaps, for example, some early mammal rose briefly to civilization building during the Paleocene epoch, about 60 million years ago. There are fossils, of course. But the fraction of life that gets fossilized is always minuscule and varies a lot depending on time and habitat. It would be easy, therefore, to miss an industrial civilization that lasted only , years—which would be times longer than our industrial civilization has made it so far.
Given that all direct evidence would be long gone after many millions of years, what kinds of evidence might then still exist? Future researchers should see this in characteristics of nitrogen showing up in sediments from our era.
Likewise our relentless hunger for the rare-Earth elements used in electronic gizmos. They might also show up in future sediments, too. For starters, we only have one planet Earth that we know of where life exists at all, and it's not like we have Suns and Earth-like planets just waiting around for us to observe them over 4. The idea that we could take a large sample of them and run a controlled experiment is simply not possible in our realistic Universe. And that's too bad, because the most straightforward way to think about probability is to do exactly this.
You take a large set of prepared samples that have all been prepared identically, you let them evolve under a controlled set of conditions, and you see what comes out.
The number of successes — whether you define success by life, intelligent life, or some other criteria — divided by the total number of attempts will give you your probability of success. Trilobites fossilized in limestone, from the Field Museum in Chicago. All extant and fossilized In mathematical terms, this is what we call a frequentist probability.
In reality, you might only have one planet around one star, but if you could know the outcomes of a very large number of systems that were made of identical planets around identical stars, you'd know what the probability was of your particular planet getting a specific outcome. Just as you know the probability of getting two six-sided dice to sum up to 7 is one-sixth, you could know the probability of life or intelligent life arising on Earth.
But in practice, we cannot take this approach for planet Earth. With only one system, we cannot perform the experiment many times and determine the frequency of the desired and undesired outcomes. That doesn't mean we're hamstrung entirely, however. There's a different approach we can take: one based on Bayesian probability. You can find the oldest deposits in panel d, which In Bayesian probability, the reasoning goes backwards instead of forwards.
What you're evaluating isn't the overall probability of outcomes, but rather the probability of one particular hypothesis being valid compared to all the possible hypotheses. It's the best tool to use when you only have one system with one outcome. This is tougher to understand, so let's give an example: planet Earth. We know that life arose on Earth relatively early on. The earliest fossils go back 3. On the other hand, complex life didn't arise until shortly before the Cambrian explosion just million years ago , and intelligent, technologically advanced life only came to be with the arrival of human beings.
We cannot responsibly call a planet 'Earth-like' or not until we understand more about what conditions lead to which outcomes. So, what are the possible hypotheses that could lead to this? Realistically, there are only four. If you were to take the frequentist's dream, you could start off with billions of planets around billions of stars that were decidedly very much like Earth, and watch what unfolded.
0コメント