TR: The Drake Equation for the Multiverse

[bystander]

The Drake Equation for the Multiverse
Technology Review: The Physics arXiv Blog - 2010 Feb 10

The famous Drake equation estimates the number of intelligent civilizations in the Milky Way. A new approach asks how many might exist in the entire multiverse.



In 1960, the astronomer Frank Drake devised an equation for estimating the number of intelligent civilizations in our galaxy. He did it by breaking down the problem into a hierarchy of various factors.

He suggested that the total number of intelligent civilizations in the Milky Way depends first on the rate of star formation. He culled this number by estimating the fraction of these stars with rocky planets, the fraction of those planets that can and do support life and the fraction of these that go on to support intelligent life capable of communicating with us. The result is this equation:



which is explained in more detail in this Wikipedia entry.

Today, Marcelo Gleiser at Dartmouth College in New Hampshire points out that cosmology has moved on since the 1960s. One of the most provocative new ideas is that the universe we see is one of many, possibly one of an infinite number. One line of thinking is that the laws of physics may be very different in these universes and that carbon-based life could only have arisen in those where conditions were fine-tuned in a particular way. This is the anthropic principle.

Consequently, says Gleiser, the Drake Equation needs updating to take the multiverse and the extra factors it introduces into account.

He begins by considering the total set of universes in the multiverse and defines the subset in which the parameters and fundamental constants are compatible with the anthropic principle. This is the subset {c-cosmo}.

He then considers the subset of these universes in which astrophysical conditions are ripe for star and galaxy formation {c-astro}. Next he looks at the subset of these in which planets form that are capable of harbouring life {c-life}. And finally he defines the subset of these in which complex life actually arises {c-complex life}.

Then the conditions for complex life to emerge in a particular universe in the multiverse must satisfy the statement at the top of this post (where the composition symbol denotes 'together with').

But there's a problem: this is not an equation. To form a true Drake-like argument, Gleiser would need to assign probabilities to each of these sets allowing him to write an equation in which the assigned probabilities multiplied together, on one side of the equation, equal the fraction of universes where complex life emerges on the other side.

Here he comes up against one of the great problems of modern cosmology--that without evidence to back up their veracity, many ideas in modern cosmology are little more than philosophy. So assigning a probability to the fraction of universes in the multiverse in which the fundamental constants and laws satisfy the anthropic principle is not just hard, but almost impossible to formulate at all.

Drake Equation For the Multiverse: From String Landscape to Complex Life - Marcelo Gleiser

• arXiv.org > hep-th > arXiv:1002.1651 > 08 Feb 2010

[RJN]

Very interesting!

[bystander]

Probability of ET Life Arbitrarily Small, Say Astrobiologists
Technology Review | The Physics arXiv Blog | kfc | 2011 July 25

Astronomers have always thought that because life emerged quickly on Earth, it must be likely to occur elsewhere. That thinking now turns out to be wrong.

The Drake equation is one of those rare mathematical beasts that has leaked into the public consciousness. It estimates the number of extraterrestrial civilisations that we might be able to detect today or in the near future.

The equation was devised by Frank Drake at the University of California, Santa Cruz in 1960. He attempted to quantify the number by asking what fraction of stars have planets, what fraction of these might be habitable, then the fraction of these on which life actually evolves and the fraction of these on which life becomes intelligent and so on.

Many of these numbers are little more than wild guesses. For example, the number of ET civilisations we can detect now is hugely sensitive to the fraction that destroy themselves with their own technology, through nuclear war for example. Obviously we have no way of knowing this figure.

Nevertheless, many scientists have attempted to come up with a figure with estimates ranging from a handful of ET civilisations to tens of thousands of them.

Of the many uncertainties in the Drake equation, one term is traditionally thought of as relatively reliable. That is the probability of life emerging on a planet in a habitable zone. On Earth, life arose about 3.8 billion years ago, just a few million years after the planet had cooled sufficiently to allow it.

Astrobiologists naturally argue that because life arose so quickly here, it must be pretty likely to emerge in other places where conditions allow.

Today, David Spiegel at Princeton University and Edwin Turner at the University of Tokyo say this thinking is wrong. They've used an entirely different kind of thinking, called Bayesian reasoning, to show that the emergence of life on Earth is consistent with life being arbitrarily rare in the universe.

At first sight, that seems rather counterintuitive. But if Bayesian reasoning tells us anything, it's that we can easily fool ourselves into thinking things are far more likely than they really are.

Spiegel and Turner point out that our thinking about the origin of life is heavily biased by the fact that we're here to observe it. They point out that it's taken about 3.5 billion years for intelligent life to evolve on Earth.

So the only way that enough time could have elapsed for us to have evolved is if life emerged very quickly. And that's a bias that is entirely independent of the actual probability of life emerging on a habitable planet.

"In other words, if evolution requires 3.5 Gyr for life to evolve from the simplest forms to sentient, questioning beings, then we had to find ourselves on a planet on which life arose relatively early, regardless of the value of [the probability of life developing in a unit time]," say Spiegel and Turner. #

When you strip out that bias, it turns out that the actual probability of life emerging is consistent with life being arbitrarily rare. In other words, the fact that life emerged at least once on Earth is entirely consistent with it only having happened here.

So we could be alone, after all.

That's a sobering argument. It's easy to be fooled by the evidence of our own existence. What Speigel and Turner have shown is the true mathematical value of this evidence.

Of course, that doesn't mean that we are alone; only that the evidence can't tell us otherwise.

And if the evidence changes then so to will the probabilities that we can infer from it.

There are two ways of finding new evidence. The first is to look for signs of life on other planets, perhaps using biogenic markers in their atmospheres. The capability to do begin this work on planets around other stars should be with us in the next few years.

The second is closer to home. If we find evidence that life emerged independently more than once on Earth, then this would be a good reason to change the figures.

Either way, this debate is set to become a major issue in science in the next few years. That's something to look forward to.

Are We Alone In the Universe? New Analysis Says Maybe
Space.com | Natalie Wolchover, Life's Little Mysteries | 2011 July 26

Still from the 2005 film '[b]Alien Planet[/b]' [i](Credit: Discovery Channel/Evergreen Films)[/i]

---

Scientists engaged in the search for extraterrestrial intelligence (SETI) work under the assumption that there is, in fact, intelligent life out there to be found. A new analysis may crush their optimism.

To calculate the likelihood that they'll make radio contact with extraterrestrials, SETI scientists use what's known as the Drake Equation. Formulated in the 1960s by Frank Drake of the SETI Institute in California, it approximates the number of radio-transmitting civilizations in our galaxy at any one time by multiplying a string of factors: the number of stars, the fraction that have planets, the fraction of those that are habitable, the probability of life arising on such planets, its likelihood of becoming intelligent and so on.

The values of almost all these factors are highly speculative. Nonetheless, Drake and others have plugged in their best guesses, and estimate that there are about 10,000 tech-savvy civilizations in the galaxy currently sending signals our way — a number that has led some scientists to predict that we'll detect alien signals within two decades.

Their optimism relies on one factor in particular: In the equation, the probability of life arising on suitably habitable planets (ones with water, rocky surfaces and atmospheres) is almost always taken to be 100 percent. As the reasoning goes, the same fundamental laws apply to the entire universe, and because those laws engendered the genesis of life on Earth — and relatively early in its history at that — they must readily spawn life elsewhere, too. As the Russian astrobiologist Andrei Finkelstein put it at a recent SETI press conference, "the genesis of life is as inevitable as the formation of atoms."

But in a new paper published on arXiv.org, astrophysicist David Spiegel at Princeton University and physicist Edwin Turner at the University of Tokyo argue that this thinking is dead wrong. Using a statistical method called Bayesian reasoning, they argue that the life here on Earth could be common, or it could be extremely rare — there's no reason to prefer one conclusion over the other. With their new analysis, Spiegel and Turner say they have erased the one Drake factor scientists felt confident about and replaced it with a question mark.

While it's true that life arose quickly on Earth (within the planet's first few hundred million years), the researchers point out that if it hadn't done so, there wouldn't have been enough time for intelligent life — humans — to have evolved. So, in effect, we're biased. It took at least 3.5 billion years for intelligent life to evolve on Earth, and the only reason we're able to contemplate the likelihood of life today is that its evolution happened to get started early. This requisite good luck is entirely independent of the actual probability of life emerging on a habitable planet.

"Although life began on this planet fairly soon after the Earth became habitable, this fact is consistent with … life being arbitrarily rare in the Universe," the authors state. In the paper, they prove this statement mathematically.

Their result doesn't mean we're alone — only that there's no reason to think otherwise. "[A] Bayesian enthusiast of extraterrestrial life should be significantly encouraged by the rapid appearance of life on the early Earth but cannot be highly confident on that basis," the authors conclude. Our own existence implies very little about how many other times life has arisen.

Two data points rather than just one would make all the difference, the researchers say. If life is found to have arisen independently on Mars, then scientists would be in a much better position to assert that, under the right conditions, the genesis of life is inevitable.

Astrophysicists apply new logic to downplay the probability of extraterrestrial life
PhysOrg | Bob Yirka | 2011 July 27

David Spiegel of Princeton University and Edwin Turner from the University of Tokyo have published a paper on arXiv that turns the Drake equation on its head. Instead of assuming that life would naturally evolve if conditions were similar to that found here on Earth, the two use Bayesian reasoning to show that just because we evolved in such conditions, doesn’t mean that the same occurrence would necessarily happen elsewhere; using evidence of our own existence doesn’t show anything they argue, other than that we are here.

The Drake equation, developed in 1960 by Frank Drake uses probability and statistics to derive the possibility of life existing elsewhere in the universe. The data for it comes from observations of the known universe, i.e. the number of stars and solar systems that can be seen, the number that are thought likely to have conditions similar to our own, etc. It’s this equation and its results that drive much of the belief that there surely must be life out there; hopefully, intelligent life.

The problem with all this though, is that so much of it is based on assumptions that have no real basis in reality. As Spiegel and Turner point out, basing our expectations of life existing on other planets, for no better reason that it exists here, is really only proof that were are more than capable of deceiving ourselves into thinking that things are much more likely than they really are.

The two argue that just because intelligent life occurred rather quickly here on Earth, once conditions were ripe, giving rise to the people we are today, that doesn’t mean it naturally would on another planet just like ours in another place in the universe. There are other factors after all, that could have contributed to us being here that we don’t yet understand. So, they contend, deriving numbers from an equation such as that put forth by Drake, only serves to bump up our belief in the existence of other alien life forms, not the actual chances of it being so.

When taken at face value, some might conclude that such arguments hold no more logic than arguments for the existence of God, i.e. it’s more about faith, than science.

At any rate, most would agree that the only concrete way to prove whether there is life out there or not is to prove it, by finding it.

Life might be rare despite its early emergence on Earth: a Bayesian analysis of the probability of abiogenesis - DS Spiegel, EL Turner

• arXiv.org > astro-ph > arXiv:1107.3835 > 19 Jul 2011

• http://asterisk.apod.com/viewtopic.php?f=23&t=19193
  http://asterisk.apod.com/viewtopic.php?f=8&t=20453
  http://asterisk.apod.com/viewtopic.php?f=8&t=22512

[Beyond]

It took at least 3.5 billion years for intelligent life to evolve on Earth....

Really?? Where's it hiding?? I've been on this planet for over a half century and have yet to discover any "intelligent" life. Although i have noticed some 'smart' people, some of which were named *aleck*, or the like.

[Ann]

I read the book "Rare Earth" once. It argued that advanced life (like, for example, earthworms) is rare in the cosmos, whereas simple lifeforms like bacteria may be common. However, the authors cautioned that their reasoning was based on an example of one, the example of the Earth, and therefore their reasoning was very uncertain. For example, it took life on Earth about three billion years to progress from "simple" to "advanced". That doesn't necessarily mean that life elsewhere may need such a long time to progress to the "earthworm" stage, and then advanced life may not be so uncommon after all.

I have often thought that the most optimistic SETI proponents are so much less humble in their reasonings. For example, I'm sure I've seen SETI enthusiasts argue that there is either a 50% or a 100% chance that life will emerge on a planet that is orbiting at a suitable distance from a G-type star. Then there is either a 50% or a 100% chance that this life will become intelligent. Then there is either a 50% or a 100% chance that this intelligent life will create a technological civilization. Then there is either a 50% or a 100% chance that these technological aliens will build spaceships and communicate with civilizations on other planets.

And how do the SETI optimists know all this? Well, there was a 100% chance that all of this would happen on the Earth - well, there must have been, since all of it actually happened - so then there should be at least a 50% chance of it happening wherever there is a moderately sun-like star that has planets in its habitable zone, right?

This new argument by David Spiegel and Edwin Turner reinforces the fact that an example of one is not that much to go on when it comes to making generalizations for trillions of planets.

Ann