Monday, October 22, 2012

936. Life, the Universe, and Everything: What are the Odds?

Dan Falk
By Dan Falk, PLOS Blogs, December 31, 2011

Have you ever wondered how likely – or unlikely – it is that you exist?  Although it may sound pie-in-the-sky, it’s really a scientific problem, though you don’t have to be a scientist to be captivated by it.
Take, for example, the wonderfully-named Cosmicomics of 20th-century Italian writer Italo Calvino – a collection of whimsical, science-fiction-flavoured short stories.  One of the stories, called “How Much Shall We Bet,” involves two characters, the narrator (with the unpronounceable name “Qfwfq”) and someone named “Dean (k)yK.”  The two men seem to have existed since the before the beginning of the universe – somehow separate from the universe, whatever that could mean – and they seem to be immortal.  All they do is make an endless series of bets regarding what sorts of things will happen in their cosmos.
As you might imagine, the series of events that they bet on, and the series of events that actually unfold, are rather familiar:  They seem to resemble the actual events that have unfolded in the history of our own universe.  Their first bet is on the formation of atoms; the narrator bets for it, while Dean bets against it.  They go on betting on the formation of various chemical elements, and, looking billions of years ahead, they bet as to whether the Assyrians will invade Mesopotamia. We’re told that Dean always bets no, “not because he believed the Assyrians wouldn’t do it, but because he refused to think there would ever be Assyrians and Mesopotamia and the Earth and a human race.”
Let’s begin with the big philosophical questions:  First there’s the issue of determinism – roughly, whether the “stuff that happens” in the universe is largely, or perhaps completely, determined by what came before.  This is something that thinkers have wrestled with for 2,500 years, and I won’t attempt to add to that discussion here; but it is worth mentioning that most versions of determinism seem to place free will in jeopardy, making them rather unappetizing (though not necessarily wrong).  (But I would say that, wouldn’t, if I were destined to say it?)
Secondly, assuming that the future is not fully determined by the present, there’s the string of probabilities associated with each development along the way to “us.” Thinking again of Calvino’s story:  Before you can have Assyrians, you have to have human beings, and before you can have human beings you have to have life, and before you can have life you have to have a habitable planet orbiting a star at just the right distance… it does sound like a leaning tower of improbabilities, doesn’t it?
In my next blog post, I’ll explore what I think is the weakest link in that chain – the appearance of intelligent life.  But first, let’s have some more fun with the ideas and the numbers.
Certainly, the more specific the outcome, more improbable it seems.  If you consider some particular state of affairs, and then ask what the odds are, starting from today and going back even a short time (let alone the 3.8 billion years to when life first appeared on this planet), that particular state will seem extraordinarily unlikely.  For example, imagine turning the clock back five years.  From that perspective, what were the odds that, on this particular day, you would be sitting in this particular room, in this city, reading this particular sentence?
And what something even more basic – say, your own existence?  A couple of months ago, a probability chart produced by Harvard Law School blogger Ali Binazir went somewhat viral, encouraging people to contemplate this very question.  In the chart, Binazir calculates just how improbable it was that the right sperm from your father hooked up with the right egg produced by your mother – by his estimate, it’s about one chance in 400 quadrillion (that number seems only slightly more tame in scientific notation: 4 x 10^17).  And that’s hardly the whole battle:  To even get to that stage, all of your ancestors, going all the way back to the beginning of life on Earth, had to survive to reproductive age.  Multiplying the string of probabilities together, he concludes that the odds of your existence are an astronomical one in 10^2,685.000.  (As you can imagine, not everyone in the blogosphere was kind to Binazir; one asked if it was painful to pull those numbers out of you-know-where.)
To be sure, we can quibble about the precise figures.  But I’m sure we can agree that the chances of anything specific happening, viewed from a remote enough point in the past, seem absurdly low.  And yet, for some reason, we often weave stories in which historical events have a flavour of inevitability to them.  Think how many science fiction stories you’ve read on the theme of time travel, in which the time traveller attempts to “change history,” only to find that what was going to happen, happens anyway.  Push history, and it pushes back.
If you’re a Stephen King fan, you’ll know that his latest book, 11-22-63, involves a time traveller who attempts to prevent the Kennedy assassination (which of course took place on the date that gives the book its title).  As you might guess, even with several years lead-time, preventing the fatal shot from being fired from the Dallas book depository is no simple task.  As filmmaker Errol Morris puts it in his review of King’s book:  “What if history is too forceful to redirect?  What if jiggering the engine produces no favourable outcome – merely a postponement of the inevitable?  If he had lived, Kennedy might not have escalated the war in Vietnam, and might have kept America out of a bloody mire.  But we don’t know.  What if we were headed there anyway?  Then our tampering might only make things worse.  It is not historical inevitability, but something close.”
These kinds of questions, about the inevitability (or otherwise) of history, have made their way into our popular culture, so I’m happy to give the last word to Lisa Simpson.  I’m thinking of a Halloween episode in which Lisa had lost a tooth; as part of an experiment for a science fair project, she leaves the tooth in a glass of cola overnight.  Sure enough, the next morning she sees a peculiar mould growing on it; and looking through her microscope, she sees that she’s crated little cave men.  Some hours later she looks again, and the little people are undergoing what appears to be the Renaissance; soon, one of the little people is seen nailing something to the cathedral door. She gasps:  “I’ve created Lutherans!”
More on likelihood of life – and intelligent life in particular – next time.

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