Un libro de ciencia que vale la pena leer

Time waits for no man. But when it comes to time, one man, Caltech physicist Sean Carroll, might be worth yours.

That's because the public-minded scientist is walking readers through his just-released From Eternity to Here: The Quest for the Ultimate Theory of Time. Every Tuesday, he guides folks through a chapter in an online book club.

Ain't the Internet grand, folks? This is the closest most of us will get to taking a Caltech class.

"Time is something we are all familiar with, but most of us don't think about," says Carroll. "Thinking about it a little more deeply can lead us to some grand speculation."

In the book, Carroll recounts the history of scientists thinking about the "arrow of time," the clock's curiously one-dimensional march ever onward (we have up and down, right and left, backwards and forwards, but time just zips along ever forward). In particular, he revisits some of the 19th century thinkers, such as statistical physicist Ludwig Boltzmann, overshadowed today by 20th century icons like Einstein.

Boltzmann and colleagues put entropy, energy's tendency toward disorder, on a statistical basis, offering a physics interpretation of time. Time results from entropy sending events, everything from the egg scrambled for your breakfast to stars running out of steam over billions of years, heading relentlessly one-way, never to unscramble themselves or restart their fires again.

One of the mysteries of the universe is its beginning in a highly-ordered low-entropy state, a hot, dense ball of energy some 13.7 billion years ago called the Big Bang. (Which was low-entropy in the sense that its energy was so useful for making stars, galaxies, planets, people and everything else, energy once spent that it couldn't be re-ordered like that breakfast egg that could be scrambled, poached or served sunny side up, but never put back together again.) "Why highly-ordered but not perfectly ordered," Carroll asks, pointing to results from NASA's WMAP probe and others showing temperature ripples in the aftermath several hundred thousand years after the Big Bang, the so-called Cosmic Microwave Background filling the sky.

WMAP's ripples also seem to confirm a 1981 theory by physicist Alan Guth of the Massachusetts Institute of Technology called "inflation." Inflation suggests that the Big Bang generated exotic matter with gravity-repulsing properties that stretched the early universe out and sent it on an expanding path. Inflation seems to answer the question of what set time a 'ticking, but Carroll says just stopping there doesn't answer the more fundamental question of why our universe started with a low-entropy kablam that allowed inflation and time to proceed.

"Cosmologists don't think about this enough," Carroll says. "I think in a sense, they are cheating."

Of course, some of the biggest names in modern physics, including Brief History of Time author Stephen Hawking, have wrestled with the arrow of time. Writing by one of Hawking's colleagues, the English physicist Roger Penrose, sparked Carroll's interest in the nature of time when he was a student. The physicist Victor Stenger argued a decade ago in Timeless Reality: Symmetry, Simplicity and Multiple Universes, that time may have started running after the Big Bang in two directions: one in our universe and the other in a mirror-image cosmos where time would seem to run backwards to us but not to its inhabitants.

Carroll goes further, suggesting the low-entropy state of our universe at its beginning may point to our corner of the cosmos (some 27.4 billion light years across and expanding at an accelerating rate in its observable expanse, where one light year is about 5.9 trillion miles) resulting from inflation playing out in a certain way after the Big Bang. Maybe beyond the universe's horizon, some other corner of the cosmos was cursed with a low-entropy origin.

Or corners. Rather than a universe, we might have a multiverse, filled with all sorts of cosmological possibilities, where the Big Bang isn't the beginning of time, Carroll suggests, but rather an inevitable offspring of an extremely low-entropy starter cosmos, which pops off new worlds repeatedly.

"I thought it would be fun to start with something as simple as mixing cream into your coffee cup and then moving into some really perplexing questions," Carroll says. "And it was. I really found myself enjoying writing for people who aren't scientists."

(On a side note, Caltech's Sean Carroll shouldn't be confused with the University of Wisconsin's Sean B. Carroll, who is also a terrific popular science writer, only one specializing in evolution. One argument for multiple universes might be the unlikelihood of two science professors with the same name stalking the popular science bookshelves. Although MIT physicist Max Tegmark has demonstrated mathematically that if the universe is roughly infinite in size, even stranger coincidences — such as the existence of an identical version of yourself — are inevitable.)

Physicist Carroll's explanation for time differs from proposals like the cyclic cosmos that physicists Neil Turok and Paul Steinhardt wrote about in 2007's Endless Universe: Beyond the Big Bang.They suggest our three-dimensional (or four if you count time) universe results from higher-dimensioned universes clashing together and splitting off measly ones such as ours as collateral damage. Instead Carroll starts with entropy as the ending and beginning of his cosmology. "I just thought it would be nice to show how something we take for granted, time, can lead us to some big questions," he says.

Speaking of time, the 448-page From Eternity to Here will take a little to finish. But, "Time is an illusion. Lunchtime doubly so," as the late Hitchhiker's Guide to the Galaxy author Douglas Adams once wrote. So, if you have a few free lunchtimes, Sean Carroll might straighten out for you a few of time's illusions.