Friday, October 11, 2013

Big Blunders: When the Multiverse is Not the Answer

Brilliant-Blunders-coverAstrophysicist Mario Livio has a gift for clear prose and something more: the ability to turn up – or track down – the odd fact, the misunderstanding, the story behind the story everybody thinks they know, the stuff that makes science come alive.

The discussion of infamous scientific goofs in Livio’s recent book, Brilliant Blunders, covers familiar ground. They include Charles Darwin’s misguided attempt, long before Mendelian genetics became known, to rescue natural selection from the persuasive “paintpot” theory of inheritance that indicated variations would soon be swamped. Darwin proposed something called pangenesis, unpersuasive even to himself.

William Thomson, Lord Kelvin, attacked Darwin by demonstrating that Earth was cooling so fast it could not have been around long enough for species to evolve; he persisted in this opinion even after the discovery of radioactivity, source of half the planet’s heat.

Linus Pauling takes his lumps for beating Watson and Crick into print with an inside-out, triple-helix model of DNA, incorporating a fundamental error of chemistry.

Fred Hoyle’s mistake was not his steady-state theory, perfectly reasonable at the time. (Livio shows that when Hoyle uttered “big bang” in a BBC interview it wasn’t derogatory, just plain speaking.) Like Kelvin, however, Hoyle refused to relent in the face of overwhelming evidence, including discovery of the cosmic microwave background, the nail in the steady state’s coffin.

What Livio concludes from these examples isn’t original – “blunders are not only inevitable but also an essential part of progress in science” – but his storytelling is both fresh and fascinating.

His final example is the “biggest blunder” we’ve heard most about in recent years. It’s what Albert Einstein, after Edwin Hubble discovered that the universe is in fact expanding, supposedly said about the cosmological constant he’d introduced into General Relativity to keep the universe stable.

“Did Einstein actually say this?” Livio asks. His original research shows the answer is probably no. The words were first recounted after Einstein’s death in reminiscences by George Gamow, in this case a very unreliable narrator.

Einstein certainly wasn’t happy with his cosmological constant, but Livio notes with some irony that his “true blunder was to remove the cosmological constant.”

In late 1997 the Supernova Cosmology Project and the High-Z Supernova Search Team independently discovered that universal expansion is accelerating. The cause would soon be called dark energy. So far, measures of expansion from supernovas and baryon oscillation are entirely consistent with dark energy being the cosmological constant.

If the cosmological constant is real, however, its physical nature – the force with which it expands space and pushes mass apart – is unknown. It’s apparently some form of the energy of the vacuum, but no good theory exists to explain its tiny but significant value.

Enter the multiverse. “Consider a universe harboring the same laws of nature as ours,” Livio suggests, “and the same values of all the ‘constants of nature’ but one.” You get the picture. In a multitude of different universes, we just happen to live in one where the cosmological constant has precisely the value needed to produce the accelerating expansion we observe.

Many scientists find this anthropic reasoning loathsome – so do I – if only because it puts so many interesting questions forever beyond hope of an answer. Yet Livio defends it. His argument has a spectacular flaw – his own contribution to his list of blunders.

Just because other universes can’t be observed doesn’t mean they aren’t real, Livio says, then adds the obligatory caveat: they have to be “predicted by a theory that gains credibility in other ways…. If a theory makes testable and falsifiable predictions in the observable parts of the universe, we should be prepared to accept its predictions [elsewhere].”

Theories that predict or at least allow a multiverse are far from proven anywhere, but that’s not the crux of the problem. Whether the value of the cosmological constant is set arbitrarily small or arbitrarily large, there’s no persuasive physical explanation. Why does it have any value?

Until there’s a testable theory of why a cosmological constant can have a range of values, the multiverse is no explanation for dark energy. It’s not enough to say that’s just the way it is.

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