David Trowbridge, Flickr, Creative Commons license

SFI Distinguished Fellow Murray Gell-Mann and External Professor James Hartle are featured in a Science News cover article, “Clash of the Quantum Titans,” about the current state of quantum physics and the differing, often debated perspectives of physicists about the oddities it presents.

“What quantum mechanics really means, where it ultimately comes from, why it denies the cause-and-effect certainty of traditional physics are all questions that haunt the deepest scientific thinkers — and divide them almost as badly as 21st century political parties,” the article says. “Physicists simply can’t agree on how to interpret quantum physics.”

After reviewing the history of quantum mechanics and the notion of decoherence, it describes some of Gell-Mann’s and Hartle’s work over the last two decades.

“In their approach, multiple realities in the quantum fog condense into various chains of events, each chain approximately observing the cause-and-effect rules of classical physics. In other words, people perceive the world as classical and predictable, rather than quantum and probabilistic, because they occupy a realm where predictable patterns have decohered from the coherent cloud of quantum possibilities. Each such chain of events would constitute a consistent history. More than one consistent history might emerge from the quantum cloud, similar to the many worlds of Everett’s interpretation.

“Using the math describing decoherence, physicists can calculate the probabilities of the various consistent histories, says Gell-Mann, of the Santa Fe Institute in New Mexico. He and Hartle, of the University of California, Santa Barbara, emphasize that these consistent histories arise naturally in any coarse-grained view of reality. Quantum weirdness persists in the fine-grained view of nature at the subatomic scale, but decoheres into ordinary physics in the coarser-grained realm of macroscopic objects. It’s much like the way coarse-grained (and predictable) properties of a gas, like temperature and pressure, emerge from the unpredictable and unobservable behavior of tiny molecules bouncing off each other.

“Viewed in this way, quantum physics can accommodate an entire universe with no reference to an outside observer — consistent histories decohere from within. ‘Our way of doing it … for a given initial condition of the universe, as well as a given unified theory, would give predictions for the probabilities of alternative coarse-grained decoherent histories of the universe,’ Gell-Mann says.

“Gell-Mann and Hartle’s approach is similar in some respects to Everett’s, and it incorporates Bohr’s view as well. Bohr’s analyses involved measurements by observers experimenting on systems within the universe. That approach wasn’t wrong, Gell-Mann says — just not general enough to deal with the universe as a whole.

"'It’s correct in a sense, but it can’t be general, it can’t be the deep way to look at quantum mechanics,’ Gell-Mann observed in a 2009 interview. ‘It’s a special case.… If you look at 13 billion years of the history of the universe, you can’t describe it that way until very recently.’

"Gell-Mann and Hartle’s view also offers natural explanations for the counterintuitive results of some quantum experiments. Multiple possible outcomes of such experiments are simply different results in different histories.

Read the Science News article