A simple question from his wife – Does physics really allow people to travel back in time? – propelled physicist Richard Muller on a quest to resolve a fundamental problem that had puzzled him throughout his 45-year career: Why does the arrow of time flow inexorably toward the future, constantly creating new "nows"?

That quest resulted in a book to be published Sept. 20, NOW: The Physics of Time (W. W. Norton), that delves into the history of philosophers' and scientists' concepts of time, uncovers a tendency physicists have to be vague about time's passage, demolishes the popular explanation for the arrow of time and proposes a totally new theory."Time has been a stumbling block to our understanding of the universe," said Muller, a UC Berkeley professor emeritus who for many years taught a popular introductory course, "Physics for Future Presidents," which he turned into a 2008 book of the same name. "Over my career, I've seen a lot of nonsense published about time, and I started thinking about it and realized I had a lot to say from having taught the subject over many decades, having thought about it, having been annoyed by it, having some really interesting ways of presenting it, and some whole new ideas that have never appeared in the literature."

In commenting on the theory and Muller's new book, astrophysicist Neil deGrasse Tyson, host of the 2014 TV miniseries “Cosmos: A Spacetime Odyssey,” wrote, "Maybe it's right. Maybe it's wrong. But along the way he's given you a master class in what time is and how and why we perceive it the way we do."

Muller’s new idea: Time is expanding because space is expanding.

"The new physics principle is that space and time are linked; when you create new space, you will create new time," Muller said.

Time kicked off by Big Bang

Ever since the Big Bang explosively set off the expansion of the universe 13.8 billion years ago, the cosmos has been growing, something physicists can measure as the Hubble expansion. They don't think of it as stars flying away from one another, however, but as stars embedded in space and space continually expanding.
Muller takes his lead from Albert Einstein, who built his theory of general relativity – the theory that explains everything from black holes to cosmic evolution – on the idea of a four-dimensional spacetime. Space is not the only thing expanding, Muller says; spacetime is expanding. And we are surfing the crest of that wave, what we call “now.”

"Every moment, the universe gets a little bigger, and there is a little more time, and it is this leading edge of time that we refer to as now," he writes. "The future does not yet exist ... it is being created. Now is at the boundary, the shock front, the new time that is coming from nothing, the leading edge of time."

Because the future doesn't yet exist, we can't travel into the future, he asserts. He argues, too, that going back in time is equally improbable, since to reverse time you would have to decrease, at least locally, the amount of space in the universe. That does happen, such as when a star explodes or a black hole evaporates. But these reduce time so infinitesimally that the effect would be hidden in the quantum uncertainty of measurement – an instance of what physicists call cosmic censorship.

"The only example I could come up with is black hole evaporation, and in that case it turns out to be censored. So I couldn't come up with any way to reverse time, and my basic conclusion is that time travel is not possible," he said.
Muller's theory explaining the flow of time led to a collaboration with Caltech theoretician Shaun Maguire and a paper posted online June 25 that explains the theory in more detail – using mathematics – and proposes a way to test it using LIGO, an experiment that detects gravitational waves created by merging black holes.
If Muller and Maguire are right, then when two black holes merge and create new space, they should also create new time, which would delay the gravitational wave signal LIGO observes from Earth.

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Video by Stephen McNally

Additional video provided by:

LIGO-SXS-R. Hurt-T. Pyle
LIGO
NASA
LIGO-T. Pyle
SXS

Music:

"Oxygen Garden" and "Out of the Skies Under the Earth" by Chris Zabriskie

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