HERE’S A FAVORITE JOKE among physicists: “Time flies like an arrow, but fruit flies like a banana.” Some attribute it to Groucho Marx (a layman). Some attribute it to the high regard that physicists have for their careers and aptitudes as opposed to those of biologists.

But maybe it’s just a case of whistling past the graveyard. Time is among the slipperiest concepts in physics. It is a dimension (number four) but not one you can visualize. It speeds up and slows down — subjectively, but also by the clock, if you go fast enough. Its exact nature has baffled philosophers for ages: “What, then, is time?” Augustine complained. “If no one ask of me, I know; if I wish to explain to him who asks, I know not.” Physicists call it t, for use in their equations (where, unlike its behavior in the real world, it is reversible), and seem unworried by its insubstantiality.

In Now: The Physics of Time, Berkeley physics professor Richard A. Muller sets out to trap this enormous will-o’-the-wisp. An experimentalist rather than a theoretician, he has seen things in the Lawrence Berkeley Laboratory that would curl your hair. He is not free to live in his mind as the theoreticians are; he has actually manipulated time by, for instance, accelerating pions to just below the speed of light.

Muller addresses the question of what gives time its flow — its real-world direction from past to future. Here he takes issue with Sir Arthur Eddington, the brilliant English physicist celebrated for his experimental proof of Einstein’s special theory of relativity. Eddington pinned the direction of the arrow of time to increasing entropy — the random disorder that persists in any particular system, including the universe. (You can’t, for instance, resurrect the egg from the omelet; what’s worse, most physicists believe that eventually every particle in the universe will drift away from every other particle, leading to a universal “heat death.”) Muller calls Eddington’s theory of time’s arrow a “jigsaw piece that has been crammed in the wrong spot,” the theory “that has most inhibited progress in understanding now.”

Be that as it may, the now does indeed move toward the future and away from the past. “Why?” asks Muller. “We can change the future,” he writes; “Why can’t we change the past? Or can we?” If you think such questions immediately answer themselves, prompting you to put his book aside, you’ll be missing a mind-bending experience. Professor Muller has a refreshing way of describing the history of our current picture of the universe. This is no small task: as he notes in his introduction, “Understanding now requires knowledge of relativity, entropy, quantum physics, antimatter, backward time travel, entanglement, the Big Bang, and dark energy.” His description of the unsettling conundrum of Schrödinger’s cat, for example, is the best I’ve come across.

Optimistically (and futilely, I think), he bolsters his discussions with equations. Most of these are well under an inch long, but they will be Greek to the mathless, in which group I include myself. He urges his readers to try various calculations — on the quirks of interstellar travel, for instance — at home, so if you don’t mind wrestling with the cube of gamma, go for it! Real mathematicians can consult the impressive-looking appendices.

He is at his most entertaining in the chapters on the quantum world — a world where, quite literally, anything goes. Among other existential horrors, experiments have demonstrated:

that a photon can go through two separate slits at once;

that measuring one particle’s spin affects, instantaneously (that is, faster than lightspeed), the spin of its “entangled” twin many miles away;

that you will never, no matter how hard you try, be able to predict the exact future location of a moving particle.

Attempts to come to terms with the astounding counterintuitiveness of quantum theory will send you “down the drain,” said Richard Feynman. Physics students are often counseled to ignore it. Einstein famously rejected the “spookiness” of quantum physics, and Muller seems to share his distaste while remaining content to compute with it. “The collapse of the wave function at infinite velocities” — as evinced by those far-distant entangled particles — “bothers me, and I take that as a hint that the formulation is wrong,” he writes.

Going public with this kind of distress is rare in the ranks of physics, and there’s something endearing about it. Endearing, too, is Muller’s insistence, in a late chapter called “Beyond Physics,” that science does not have all the answers. He rejects “physicalism,” which he defines as a “dogma that physics encompasses all reality,” arguing that “when this discipline is presumed to represent all of reality, it takes on aspects of religion.” (That should raise a few hackles in physics departments across the country.) To illustrate the limits of physicalism, he invokes the phenomenon of consciousness. “When you see blue, and I see blue, are we seeing the same color?” he asks, in line with the longstanding debate on “qualia.” “Or could it be that when you see blue, you are really seeing what I see when I see red?” Here he risks going down the drain himself, into outright solipsism.

In the same chapter, he uses the teleporting machine in Star Trek to elaborate on the classic philosophical trope of the Ship of Theseus, whose every plank is replaced over time. Is it then the same ship? And should we let Scotty beam us up?

I wouldn’t. Why? I worry that the new person appearing at the end of the beam might not be me. I accept the premise that the created human would have all of my memories, all of my characteristics, all of my foibles and loves and likes, and would be indistinguishable from me by any physical measurement. But would he be me? […] Certainly physics couldn’t tell us apart. But is there a reality beyond physics? Put into the old language of religion, how do we know that my soul would be transported along with my body?

Uh-oh. Well, we don’t know. Nor do we know that people have souls. But Muller is sure he has one, and says so:

I know I have a soul. You can’t talk me out of that. It’s that thing that goes beyond physics, that is beyond the body and past the brain and sees what things and colors look like. […] My soul is blatantly obvious to me, even though I have a hard time expressing what I mean by it.

Some might argue that if something is ineffable, it’s a waste of time trying to talk about it, but they will risk being labeled (correctly) a physicalist by Professor Muller. As for the sensation you experience when you see blue, or, indeed, the self-awareness that suggests to you that you have a soul — these are described by physicalists as having “emerged” from physical interactions in the brain. While “emergence” certainly has its own ineffable aspects, there are ongoing attempts in the labs of neuroscientists to get rid of them.

¤

Returning to Muller’s elucidation of now.

Besides Eddington’s emphasis on increasing entropy, there are two other “jigsaw pieces jammed into the wrong places” in our picture of the universe. One is the misinterpretation of 4D space-time diagrams. Such diagrams “make no reference to the fact that time flows or that a now exists,” giving physicists “a ready excuse to avoid these issues. […] The mistake of this vision is in interpreting a computational tool as a deep truth. It is fundamentally the error of physicalism: if it isn’t quantifiable, it isn’t real.”

The third misplaced piece is “the assumption, made by Einstein and others, that the past can be, must be, able to determine the future completely.” Muller, who convincingly refutes unpleasant physics arguments denying free will, is a champion of the open-endedness of our existence:

Free will is the ability to use nonphysics knowledge to […] choose among the accessible futures. It doesn’t stop the increase of entropy, but it can exercise control over accessible states, and that gives entropy direction.

Let’s hope so. Might we, or persons like us, manage, in the far future, to avoid or postpone the predicted bleak universal heat death? Muller doesn’t go this far, but he suggests that now can save us:

As space expands, so does time. […] Now is that special moment in time that has just been created in the expansion of the 4D universe, as part of the continuing 4D Big Bang. By the flow of time, we mean the continual addition of new moments, moments that give us the sense that time moves forward, in the continual creation of new nows.

Most important, he offers an experimental test to demonstrate this ongoing creation of time. In September 2015, LIGO (the Laser Interferometer Gravitational-Wave Observatory) for the first time observed ripples in space-time predicted by Einstein’s general theory of relativity. They were gravitational waves, caused by the merger of two black holes 1.3 billion years ago. Muller writes:

As two black holes collapse into each other, new time should be created locally, observable as an increasing delay between the predicted and observed signal. The one wave seen so far [by LIGO] is insufficiently precise to test this prediction, but if many events are seen, or a closer one with a stronger signal, then the presence or absence of this lag could confirm or falsify the now theory.

LIGO, after several upgrades, has just gone back to work searching for more gravitational waves. Muller may eventually get experimental proof of his comforting idea that tomorrow really is a new day, and that we will have something to say about how it turns out.

¤

Sara Lippincott, a former nonfiction editor at The New Yorker, is a freelance book editor in Los Angeles, specializing in science.