“Now—that enigmatic and ephemeral moment that changes its meaning every instant—has confounded priests, philosophers, and physicists, and with good reason.” So writes Richard Muller, a physicist at the University of California, Berkeley, in the opening lines of Now: The Physics of Time (W. W. Norton, 2016). But Muller thinks he might have a solution to the puzzle—one that can be experimentally tested. In the February issue of Physics Today, reviewer Martin Bojowald calls Muller’s theory “original and intriguing” and says that Now “presents important lessons in physics and beyond.”



Credit: Rahal Waladi

Physics Today recently interviewed Muller about space, time, climate change, and how to talk to future world leaders about physics.

PT: In the introduction to Now, you write that your goal is to “bring together the essential physics” that will enable us to construct “a clear picture of now.” Can you briefly summarize your now theory?

MULLER: In the general theory of relativity, space is not a conserved quantity; for example, when two black holes combine, the amount of space in the vicinity of the holes―the proper volume―is significantly increased. In the recent observation of colliding black holes by the Laser Interferometer Gravitational-Wave Observatory, we estimate that several million cubic kilometers of space was created.

The basic postulate of the now theory is that creation of space is accompanied by the creation of new time. In the now theory, the moment we call “now” is the time that was just created to match the new space created from the expansion of the universe. In the LIGO event, about a millisecond of new time should have been created locally, just barely below the observable threshold for that event. When LIGO sees a stronger event―it’s seen some weaker ones―then the theory should be falsifiable.

PT: You mention in your introduction that some physicists have concluded that the flow of time is an illusion. Why do you think that’s not the case?

MULLER: The flow of time does not exist in the usual spacetime diagram of physics. Time is mysterious; in any relativistic coordinate system, it is linked to space. And yet time is different—and I mean much more than simply a sign in the metric. Time flows. Choose any coordinate system and you can stand still in space but not in time. That different behavior breaks the otherwise glorious spacetime symmetry. Moreover, there is a special moment in time we call “now.” No such special location exists in the dimensions of space.

Einstein considered his inability to account for the flow of time and the meaning of “now” as a failure. Some modern theorists aren’t up to his standard; they think that anything they can’t explain with their current theories must be dismissed as illusionary. They address the time–space asymmetry―one flows, the other doesn’t―by denying it. They call the flow of time an illusion. But if the flow of time does not appear in their theory, that doesn’t mean that the flow should be dismissed; it means that their theory is incomplete. The legitimate goal of physics is to account for reality, not to deny it. The fact that standard physics doesn’t incorporate the flow of time is a clue, a clue to new physics.

I’ve been working with another physicist to set up an experiment that could test the now theory, not just with future LIGO discoveries but with a laboratory experiment. I’m not mentioning the name of the other physicist because that might give a clue to our approach, and we want to be first. We might be able, in the next year or two, to create enough new time in the laboratory to be able to measure it.

PT: One of your best-known books, Physics for Future Presidents: The Science Behind the Headlines (W. W. Norton, 2008), is based on a course for nonmajors you teach at Berkeley. How did you approach the task of turning an introductory class into a general-audience book?

MULLER: When teaching the course, I always imagined that one of my students would someday be US president. I would pause before entering the lecture room and ask myself, “Twenty years from now, when one of these students is the most powerful person on Earth, what will I regret not having taught? How will I consider today’s lecture a failure?” Then I would think of the day’s message and make sure it was well understood by all.

I first wrote the book as a textbook, Physics and Technology for Future Presidents, to help my students. Then I drew from the textbook to make the popular book. It is not simple material. The five sections cover terrorism and counterterrorism, energy, nukes, space, and climate change. Some issues were updated in the sequel Energy for Future Presidents. There is nothing in those books that a president can afford to ignore.

PT: When talking about global warming, you’ve described yourself as a “converted skeptic.” What persuaded you to move from skeptic to believer? Does your experience suggest strategies for talking to current climate skeptics?

MULLER: I was a skeptic because there were five major issues that the Intergovernmental Panel on Climate Change (IPCC) was not adequately addressing. My daughter Elizabeth and I formed the nonprofit Berkeley Earth to examine those, and remarkably, we were able to address all five issues. We enlisted some great team members who were experts at objective analysis of big data, including [Nobel-winning astrophysicist] Saul Perlmutter and [the late Berkeley physicist] Art Rosenfeld.

All of the issues legitimately raised by skeptics were potential biases: data selection, temperature-station siting, data adjustment, and heat island. The fifth was potential bias from the large number of adjusted parameters that were used in the global climate models, and from the instability of those enormous simulations. We came up with a solid analysis of each of the biases and were able to conclude, using our independent work, that global warming was real and caused by humans. We can go farther than the IPCC by attributing 90% of the warming of the past 260 years to humans. We’ve kept our work open and transparent.

I get along very well with skeptics, largely because I respect them. Most of their complaints against climate change are legitimate. Most headlines and most comments made by politicians―and by many scientists!―on this subject are either exaggerated, misleading, or false; that’s why there are so many skeptics. I’ve talked privately to very prominent scientists who admitted to me that they exaggerate on purpose to garner public concern and action. But I think such exaggerations are counterproductive; they lead to a mistrust in science.

PT: What is your next project?

MULLER: Nuclear waste. I am convinced that commercial spent fuel from nuclear reactors can most cheaply and safely be stored and disposed of in horizontal boreholes drilled at depths of a mile or more into shale. Such rock layers have already held volatiles such as methane for tens of millions of years. Yucca Mountain may be reopened, but it cannot compete on a level playing field with deep shale storage.