The Accidental Universe

When asked the question, "What is time?", Einstein gave a pragmatic response: "Time," he said, "is what clocks measure and nothing more." Many physicists would agree with him. But for FQXi member Andreas Albrecht, the statement raises another question: "What is a clock?" And in trying to answer that question, he has stumbled on a shocking result: Our choice of the "clock" used to mark off time in the early universe may impinge on the fundamental laws that govern nature and reality.

Albrecht has christened this troubling paradox the "clock ambiguity." It’s much more than a strange oddity of the early universe; it casts doubt on the viability of the Holy Grail of physics—an all-encompassing theory of everything that reduces our universe to a set of basic laws.

Time is fundamental to our interpretation of the world yet we very rarely question our choice of timekeeper. All that is required is something that can serve as a clock, preferably with a regular rhythm, whether that is the steady trickle of sand in an hourglass, the gentle swing of a pendulum or the quantum vibrations of an atom. Our decision to choose one clock over another is often born of convenience and has little effect on our lives, let alone on the state of the universe; after all, the fundamental laws of nature are assumed to be the same, whether you choose to look at the clock on the wall, or the watch on your wrist. But could those choices have a much more profound effect when examined at the quantum level of the very early universe?

This is the quandary preoccupying Albrecht, at the University of California at Davis, whose research into time in the early universe revealed a major surprise: "If you give yourself a lot of freedom about choosing the clock, the time evolution of the physical world would look completely different under different choices," says Albrecht. "If that is the case, how fundamental are the laws of physics anyway?"

It was not Albrecht’s intention to shake the foundations of physics. His original dream at high school had been to become a concert violinist, but an inspiring teacher and a compelling curriculum born of the space race soon hooked him onto physics. His father —a chemist—nurtured this interest and Albrecht was soon fascinated by the way practical thinking and big adventurous ideas played off each other in physics research. An undergraduate degree at Cornell and a PhD at the University of Pennsylvania soon followed, as he found himself being drawn ever more to cosmology as the purest expression of this dynamic interplay. "Most humble efforts to understand something simple about the universe can lead to profound questions and challenges," says Albrecht.

It was a wise choice. Almost immediately, Albrecht earned a reputation as someone who was not afraid to rewrite the history of the universe. Together with his PhD supervisor, Paul Steinhardt, Albrecht proposed a new theory of cosmological inflation—dubbed "new inflation">—building on earlier work by FQXi member Alan Guth to explain why the universe looks so similar in every direction. The problem is that if you measure the temperature at two opposite ends of the cosmic microwave background—the relic radiation of the Big Bang that stretches across the sky—you’ll find that it is remarkably similar, wherever you look. The furthest you can see in each direction is the horizon, which is at a distance determined by the speed that light can travel from this point to you over the entire lifetime of the universe—about 15 billion light years. That means that radiation from one end would take double the age of the universe to reach the other end. The puzzle was: How can these completely disconnected parts of the universe be essentially identical in temperature?

New inflation (also independently derived by FQXi member Andrei Linde) describes a period of rapid exponential expansion of the universe in the aftermath of the Big Bang and solves the problem because it states that, prior to inflation, far flung reaches of the universe would have been in contact, evening out their temperature. Though it is now an accepted facet of the cosmological story, it was radical at the time and is an example of the tension between the practical and the profound in physics that drove Albrecht’s research.

"If you close your eyes and believe a very simple version of cosmic inflation theory, it seems to be a resounding success, in terms of tests against astronomical data," says Albrecht. "But dig a little deeper and you discover all kinds of puzzles that make you feel a lot still needs to be understood before we know what cosmic inflation really is." Albrecht is certainly not one to rest on his laurels, continuing to investigate inflation and also proposing alternative models (see "Faster Than Light" for more about another theory Albrecht co-developed.) But while trying to resolve these kinds of fundamental problems about the early universe, questions about time reared up.

Quantum Clocks

In the mid 1990s, Albrecht was looking into quantum mechanics, the theory that describes the behavior of subatomic particles. In particular, he was investigating a simple model to understand more clearly what happens when you measure a quantum system. Quantum theory assumes that, before you before measure it, all the possible states of a quantum particle—whether it is located here or there, for instance, or whether it is spinning in one direction or another—co-exist at once. The state is only set when the system is measured, and you cannot predict with certainty what state you’ll find it in —though you can calculate the probability that you will find it in a certain state. These probabilities are encoded in what physicists term the particle or system’s "wavefunction."

Albrecht constructed a simple model to look at how two basic quantum systems interact with each other. It seemed like a simple enough project with nothing too surprising at first, but he immediately ran into a problem when he introduced time into the mix. According to quantum mechanics, time is an external absolute entity that marches to the same beat for everything. Einstein’s classical theory of gravity, general relativity, takes a different view, however. Relativity states that time is not absolute but instead a subjective entity. Observers in different gravitational fields, or in motion relative to each other, for instance, will experience time flowing at different rates. The clash between these two representations of time is one of the key issues at the heart of reconciling the twin worlds of the classical and the quantum.

To assert the passage of time in his simple system and construct an evolving story of what happens, Albrecht realized, one must choose a clock. However, that’s easier said than done in an isolated quantum system or for that matter, in the early universe—after all, there are no handy wristwatches floating through space and no daily journey of the sun across the sky to look out for. In the absence of an external clock, Albrecht decided to use the "internal time" concept of general relativity and define time relative to his quantum components. The simplicity of his computer model meant the clock was nothing more than a list of numbers indicating the progress of time. The physics and evolution of the quantum system were driven entirely by how those states of the clock were correlated with the other evolving parts.

But now, Albrecht ran into an unexpected problem. It was entirely up to him to set up these correlations in his simple computer model. Depending on his choice of correlations, he could hypothesise different clocks and radically change the physical behaviour that the quantum system would then follow, all without changing the overall wavefunction.

Later work and an extensive collaboration with Alberto Iglesias, formerly at New York University and now a researcher at J.P. Morgan, on the kinds of quantum conditions dominant in the very early universe soon confirmed that the same applied in the infant cosmos. If you choose to measure time using one type of clock in the early universe, the history of the cosmos would pan out very differently, than if you chose another. In other words, when considered at the quantum level, different clocks led to arbitrarily different physical laws.

Targeting the Weak

For example, our current understanding of the fundamental forces includes the weak interaction, which is responsible for diverse phenomena such as the hydrogen fusion reaction that fuels stars as well as the radioactive decay of subatomic particles. It is also the fundamental force responsible for giving us exotic particles like neutrinos and the Higgs boson. However, a different choice of clock could easily alter the fundamental laws of nature to describe a universe with the same quantum wavefunction but where no weak interaction exists. In such a universe, no neutrinos can be created through phenomena such as supernovae. In their absence, the debris of heavier elements such as oxygen that are needed to create planets and ultimately, the conditions for life might never have existed.

This led Albrecht to a dramatic realization: We may think that we understand the fundamental physical laws around us—gravity, electromagnetism and the strong and weak forces—but we may have been fooled into seeing one form of reality based on the way that we have chosen to measure time. If so, perhaps we are wasting time trying to find a single unifying law to bring them together because, deep down, there are no universal fundamental laws at all.

"Questions like the clock ambiguity show we are not quite sure what a fundamental theory means," explains Albrecht. "My research suggests that the most we can say is that here are the laws of physics that are strongly preferred for us to observe, when drawn in some way from a statistical ensemble."

The implications are groundbreaking: Our universe did not arise with one single set of fundamental laws prewritten, which governed the evolution of particles, planets, galaxies and people, or even the dimensionality of space. Instead, the basis of our universe would be an underlying random structure through which the observed laws of nature emerge as probabilities.

The ideas are exciting and Albrecht’s peers have a high regard for his skills. "It is quite astonishing how much he can extract from such a general discussion of the form of physical laws, leaving their specific realization pretty much unrestricted," says Martin Bojowald at Pennsylvania State University, an expert on quantum gravity and quantum cosmology. "He combines deep and long-standing problems, such as the role of time in nature and the fact that we can make and test predictions in an overwhelmingly large universe around us, to draw conclusions about the conservation of physical laws and possibly the dimensionality of space."

Alan Kostelecky at Indiana University, a string theorist who has examined how the space-time symmetries at the heart of general relativity might break down, agrees: "The work is interesting and imaginative, and it is a serious contribution addressing the mystery of time."

Einstein’s Emergence

If the thought that the universe could quite easily contain vastly different laws leaves you reeling, don’t worry—there are signs that the laws we know and love would have more of a chance of emerging than others. One important recent milestone for Albrecht and Iglesias has been the prediction that these statistical arguments strongly prefer laws of physics that exhibit a key property of spacetime that forms the cornerstone of Einstein’s theory of relativity, known as "Lorentz symmetry," for instance.

But the proposal remains controversial and not everyone is convinced—even if they do appreciate Albrecht’s past triumphs. "Albrecht’s impressive citation index indicates the community’s appreciation of his contributions over the years to cosmology," says Julian Barbour, an FQXi member, at the University of Oxford who has long investigated the nature of time. "However, I question his radical ideas about time," Barbour adds.

Barbour has long argued that time is an illusion and that we perceive its flow as our brains process successive snapshots of events, like a movie made up of separate frames. According to him, if you were to make a movie of a swarm of bees, you can choose the separation between any pair of bees as the "time" that governs the speed at which the movie is projected. However, the objective facts represented by the successive relative configurations of the bees are unchanged, irrespective of the time variable chosen at this classical level. (See "The Non-Expanding Universe" for more on Barbour’s FQXi-funded work.) "Since the ambiguity of time variable has no objective consequences in the classical swarm and quantum theory reflects to a large degree the classical theory that gives rise to it, I cannot see that wildly discordant quantum theories will arise," says Barbour.

For his part, Albrecht is delighted by the debate. "Given how far off the beaten path these ideas are, I am impressed by how open and curious many of my colleagues are to these ideas," he says. "But others are very resistant and critical, so a pretty healthy mix, I think."