Not to mention the question of which way it goes...No one keeps track of time better than Ferenc Krausz. In his lab at the Max Planck Institute of Quantum Optics in Garching, Germany, he has clocked the shortest time intervals ever observed. Krausz uses ultraviolet laser pulses to track the absurdly brief quantum leaps of electrons within atoms. The events he probes last for about 100 attoseconds, or 100 quintillionths of a second. For a little perspective, 100 attoseconds is to one second as a second is to 300 million years.
But even Krausz works far from the frontier of time. There is a temporal realm called the Planck scale, where even attoseconds drag by like eons. It marks the edge of known physics, a region where distances and intervals are so short that the very concepts of time and space start to break down. Planck time—the smallest unit of time that has any physical meaning—is 10-43 second, less than a trillionth of a trillionth of an attosecond. Beyond that? Tempus incognito. At least for now.
Efforts to understand time below the Planck scale have led to an exceedingly strange juncture in physics. The problem, in brief, is that time may not exist at the most fundamental level of physical reality. If so, then what is time? And why is it so obviously and tyrannically omnipresent in our own experience? “The meaning of time has become terribly problematic in contemporary physics,” says Simon Saunders, a philosopher of physics at the University of Oxford. “The situation is so uncomfortable that by far the best thing to do is declare oneself an agnostic.”
The trouble with time started a century ago, when Einstein’s special and general theories of relativity demolished the idea of time as a universal constant. One consequence is that the past, present, and future are not absolutes. Einstein’s theories also opened a rift in physics because the rules of general relativity (which describe gravity and the large-scale structure of the cosmos) seem incompatible with those of quantum physics (which govern the realm of the tiny). Some four decades ago, the renowned physicist John Wheeler, then at Princeton, and the late Bryce DeWitt, then at the University of North Carolina, developed an extraordinary equation that provides a possible framework for unifying relativity and quantum mechanics. But the Wheeler-DeWitt equation has always been controversial, in part because it adds yet another, even more baffling twist to our understanding of time.
“One finds that time just disappears from the Wheeler-DeWitt equation,” says Carlo Rovelli, a physicist at the University of the Mediterranean in Marseille, France. “It is an issue that many theorists have puzzled about. It may be that the best way to think about quantum reality is to give up the notion of time—that the fundamental description of the universe must be timeless.”
No one has yet succeeded in using the Wheeler-DeWitt equation to integrate quantum theory with general relativity. Nevertheless, a sizable minority of physicists, Rovelli included, believe that any successful merger of the two great masterpieces of 20th-century physics will inevitably describe a universe in which, ultimately, there is no time.
The possibility that time may not exist is known among physicists as the “problem of time.” It may be the biggest, but it is far from the only temporal conundrum. Vying for second place is this strange fact: The laws of physics don’t explain why time always points to the future. All the laws—whether Newton’s, Einstein’s, or the quirky quantum rules—would work equally well if time ran backward. As far as we can tell, though, time is a one-way process; it never reverses, even though no laws restrict it.
“It’s quite mysterious why we have such an obvious arrow of time,” says Seth Lloyd, a quantum mechanical engineer at MIT. (When I ask him what time it is, he answers, “Beats me. Are we done?”) “The usual explanation of this is that in order to specify what happens to a system, you not only have to specify the physical laws, but you have to specify some initial or final condition.”
The mother of all initial conditions, Lloyd says, was the Big Bang. Physicists believe that the universe started as a very simple, extremely compact ball of energy. Although the laws of physics themselves don’t provide for an arrow of time, the ongoing expansion of the universe does. As the universe expands, it becomes ever more complex and disorderly. The growing disorder—physicists call it an increase in entropy—is driven by the expansion of the universe, which may be the origin of what we think of as the ceaseless forward march of time.