TIME ain't what it used to be.

A hundred years or so ago, we thought that the seconds ticked away predictably. Tick followed tock, followed tick.

And clocks ran...well, like clockwork. Then along came Einstein and everything changed.

His theories of relativity dealt a blow to our naive ideas about time. Hitch a ride on a rocket travelling close to the speed of light, and time slows to a virtual standstill. The same happens if you park near a black hole and feel its awesome gravity. Even worse, space-time becomes so warped inside a black hole that space and time actually switch places.

Now just as we're getting to grips with time's weirdness, one daring physicist has dropped another bombshell. "There isn't just one dimension of time," says Itzhak Bars of the University of Southern California in Los Angeles. "There are two. One whole dimension has until now gone entirely unnoticed by us."

Does this mean we can look forward to extra hours and seconds" Or will time's second dimension play havoc with our notions of the past, present and future" Or is Bars, in fact, a few quarks short of a proton" One thing Bars's extra time dimension does appear to reveal is the existence of deep and unexpected connections between disparate systems, such as atoms and the expanding universe. Such connections could point the way to a "theory of everything" that unites all the physical laws of the universe into one. Even better, Bars claims his theory has true predictive power and can be tested in upcoming particle physics experiments.

Physicists are no strangers to extra dimensions. For decades, theorists attempting to unify the forces of nature have been adding extra dimensions of space to their equations. As early as the 1920s, mathematicians found that moving up to four dimensions of space, instead of the three we experience, helped in their quest to reconcile electromagnetism and gravity. Later, in the 1980s, came various superstring theories, which describe the universe in terms of tiny one-dimensional strings vibrating against a backdrop of nine space dimensions, six of which are curled up so tightly we cannot see them. A decade or so on, theorists recognised the assorted string theories as different facets of a single idea called M-theory that adds yet another dimension, taking the total to 11: 10 of space and one of time.

Meddling with space, at least, is fair game. So how come so few have dared to tinker with time" There are two good reasons why adding extra time dimensions makes theorists queasy. For a start, when you insert time into your equations it tends to come with a negative rather than a positive sign. A second time dimension only makes this problem more severe and leads to events happening with a negative probability, a concept which is meaningless, says Bars.

Worse, it gives the green light to the idea of time travel. If time is one-dimensional, like a straight line, the route linking the past, present and future is clearly defined. Adding another dimension transforms time into a two-dimensional plane, like a flat sheet of paper. On such a plane, the path between the past and future would loop back on itself, allowing you to travel back and forwards in time (see Diagram, page 39). That would permit all kinds of absurd situations, such as the famous grandfather paradox. In this scenario, you could go back and kill your grandfather before your mother was a twinkle in his eye, thereby preventing your own birth.

Two-dimensional time gives every appearance of being a non-starter. Yet when Bars found hints of an extra time dimension in M-theory in 1995, he was determined to take a closer look. When he did, Bars found that a key mathematical structure common to all 11 dimensions remained intact when he added an extra dimension. "On one condition," says Bars. "The extra dimension had to be time-like."

Of course, Bars knew all about the horrors that emerge when you start to mess around with time. Undeterred, he wondered if negative probability and time travel would disappear if movement in the new space-time was severely constrained. But what kind of constraint" Bars guessed it had to be a hitherto unsuspected symmetry of nature.

Symmetry concerns the properties of objects that stay the same even when you do something to them - a cube looks the same no matter which face you view. And symmetry applies to the laws of physics too. So if you conduct an experiment it makes no difference to the results whether your laboratory is standing still, being carried along in the train or being whirled round on a fairground ride.

This connection between physics and symmetry was first recognised by the German mathematician Emmy Noether. In a remarkable paper published in 1918, she showed that many of the fundamental conservation laws of physics are nothing more than consequences of underlying symmetries. For instance, the law of conservation of energy - that energy cannot be created or destroyed, merely transformed from one form into another - is a consequence of "time translational symmetry", the fact that if you do an experiment today or tomorrow or next month, you will get the same result, all things being equal. Both general relativity and the standard model of particle physics are based on symmetries that hold independently at every point of space and time, so-called "gauge symmetries".

Noether's powerful insight was a powerful driving force behind modern fundamental physics, which is constantly looking for the deep and simple symmetries that spawn the complex phenomena we observe in the universe. Bars wondered whether an unsuspected gauge symmetry would make sense of an extra time dimension. And, in 1995, he began looking for one.

The clue came from quantum theory. Quantum uncertainty limits how well you can ever know certain properties of a physical system. Pin down the position of an atom and you can never measure its momentum, and vice versa. Bars wondered whether this result hints of a deeper relationship between position and momentum. Could there be an underlying symmetry between the two"

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