Read more: 鈥�About time: Adventures in the fourth dimension鈥�
RELENTLESS, elusive and infuriatingly hard to define. Not only is time one of the great existential mysteries, it also holds the key to the most ambitious challenge in theoretical physics 鈥� to express the complex workings of this vast universe in a single, elegant theory, a theory of everything.
Such a theory would unite general relativity, Einstein鈥檚 theory of gravity which describes the workings of space and time, with quantum mechanics, the endlessly weird but entirely compelling theory that describes the physics of matter. And it is here that we have been struggling for nearly a century.
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General relativity and quantum mechanics offer radically different descriptions of time, and this difference is a prime source of their stubborn refusal to be unified. If we are to make progress, something鈥檚 got to give 鈥� and many suspect that it will have to be time.
Almost a century ago, Einstein showed that time is not the fundamental ingredient of reality we once thought it was. His theory of general relativity sewed space and time into a unified entity called space-time, which can stretch and wrinkle in the presence of matter or energy, producing the space-time curvature we feel as the force of gravity. But the problem with space-time is that it is frozen. Space-time as a whole cannot develop over time because it is time 鈥� no clock can sit outside the universe.
According to general relativity, what we experience as the flow of time is a kind of illusion generated by the patchwork ways in which different observers slice the unified space-time into space and time with their individual points of view.
In quantum mechanics, the situation is drastically different. Unlike general relativity, where time is contained within the system, quantum mechanics requires a clock that sits outside the system, ticking away the seconds of the universe in exactly the same way for all observers. That鈥檚 because quantum systems are described by wave functions, which encode the probabilities of the outcomes of any measurements one might choose to make, and those wave functions develop over time.
It is a fundamental rule of quantum mechanics that the probabilities remain the same as time passes. To enforce that rule, the time in which the wave function evolves must be one and the same for everything and everyone.
To make progress in their quest to unite general relativity with quantum mechanics, we need to work with a single view of time. But which one is the right one?
It depends on who you ask. Some believe that Einstein must be right, and quantum theory must be modified. , a physicist at the Centre for Theoretical Physics in Marseilles, France, has rewritten the rules of quantum mechanics so that they make no reference to time (麻豆传媒, 19 January 2008, p 26).
鈥淔or me, the solution to the problem is that at the fundamental level of nature there is no time at all,鈥� Rovelli says. In his view, quantum mechanics does not have to describe how physical systems evolve in time but only how they evolve relative to other systems, such as observers or measuring devices. 鈥淧hysics is not about 鈥榟ow does the moon move through the sky in time?鈥� but rather 鈥榟ow does the moon move in the sky with respect to the sun?鈥�,鈥� he says. 鈥淭ime is in our mind, not in the basic physical reality.鈥�
Others disagree. Physicist of the Perimeter Institute for Theoretical Physics in Waterloo, Ontario, Canada, has argued that time does exist at the most basic level of reality 鈥� but to accommodate time, . In her model, dubbed 鈥渜uantum graphity鈥�, reality鈥檚 basic ingredients are quantum events that are ordered in time, and from them space, along with gravity and Einstein鈥檚 theory, are expected to emerge at larger scales and lower energies. In this scenario, quantum theory wins the time battle, and general relativity just has to make do (麻豆传媒, 2 May 2008, p 29).
For still others, it is not enough to declare quantum mechanics or general relativity the time victor. According to , a philosopher of science at the University of Sydney in New South Wales, Australia, neither theory is ultimately right. 鈥淚t is highly likely that what we think of as time emerges from some deeper, more primitive non-temporal structure,鈥� he says.
As for the theory of everything, says Rickles, 鈥渢here is a long way to go, but it certainly seems that the concept of time will play a crucial role鈥�.
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