TCS Daily

Dismantling Space and Time

By Kenneth Silber - March 9, 2004 12:00 AM

Space and time are pervasive in our everyday experience, and yet it is hard to say exactly what they are. They resist definition in terms other than themselves. Moreover, they have various subtle and elusive properties, with which science continues to grapple. Relativity and quantum mechanics, the physics breakthroughs of the 20th century, revolutionized scientific thinking about these subjects. And this revolution has not played itself out, since cutting-edge physics today involves further radical rethinking of time and space.

The Fabric of the Cosmos: Space, Time, and the Texture of Reality, by Brian Greene (Knopf, $28.95), is an excellent guide to the science of space and time, and to modern physics more generally. The book provides lucid expositions of arcane topics that often are either oversimplified or obfuscated in physics popularizations. Greene is adept at placing complex ideas into context, explaining how they relate to each other and distinguishing the various degrees between well-established fact and wild speculation.

Greene, a Columbia University professor, has emerged as a high-profile expositor of physics in recent years. Indeed, he is a physicist with a certain cultural cachet; among other activities, he has hosted a documentary based on his previous book The Elegant Universe, performed a cameo role in the movie Frequency, and collaborated with the Emerson String Quartet on a project combining superstring theory with string music.

Is space "something" -- a physical entity -- or just a concept for describing relations among objects? Greene sorts through several centuries of thought about this question. In the late 1600s, Isaac Newton argued that space has an absolute, independent existence, while his rival Gottfried Wilhelm von Leibniz took the relationist view. In the 19th century, physicist Ernst Mach undermined the prevailing Newtonian position, setting motion against a backdrop not of absolute space but of widely distributed matter. The forces we feel when we spin or accelerate, Mach argued, are exerted on us by all the other objects near and far; if you were alone in the cosmos, you wouldn't feel a thing.

Then came Einstein, whose relativity theories both reaffirmed and refuted aspects of the relationist view. It became clear that Newton's absolute space does not exist; space and time depend on the relative motion of observers. Then again, space was evidently "something," since it could be curved and warped by matter and energy. Moreover, spacetime, the union of what were previously seen as distinct entities, was absolute. In Greene's analogy, spacetime is like a loaf of bread; it can be sliced different ways by observers at different locations, but the overall shape is independent and unchanging.

Quantum mechanics transformed science's understanding of matter and energy, and brought new implications for space and time. There was now seen to be genuine randomness in physical phenomena, unlike the deterministic laws of Newton and Einstein. Particles exist in a haze of probability, and interfere with one another like waves. Moreover, space no longer reliably performs its basic role of separating objects. Rather, particles can be "entangled," or correlated with each other over large distances even though nothing passes between them; they behave, in a sense, as one object. Such entanglement is a very difficult concept to explain, and Greene does a laudable job of it.

Time presents further puzzles. For one, does time "flow"? Physics provides no clear basis for our sense of a shifting present that is distinct from past and future. Indeed, relativity runs counter to that everyday perception, since observers at widely different locations and speeds will disagree on what is happening "now." Greene leans toward the view that time's flow is a function of the human mind rather than of fundamental physics. But he notes, sensibly, that our understanding of this matter may be far from complete.

The arrow of time, or direction in which things change, poses another question. We are accustomed to eggs scrambling but not unscrambling, and to black coffee and cream being stirred into light coffee but never the reverse. Yet at a more fundamental level, physics appears symmetrical between past and future; a movie of the particles that compose the egg or coffee could be run in reverse and you wouldn't know the difference.

The time asymmetry of everyday life arises from increasing entropy, a measure of the disorder in a system. But what explains the drive toward higher entropy? A plausible answer is provided by the cosmological theory of inflation, in which space underwent a phase of extremely rapid expansion. Remarkably, as Greene points out, understanding why eggs get scrambled requires consideration of the earliest moments of the universe.

Yet science's picture of the early universe (and of certain cosmic features such as black holes) remains fuzzy. The reason is that the relevant theories, quantum mechanics and general relativity, are incompatible with each other. General relativity posits a spacetime that is geometrically smooth. Quantum mechanics suggests spacetime, at the smallest scale, is wildly tumultuous. Hence, physicists search for a theory that will reconcile such differences. This was the impetus for superstring theory, which later morphed into M-theory (the M can stand for various things, including mystery, matrix, and membrane).

The superstring/M-theory approach raises the possibility that the familiar three dimensions of space (and one dimension of time) are not all there is to reality. Instead, there are higher dimensions, which go unnoticed because they are curled up at small scales or impervious to the electromagnetic radiation with which we normally see things. In fact, space as we know it may be a "something" that is as real as any object: a three-dimensional membrane, or "brane," embedded in a higher-dimensional spacetime.

Research continues on M-theory, and on a competing approach called loop quantum gravity (which can be understood, roughly speaking, as involving little loops of space). Experimental verification of such cutting-edge physics is difficult, but Greene expresses optimism that experiments in the not-distant future will provide a window into the nature of space and time. There are indications in current theory that space and time arise from something yet more fundamental -- but what that something is, nobody knows.


TCS Daily Archives