The Hottest Thing in Theoretical Physics

The Unraveling of String Theory — Michael D. Lemonick— — August 2006— (Time Magazine)     Everyone knows that string theory is the hottest thing in theoretical physics. But string theory hasn't been embraced by everyone.     Physicists Peter Woit in Not Even Wrong and Lee Smolin in The Trouble with Physics both argue that string theory is largely a fad propped up by practitioners who tend to be arrogantly dismissive of anyone who dare suggest that the emperor has no clothes.       The two most important ideas of 20th century physics, relativity and quantum theory, were known to be fundamentally incompatible.         Quantum theory describes the universe as intrinsically discontinuous: energy, for example, can come in bits just so small, but no smaller. Relativity treats time and space and gravity as a smooth, unbroken continuum.             The solution: to think of the basic units of matter and energy not as particles but as minuscule, vibrating loops and snippets of stuff resembling string, which turn out to exist not just in our familiar four dimensions of space and time but in 10 or more dimensions.         This bizarre-seeming scheme appeared on first blush to explain why particles have the characteristics they do. It also included a quantum version of gravity and thus of relativity.         But: superstrings have proved a lot more complex than anyone expected. The mathematics is excruciatingly tough, and when problems arise, the solutions often introduce yet another layer of complexity.         The new, improved theory posits a nearly infinite number of different possible universes, with no way of showing that ours is more likely than any of the others.                The string theory’s idea of infinite universes is currently in vogue among some astronomers as well.           String theorists seem ready to abandon the essential definition of science. Is string theory too important to be hampered by old-fashioned notions of experimental proof?       In science, slow accretion of data and evidence eventually eliminates reasonable doubt, but not so with strings.       Nobody has any good idea of how to test string theory. Woit says, 'proposing speculative ideas is fine, but if they can't be tested, they're not science'.

General Relativity and Quantum Mechanics

The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory — Brian Greene— (Chapter 1: Tied Up with String)         There are two foundational pillars upon which modern physics rests.       One is Albert Einstein's general relativity, which provides a theoretical framework for understanding the universe on the largest of scales: stars, galaxies, clusters of galaxies, and beyond to immense expanse of the universe itself.       The other is quantum mechanics, which provides a theoretical framework for understanding the universe on the smallest of scales: molecules, atoms, and all the way down to subatomic particles like electrons and quarks.       Through years of research, physicists have experimentally confirmed to almost unimaginable accuracy virtually all predictions made by each of these theories.         But: as they are currently formulated, general relativity and quantum mechanics cannot both be right.         The two theories underlying the tremendous progress of physics during the last hundred years are mutually incompatible.         In the central depths of a black hole an enormous mass is crushed to a minuscule size.       At the moment of big bang the whole of the universe erupted from a microscopic nugget whose size makes a grain of sand look colossal.              There are realms that are tiny and yet incredibly massive, therefore requiring that both quantum mechanics and general relativity simultaneously be brought to bear.         Well-posed physical questions elicit nonsensical answers from the unhappy amalgam of these two theories.         Can it really be that the universe at its most fundamental level is divided, requiring one set of laws when things are large and a different, incompatible set when things are small?         Superstring theory: this new approach to describing matter at its most fundamental level resolves the tension between general relativity and quantum mechanics. Within this new framework, general relativity and quantum mechanics require one another for the theory to make sense.