If successful scientific theories can be thought of as cures for stubborn problems, quantum physics was the wonder drug of the 20th century. It successfully explained phenomena such as radioactivity and antimatter, and no other theory can match its description of how light and particles behave on small scales. But it can also be mind-bending. Quantum objects can exist in multiple states and places at the same time, requiring a mastery of statistics to describe them. Rife with uncertainty and riddled with paradoxes, the theory has been criticised for casting doubt on the notion of an objective reality - a concept many physicists, including Albert Einstein, have found hard to swallow. Today, scientists are grappling with these philosophical conundrums, trying to harness quantum's bizarre properties to advance technology, and struggling to weave quantum physics and general relativity into a seamless theory of quantum gravity.

It's the theory everyone loves to hate. Depending on who you ask these days, string theory is either untestable, disconnected from reality or not even science. Right? Not so fast. While critics have been chipping away at its claim to be a "theory of everything", string theorists themselves have realised they must find ways to put their models to the test. They may still be far from being able to observe a string in a laboratory, but experiments planned for the near future - and even one currently under way - could provide tantalising evidence either for or against string theory.

In his later years, Einstein sought a unified theory that would extend general relativity and provide an alternative to quantum theory. There is now talk of a 'theory of everything' (although Einstein himself never used the phrase). Fifty years after his death, how close are we to such a theory?

By now, just about everyone has heard of string theory. Even those who don't really understand it--which is to say, just about everyone--know that it's the hottest thing in theoretical physics. Any university that doesn't have at least one string theorist on the payroll is considered a scientific backwater. The public, meanwhile, has been regaled for years with magazine articles breathlessly touting it as "the theory of everything." Brian Greene's 1999 book on the topic, The Elegant Universe, has sold more than a million copies, and his Nova series of the same name has captivated millions of TV viewers.

Two recently published books are riling the small but influential community of string theorists, by arguing that the field is wandering dangerously far from the mainstream.