The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory

The 20th century left physics with two extraordinary theories: general relativity, from Einstein, which describes gravity on a gigantic scale, and quantum mechanics, which describes the world of atoms and of particles smaller than an atom. Each one, on its own, is the best-tested theory science has ever produced, and in more than a hundred years no experiment has been able to contradict either of them.

The problem shows up when someone tries to combine the two calculations in a situation where gravity gets strong and space gets tiny at the same time, like inside a black hole or at the instant the universe began: the math of the two theories refuses to work together, and the calculation spits out infinity. It's like having two perfect apps on your phone that crash the device when you open them together. Brian Greene's book tells the story of a candidate to resolve this standoff.

Before getting to strings, the book reconstructs what relativity has already solved. Einstein showed, in special relativity, that space and time aren't fixed: they depend on how fast you're moving. Ten years later, in general relativity, he took the bigger step: gravity is not a force pulling from a distance, as people had thought since Newton, but the effect of mass curving space-time around it.

A simple way to picture it: a stretched trampoline with a bowling ball sinking into the fabric; a marble rolling nearby isn't pulled by any invisible rope, it just follows the curve of the fabric. That's how the Sun holds the Earth in orbit. This idea has been confirmed many times over, including by the GPS in your phone, which has to correct the satellites' clocks because of this curvature of space-time. The problem is what happens when this same physics has to deal with the scale of an atom.

While relativity takes care of the very large, quantum mechanics takes care of the very small and behaves in a way that makes no sense to an intuition trained in the visible world. An electron has no definite position before it's measured: it exists as a cloud of probabilities until the moment someone observes it. Another hallmark of this theory is the uncertainty principle: the more precisely you know a particle's position, the less you know about its speed, and vice versa, a feature of nature itself, not of the measuring instrument.

It's like photographing someone running through a packed neighborhood party: focusing on the face blurs the motion, and no better camera fixes that, because the mess is in the reality of that scale itself. This randomness bothered Einstein himself, but the experimental record, repeated in labs around the world, confirms this probabilistic behavior with no known exception to this day. It's established consensus, not an open hypothesis.