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.