“Theories of everything” go back to the ancient Greeks and their idea that indivisible atoms are the building blocks of reality. This conjecture grew into modern atomic theory, which has been perfected by relativity theory and quantum theory. But there’s never been anything like string theory. Designed to describe the universe at the largest and smallest scales, string theory is the most ambitious and far-reaching theory of everything yet devised, leading to intriguing speculations, such as:

• Strings, Not Particles: In string theory, the fundamental units of matter and energy are inconceivably tiny strings, not point-like particles as in conventional theories. The advantage is a wide range of different modes that can represent diverse phenomena.
• Extra Dimensions: Strings can’t do their job without existing in a multitude of dimensions—10 according to the standard version of the theory. Why don’t we perceive these extra dimensions? They are “compactified” and thus invisible at our human scale.
• Multiple Universes: The versatility of string theory allows for different laws of physics that may have developed in separate regions of the rapidly expanding universe in the moments after the Big Bang. We might live in one isolated bubble of this multiverse.