The key ideas

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

Brian Greene, 1999

Physics has two extraordinarily accurate theories, and they contradict each other. Brian Greene shows exactly where they collide and how string theory tries to reconcile them: particles as the notes of tiny strings, hidden dimensions, space that can tear. Beautiful, bold, and still not confirmed by experiment.

Who it is for

For anyone curious about what the world is made of at its very deepest level, who wants to understand string theory without equations and tell a beautiful idea from a tested fact.

As they are currently formulated, general relativity and quantum mechanics cannot both be right.

What the book is about

Twentieth-century physics has two great theories. Einstein's general relativity describes gravity, stars and the expanding universe. Quantum mechanics describes atoms and particles. Both have been tested to astonishing precision. And yet, as Brian Greene shows, in their current form they cannot both be right: wherever you need both at once, the calculations stop giving answers that make sense.

Greene, a physicist at Columbia University, works on string theory himself. It is the leading candidate for reconciling the two. He guides you almost without equations, through images: a violin string, a garden hose, an ant.

One thing matters from the very first page: string theory is a hypothesis. When the book came out in 1999, no experiment had confirmed it, and none has since. Greene's analogies help you understand what the theory proposes, but they prove nothing. That is why each idea here ends with a line on its status.

Idea 1. Two truths that cannot live together

For Einstein, space is a smooth fabric that mass gently bends. For quantum mechanics, the world at small distances jitters: the smaller the patch you look at, the wilder the random swings of energy inside it. The uncertainty principle demands it.

Greene invites you to zoom in on space, as if with a magnifying glass. At first everything is smooth. Then ripples appear. And at distances around 10⁻³³ centimeters, the so-called Planck length, the ripples turn into a violent churning. The physicist John Wheeler called this picture "quantum foam". In this picture, the smooth fabric Einstein's theory is built on may simply not exist down there.

In everyday life the conflict goes unnoticed: stars are big and heavy, particles are small and light. But at the center of a black hole, and at the very beginning of the universe, an enormous mass is squeezed into a tiny volume. You need both theories at once, and together they give meaningless infinities.

Both theories have been confirmed by observation, each in its own domain, and the clash between them is a widely recognized problem. The "foam" remains an image: no one has observed such scales directly.

A table looks smooth, but under a microscope it is full of grooves. Greene asks you to keep zooming in: nothing stays flat anywhere, all the way down.

The conflict does not mean one of the theories is wrong. More likely, both are approximations of something deeper.

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About the author

Brian Greene (born 1963) is an American theoretical physicist, a professor at Columbia University and a specialist in string theory. He is one of the best known popularizers of physics. The Elegant Universe was his first popular science book.

All books by this author: Brian Greene

Terms from the book

  • Einstein
  • Hawking
  • Wheeler
  • Entropy
  • Morrison
  • Quantum mechanics
  • Quark
  • Photon
  • General relativity
  • String theory
  • Uncertainty principle: A law of quantum mechanics stated by Werner Heisenberg in 1927: you cannot know exactly both where a particle is and how fast it is moving. It follows that at small scales energy is constantly jittering.
  • Planck length: About 10⁻³³ centimeters. The scale at which, on current understanding, the quantum effects of gravity become significant.
  • Quantum foam: John Wheeler's image: because of quantum fluctuations, space at the very smallest scales may be not smooth but churning. An image to aid understanding, not an observation.
  • Veneziano: Gabriele Veneziano, an Italian theoretical physicist. His 1968 formula for particle collisions was the first step toward string theory.
  • Strong nuclear force: The most powerful of the four fundamental forces of nature. It holds quarks together inside protons and neutrons, and holds those together in the atomic nucleus.
  • Euler: Leonhard Euler, an eighteenth-century Swiss mathematician and one of the most prolific in history. Veneziano's formula rested on a function Euler had studied two centuries earlier.
  • Scherk: Joël Scherk, a French theoretical physicist. In 1974, with John Schwarz, he proposed treating strings as a theory that includes gravity.
  • Schwarz: John Schwarz, an American theoretical physicist and one of the founders of string theory. In 1974, with Joël Scherk, he spotted the particle of gravity among the vibrations of strings.
  • Graviton: A hypothetical particle that carries gravity, the way the photon carries light. It has not been detected yet.
  • Kaluza: Theodor Kaluza, a German mathematician and physicist. In 1919 he proposed adding one more dimension to space in order to unite gravity and electromagnetism.
  • Klein: Oskar Klein, a Swedish physicist. In 1926 he explained why Kaluza's extra dimension goes unnoticed: it is curled up into a tiny circle.
  • Curled-up dimensions: The extra dimensions of space that string theory requires. The hypothesis is that they are curled up so tightly that they cannot be noticed. No sign of them has been found yet.
  • Calabi-Yau: Intricate six-dimensional shapes named after the mathematicians Eugenio Calabi and Shing-Tung Yau. In string theory they are the candidates for the shape of the hidden dimensions.
  • Aspinwall: Paul Aspinwall, a theoretical physicist who coauthored with Greene the 1993 work showing that in string theory space can tear without catastrophe.
  • Witten: Edward Witten, an American theoretical physicist and one of the leading experts in string theory. The only physicist to have won the Fields Medal, the highest award in mathematics. In 1995 he proposed M-theory.
  • M-theory: A deeper theory proposed by Edward Witten in 1995, in which the five versions of string theory may turn out to be special cases. It has not been fully formulated.
  • Strominger: Andrew Strominger, an American theoretical physicist. In 1996, with Cumrun Vafa, he used string theory to calculate the entropy of certain black holes.
  • Vafa: Cumrun Vafa, an Iranian-born American theoretical physicist, who worked with Andrew Strominger on the 1996 calculation of black hole entropy.
  • Bekenstein: Jacob Bekenstein, an Israeli physicist. In 1972 he suggested that a black hole has entropy that depends on the area of its surface.
  • Superpartners: Hypothetical heavy twins of the known particles, predicted by supersymmetry. They have not been found at the Large Hadron Collider so far.
  • Supersymmetry: The hypothesis that every known particle has a heavier partner. It is part of superstring theory but has not been confirmed by experiment.
  • Large Hadron Collider: The world's most powerful particle accelerator: an underground ring about 27 km long near Geneva, at CERN. It started running in 2008, and in 2012 it found the Higgs boson.

All terms

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