Earth 1

Stanford · string theory · black holes

Leonard Susskind

Born 1940 · South Bronx, New York

A plumber from the Bronx who found strings inside a formula, out-argued Hawking, and says the universe may be a hologram.

n = 2

level

4

α′ M²

4

top spin

mode
Pick a vibration. In string theory every particle is one string playing one note: mass² climbs in equal steps.

Particles as strings

In 1968 Gabriele Veneziano found a formula that fitted how protons and mesons scatter, without anyone knowing why. Susskind saw that it was the maths of a tiny vibrating string. Each way the string can vibrate is a different particle. One vibration of a closed string has spin 2 and no mass: the graviton, the particle of gravity.

  • M: the mass of the particle the string makes
  • n: how excited the vibration is, 1, 2, 3 …
  • α′: the string's slack, set by its tension: one over 2π times the tension
  • 4 (n − 1): mass² rises in equal steps; n = 1 is massless

The black hole war

Hawking showed in 1974 that black holes glow and slowly evaporate. If the glow is random, whatever fell in is gone for ever, which would break quantum mechanics. Susskind spent 23 years arguing that the information comes back out in the radiation, scrambled. Complementarity was his answer to the paradox: to someone falling in nothing strange happens at the horizon; to someone outside the infalling book is smeared across it and later radiated. Both are true, and no observer can compare them.

2.95 km

horizon radius

1.5 × 10⁷⁷

bits on it

6.2 × 10⁻⁸ K

glows at

Everything that falls in is stored, scrambled, on the surface: one bit for about every 2.8 Planck areas (4 ln 2). Bits grow with area, not volume.

  • S: the black hole's entropy: how much information it hides
  • A: the area of the horizon, not the volume inside
  • G: Newton's constant of gravity
  • ħ: Planck's constant, the size of quantum steps
  • c: the speed of light
  • k_B: Boltzmann's constant, turning information into heat units

This is Bekenstein and Hawking's formula. Susskind and 't Hooft took it as a law of nature: the most information any region can hold grows with its surface. A 3D world could then be fully described on a 2D boundary, like a hologram.

ER = EPR

In 1935 Einstein wrote two papers: one with Podolsky and Rosen on entanglement (EPR), one with Rosen on bridges through spacetime (ER). In 2013 Maldacena and Susskind proposed they are the same thing: entangled black holes are joined by a wormhole, and perhaps every entangled pair by a tiny one. Nothing can be sent through, matching the rule that entanglement carries no signal.

0

pairs made

none

signals through

Each pair is one entangled particle at each black hole. Maldacena and Susskind's conjecture: the entanglement and the bridge between the holes are the same thing.

The teacher

His Stanford continuing-studies lectures, recorded from the late 2000s and free online, became The Theoretical Minimum books: classical mechanics, quantum mechanics, special relativity and fields, taught to anyone who wants to learn real physics. It is on the books shelf.

Timeline

  1. 16 Jun 1940

    Born in the South Bronx, New York. Works as a plumber with his father before and during college.

  2. 1962

    Graduates in physics from the City College of New York, having switched from engineering.

  3. 1965

    PhD at Cornell. Then Yeshiva University in New York, with a spell at Tel Aviv University.

  4. 1969 – 70

    Sees that the Veneziano formula for strongly interacting particles describes a vibrating string. Yoichiro Nambu and Holger Nielsen see it independently: string theory begins.

  5. 1979

    Joins Stanford. Proposes technicolor, a way to give particles mass without a Higgs, at the same time as Steven Weinberg.

  6. 1981

    At a small meeting in San Francisco, Stephen Hawking argues that black holes destroy information. Susskind and Gerard 't Hooft refuse to accept it.

  7. 1993

    Black hole complementarity, with Lárus Thorlacius and John Uglum: the outside view and the infalling view are both right, and no one can see both.

  8. 1995

    "The World as a Hologram": building on 't Hooft, he argues all the information in a region fits on its boundary.

  9. 1997

    Juan Maldacena's AdS/CFT correspondence gives the first exact hologram.

  10. 21 Jul 2004

    Hawking concedes in Dublin that information is not lost.

  11. 2003 – 05

    The string landscape: a vast number of possible vacua, and the anthropic argument. The Cosmic Landscape (2005).

  12. 2008

    The Black Hole War, his account of the fight with Hawking.

  13. 2013

    ER = EPR with Maldacena. The Theoretical Minimum: Classical Mechanics, from his Stanford lectures, free online.

Sources