by: Phillip Choi
Most people don’t know or even care too much about black holes. The basic premise of a black hole is easy to grasp: A black hole is an incredibly compact mass that has so much gravity that nothing, even light, can escape from it. However, what happens to something when it does get sucked into a black hole? The answer to this seemingly innocuous question was debated by Leonard Susskind and Stephen Hawking for more than two decades. Leonard Susskind’s account of his twenty three year battle with Stephen Hawking over the fundamental nature of black holes is brilliantly recounted in his “Black Hole War”.
Most people don’t know or even care too much about black holes. The basic premise of a black hole is easy to grasp: A black hole is an incredibly compact mass that has so much gravity that nothing, even light, can escape from it. However, what happens to something when it does get sucked into a black hole? The answer to this seemingly innocuous question was debated by Leonard Susskind and Stephen Hawking for more than two decades. Leonard Susskind’s account of his twenty three year battle with Stephen Hawking over the fundamental nature of black holes is brilliantly recounted in his The Black Hole War.
The Black Hole War started in 1981 during an informal meeting of eminent physicists in San Francisco at the mansion of a rich New Age self help guru. During his presentation, Stephen Hawking proposed that information that falls into a black hole is eventually lost permanently in black hole evaporation. If information really was lost forever within black holes, one of the key principles holding quantum mechanics together, information conservation, would be violated. Information conservation is a simple law that states information cannot be lost or gained in the universe. That day, only Susskind and Gerard ‘t Hooft, a preeminent Dutch physicist, were troubled by Hawking’s conclusion. Despite all the mental acrobatics normally required for theoretical physics, Susskind and ‘t Hooft felt there was something intuitively wrong with losing information within a black hole because losing information is the same as increasing entropy. This entropy would become heat, which meant, “if Stephen was right, empty space would heat up to a thousand billion billion billion degrees in a tiny fraction of a second.” This bit of reasoning was the beginning of twenty three years of work to conclusively prove Hawking wrong.
Susskind recognizes that as a theoretical physicist writing for a lay audience, he must provide a significant amount of background knowledge to make the Black Hole War somewhat intelligible. He starts at the most basic level possible by talking about the scale of numbers that the reader will be exposed to in the book. Then, Susskind deftly explains numerous concepts important to gravity, thermodynamics, relativity, and quantum mechanics, such as tidal forces, the Uncertainty Principle, the Equivalence Principle, time dilation, entropy etc. The presence of many illustrations and the absence of unnecessary equations are greatly appreciated.
The term “black hole” only came into the physics lexicon during the mid 1960s; the term is widely credited to John Wheeler. Before then, black holes were called gravitationally completely collapsed stars. The existence of black holes was first postulated by John Mitchell and Pierre-Simon Laplace in the late 18th century. However, it was not until the latter half of the 20th century that black holes were became more than extremely dense dead stars. It was agreed upon that there were only two key components when describing a black hole: the singularity and the horizon. The singularity is the center of the black hole where the mass of infinitely high density resides. The horizon is the invisible spherical boundary between the black star and the rest of the universe; once an object crosses the horizon, it will be pulled into the singularity and destroyed. Black holes were thought to be cold balls of mass that would be permanent fixtures in the cosmos. Now, it is recognized that black holes have can radiate heat, split into smaller black holes, evaporate into nothing and a whole bunch of other lively activities.
Susskind’s quest to prove Hawking wrong about the destruction of information in black holes results in two major theories: Black Hole Complementarity and the Holographic Principle. It should be noted that he worked with ‘t Hooft on the Holographic Principle. Both theories get around Hawking’s assertion by stating that even information that is sucked into a black hole is not actually “in” the black hole.
At the most basic level, Black Hole Complementarity states that what one observes while inside a black hole is different from what one observes outside the same black hole. For example, let’s say a person falls into a black hole while his friend watches in horror from a nearby spaceship. The friend outside the black hole in the spaceship will see his friend swallowed up and disintegrated as he passes the horizon. However, the person falling into the black hole will experience something entirely different: He will cross the horizon of the black hole without noticing any changes and continues living. He won’t feel his body being torn to bits by the black hole. (That happens when he reaches the singularity.) There seems to be a paradox here as two different events take place at the same moment. However, this paradox can be reconciled by the fact that, even theoretically, there is no way to experimentally prove there is a paradox because it would be impossible for the two observers to ever come together and compare their observations about crossing the horizon. Black Hole Complementarity is analogous to the wave-particle duality of light. In the same way that light is a particle and light is a wave, the observer crossing the horizon is destroyed and he is not. Black Hole Complementarity circumvents Hawking’s claim because even though we on the outside might see the bit of information being sucked into the black hole and evaporating as Hawking radiation, that bit of information actually still exists in the black hole.
The Holographic Principle is the second major development that came from Susskind’s war with Hawking. A hologram is a three dimensional image created by focusing light projected from a two-dimensional film surrounding the image. Susskind and ‘t Hooft’s Holographic Principle states that “everything inside a region of space can be described by bits of information restricted to the boundary.” This means that any object in the universe, even the largest ones like stars, galaxies, and the universe itself are simply holograms created from a massive, information-containing shell that surrounds the object. Whenever an object enters the boundary of one of these shells, the information regarding the object is encoded into the boundary and the boundary expands in proportion to the amount of information added. Incredible as it sounds, the Holographic Principle is now an accepted tool of theoretical physics. The Holographic principle solidly proves Hawking’s claim to be incorrect. If the holographic shell of a black hole is taken to be its boundary and the black hole itself is the object, any object or bit of information that enters the black hole will encode its information into the shell before entering the black hole. Thus, the information is conserved and not lost.
As Susskind and ‘t Hooft develop a the tools necessary to prove that black holes don’t swallow up and obliterate information, the nature of modern research becomes increasingly clear. Although Susskind and ‘t Hooft thought up the concepts of Black Hole Complementarity and the Holographic Principle, they must collaborate with many individuals to make the theories mathematically consistent with the physics establishment. Contributions from Cumrun Vafa, Ashoke Sen, Joseph Polchinski, Andrew, Strominger, Juan Maldacena, and Claudio Bunster were necessary to making Susskind and ‘t Hooft’s ideas sound. Brainstorming often took place at conferences where the various physicists would have relatively casual conversations about the problems they were having in their research. The general consensus regarding theories was taken via a straw poll at many of these conventions. Research didn’t take place in the offices of individual professors. Instead, it seems like a large family bickering and working together to find the “truth.”
Throughout the book, Susskind’s personality comes through loud and clear when he isn’t straightforwardly explaining the intricacies of his theories. It is very apparent that he is a grizzled physics professor willing to tell the war as he saw it without censoring himself. He recounts his encounters with other giants of theoretical physics, such as Richard Feynman, as a young student. When explaining the nature of black holes, he takes some time to discuss the reality behind some popular uses of black holes, such as time travel and teleportation. The consensus is that we can travel forward in time, but not backward, and worms holes, if they exist, could never actually be used to travel to an alternate universe or part of this universe. What’s most interesting is his characterization of Stephen Hawking. Very few people would have the audacity to call Hawking dense or get annoyed at how slowly it takes him to formulate answers using his synthesizer, but Susskind does. He’s willing to see the situation for what it is, not what it should be.
The Black Hole War is both highly informative and entertaining. Patience is required to understand the physics taking place, but it is rewarding to see how Susskind systematically attacks Hawking’s claim. I highly recommend The Black Hole War is you are looking to learn a little bit about how black holes work and how physicists wage war with each other.