Peeking Inside a Black Hole

This post is a review of What is Inside a Black Hole? by Stephen Hawking. It is a 67-page reissue of two of his essays from his longer book Brief Answers to the Big Questions. Apart from the title essay, it includes the essay Is Time Travel Possible?

1. Inside a black hole : The essay recalls the early black hole thoughts of John Mitchell, the grand impetus provided by Einstein's general theory of relativity, the quantitative work of Karl Schwarzschild, the pioneering contributions of John Wheeler, the papers of Oppenheimer, and the results of Chandrasekhar and Landau.

   
   

   The picture that emerged from these advances: stars above a certain mass (the 'Chandrasekhar limit') eventually collapse into a black hole; the simplest black hole is spherical; its radius (the 'event horizon') is where gravity becomes strong enough to trap light; at the center of the sphere is a singularity.

   
   

   The laws of physics break down at the singularity - space and time cease to exist there. In response to this catastrophic scenario, Roger Penrose proposed the Cosmic Censorship Principle: nature does not allow naked singularities: they are always hidden behind event horizons, so physics is well behaved outside of black holes (which is where all live observers are located). The Principle has yet to be proved.

   
   

   At this point Hawking describes a discovery, which stemmed from his trying to bring quantum mechanics to the study of black holes. He found that a black hole not only allows something to escape, it in fact radiates stuff continuously. A typical process involves particle-antiparticle pairs popping out of the vacuum just outside the black hole. One of the particles is swallowed by the black hole; the other escapes and appears to an observer as radiation coming from the black hole.

   
   

   The smaller the black hole, the faster it radiates. Hawking speculates about trapping one in orbit around Earth to power civilization. He also rues no one has found one yet, or he would have surely received a Nobel prize for his prediction.

   
   

   The biggest worry he states is the implication that when something falls into it, the black hole swallows up all of its information (the emission seems to be random and does not contain any information). This information loss could mean we cannot be certain about the past or the future of the universe as black holes can appear and cycle information into nonsense. This apparent loss of determinism is still one of the open problems in physics.

2. Is time travel possible? Hawking starts this essay by pointing out that, as per special relativity, time slows down for moving observers. But the fastest one can move is the speed of light. At that velocity time comes to a stop. But that is not good enough - to travel into the past time needs to go backwards.

   
   

   While special relativity does not seem to allow for time travel, Hawking points out that general relativity does. This follows from Einstein's discovery that spacetime can be curved by massive objects (special relativity assumes a flat spacetime). In this context, Godel's universe and string theory offer (presently impractical) possibilities for time travel. Hawking discusses the use of wormholes for time travel. This would require warping spacetime using negative energy (from the vacuum via the Casimir effect), not very practicable currently.

   
   

   He concludes by discussing conceptual issues associated with time travel - Why haven't we been visited by people from the future? How does time travel jive with free will? Are there other universe which we can visit going back in time? He suggests the Chronology Protection Conjecture: the laws of physics prevent time travel so we can keep history straight (and historians employable).

   
   

   Summary

   Both essays are lucid and thoughtful; one can hear the master think. A highly recommended read.