Exotic idea: Prof. Ashtekar explains space-time at the lowest level

Black holes, the cosmic beasts whose deadly grip allows nothing ? not even light ?to escape have fascinated both physicists and science fiction writers. To quote the Nobel laureate physicist Subrahmanyam Chandrasekhar, they are ?the most perfect macroscopic objects in the Universe.? Experts marvel on their beauty for more reasons than one. Time and again they have dared experts to visualise them, mustering all the conceptual tools they have. In fact, over the years, they have often been considered improbable, only to be resurrected again with the help of fresh insights. In the bargain, physics has become the winner.

Prof. Abhay Ashtekar, director, Institute for Gravitational Physics and Geometry, and Eberly Professor of Physics at the Pennsylvania State University, US, recounted that story in great detail while delivering the 16th S.N. Bose Memorial Lecture on January 3. Titled ?Black Hole: Surprises, Puzzles and Clues for Fundamental Physics?, it was organised by the S.N. Bose National Centre for Basic Sciences.

Ashtekar began his lecture by explaining how black holes caught physicists? fancy initially very smoothly. Isaac Newton?s laws of gravitational attraction can be used to calculate what is called the escape velocity of a projectile hurled upward from any massive object. It?s the minimum speed that the projectile must have to overcome the object?s gravitational pull, or else it will fall down after being thrown up. Newton?s law made it clear that the heavier the object, the bigger would be the escape velocity of a projectile intending to break free from it.

According to Ashtekar, it was John Mitchel, a British cleric, who in 1784 came to the conclusion that there could be stars so massive that the escape velocity from them might be faster than even the speed of light. This meant that the light emitted by such stars would not be able to escape from them. In 1796, the French philosopher Pierre Simon de Laplace wrote that there were many massive bodies in the sky that were not visible to us.

According to Ashtekar, although Newtonian ideas led to such a notion, it was wrong, simply because light could not be viewed as any other projectile which would become slower as it went upward, unless its initial speed was more than the escape velocity. Light?s velocity was found to be independent of the speed of any observer who was measuring it. So, Newtonian mechanics did not allow the existence of black holes.

They became a reality, explained Ashtekar, when Albert Einstein married time with space in his theory of general relativity, showing that the former was merely a new dimension of space, just like the three other it already had. ?The geometry of space-time resurrected the idea of black holes,? commented Ashtekar. ?It showed that matter could warp space around it, and if the warping was right, light could not escape a volume of space ? a region labelled as a black hole.? Ashtekar described how even Prof. Arthur Eddington, proclaimed to be an authority on relativity theories, resisted the idea of dead stars collapsing to form black holes when he challenged Chandrasekhar?s premise in 1931. ?If he didn?t do that, he would have been the first scientist to visualise black holes,? Ashtekar said.

He discussed the spectacular realisation of Prof. Stephen Hawking of Cambridge University in 1974 that black holes, too, radiate energy, and so, eventually, evaporate. This led to the now-famous bet ? between Hawking and Prof. Kip Thorn of Caltech on one side and Thorn?s colleague, Dr John Preskill, on the other ? on whether all the information of the matter gobbled up by black holes actually vanishes from the cosmos. Ashtekar lamented that Hawking?s conceding the bet last September was ?reported in the press, instead of a science journal.? He made it clear that he wasn?t convinced by Hawking?s arguments from whatever he could find on the topic on Internet.

According to Ashtekar, the efforts to analyse black hole evaporation by mixing general relativity with the two other pillars of physics, statistical mechanics and quantum mechanics, have indicated a startling possibility ? that space may not be a continuous, smooth entity. It may actually consist of unimaginably tiny units, or quanta. ?The science of black holes has been a cyclic story of surprises to puzzles to solutions to puzzles again,? Ashtekar concluded.