Stephen Hawking, the well-known cosmologist and theoretical physicist, has focused his intellect on an elusive problem: How can Einstein’s relativistic universe be reconciled with our quantum understanding of the world of elementary particles?
Both of these lines of inquiry have been developed into full-fledged theories that are currently supported by no small amount of experimental and observational evidence. Nevertheless, reconciliation has been problematic. In the face of this enormous challenge, Hawking’s work has set the stage for further advances, such as the still unverified String Theory, which may eventually provide the much-sought answer.
Hawking emerged as an intellectual giant when, in 1965, he was inspired by Robert Penrose’s notion that there exist, in the centers of black holes, space-time singularities. Hawking proposed an analogous structure for the entire universe. For his doctoral essay, he wrote Singularities and the Geometry of Space-Time. In the years that followed, Hawking joined with Penrose to publish an essay postulating that the universe began as a singularity. Subsequently, they proposed that if the universe conforms to Einstein’s General Theory of Relativity and also to any of Alexander Friedmann’s cosmological models, then it assuredly began as a singularity. This definitively contradicted the Steady-State Theory, which is no longer considered viable.
Hawking went on, in 1970, to postulate the second law of black-hole dynamics, which states that the event horizon can never become smaller. A further stage in the line of reasoning concluded (this is astonishing) that a black hole can be completely described by stating its properties of mass, electrical charge and rotation.
In 1973, with George Ellis, Hawking’s first full-scale book appeared, titled The Large-Scale Structure of Space-Time. The heavy volume is enormously instructive. Its hefty price on Amazon.com makes it inaccessible to many, but a substantial preview may be seen on Google books. This articulate and thorough exposition of Hawking’s view of the universe at that time is an excellent gateway for understanding the great issues that existed then in physics and cosmology.
In 1974, Hawking demonstrated that, contrary to previous doctrine, black holes emit radiation. This so-called Hawking radiation, difficult to discern amid the violent fireworks just outside the event horizon, would nevertheless be sufficient to eventually extinguish all black holes. The question arose: What happens to all the information consumed by the black hole during its period of gravity-driven consumption?
Though Hawking has been by far the most imminent and well-known black hole theoretician, the existence of these enigmatic astronomical objects was predicted long before he was born in 1942. The idea first appeared in a paper presented by John Michell before the Royal Society in London in 1783. He stated that a star, if sufficiently massive, would exert so strong a gravitational attraction that its own light would not be able to escape. Such a body, which he called a dark star, would be invisible to outside observers.
A few years later, Pierre-Simon La Place, as an aside in his discussion of the solar system, wrote “The gravitational attraction of a star with a diameter 250 times that of the sun and comparable in density to the earth would be so great no light could escape from its surface. The largest bodies in the universe may thus be invisible by reason of their magnitude.”
These early theoreticians were correct in assuming the existence of such invisible objects, but they had no idea of the awesome dynamic that would make possible their creation and growth. Moreover, in making mention of these bodies they seemed to be suggesting that they were passive and incidental phenomena, invisible and not interacting with the rest of the universe.
If we fast forward to our time and listen to Hawking and colleagues, a far more dynamic and energetic picture emerges. Now known as black holes, these objects have such awesome gravitational pull that nearby objects, stars, planets, dust, other black holes, are pulled into the dense body contributing to its mass.
Due to its intense mass and as a consequence of both relativistic and quantum characteristics possessed by the black hole and in the space surrounding it, strange interactions arise. Hawking in 1975 stated that a black hole would actually glow due to the emission of photons, neutrinos and other particles. Its glow would be the same as a black body of a temperature in Kelvin degrees equal to 6 x 10-8/M
where M is the number of solar masses possessed by the black hole.
The presence of M in the denominator means the larger (more massive) the black hole, the fainter its glow. Small micro-black holes, conceivably produced in the super-sized particle colliders, would therefore be unable to consume matter around them, eventually destroying the earth, because they would instantly wink out of existence, their entire mass emitted by way of Hawking radiation.
A further implication is that a black hole would begin to shrink due to Hawking radiation as it swallowed up all the matter close enough to be attracted to it. The process would happen gradually at first. The black hole would lose mass until, as it became smaller, it would diminish more rapidly until it would cease to exist. However, for a large black hole, this process could require more time than the present age of the universe.
One proposed mechanism that may underlie Hawking radiation depends upon the fact that virtual particles (particle and antiparticle) are spontaneously created throughout all space including near the black hole’s event horizon. The usual scenario is that the pair annihilates one another, but there will be instances where one member of the pair will vanish into the black hole before the annihilation can happen. Cast loose, the other particle escapes as Hawking radiation.
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