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1、Outline: The scientist Information and black holes Hawking radiation Detection of black holes Bets on black holes,3 June 2006 Hawking and Black Holes,The Scientist,Born 1942 1st class honours from Oxford, after “not very much work” Symptoms of ALS during Oxford years PhD and Research Fellow in Cambr
2、idge Discovered Hawking radiation in 1974 “A Brief History of Time” published in 1987 Numerous honorary degrees and awards Outspoken for world peace, welfare of the handicapped, and other current issues,Amyotrophic Lateral Sclerosis,肌萎縮性脊髓側索硬化症 Also called Lou Gehrigs disease Symptoms: Difficulty st
3、anding, walking, or running Clumsiness Frequent tripping or falls Difficulty with fine hand motions such as buttoning, writing, turning a key in a lock Atrophy of hand muscles Atrophy of tongue Difficulty chewing food Difficulty swallowing (dysphagia) Difficulty speaking Muscle cramp,4,Black Hole,Bl
4、ack holes represent the final victory of gravity. A black hole is black because gravity is so strong that light cannot escape. The escape velocity at a distance r from the center of an object with mass M is,The escape velocity increases with mass and decreases with radius. If vescape c, then light c
5、annot escape and we have a black hole.,5,Space Warps,If we imagine the spacetime as a “rubber sheet”, then any mass would produce warpings in it. Since black holes produce very strong gravity, spacetime is significantly warped (curved) around them. We see strong light-bending and gravitational redsh
6、ift.,6,The Black Hole Radius,For any mass, there is a smallest radius beyond which the object becomes a black hole. This smallest radius rs is called the Schwarzschild radius. Hawking: This defines the size of a black hole, and it depends on the mass only. Anything smaller than its corresponding Sch
7、warzschild radius becomes a black hole.,7,Dissecting a Black Hole,A non-rotating black hole is particularly simple. There is a point at the center called the singularity (奇點). It has zero size and infinite density. In fact, its properties cannot be described by currently known physics. The event hor
8、izon (穹界) is a sphere centered at the singularity with radius equal to the Schwarzschild radius of the black hole. What is inside the event horizon cannot be known by anyone outside because even light cannot escape out.,8,Event Horizon and Singularity,9,No-hair Theorem,Hair here means something comp
9、licated (e.g. different styles, colors, perms, etc). Black holes have no hair because they are simple. Only three things completely characterize a black hole (Hawking 1972): mass angular momentum electric charge,10,Under general physical conditions, the singularity is enclosed by the event horizon.
10、Information within the event horizon cannot be transmitted to the external world. We say the singularity is concealed or dressed. Those which are not dressed are called naked singularities. Mathematically, naked singularities can exist, but physical considerations suggest cosmic censorship: all sing
11、ularities are enclosed (Roger Penrose). Hawking bet on cosmic censorship (and conceded too early in 1997).,Cosmic Censorship Conjecture: Nature Forbids Naked Singularity,11,One way traffic in Nature? 1. The disintegration of the egg will never happen in the reverse direction (re-integration). 2. Air
12、 molecules diffusing out of the bottle will never progress in the reverse direction (infusion).,Time Arrow,12,There is a very important law in physics, which governs the direction of any process in a physical system. This is called the second law of thermodynamics: The entropy of an isolated system
13、never decrease.,Second Law of Thermodynamics,13,If we throw complicated objects (with low entropy) into a black hole, where has the entropy gone? Where has the information escaped from the black hole?,The Information Paradox,14,Four Laws of Black Hole Thermodynamics,Bardeen, Carter and Hawking (1973
14、) formulated the four laws of black hole physics, analogous to the four laws of thermodynamics. Second Law The total surface area of black holes is always the same or greater than before. When we throw matter into a black hole, or allow two black holes to merge, the total area of the event horizons
15、will never decrease.,15,Area Theorem,This implies that the surface area of a black hole is a measure of the entropy. If an object has nonzero entropy, then it has a temperature, and it must radiate! At first, Hawking himself could not accept this implication.,16,17,General Relativity and Quantum Mec
16、hanics,General relativity and quantum mechanics are two major achievements of 20th century physics. General relativity deals with the very large. Quantum mechanics deals with the very small. Physicists attempted to unify the two.,18,Hawking Radiation (1974),When Hawking considered quantum mechanics, many of his ideas of black holes need to be changed. Black holes may actually radiate! Near the horizon, particle-anti-particle pairs can be created so that one escapes and the other falls in.
