Presentation on theme: "Perspectives on the Origin of the Universe"— Presentation transcript:
1Perspectives on the Origin of the Universe 3 June 2006Hawking and Black HolesProf. K . Y. Michael WongOutline:The scientistInformation and black holesHawking radiationDetection of black holesBets on black holes
2The ScientistBorn 19421st class honours from Oxford, after “not very much work”Symptoms of ALS during Oxford yearsPhD and Research Fellow in CambridgeDiscovered Hawking radiation in 1974“A Brief History of Time” published in 1987Numerous honorary degrees and awardsOutspoken for world peace, welfare of the handicapped, and other current issues
3Amyotrophic Lateral Sclerosis 肌萎縮性脊髓側索硬化症Also called Lou Gehrig’s diseaseSymptoms:Difficulty standing, walking, or runningClumsiness – Frequent tripping or fallsDifficulty with fine hand motions such as buttoning, writing, turning a key in a lockAtrophy of hand musclesAtrophy of tongueDifficulty chewing foodDifficulty swallowing (dysphagia)Difficulty speakingMuscle cramp
4Black HoleBlack 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 isLight raysThe escape velocity increases with mass and decreases with radius.If vescape> c, then light cannot escape and we have a black hole.
5Space WarpsIf 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 redshift.
6The Black Hole RadiusFor 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 Schwarzschild radius becomes a black hole.
7Dissecting 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 horizon (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.event horizonsingularity
9No-hair TheoremHair here means something complicated (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):massangular momentumelectric charge
10Cosmic Censorship Conjecture: Nature Forbids Naked Singularity Under general physical conditions, the singularity is enclosed by the event horizon. 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 singularities are enclosed (Roger Penrose).Hawking bet on cosmic censorship (and conceded too early in 1997).
11Time ArrowOne way traffic in Nature? 1. The disintegration of the egg will never happen in the reverse direction (re-integration). 2. Air molecules diffusing out of the bottle will never progress in the reverse direction (infusion).
12Second Law of Thermodynamics 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 never decrease.Examplethe reverse is not allowedbecomesSmaller entropyLarger entropy
13The Information Paradox 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?
14Four Laws of Black Hole Thermodynamics Bardeen, Carter and Hawking (1973) formulated the four laws of black hole physics, analogous to the four laws of thermodynamics.Second LawThe 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 will never decrease.
15Area Theorem time space 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.
17General Relativity and Quantum Mechanics 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.
18Hawking 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.Radiation energy follows the blackbody distribution.
19Virtual ParticlesIn classical physics, vacuum means nothing exists. However, in quantum mechanics, vacuum is actually a sea of virtual particles.In quantum mechanics, there is a concept called vacuum fluctuations.Although the average energy of space is zero, local fluctuations of energy are allowed by the Heisenberg uncertainty principle.Energy fluctuations create pairs of particles and antiparticles (e.g. 2 photons). A pair can exist momentarily and is therefore virtual. They annihilate quickly. However, the virtual particles can become real, if the intense curvature of the spacetime of the black hole puts energy to the pair.
20Real Pairs CreatedNear a black hole, the tidal force is so strong that the virtualpairs are pulled apart. The two virtual particles can become real.
21Hawking TemperatureSince the photons can be formed outside the event horizon, they can be emitted away from the black hole. This is called Hawking radiation.The Hawking radiation has a blackbody spectrum, with the temperature given byThe temperature is called the Hawking temperature. It decreases with the mass of the black hole.
22Humour: Hawking StyleEinstein (on quantum mechanics): God does not play dice with the universe.Bohr (defending quantum mechanics): Einstein should not tell God what to do!Hawking (on radiation from black holes): God not only plays dice but also sometimes throws them where they cannot be seen.
23Evaporation of Black Holes The energy needed to create the real particles comes from the gravitational field of the black hole.Hence, the emission of Hawking radiation reduces the mass of the black hole. As the process continues, the black hole will finally disappear. This is called black hole evaporation.Small black holes have a large tidal force near the event horizon, and the creation of the real particles is easier. Hawking radiation will be more significant.In fact, the time required for evaporation is given by
24Typical Time of Evaporation Black hole with mass aboutTime for evaporationA man10-12 secondsA building4 secondsThe Earth1049 yearsThe Sun1066 yearsA galaxy1099 yearsFor reference, the age of the universe is about 1010 years.
25Luminosity of Radiation For most black holes, the Hawking radiation is too small to be detected.A black hole with detectable Hawking radiation must be very small, with Schwarzschild radius comparable to atomic nucleus. There is no real observational evidence for this kind of black holes so far. These tiny black holes are called primordial black holes, which is believed to be formed in the very early universe.The emission of Hawking radiation reduces the entropy of a black hole. However, the second law of thermodynamics is not violated, since we have to count the entropy of the radiation as well.
26Size DependenceHawking radiation suggests that black holes must have a finite temperature. The temperature of a black hole is given byThe smaller a black hole, the higher the temperature, and therefore the stronger the Hawking radiation.For the primordial black holes, the temperature is extremely high.
27Detection of Black Holes If light cannot escape a black hole, how can we ever find them?Improvements in observational astronomy render the detection of black holes more than a theoretical speculation.Nowadays, we have found many candidates of black holes. Many of them are X-ray binaries.
35Baby Universes in Black Holes In 1980s, Thorne thought about wormholes, and Hawking thought about baby universes.If an object falls into a black hole, it could go to an independent baby universe.Science fiction? Can one travel to the past or another universe?Beware of spaghettification!Implication: there can be information loss in black holes.In 1992, Hawking concluded that the universe is “safe for historians”.
36Another BetIn 1997, Hawking and Thorne bet with Preskill on the black hole information paradox.Hawking and Thorne: The information crossing into the event horizon of a black hole is lost to our universe; the black hole emits the same radiation regardless of what falls into it.Preskill: The information will be eventually released.In 2004, Hawking conceded and admitted that black holes eventually transmit, in a garbled form, information about all matter they swallow.Preskill was awarded an encyclopedia of baseball, from which “information can be recovered at will”.
37ConclusionHawking’s contribution to the theory of black holes (structure, no hair theorem, radiation, information and entropy).Hawking’s work is confirmed by experiments (Cygnus X-1).Hawking’s openmindedness (bet concessions).Hawking’s attitude towards life (adversity, science, humour).Hawking’s eagerness to popularize science.