BlackHole Thermodynamics PHYS 4315 R S Rubins Fall

Black-Hole Thermodynamics PHYS 4315 R. S. Rubins, Fall 2009 1

Quantum Fluctuations of the Vacuum • The uncertainty principle applied to electromagnetic fields indicates that it is impossible to find both E and B fields to be zero at the same time. • The quantum fluctuations of the vacuum so produced cannot be detected by normal instruments, because they carry no energy. • However, they may be detected by an accelerating detector, which provides a source of energy. • The accelerating observer would measure a temperature of the vacuum (the Unruh temperature), given by TU = aħ/2πc. Notes i. For an acceleration of 1019 m/s 2, TU ~ 1 K. ii. TU = 0 if either ħ =0 or c = ∞, which is the classical result. 2

Zeroth Law of Black-Hole Mechanics Zeroth law • The horizon of a stationary black hole has a uniform surface gravity κ. Thermodynamic analogy • An object in thermal equilibrium with a heat reservoir has a uniform temperature T. Relationship between κ and T • Analogous to the Unruh effect , Hawking showed that black holes emit Hawking radiation at a temperature TH, given by TH = ħκ/2πc, where κ may be thought of as the magnitude of the acceleration needed by a spaceship to just counteract the gravitational acceleration just outside the event horizon. 3

Entropy of a Black Hole • Black holes must carry entropy, because the 2 nd law of thermodynamics requires that the loss of entropy of an object falling into a black hole must at least be compensated by the increase of entropy of the black hole. • The expression for the entropy of a black hole, obtained by Beckenstein, and later confirmed by Hawking is SBH = k. Ac 3/4 Għ, where k is Boltzmann’s constant, A is the area of the black hole’s horizon, and BH could stand for black hole or Beckenstein-Hawking. • A system of units with c=1 gives SBH = k. A/4 Għ, while one in which c=1, ħ=1, k=1 and G=1 gives SBH = A/4, showing that a 4 black-hole’s entropy is proportional to the area of its horizon.

First Law of Black-Hole Mechanics 1 st law d. M = (κ/8π) d. A + Ω d. J + Φ d. Q, where M is the mass, Ω is the angular velocity, J is the angular momentum, Φ is the electric potential, Q is the charge, and the constants c, ħ, k, and G are all made equal to unity. Thermodynamic analogy d. U = T d. S – P d. V Relationship between (κ/8π)d. A and Td. S • Since TH = κ/2π and SBH = A/4, (κ/8π) d. A = (2πTH)(1/8π)(4 d. SBH) = THd. SBH; i. e. the first term is just the product of the black-hole temperature and its change of entropy. 5

Second Law of Black-Hole Mechanics 2 nd law • The area A of the horizon of a black hole is a non-decreasing function of time; i. e. ΔA ≥ 0. Thermodynamic analogy • The entropy of an isolated system is a non-decreasing function of time; i. e. ΔS ≥ 0. Hawking radiation • If the quantum fluctuations of the vacuum produces a particleantiparticle pair near the horizon of a black hole, and the antiparticle drops into the hole, the particle will appear to have come from the black hole, which loses entropy. • This leads to a generalized 2 nd law: Δ[Soutside + (A/4)] ≥ 0. 6