arXiv subjects

A. N. St. J. Farley

Publications and source records attributed to A. N. St. J. Farley.

13 recordsLinked to original sources

arXiv2010

Quantum Amplitudes in Black-Hole Evaporation: Complex Approach and Spin-0 Amplitude

We consider the quantum-mechanical decay of a Schwarzschild-like black hole formed by gravitational collapse into almost-flat space-time and weak radiation at a late time. We evaluate quantum amplitudes (not just probabilities) for transitions from initial to final states, and show that no information is lost in collapse to a black hole. Boundary data for the gravitational field and a scalar field are posed on an initial space-like hypersurface $Σ_I$ and a final surface $Σ_F$. These asymptotically-flat 3-surfaces are separated by a Lorentzian proper-time interval $T$, measured at spatial infinity. The boundary-value problem is made well-posed, classically and quantum-mechanically, by rotating $T$ into the lower-half complex plane: $T\to {\mid}T{\mid}\exp(-iθ), 0<θ\leqπ/2$. This corresponds to Feynman's $+iε$ prescription. For the classical boundary-value problem, we calculate the second-variation classical Lorentzian action $S^{(2)}_{class}$ as a functional of the boundary data. Following Feynman, the Lorentzian quantum amplitude is recovered in the limit $θ\to 0_{+}$ from the well-defined complex-$T$ amplitude. Dirac's canonical approach to the quantisation of constrained systems shows that for locally-supersymmetric theories of gravity the amplitude is exactly semi-classical: $\exp(i S^{(2)}_{class})$ for weak perturbations, apart from delta-functionals of the supersymmetry constraints. We treat such quantum amplitudes for weak scalar-field configurations on $Σ_F$, taking the weak final gravitational field to be spherically symmetric. The treatment involves adiabatic solutions of the scalar wave equation. This extends our previous work, giving explicit expressions for the real and imaginary parts of such quantum amplitudes.

gr-qc↗

arXiv2007

Quantum amplitudes in black-hole evaporation: Spins 1 and 2

Quantum amplitudes for $s=1$ at Maxwell fields and for $s=2$ linearised gravitational wave perturbations of a spherically symmetric Einstein/massless scalar background, describing gravitational collapse to a black hole, are treated by analogy with a previous treatment of $s=0$ scalar-field perturbations of gravitational collapse at late times. In both the $s=1$ and $s=2$ cases, we isolate suitable 'co-ordinate' variables which can be taken as boundary data on a final space-like hypersurface $Σ_F$. For simplicity, we take the data on an initial pre-collapse surface $Σ_I$ to be exactly spherically symmetric. The (large) Lorentzian proper-time interval between $Σ_{I}, Σ_{F}$, measured at spatial infinity, is denoted by $T$. The complexified classical boundary-value problem is expected to be well-posed, provide that the time interval $T$ has been rotated into the complex: $T\to{\mid}T{\mid}\exp(-iθ)$, for $0<θ\leqπ/2$. We calculate the second-variation classical Lorenztian action $S ^{(2)}_{\rm class}$. Following Feynman, we recover the Lorentzian quantum amplitude by taking the limit as $θ\to 0_+$ of the semi-classical amplitude $\exp(iS^{(2)}_{\rm class})$. The boundary data for $ s=1$ involve the Maxwell magnetic field; the data for $s=2$ involve the magnetic part of the Weyl curvature tensor. The magnetic boundary conditions are related to each other and to the natural $s={1 \over 2}$ boundary conditions by supersymmetry.

gr-qc↗

arXiv2007

Quantum amplitudes in black-hole evaporation: coherent and squeezed states

The quantum amplitude for processes involving the formation and evaporation of black holes was previously calculated by means of a complex-time approach. In that treatment, we followed Feynman's $+iε$ approach in quantum field theory. The Lorentzian time interval $T$, measured at spatial infinity between a pair of asymptotically flat spacelike hypersurfaces $Σ_I$ and $Σ_F$ carrying initial and final boundary data for the gravitational and other fields, is rotated: $T\to{\mid}T{\mid}\exp(-iδ)$, where $0< δ\leqπ/2$. Classically and quantum mechanically, this procedure is expected to lead to a well-posed boundary-value problem. Thus, we have found quantum amplitudes (not just probability densities) relating to a pure state at late times following gravitational collapse of matter to a black hole. Such pure states, arising from gravitational collapse, admit a description in terms of coherent and squeezed states. Indeed, this description is not so different from that arising in a well-known context, namely, the highly-squeezed final state of the relic radiation background in inflationary cosmology. For definiteness, we study the simplest model of collapse, based on Einstein gravity with a massless scalar field. Following the complex rotation above, one finds that, in an adiabatic approximation, the resulting quantum amplitude may be expressed in terms of generalized coherent states of the harmonic oscillator. A physical interpretation is given; further, a squeezed-state representation follows.

gr-qc↗

arXiv2007

Relic Radiation from an Evaporating Black Hole

We present a non-string-theoretic calculation of the microcanonical entropy of relic integer-spin Hawking radiation -- at fixed total energy $E$. The only conserved macroscopic quantity is the total energy $E$ (the total energy of the relic radiation). Data for a boundary-value approach, with massless, integer-spin perturbations, are set on initial and final space-like hypersurfaces. In the resulting 1-dimensional statistical-mechanics problem, the real part of the (complex) time separation at spatial infinity, $T = {\mid}T{\mid}\exp(-iδ), δ>0$, is the variable conjugate to the total energy. We count the number of weak-field configurations on the final space-like hypersurface with energy $E$. One recovers the Cardy formula and the Bekenstein-Hawking entropy, if Re(T) is of the order of the black-hole life- time, leading to a statistical interpretation of black-hole entropy. The microcanonical entropy includes a logarithmic correction to the black-hole area law, which is {\it universal} (independent of black-hole parameters). Here, the discreteness of the energy levels is crucial. This approach is compared with that of string theory for the transition to the fundamental-string régime in the final stages of evaporation. The squared coupling, $g^2$, regulating the transition to a highly-excited string state and {\it vice versa}, can be related to the angle, $δ$, of complex-time rotation above. The strong-coupling régime corresponds to a Euclidean black hole, while the physical limit of a Lorentzian space-time (as $ δ\to 0_+$) corresponds to the weak-coupling régime. This resembles the transition to a highly-excited string-like state which subsequently decays into massless particles, thereby avoiding the naked singularity.

hep-th↗

arXiv2006

Coherent and squeezed states in black-hole evaporation

In earlier Letters, we adopted a complex approach to quantum processes in the formation and evaporation of black holes. Taking Feynman's $+iε$ prescription, rather than than one of the more usual approaches, we calculated the quantum amplitude (not just the probability density) for final weak-field configurations following gravitational collapse to a black hole with subsequent evaporation. What we have done is to find quantum amplitudes relating to a pure state at late times following black-hole matter collapse. Such pure states are then shown to be susceptible to a description in terms of coherent and squeezed states - in practice, this description is not very different from that for the well-known highly-squeezed final state of the relic radiation background in inflationary cosmology. The simplest such collapse model involves Einstein gravity with a massless scalar field. The Feynman approach involves making the boundary-value problem for gravity and a massless scalar field well-posed. To define this, let T be the proper-time separation, measured at spatial infinity, between two space-like hypersurfaces on which initial (collapse) and final (evaporation) data are posed. Then, in this approach, one rotates T by a complex phase exp(-iδ) into the lower half-plane. In an adiabatic approximation, the resulting quantum amplitude may be expressed in terms of generalised coherent states of the quantum oscillator, and a physical interpretation is given. A squeezed-state representation, as above, then follows.

gr-qc↗

arXiv2005

Bogoliubov transformations in black-hole evaporation

Our boundary-value approach to quantum processes in the gravitational collapse to a black hole leads to quantum amplitudes (not just probabilities) for transitions between data posed on initial and final hypersurfaces $Σ_{I,F}$, separated by a Lorentzian proper-time interval $T$, measured at spatial infinity. Following Feynman's $+iε$ approach, we rotate: $T\to {\mid}T{\mid} \exp(-iθ)$, for $0<θ\leqπ/2$. The {\it classical} complexified boundary-value problem is expected to be well-posed for $0<θ\leqπ/2$, with classical action $S_{\rm class}$. For a locally supersymmetric Lagrangian, containing supergravity, possibly coupled to supermatter, the resulting quantum amplitude will be proportional to $\exp(iS_{\rm class})$, apart from possible loop corrections which are negligible for boundary data with frequencies below the Planck scale. The Lorentzian quantum amplitude is recovered by taking the limit as $θ\to 0_+$ of this amplitude. In the present paper, a connection is made between this boundary value approach and the original approach to quantum evaporation in gravitational collapse to a black hole, {\it via} Bogoliubov coefficients. This connection is developed through consideration of the radial equation obeyed by the (adiabatic) non-spherical classical perturbations. When one studies the resulting final probability distribution, based on our quantum amplitudes above, one finds that this distribution can also be interpreted in terms of the Wigner quasi-probability distribution for harmonic oscillators.

gr-qc↗

arXiv2005

Vaidya Space-Time in Black-Hole Evaporation

Recently we have studied, using a boundary-value approach, quantum amplitudes resulting from gravitational collapse to a black hole. Suitable boundary data for all fields present are posed on initial and final space-like asymptotically flat hypersurfaces $Σ_{I,F}$. The Lorentzian proper-time separation between the surfaces, as measured at spatial infinity, is denoted by $T$. Following Feynman's $+iε$ approach, we rotate $T$ into the complex: $T\to {\mid}T{\mid} \exp(-iθ)$, where $0<θ\leqπ/2$. The corresponding {\it classical} complex boundary-value problem is expected to be well-posed for $θ> 0$. The Lorentzian amplitude is found by taking the limit $θ\to 0_+$ of the quantum amplitude, itself closely approximated by the semi-classical expression $\exp(iS_{\rm class})$, where $S_{\rm class}$ is the classical action. For given weak anisotropic spin-0 and spin-2 boundary data on $Σ_F$, one can compute an effective classical energy-momentum tensor in the interior, which has been averaged over several wave-lengths of the radiation. This averaged extra contribution will be spherically symmetric, equivalent to a null fluid, and describing the radial outward streaming of the radiation (of quantum origin). The corresponding space-time metric, in this region containing radially-outgoing radiation, is of the Vaidya form. This, in turn, justifies the treatment of the adiabatic radial mode equations, for spins $s=0$ and $s=2$, which is used throughout this larger project.

gr-qc↗

arXiv2005

Spin-2 Amplitudes in Black-Hole Evaporation

Quantum amplitudes for $s=2$ gravitational-wave perturbations of Einstein/scalar collapse to a black hole are treated by analogy with $s=1$ Maxwell perturbations. The spin-2 perturbations split into parts with odd and even parity. We use the Regge-Wheeler gauge; at a certain point we make a gauge transformation to an asymptotically-flat gauge, such that the metric perturbations have the expected falloff behaviour at large radii. By analogy with $s=1$, for $s=2$ natural 'coordinate' variables are given by the magnetic part $H_{ij} (i,j=1,2,3)$ of the Weyl tensor, which can be taken as boundary data on a final space-like hypersurface $Σ_F$. For simplicity, we take the data on the initial surface $Σ_I$ to be exactly spherically-symmetric. The (large) Lorentzian proper-time interval between $Σ_I$ and $Σ_F$, measured at spatial infinity, is denoted by $T$. We follow Feynman's $+iε$ prescription and rotate $T$ into the complex: $T\to{\mid}T{\mid} \exp(-iθ)$, for $0<θ\leqπ/2$. The corresponding complexified {\it classical} boundary-value problem is expected to be well-posed. The Lorentzian quantum amplitude is recovered by taking the limit as $θ\to 0_+$. For boundary data well below the Planck scale, and for a locally supersymmetric theory, this involves only the semi-classical amplitude $\exp(iS^{(2)}_{\rm class}$, where $S^{(2)}_{\rm class}$ denotes the second-variation classical action. The relations between the $s=1$ and $s=2$ natural boundary data, involving supersymmetry, are investigated using 2-component spinor language in terms of the Maxwell field strength $ϕ_{AB}=ϕ_{(AB)}$ and the Weyl spinor $Ψ_{ABCD}=Ψ_{(ABCD)}$.

gr-qc↗

arXiv2005

Quantum Amplitudes in Black-Hole Evaporation I. Complex Approach

Here we examine the quantum-mechanical decay of a Schwarzschild-like black hole, formed by gravitational collapse, into almost-flat space-time and weak radiation at a very late time, in order to evaluate quantum amplitudes (not just probabilities) for final states. No information is lost in collapse to a black hole. Boundary data are specified on initial and final hypersurfaces $Σ_{I, F}$, separated by a Lorentzian proper-time interval $T$, as measured at spatial infinity. For simplicity, consider Einstein gravity coupled minimally to a massless scalar field $ϕ$. In Lorentzian signature, the classical Dirichlet boundary-value problem, corresponding to specification of the intrinsic spatial metric $h_{ij} (i,j =1,2,3)$ and $ϕ$ on the bounding surfaces, is badly posed, being a boundary-value problem for a wave-like (hyperbolic) set of equations. Following Feynman's $+iε$ prescription, the problem is made well-posed by rotating the asymptotic time interval $T$ into the complex: $T\to{\mid} T{\mid}\exp(-iθ)$, with $0<θ\leqπ/2$. After calculating the amplitude for $θ>0$, one takes the 'Lorentzian limit' $θ\to 0_+$ to obtain the Lorentzian quantum amplitude.

gr-qc↗

arXiv2005

Quantum Amplitudes in Black-Hole Evaporation II. Spin-0 Amplitude

This work on spin-0 amplitudes in black-hole evaporation is based on the underlying results and methods outlined in our first paper, "I. Complex Approach". The main result here, and the model calculation for work on all higher spins, as described in several further papers, is the computation of the quantum amplitude (rather than merely the probability) for a given slightly anisotropic configuration of a scalar field $ϕ$ on a space-like hypersurface $Σ_F$ at a very late time $T$. For simplicity, one may take the initial data for gravity and the massless scalar field at an initial surface $Σ_I$ to be spherically symmetric. This applies to perturbations of spherically-symmetric collapse to a black hole, starting from a diffuse, nearly-stationary configuration, where the bosonic part of the Lagrangian consists of Einstein gravity and the massless scalar field. As in Paper I, Feynman's $+iε$ approach is taken; this involves a rotation into the complex: $T\to {\mid}T{\mid} \exp (-iθ)$, with $0<θ\leqπ/2$. A complex solution of the classical boundary-value problem is expected to exist, provided $θ>0$; although for $θ=0$ (Lorentzian time-separation), the classical boundary-value problem is badly posed. Once the amplitude is found for $θ>0$, one can take the limit $θ\to 0_+$ to find the Lorentzian amplitude. The paper also includes a discussion of adiabatic solutions of the scalar wave equation, needed for the spin-0 calculation.

gr-qc↗

arXiv2005

Scalar--Field Amplitudes in Black--Hole Evaporation

We study the quantum-mechanical decay of a Schwarzschild-like black hole into almost-flat space and weak radiation at a very late time, evaluating quantum amplitudes (not just probabilities) for transitions from initial to final states. No information is lost. The model contains gravity and a massless scalar field. The quantum amplitude to go from given initial to final bosonic data in a slightly complexified time-interval $T=τ{\exp}(-iθ)$ at infinity is approximately $\exp(-I)$, where $I$ is the (complex) Euclidean action of the classical solution filling in between the boundary data. And in a locally supersymmetric (supergravity) theory, the amplitude const. exp(-I) is exact. Dirichlet boundary data for gravity and the scalar field are posed on an initial spacelike hypersurface extending to spatial infinity, just prior to collapse, and on a corresponding final spacelike surface, sufficiently far to the future of the initial surface to catch all the Hawking radiation. In an averaged sense this radiation has an approximately spherically-symmetric distribution. If the time-interval $T$ were exactly real, the resulting `hyperbolic Dirichlet boundary-value problem' would not be well posed. If instead (`Euclidean strategy'), one takes $T$ complex, as above ($0<θ{\leq}π/2$), the field equations become strongly elliptic, with a unique solution to the classical boundary-value problem. Expanding the bosonic part of the action to quadratic order in perturbations about the classical solution gives the quantum amplitude for weak-field final configurations, up to normalization. Such amplitudes are calculated for weak final scalar fields.

gr-qc↗

arXiv2005

Bogoliubov transformations for amplitudes in black-hole evaporation

The familiar approach to quantum radiation following collapse to a black hole proceeds via Bogoliubov transformations, and yields probabilities for final outcomes. In our (complex) approach, we find quantum amplitudes, not just probabilities, by following Feynman's $+iε$ prescription. Initial and final data for Einstein gravity and (say) a massless scalar field are specified on a pair of asymptotically-flat space-like hypersurfaces $Σ_I$ and $Σ_F$; both are diffeomorphic to ${\Bbb R}^3$. Denote by $T$ the (real) Lorentzian proper-time interval between the surfaces, as measured at spatial infinity. Then rotate: $T\to{\mid}T{\mid}\exp(-iθ),0<θ\leq π/2$. The {\it classical} boundary-value problem is expected to be well-posed on a region of topology $I\times{\Bbb R}^3$, where $I$ is a closed interval. For a locally-supersymmetric theory, the quantum amplitude should be dominated by the semi-classical expression $\exp(iS_{\rm class})$, where $S_{\rm class}$ is the classical action. One finds the Lorentzian quantum amplitude from the limit $θ\to 0_+$. In the usual approach, the only possible such final surfaces are in the strong-field region shortly before the curvature singularity. In our approach one can put arbitrary smooth gravitational data on $Σ_F$, provided that it has the correct mass $M$ -- the singularity is by-passed in the analytic continuation. Here, we consider Bogoliubov transformations and their possible relation to the probability distribution and density matrix in the traditional approach. We find that our probability distribution for configurations of the final scalar field cannot be expressed in terms of the diagonal elements of some non-trivial density-matrix distribution.

gr-qc↗

arXiv2005

Spin-1 Amplitudes in Black-Hole Evaporation

Our earlier work on the quantum amplitude for a scalar field in black-hole evaporation, following gravitational collapse, is here extended to Maxwell theory. Boundary data are specified on initial and final space-like hypersurfaces $Σ_{I,F}$, separated by a large Lorentzian proper-time interval $T$, as measured at spatial infinity. The initial boundary data may be chosen (say) to be spherically symmetric, corresponding to a nearly-spherical configuration prior to gravitational collapse. The final data include the intrinsic 3-metric and scalar field, restricted to $Σ_F$, in addition to spin-1 data, naturally taken to be the magnetic field $B_i$ on $Σ_{I,F} (i=1,2,3)$. For a locally-supersymmetric theory, the quantum amplitude should be proportional to $\exp(iS_{\rm class})$, apart from corrections which are very small when the frequencies in the boundary data are small compared to the Planck scale. Here, $S_{\rm class}$ is the action of the classical solution. The Lorentzian amplitude is found by taking the limit $θ\to 0_+$. By a method similar to that used in the spin-0 case, one obtains the quantum amplitude for photon data on $Σ_F$. The magnetic boundary conditions are related by supersymmetry to the natural spin-2 (gravitational-wave) boundary conditions, which involve fixing the magnetic part of the Weyl tensor.

gr-qc↗