Fay Dowker

Pair creation of extremal black holes and Kaluza-Klein monopoles.

Classical solutions describing charged dilaton black holes accelerating in a background magnetic field have recently been found. They include the Ernst metric of the Einstein-Maxwell theory as a special case. We study the extremal limit of these solutions in detail, both at the classical and quantum levels. It is shown that near the event horizon the extremal solutions reduce precisely to the static extremal black hole solutions. For a particular value of the dilaton coupling, these extremal black holes are five-dimensional Kaluza-Klein monopoles. The Euclidean sections of these solutions can be interpreted as instantons describing the pair creation of extremal black holes and/or Kaluza-Klein monopoles in a magnetic field. The action of these instantons is calculated and found to agree with the Schwinger result in the weak-field limit. For the Euclidean Ernst solution, the action for the extremal solution differs from that of the previously discussed wormhole instanton by the Bekenstein-Hawking entropy. However, in many cases quantum corrections become large in the vicinity of the black hole, and the precise description of the creation process is unknown.

Properties of consistent histories.

We describe some properties of consistent sets of histories in the Gell-Mann--Hartle formalism, and give an example to illustrate that one cannot recover the standard predictions, retrodictions, and inferences of quasiclassical physics using the criterion of consistency alone.

Causal sets as discrete spacetime

Causal set theory is an approach to the problem of quantum gravity in which spacetime is fundamentally discrete and in which causality is a primary concept. The concrete kinematics of the approach makes it relatively straightforward to build phenomenological models that produce observable effects of the underlying discreteness. An example is a model of a massive free particle moving on a causal set background which gives rise to a Lorentz invariant diffusion in the particles momentum, a novel and potentially fruitful effect.

The Status of the Wave Function in Dynamical Collapse Models

No HeadingThe idea that in dynamical wave function collapse models the wave function is superfluous is investigated. Evidence is presented for the conjecture that, in a model of a field theory on a 1+1 lightcone lattice, knowing the field configuration on the lattice back to some time in the past, allows the wave function or quantum state at the present moment to be calculated, to arbitrary accuracy so long as enough of the past field configuration is known.

Spontaneous Collapse Models on a Lattice

We present spontaneous collapse models of field theories on a 1+1 null lattice, in which the causal structure of the lattice plays a central role. Issues such as “locality,” “nonlocality,” and superluminal signaling are addressed in the context of the models which have the virtue of extreme simplicity. The formalism of the models is related to that of the consistent histories approach to quantum mechanics.

Euclidean black-hole vortices

We argue the existence of solutions of the Euclidean Einstein equations that correspond to a vortex sitting at the horizon of a black hole. We find the asymptotic behaviors, at the horizon and at infinity, of vortex solutions for the gauge and scalar fields in an Abelian Higgs model on a Euclidean Schwarzschild background and interpolate between them by integrating the equations numerically. Calculating the back reaction shows that the effect of the vortex is to cut a slice out of the Euclidean Schwarzschild geometry. The consequences of these solutions for black-hole thermodynamics are discussed.

U duality, D-branes, and black hole emission rates: Agreements and disagreements

An expression for the spacetime absorption coefficient of a scalar field in a five dimensional, near extremal black hole background is derived, which has the same form as that presented by Maldacena and Strominger, but is valid over a larger, U-duality invariant region of parameter space and in general disagrees with the corresponding D-brane result. We develop an argument, based on D-brane thermodynamics, which specifies the range of parameters over which agreement should be expected. For neutral emission, the spacetime and D-brane results agree over this range. However, for charged emission, we find disagreement in the ‘Fat Black Hole’ regime, in which charge is quantized in smaller units on the brane, than in the bulk of spacetime. We indicate a possible problem with the D-brane model in this regime. We also use the Born approximation to study the high frequency limit of the absorption coefficient and find that it approaches unity, for large black hole backgrounds, at frequencies still below the string scale, again in disagreement with D-brane results.

Simulating causal wavefunction collapse models

We present simulations of causal dynamical wavefunction collapse models of field theories on a 1 + 1 null lattice. We use our simulations to compare and contrast two possible interpretations of the models, one in which the field values are real and the other in which the state vector is real. We suggest that a procedure of coarse graining and renormalizing the fundamental field can overcome its noisiness and argue that this coarse grained renormalized field will show interesting structure if the state vector does on the coarse grained scale. We speculate on the implications for quantum gravity.

Causal Sets and Discrete Spacetime

Causal set theory is an approach to the problem of quantum gravity in which spacetime is fundamentally discrete and in which causal structure is a primary concept. Causal set quantum gravity was used successfully to predict the current order of magnitude of the Cosmological Term, Λ. The concrete kinematics of the approach makes it relatively straightforward to build further phenomenological models that produce observable effects of the underlying discreteness. As an example, a model of a massive free particle moving on a causal set background is given. This gives rise to a novel Lorentz invariant effect: diffusion in the particle’s momentum which may have phenomenological consequences.

Noncontextuality in Quantum Measure Theory

The Clauser‐Horne‐Shimony‐Holt‐Bell inequalities are necessary conditions for a set of no‐signalling probabilities for two measurers each with two alternative experiments each with two possible outcomes to admit a joint probability distribution. An analogue of these inequalities in the context of quantum measure theory is presented. Talk given by Fay Dowker.