William G. Unruh

300 Years of Gravitation

The invention relates to a novel filtering circuit whereby a phase detected output control signal is filtered and utilized to provide greater stability in a phase-locked loop. The filtering circuit is placed in the connecting path between a phase detector and an oscillator; the oscillator supplies output signals, having a repetition rate which depends on the instantaneous values of the filtered control signal. The filtering circuit comprises an adder which adds the phase detected control signal to a signal corresponding to the preceding filtered control signal multiplied by a constant factor which is less than unity (<1).

Sonic analogue of black holes and the effects of high frequencies on black hole evaporation.

The naive calculation of black hole evaporation makes the thermal emission depend on the arbitrary high frequency behavior of the theory where the theory is certainly wrong. Using the sonic analogue to black holes, ‘‘dumb holes,’’ I show numerically that a change in the dispersion relation at high frequencies does not seem to alter the evaporation process, lending weight to the reality of the black hole evaporation process. I also suggest a reason for the insensitivity of the process to the high frequency regime.

Maintaining coherence in quantum computers

The effects of the inevitable coupling to external degrees of freedom of a quantum computer are examined. It is found that for quantum calculations (in which the maintenance of coherence over a large number of states is important), not only must the coupling be small, but the time taken in the quantum calculation must be less than the thermal time scale \ensuremath{\Elzxh}/

Measurement of stimulated Hawking emission in an analogue system

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Gravity wave analogues of black holes

T. For longer times the condition on the strength of the coupling to the external world becomes much more stringent.

Quantum analogues : from phase transitions to black holes and cosmology

Hawking argued that black holes emit thermal radiation via a quantum spontaneous emission. To address this issue experimentally, we utilize the analogy between the propagation of fields around black holes and surface waves on moving water. By placing a streamlined obstacle into an open channel flow we create a region of high velocity over the obstacle that can include surface wave horizons. Long waves propagating upstream towards this region are blocked and converted into short (deep-water) waves. This is the analogue of the stimulated emission by a white hole (the time inverse of a black hole), and our measurements of the amplitudes of the converted waves demonstrate the thermal nature of the conversion process for this system. Given the close relationship between stimulated and spontaneous emission, our findings attest to the generality of the Hawking process.

Universality of the Hawking effect

It is demonstrated that gravity waves of a flowing fluid in a shallow basin can be used to simulate phenomena around black holes in the laboratory. Since the speed of the gravity waves as well as their high-wave-number dispersion (subluminal vs superluminal) can be adjusted easily by varying the height of the fluid (and its surface tension) this scenario has certain advantages over the sonic and dielectric black hole analogs, for example, although its use in testing quantum effects is dubious. It can be used to investigate the various classical instabilities associated with black (and white) holes experimentally, including positive and negative norm mode mixing at horizons.

Measurement of time of arrival in quantum mechanics

The Analogue Between Rimfall and Black Holes.- Effective Horizons in the Laboratory.- Quantum Phase Transitions from Topology in Momentum Space.- Superfluid 3He as a Model System for Cosmology - Experimental Point of View.- Dynamical Aspects of Analogue Gravity: The Backreaction of Quantum Fluctuations in Dilute Bose-Einstein Condensates.- Analogue Space-time Based on 2-Component Bose-Einstein Condensates.- Links. Relating Different Physical Systems Through the Common QFT Algebraic Structure.- The Classical and Quantum Roots of Paulis Spin-statistics Relation.- Black Hole Lasers Revisited.- Cosmic Strings.

Hawking radiation in an electromagnetic waveguide

Addressing the question of whether the Hawking effect depends on degrees of freedom at ultrahigh (e.g., Planckian) energies/momenta, we propose three rather general conditions on these degrees of freedom under which the Hawking effect is reproduced to lowest order. As a generalization of Corleys results, we present a rather general model based on nonlinear dispersion relations satisfying these conditions together with a derivation of the Hawking effect for that model. However, we also demonstrate counter-examples, which do not appear to be unphysical or artificial, displaying strong deviations from Hawkings result. Therefore, whether real black holes emit Hawking radiation remains an open question and could give nontrivial information about Planckian physics.

Cosmological density perturbations with modified gravity

It is argued that the time of arrival cannot be precisely defined and measured in quantum mechanics. By constructing explicit toy models of a measurement, we show that for a free particle it cannot be measured more accurately then