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W. G. Unruh

Publications and source records attributed to W. G. Unruh.

35 records · Page 2Linked to original sources
arXiv2003

On Slow Light as a Black Hole Analogue

Although slow light (electromagnetically induced transparency) would seem an ideal medium in which to institute a ``dumb hole'' (black hole analog), it suffers from a number of problems. We show that the high phase velocity in the slow light regime ensures that the system cannot be used as an analog displaying Hawking radiation. Even though an appropriately designed slow-light set-up may simulate classical features of black holes -- such as horizon, mode mixing, Bogoliubov coefficients, etc. -- it does not reproduce the related quantum effects. PACS: 04.70.Dy, 04.80.-y, 42.50.Gy, 04.60.-m.

gr-qc↗
arXiv2002

Large Universe as the initial condition for a Collapsing Universe

Using the extension of the standard Hawking-Hartle prescription for defining a wave function for the universe, we show that it is possible, given a suitable form for the scalar field potential, to have the universe begin at its largest size and thereafter contract, with the growth of perturbations proceeding from small, at the largest size, to largest when the universe is small. This solution would dominate the wave function by an exponentially large amount if one chooses the Hartle Hawking prescription for the wave-function, but is exponentially sub-dominant for the Linde-Vilenkin prescription.

gr-qc↗
arXiv1999

Dumb 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 behaviour of the theory where the theory is certainly wrong. Using the sonic analog 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 high frequency regime.

gr-qc↗
arXiv1998

Time as an Observable

The role of time in quantum mechanics is discussed. The differences between ordinary observables and an observable which corresponds to the time of an event is examined. In particular, the time-of-arrival of a particle to a fixed location is not an ordinary quantum mechanical observable. While we can measure if the particle arrives, we argue that the time at which it arrives always has an inherent ambiguity. The minimum inaccuracy of time-of-arrival measurements is given by dt>1/E where E is the kinetic energy of the particle. The use of time-of-arrival operators, as well as current operators, is examined critically.

quant-ph↗
arXiv1998

Time-of-Arrival States

Although one can show formally that a time-of-arrival operator cannot exist, one can modify the low momentum behaviour of the operator slightly so that it is self-adjoint. We show that such a modification results in the difficulty that the eigenstates are drastically altered. In an eigenstate of the modified time-of-arrival operator, the particle, at the predicted time-of-arrival, is found far away from the point of arrival with probability 1/2.

quant-ph↗
arXiv1998

When does a Measurement or Event Occur

Within quantum mechanics it is possible to assign a probability to the chance that a measurement has been made at a specific time t. However, the interpretation of such a probability is far from clear. We argue that a recent measuring scheme of Rovelli's (quant-ph/9802020) yields probabilities which do not correspond to the conventional probabilities usually assigned in quantum mechanics. The same arguments also apply to attempts to use the probability current to measure the time at which a particle arrives at a given location.

quant-ph↗
arXiv1998

Minimum Inaccuracy for Traversal-Time

Using various model clocks it has been shown that the time-of-arrival cannot be measured more accurately than 1/E where E is the kinetic energy of a free particle. However, this result has never been proved. In this paper, we show that a violation of the above limitation for the transit-time, implies a violation of the Heisenberg uncertainty relation.

quant-ph↗
arXiv1998

Is Quantum Mechaincs Non-Local?

Stapp has recently argued from a version of the Hardy type experiments that quantum mechanics must be non-local, independent of any additional assumptions like realism or hidden variables. I argue that either his conclusions do not follow from his assumptions, or that his assumptions are not true of quantum mechanics and can be interpreted as assigning an unwarranted level of reality to the value of certain quantum attributes.

quant-ph↗
arXiv1998

Acceleration Radiation for Orbiting Electrons

This paper presents an analysis of the radiation seen by an observer in circular acceleration, for a magnetic spin. This is applied to an electron in a storage ring, and the subtilty of the interaction of the spin with the spatial motion of the electron is explicated. This interaction is shown to be time dependent (in the radiating frame), which explains the strange results found for the electron's residual polarisation in the literature. Finally, some brief comments about the radiation emitted by an accelerating detector are made where it is shown that the spectrum is correlated in that particles are emitted in pairs.

hep-th↗
arXiv1998

Radiation Reaction fields for an accelerated dipole for scalar and electromagnetic radiation

The radiation reaction fields are calculated for an accelerated changing dipole in scalar and electromagnetic radiation fields. The acceleration reaction is shown to alter the damping of a time varying dipole in the EM case, but not the scalar case. In the EM case, the dipole radiation reaction field can exert a force on an accelerated monopole charge associated with the accelerated dipole. The radiation reaction of an accelerated charge does not exert a torque on an accelerated magnetic dipole, but an accelerated dipole does exert a force on the charge. The technique used is that originally developed by Penrose for non-singular fields and extended by the author for an accelerated monopole charge.

physics.class-ph↗
arXiv1998

Measurement of Time-of-Arrival in Quantum Mechanics

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 $Δt_A \sim 1/E_k$, where $E_k$ is the initial kinetic energy of the particle. With a better accuracy, particles reflect off the measuring device, and the resulting probability distribution becomes distorted. It is shown that a time-of-arrival operator cannot exist, and that approximate time-of-arrival operators do not correspond to the measurements considered here.

quant-ph↗
arXiv1994

Varieties of Quantum Measurement

Quantum measurement theory has fallen under the resticting influence of the attempt to explain the fundamental axioms of quantum theory in terms of the theory itself. This has not only led to confusion but has also restricted our attention to a limited class of measurements. This paper outlines some of the novel types of measurements which fall outside the usual textbook description.

hep-th↗
arXiv1994

Maintaining coherence in Quantum Computers

The effect 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, $\hbar/k_B T$. For longer times the condition on the strength of the coupling to the external world becomes much more stringent.

hep-th↗
arXiv1993

Time Gravity and Quantum Mechanics

Time plays different roles in quantum mechanics and gravity. These roles are examined and the problems that the conflict in the roles presents for quantum gravity are briefly summarised.

gr-qc↗
arXiv1993

The Reality and Measurement of the Wavefunction

Using a simple version of the model for the quantum measurement of a two level system, the contention of Aharonov, Anandan, and Vaidman that one must in certain circumstances give the wavefunction an ontological as well as an epistemological significance is examined. I decide that their argument that the wave function of a system can be measured on a single system fails to establish the key point and that what they demonstrate is the ontological significance of certain operators in the theory, with the wave function playing its usual epistemological role.

hep-th↗
arXiv1993

Solution to 2+1 Gravity in DreiBein formalism

The reduction of the dreibein formalism of 2+1 General Relativity to the holonomies is explicitly performed. We also show explicitly how to relate these holonomies to a geometry classically, and how to generate these holonomies from any initial data for 2+1 gravity obeying the constraints.

gr-qc↗