W. G. Unruh

University of British Columbia - Vancouver, Vancouver, British Columbia, Canada

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Publications (57)177.05 Total impact

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    Cisco Gooding, William G. Unruh
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    ABSTRACT: To investigate the possibility that intrinsic gravitational decoherence can be theoretically demonstrated within canonical quantum gravity, we develop a model of a self-gravitating interferometer. We search for evidence in the resulting interference pattern that would indicate coherence is fundamentally limited due to general relativistic effects. To eliminate the occurence of gravitational waves, we work in spherical symmetry, and construct the "beam" of the interferometer out of WKB states for an infinitesimally thin shell of matter. For internal consistency, we encode information about the beam optics within the dynamics of the shell itself, by arranging an ideal fluid on the surface of the shell with an equation of state that enforces beam-splitting and reflections. We then determine sufficient conditions for (interferometric) coherence to be fully present even after general relativistic corrections are introduced, test whether or not they can be satisfied, and remark on the implications of the results.
    07/2014;
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    W. G. Unruh
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    ABSTRACT: It is argued that Hawking radiation has indeed been measured and shown to posses a thermal spectrum, as predicted. This contention is based on three separate legs. The first is that the essential physics of the Hawking process for black holes can be modelled in other physical systems. The second is the white hole horizons are the time inverse of black hole horizons, and thus the physics of both is the same. The third is that the quantum emission, which is the Hawking process, is completely determined by measurements of the classical parameters of a linear physical system. The experiment conducted in 2010 fulfills all of these requirements, and is thus a true measurement of Hawking radiation.
    01/2014;
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    W G Unruh
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    ABSTRACT: A variety of historical coordinates in which the Schwarzschild metric is regular over the whole of the extended spacetime are compared and the hypersurfaces of constant coordinate are graphically presented. While the Kruscal form (one of the later forms) is probably the simplest, each of the others has some interesting features.
    01/2014;
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    R. Schützhold, W. G. Unruh
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    ABSTRACT: Inspired by the condensed matter analogues of black holes (a.k.a. dumb holes), we study Hawking radiation in the presence of a modified dispersion relation which becomes super-luminal at large wave-numbers. In the usual stationary coordinates $(t,x)$, one can describe the asymptotic evolution of the wave-packets in WKB, but this WKB approximation breaks down in the vicinity of the horizon, thereby allowing for a mixing between initial and final creation and annihilation operators. Thus, one might be tempted to identify this point where WKB breaks down with the moment of particle creation. However, using different coordinates $(\tau,U)$, we find that one can evolve the waves so that WKB in these coordinates is valid throughout this transition region -- which contradicts the above identification of the breakdown of WKB as the cause of the radiation. Instead, our analysis suggests that the tearing apart of the waves into two different asymptotic regions (inside and outside the horizon) is the major ingredient of Hawking radiation.
    Physical Review D 08/2013; · 4.69 Impact Factor
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    ABSTRACT: There is an analogy between the propagation of fields on a curved spacetime and shallow water waves in an open channel flow. By placing a streamlined obstacle into an open channel flow we create a region of high velocity over the obstacle that can include wave horizons. Long (shallow water) waves propagating upstream towards this region are blocked and converted into short (deep water) waves. This is the analogue of the stimulated Hawking emission by a white hole (the time inverse of a black hole). The measurements of amplitudes of the converted waves demonstrate that they appear in pairs and are classically correlated; the spectra of the conversion process is described by a Boltzmann-distribution; and the Boltzmann-distribution is determined by the determined by the change in flow across the white hole horizon.
    02/2013;
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    W G Unruh
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    ABSTRACT: The temperature of an oscillator coupled to the vacuum state of a heat bath via Ohmic coupling is non-zero, as measured by the reduced density matrix of the oscillator. This study shows that the actual temperature, as measured by a thermometer, is still zero (or, in the thermal state of the bath, the temperature of the bath). The decoherence temperature is due to 'false-decoherence', with a correlation between the oscillator and the heat bath causing the decoherence, but the heat baths state dragged along with the state of the oscillator.
    Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 09/2012; 370(1975):4460-8. · 2.89 Impact Factor
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    W G Unruh
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    ABSTRACT: That decoherence can take place in the presence of energy conservation seems to be a poorly known fact. That lack of knowledge has, for example, bedevilled the discussion of the 'black hole information' problem. I present a simple model that illustrates such energy-free decoherence.
    Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 09/2012; 370(1975):4454-9. · 2.89 Impact Factor
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    W. G. Unruh
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    ABSTRACT: The equations for waves on the surface of an irrotational incompressible fluid are derived in the coordinates of the velocity potential/stream function. The low frequency shallow water approximation for these waves is derived for a varying bottom topography. Most importantly, the conserved norm for the surface waves is derived, important for quantisation of these waves and their use in analog models for black holes.
    05/2012;
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    Ralf Schützhold, William G. Unruh
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    ABSTRACT: One of the most striking examples for the production of particles out of the quantum vacuum due to external conditions is cosmological particle creation, which is caused by the expansion or contraction of the Universe. Already in 1939, Schr\"odinger understood that the cosmic evolution could lead to a mixing of positive and negative frequencies and that this "would mean production or annihilation of matter, merely by the expansion". Later this phenomenon was derived via more modern techniques of quantum field theory in curved space-times by Parker (who apparently was not aware of Schr\"odinger's work) and subsequently has been studied in numerous publications. Even though cosmological particle creation typically occurs on extremely large length scales, it is one of the very few examples for such fundamental effects where we actually may have observational evidence: According to the inflationary model of cosmology, the seeds for the anisotropies in the cosmic microwave background (CMB) and basically all large scale structures stem from this effect. In this Chapter, we shall provide a brief discussion of this phenomenon and sketch a possibility for an experimental realization via an analogue in the laboratory.
    03/2012;
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    W. G. Unruh, R. Schützhold
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    ABSTRACT: Belgiorno et al have reported on experiments aiming at the detection of (the analogue of) Hawking radiation using laser filaments [F. Belgiorno et al, Phys. Rev. Lett. 105, 203901 (2010)]. They sent intense focused Bessel pulses into a non-linear dielectric medium in order to change its refractive index via the Kerr effect and saw creation of photons orthogonal to the direction of travel of the pluses. Since the refractive index change in the pulse generated a "phase horizon" (where the phase velocity of these photons equals the pulse speed), they concluded that they observed the analogue of Hawking radiation. We study this scenario in a model with a phase horizon and a phase velocity very similar to that of their experiment and find that the effective metric does not quite correspond to a black hole. The photons created in this model are not due to the analogue of black hole evaporation but have more similarities to cosmological particle creation. Nevertheless, even this effect cannot explain the observations -- unless the pulse has significant small scale structure in both the longitudinal and transverse dimensions.
    Physical review D: Particles and fields 02/2012; 86(6).
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    W. G. Unruh
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    ABSTRACT: The slinky, released from rest hanging under its own weight, falls in a peculiar manner. The bottom stays at rest until a wave hits it from above. Two cases - one unphysical one where the slinky is able to pass through itself, and the other where the coils of the slinky collide creating a shock wave travelling down the slinky are analysed. In the former case, the bottom begins to move much later than in the latter.
    10/2011;
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    William G. Unruh
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    ABSTRACT: The loss of coherence of a quantum system coupled to a heat bath as expressed by the reduced density matrix is shown to lead to the miss-characterization of some systems as being incoherent when they are not. The spin boson problem and the harmonic oscillator with massive scalar field heat baths are given as examples of reduced incoherent density matrices which nevertheless still represent perfectly coherent systems.
    10/2011;
  • Ralf Schützhold, William G Unruh
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    ABSTRACT: A Comment on the Letter by F. Belgiorno et al., Phys. Rev. Lett. 105, 203901 (2010). The authors of the Letter offer a Reply.
    Physical Review Letters 09/2011; 107(14):149401; author reply 149402. · 7.73 Impact Factor
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    W. G. Unruh
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    ABSTRACT: The quantum noise in a linear amplifier is shown to be thermal noise. The theory of linear amplifiers is applied first to the simplest, single or double oscillator model of an amplifier, and then to linear model of an amplifier with continuous fields and input and outputs. Finally it is shown that the thermal noise emitted by black holes first demonstrated by Hawking, and of dumb holes (sonic and other analogs to black holes), arises from the same analysis as for linear amplifiers. The amplifier noise of black holes acting as amplifiers on the quantum fields living in the spacetime surrounding the black hole is the radiation discovered by Hawking. For any amplifier, that quantum noise is completely characterized by the attributes of the system regarded as a classical amplifier, and arises out of those classical amplification factors and the commutation relations of quantum mechanics.
    07/2011;
  • B Losic, W G Unruh
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    ABSTRACT: We summarize the results of a calculation (B. Losic and W.G. Unruh. Phys. Rev. D, 72, 123510 (2005).) of cosmological back reactions about a slowly rolling background space-time. In particular, we evaluate the effect of the superhorizon second-order corrections on the (superhorizon) inhomogeneous modes of the linearized fluctuations. Their physical significance is quantified by studying their effective equation of state, where the isotropic pressure and energy density at second order are defined in terms of the averaged eigenvalues associated with timelike (spacelike) eigenvectors of a total stress energy for the metric and matter fluctuations. Given a well-defined gauge fixing at second order, we find that the higher order corrections may dominate those of the linear terms and, furthermore, that this result holds for other reasonable gauge-fixing procedures. Our work suggests that for many parameters of slow-roll inflation, the second-order effects may dominate over the first-order effects for the super-Hubble evolution. We also find that the contribution to the equation of state due to the back reactions is that of a negative cosmological constant in this coordinate gauge, confirming earlier work.PACS Nos.: 31.15.Pf, 31.30.Jv, 32.10.Hq
    Canadian Journal of Physics 02/2011; 84:599-605. · 0.90 Impact Factor
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    ABSTRACT: 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.
    Physical Review Letters 01/2011; 106(2):021302. · 7.73 Impact Factor
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    Ralf Schützhold, William G. Unruh
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    ABSTRACT: In a recent paper Belgiorno {\em et al} claimed to have observed the analog of the Hawking effect because of the detection of radiation in a frequency range in which what they called "phase horizons" existed. They created rapidly moving pulses of light in a silica glass whose Kerr effect altered the refractive index to create those horizons. Unfortunately, while the observations are very interesting, the cause of the radiation is not understood, and we feel it is not justified to call this a detection of the Hawking effect in an analog system.
    Physical Review Letters - PHYS REV LETT. 12/2010; 107.
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    Ralf Schützhold, William G. Unruh
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    ABSTRACT: Inspired by the condensed-matter analogues of black holes, we study the quantum correlations across the event horizon reflecting the entanglement between the outgoing particles of the Hawking radiation and their in-falling partners. For a perfectly covariant theory, the total correlation is conserved in time and piles up arbitrary close to the horizon in the past, where it merges into the singularity of the vacuum two-point function at the light cone. After modifying the dispersion relation (i.e., breaking Lorentz invariance) for large $k$, on the other hand, the light cone is smeared out and the entanglement is not conserved but actually created in a given rate per unit time. Comment: 5 pages RevTeX, 1 figure
    Physical review D: Particles and fields 02/2010;
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    W. G. Unruh
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    ABSTRACT: Minkowski’s conception of spacetime has had a large impact on our interpretation of quantum theory. Time in quantum mechanics plays a a role in the interpretation distinct from space, in contrast with the apparent unity of space and time encapsulate in the notion of spacetime. It clearly ruled out any quantum mechanics and showed that quantum field theory was the only possible consistent theory. And within quantum field theory, it also rules out the realistic interpretation of the wave function. Fortunately, it is not incompatible with all approaches to quantum theory. Whether or not it is compatible with quantum theory once the metric, the structure which defines the causal structure of the primitive notion of the manifold, is still an open question. Minkowski spacetime-Quantum mechanics-Schroedinger representation-Heisenberg representation-Locality-Bell’s theorem
    12/2009: pages 133-148;
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    ABSTRACT: We analyze the necessary and sufficient conditions for the occurrence of superradiance. Starting with a wave equation we examine the possibility of superradiance in terms of an effective potential and boundary conditions. In particular, we show that the existence of an ergoregion is not sufficient; an appropriate boundary condition, e.g. only ingoing group velocity waves at an event horizon, is also crucial. After applying our scheme to the standard examples of superradiance, we show that analogue models of gravity without an event horizon do not necessarily exhibit superradiance. Particularly, we show that the superradiant phenomenon is absent in purely rotating inviscid fluids with vorticity. We argue that there should be a catalogue of superradiant systems that can be found by focusing on the necessary and sufficient conditions outlined below.
    Physical review D: Particles and fields 12/2009; 80(12).

Publication Stats

918 Citations
177.05 Total Impact Points

Institutions

  • 1997–2012
    • University of British Columbia - Vancouver
      • Department of Physics and Astronomy
      Vancouver, British Columbia, Canada
  • 2011
    • University of Duisburg-Essen
      • Fakultät für Physik
      Duisburg, North Rhine-Westphalia, Germany
  • 2008
    • Princeton University
      • Department of Physics
      Princeton, NJ, United States
    • University of Alberta
      Edmonton, Alberta, Canada
  • 2005–2007
    • Technische Universität Dresden
      • Institut für theoretische Physik
      Dresden, Saxony, Germany
  • 2000
    • Tel Aviv University
      • Department of Physics and Astronomy
      Tell Afif, Tel Aviv, Israel