W. G. Unruh

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

Are you W. G. Unruh?

Claim your profile

Publications (92)396.85 Total impact

  • Qingdi Wang, William G. Unruh
    [Show abstract] [Hide abstract]
    ABSTRACT: We present a mirror model moving in the quantum vacuum of a massive scalar field and study its motion under infinitely fluctuating quantum vacuum stress. The model is similar to the one in \cite{PhysRevD.89.085009}, but this time there is no divergent effective mass to weaken the effect of divergent vacuum energy density. We show that this kind of weakening is not necessary. The vacuum friction and strong anti-correlation property of the quantum vacuum are enough to confine the mirror's position fluctuations.
  • Source
    M. Hotta, R. Schützhold, W. G. Unruh
    [Show abstract] [Hide abstract]
    ABSTRACT: The partner mode with respect to a vacuum state for a given mode (like that corresponding to one of the thermal particles emitted by a black hole) is defined and calculated. The partner modes are explicitly calculated for a number of cases, in particular for the modes corresponding to a particle detector being excited by turn-on/turn-off transients, or with the thermal particles emitted by the accelerated mirror model for black hole evaporation. One of the key results is that the partner mode in general is just a vacuum fluctuation, and one can have the partner mode be located in a region where the state cannot be distinguished from the vacuum state by any series of local measurements, including the energy density. I.e., "information" (the correlations with the thermal emissions) need not be associated with any energy transport. The idea that black holes emit huge amounts of energy in their last stages because of all the information which must be emitted under the assumption of black-hole unitarity is found not necessarily to be the case.
  • Source
    Cisco Gooding, William G. Unruh
    [Show abstract] [Hide abstract]
    ABSTRACT: We present a general relativistic model of a spherical shell of matter with a perfect fluid on its surface coupled to an internal oscillator, which generalizes a model recently introduced by the authors to construct a self-gravitating interferometer [1]. The internal oscillator evolution is defined with respect to the local proper time of the shell, allowing the oscillator to serve as a local clock that ticks differently depending on the shell's position and momentum. A Hamiltonian reduction is performed on the system, and an approximate quantum description is given to the reduced phase space. If we focus only on the external dynamics, we must trace out the clock degree of freedom, and this results in a form of intrinsic decoherence that shares some features with a proposed "universal" decoherence mechanism attributed to gravitational time dilation [2]. We show that the proposed decoherence remains present in the (gravity-free) limit of flat spacetime, indicating that the effect can be attributed entirely to proper time differences, and thus is not necessarily related to gravity. Finally, we point out a way to bootstrap the gravitational contribution to the time dilation decoherence by including self-interaction, and comment on how this can be considered a fundamentally gravitational intrinsic decoherence effect.
  • Source
    Cisco Gooding, William G. Unruh
    [Show abstract] [Hide abstract]
    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.
    Physical Review D 07/2014; 90(4). DOI:10.1103/PhysRevD.90.044071 · 4.86 Impact Factor
  • Source
    W. G. Unruh
    [Show abstract] [Hide abstract]
    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.
    Foundations of Physics 01/2014; 44(5). DOI:10.1007/s10701-014-9778-0 · 1.14 Impact Factor
  • Source
    W G Unruh
    [Show abstract] [Hide abstract]
    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.
  • Source
    Jonathan Oppenheim, William G. Unruh
    [Show abstract] [Hide abstract]
    ABSTRACT: If quantum gravity does not lead to a breakdown of predictability, then Almheiri, Marolf, Polchinski and Sully (AMPS) have argued that an observer falling into a black hole can perform an experiment which verifies a violation of entanglement monogamy - that late time Hawking radiation is maximally entangled with early time Hawking radiation and also with infalling radiation - something impossible in quantum field theory. However, as pointed out by Hayden and Harlow, this experiment is infeasible, as the time required to perform the experiment is almost certainly longer than the lifetime of the black hole. Here we propose an alternative firewall experiment which could actually be performed within the black hole's lifetime. The alternative experiment involves forming an entangled black hole in which the unscrambling of information is precomputed on a quantum memory prior to the creation of the black hole and without acting on the matter which forms the black hole or emerges from it. This would allow an observer near a black hole to signal faster than light. As another application of our precomputing strategy, we show how one can produce entangled black holes which acts like ``flat mirrors'', in the sense that information comes out almost instantly (as in the Hayden-Preskill scenario), but also emerge unscrambled, so that it can actually be observed instantly as well. Finally, we prove that a black hole in thermal equilibrium with its own radiation, is uncorrelated with this radiation.
    Journal of High Energy Physics 01/2014; 2014(3). DOI:10.1007/JHEP03(2014)120 · 6.22 Impact Factor
  • Source
    Qingdi Wang, William G. Unruh
    [Show abstract] [Hide abstract]
    ABSTRACT: The actual value of the quantum vacuum energy density is generally regarded as irrelevant in non-gravitational physics. However, this paper gives a non-gravitational system where this value does have physical significance. The system is a mirror with an internal degree of freedom which interacts with a scalar field. We find that the force exerted on the mirror by the field vacuum undergoes wild fluctuations with a magnitude proportional to the value of the vacuum energy density, which is mathematically infinite. This infinite fluctuating force gives infinite instantaneous acceleration of the mirror. We show that this infinite fluctuating force and infinite instantaneous acceleration make sense because they will not result in infinite fluctuation of the mirror's position. On the contrary, the mirror's fluctuating motion will be confined in a small region due to two special properties of the quantum vacuum: (1) the vacuum friction which resists the mirror's motion and (2) the strong anti-correlation of vacuum fluctuations which constantly changes the direction of the mirror's infinite instantaneous acceleration and thus cancels the effect of infinities to make the fluctuation of the mirror's position finite.
    Physical Review D 12/2013; 89(8). DOI:10.1103/PhysRevD.89.085009 · 4.86 Impact Factor
  • Source
    R. Schützhold, W. G. Unruh
    [Show abstract] [Hide abstract]
    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; 88(12). DOI:10.1103/PhysRevD.88.124009 · 4.86 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    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.
  • Source
    William G. Unruh
    [Show abstract] [Hide abstract]
    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 paper 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 the heat bath state being dragged along with the oscillator.
    Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 09/2012; 370(1975):4460-8. DOI:10.1098/rsta.2011.0495 · 2.86 Impact Factor
  • Source
    W G Unruh
    [Show abstract] [Hide abstract]
    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 which illustrates such energy free decoherence.
    Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 09/2012; 370(1975):4454-9. DOI:10.1098/rsta.2012.0163 · 2.86 Impact Factor
  • Source
    W. G. Unruh
    [Show abstract] [Hide abstract]
    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.
  • Source
    Ralf Schützhold, William G. Unruh
    [Show abstract] [Hide abstract]
    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.
  • Source
    W. G. Unruh, R. Schützhold
    [Show abstract] [Hide abstract]
    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). DOI:10.1103/PhysRevD.86.064006 · 4.86 Impact Factor
  • Source
    W. G. Unruh
    [Show abstract] [Hide abstract]
    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.
  • Source
    William G. Unruh
    [Show abstract] [Hide abstract]
    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.
  • Source
    Ralf Schützhold, William G Unruh
    [Show abstract] [Hide abstract]
    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 09/2011; 107(14):149401; author reply 149402. DOI:10.1103/PhysRevLett.107.149401 · 7.51 Impact Factor
  • Source
    W. G. Unruh
    [Show abstract] [Hide abstract]
    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.
  • B Losic, W G Unruh
    [Show abstract] [Hide abstract]
    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. DOI:10.1139/p06-020 · 0.93 Impact Factor

Publication Stats

6k Citations
396.85 Total Impact Points

Institutions

  • 1981–2014
    • University of British Columbia - Vancouver
      • Department of Physics and Astronomy
      Vancouver, British Columbia, Canada
  • 2011
    • Utrecht University
      Utrecht, Utrecht, Netherlands
  • 2008
    • Princeton University
      • Department of Physics
      Princeton, NJ, United States
  • 2000
    • Tel Aviv University
      • Department of Physics and Astronomy
      Tell Afif, Tel Aviv, Israel
  • 1989
    • University of California, Santa Barbara
      • Kavli Institute for Theoretical Physics
      Santa Barbara, California, United States
  • 1979
    • Government of British Columbia, Canada
      Vancouver, British Columbia, Canada
  • 1976
    • McMaster University
      Hamilton, Ontario, Canada
  • 1974
    • University of California, Berkeley
      • Department of Physics
      Berkeley, California, United States