327 research outputs found
Gravitational Thermodynamics of Causal Diamonds in (A)dS
The static patch of de Sitter spacetime and the Rindler wedge of Minkowski
spacetime are causal diamonds admitting a true Killing field, and they behave
as thermodynamic equilibrium states under gravitational perturbations. We
explore the extension of this gravitational thermodynamics to all causal
diamonds in maximally symmetric spacetimes. Although such diamonds generally
admit only a conformal Killing vector, that seems in all respects to be
sufficient. We establish a Smarr formula for such diamonds and a "first law"
for variations to nearby solutions. The latter relates the variations of the
bounding area, spatial volume of the maximal slice, cosmological constant, and
matter Hamiltonian. The total Hamiltonian is the generator of evolution along
the conformal Killing vector that preserves the diamond. To interpret the first
law as a thermodynamic relation, it appears necessary to attribute a negative
temperature to the diamond, as has been previously suggested for the special
case of the static patch of de Sitter spacetime. With quantum corrections
included, for small diamonds we recover the "entanglement equilibrium" result
that the generalized entropy is stationary at the maximally symmetric vacuum at
fixed volume, and we reformulate this as the stationarity of free conformal
energy with the volume not fixed.Comment: v3: 64 pages, 6 appendices, 8 figures; matches published versio
Hints towards the Emergent Nature of Gravity
A possible way out of the conundrum of quantum gravity is the proposal that
general relativity (GR) is not a fundamental theory but emerges from an
underlying microscopic description. Despite recent interest in the emergent
gravity program within the physics as well as the philosophy community, an
assessment of the theoretical evidence for this idea is lacking at the moment.
We intend to fill this gap in the literature by discussing the main arguments
in favour of the hypothesis that the metric field and its dynamics are
emergent. First, we distinguish between microstructure inspired from GR, such
as through quantization or discretization, and microstructure that is not
directly motivated from GR, such as strings, quantum bits or condensed matter
fields. The emergent gravity approach can then be defined as the view that the
metric field and its dynamics are derivable from the latter type of
microstructure. Subsequently, we assess in how far the following properties of
(semi-classical) GR are suggestive of underlying microstructure: (1) the
metric's universal coupling to matter fields, (2) perturbative
non-renormalizability, (3) black hole thermodynamics, and (4) the holographic
principle. In the conclusion we formalize the general structure of the
plausibility arguments put forward.Comment: 36 pages, v2: minor additions, references added. Journal version in
Studies in History and Philosophy of Modern Physic
The thin film microwave iris
Development of waveguide iris for microwave coupling applications using thin film techniques is discussed. Production process and installation of iris are described. Iris improves power transmission properties of waveguide window
Holographic Thermodynamics Requires a Chemical Potential for Color
The thermodynamic Euler equation for high-energy states of large- gauge
theories is derived from the dependence of the extensive quantities on the
number of colors . This Euler equation relates the energy of the state to
the temperature, entropy, number of degrees of freedom and its chemical
potential, but not to the volume or pressure. In the context of the
gauge/gravity duality we show that the Euler equation is dual to the
generalized Smarr formula for black holes in the presence of a negative
cosmological constant. We also match the fundamental variational equation of
thermodynamics to the first law of black hole mechanics, when extended to
include variations of the cosmological constant and Newton's constant.Comment: 9 pages, v2: corrected reference to Karch-Robinson, clarified regime
of validity of holographic Euler equation, and fixed typos; v3: reorganized
and rewritten, added a brief discussion about Lifshitz black holes and a new
section on comparison with previous literatur
Entropy of causal diamond ensembles
We define a canonical ensemble for a gravitational causal diamond by
introducing an artificial York boundary inside the diamond with a fixed induced
metric and temperature, and evaluate the partition function using a saddle
point approximation. For Einstein gravity with zero cosmological constant there
is no exact saddle with a horizon, however the portion of the Euclidean diamond
enclosed by the boundary arises as an approximate saddle in the
high-temperature limit, in which the saddle horizon approaches the boundary.
This high-temperature partition function provides a statistical interpretation
of the recent calculation of Banks, Draper and Farkas, in which the entropy of
causal diamonds is recovered from a boundary term in the on-shell Euclidean
action. In contrast, with a positive cosmological constant, as well as in
Jackiw-Teitelboim gravity with or without a cosmological constant, an exact
saddle exists with a finite boundary temperature, but in these cases the causal
diamond is determined by the saddle rather than being selected a priori.Comment: 15 pages, 3 figures, v3: added references and edited introductio
Partition function for a volume of space
We consider the quantum gravity partition function that counts the dimension
of the Hilbert space of a spatial region with topology of a ball and fixed
proper volume, and evaluate it in the leading order saddle point approximation.
The result is the exponential of the Bekenstein-Hawking entropy associated with
the area of the saddle ball boundary, and is reliable within effective field
theory provided the mild curvature singularity at the ball boundary is
regulated by higher curvature terms. This generalizes the classic
Gibbons-Hawking computation of the de Sitter entropy for the case of positive
cosmological constant and unconstrained volume, and hence exhibits the
holographic nature of nonperturbative quantum gravity in generic finite volumes
of space.Comment: 10 pages plus appendices, 2 figures, v2: added figure and improved
presentatio
Nanomechanical resonators operating as charge detectors in the nonlinear regime
We present measurements on nanomechanical resonators machined from
Silicon-on-Insulator substrates. The resonators are designed as freely
suspended Au/Si beams of lengths on the order of 1 - 4 um and a thickness of
200 nm. The beams are driven into nonlinear response by an applied modulation
at radio frequencies and a magnetic field in plane. The strong hysteresis of
the magnetomotive response allows sensitive charge detection by varying the
electrostatic potential of a gate electrode.Comment: 8 pages, 6 figure
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