lundi 29 août 2016

(Reflections on and off a) GoldenEye (or rather black hole horizon)


Imaging a Black Hole. At left is a model image for Sgr A* using a semi-analytic accretion flow (Broderick et al. 2011). Light is gravitationally lensed by the black hole to form a distinctive “ring” encircling the black hole’s “shadow” (Falcke et al. 2000). The ring diameter is ~5 Schwarzschild radii . The image is bright on the approaching side of the accretion disk and faint on the receding side because of Doppler effects. At right, a sample image shows expected EHT performance in 2017-2018 (Fish, Johnson, et al. 2014). (Source: www.scintillatingastronomy.com/eht/)

The Event Horizon Telescope is an international collaboration to create a worldwide very long baseline interferometry array observing at 1.3-millimeter wavelength. When the EHT is complete, it will be able to make images of black holes with a resolution of 10-20 microarcseconds. This resolution is fine enough to resolve the event horizons of the supermassive black holes in the center of our own Milky Way and of M87. You can see the official EHT website here; a Scientific American blog, written by Seth Fletcher, provides many great articles here. For a lot of great answers to common questions about the EHT, check out this reddit AMA.

Michael D. Johnson, Scintillating Astronomy



Great or temperate quantum gravity forecasts

Quantum information transfer necessary to reconcile black hole evaporation with quantum mechanics, while approximately preserving regular near-horizon geometry, can be simply parameterized in terms of couplings of the black hole internal state to quantum fields of the black hole atmosphere. The necessity of transferring sufficient information for unitarization sets the strengths of these couplings. Such couplings via the stress tensor offer apparently significant advantages, and behave like quantum fluctuations of the effective metric near the horizon. At the requisite strength, these fluctuations, while soft (low energy/momentum), have significant magnitude, and so can deflect near-horizon geodesics that span distances of order the black hole radius. Thus, the presence of such couplings can result in effects that could be detected or constrained by observation: disruption of near-horizon accretion flows, scintillation of light passing close to the black hole, and alteration of gravitational wave emission from inspirals. These effects could in particular distort features of Sgr A* expected to be observed, e.g., by the Event Horizon Telescope, such as the black hole shadow and photon ring.
(Submitted on 26 Jun 2014 (v1), last revised 28 Oct 2014 (this version, v3))

Classical collapse models suggest that a sufficiently massive self-gravitating system will undergo continued collapse until a singularity forms. In 1975, Hawking [1] pointed out that if an event horizon forms and if effective field theory is valid away from a stretched horizon, then radiation from the black hole is produced in a mixed state from the point of view of the observer who remains outside the black hole provided that the freely falling observer detects nothing unusual (“no drama”) while crossing the horizon. Under these conditions information is lost if the black hole evaporates completely, which violates unitarity and led Hawking to propose that quantum mechanics should be modified [2] (he has since changed his mind). To preserve unitarity in quantum mechanics one of two possibilities must be true: (a) Hawking radiation is in fact pure or (b) the evaporation leaves behind a long lived remnant, which preserves all the information that collapsed into the black hole. However, if quantum gravity is CPT invariant then remnants are ruled out and only the first of the two options above remains viable. In 1993, building on the work of ’t Hooft [3] and Preskill [4], Susskind et. al. [5] proposed that unitarity could be preserved if information is both emitted at the horizon and passes through the horizon so that an observer outside would see it in the Hawking radiation and an observer who flies into the black hole would see it inside. No single observer would be able to confirm both pictures: one simply cannot say where the information in the Hilbert space is located, so quantum mechanics is saved at the cost of locality. This is the principle of Black Hole Complementarity 

Recently, A. Almheiri  D. Marolf, J. Polchinski and  J. Sully  (AMPS) [6] suggested that the three assumptions of Black Hole Complementarity viz., (a) unitarity of Hawking evaporation, (b) validity of effective field theory outside a stretched horizon and (c) “no drama” at the horizon for a freely falling observer are not self-consistent. Briefly, their argument can be stated as follows. Consider a very large black hole so that a freely falling observer crossing the horizon sees an effectively flat spacetime (on scales much smaller than the horizon length). From the point of view of an observer who stays outside the horizon, the purity of the Hawking radiation implies that late time photons are maximally entangled with some subset of the early radiation. However, these late photons when propagated back from infinity to the near horizon region using effective field theory must be maximally entangled with modes inside the horizon from the point of view of the freely falling observer (this is simply a property of the Minkowski vacuum, appropriate to a freely falling observer). This is not permitted by the strong additivity of entanglement entropy. Assuming then that effective field theory is valid and that Hawking radiation is pure, the paradox can only be avoided if the backward propagated photon is not entangled with a mode behind the horizon. But this would lead to a divergent stress tensor near the horizon, so AMPS concluded that the freely falling observer would burn up before she could cross it. This is the “firewall”.  
Considerable interest has surrounded the proposed firewall ..., all of it assuming that continued collapse will occur, leading to black holes with event horizons. But Hawking has recently raised several objections to the firewall and suggested that the correct resolution of the AMPS paradox is that event horizons do not form, only apparent horizons form [8]. Radiation from the black hole is then deterministic, but chaotic.
... in view of the AMPS paradox, an entirely new picture of the black hole has emerged. Instead of a spacetime singularity covered by an event horizon we will have an essentially quantum object, an extremely compact dark star, which is held up not by any degeneracy pressure but by quantum gravity just as ordinary atoms are sustained by quantum mechanics. Astronomical observations [18, 19] indicate that astrophysical black holes possess dark surfaces and this is consistent with the picture we have just described.
Cenalo Vaz  (Submitted on 19 May 2014)

Whatever EHT might see, a black hole may definitely be a true attom of quantum gravity... 
... with the double t to underline the pairs of antipodal entangled Hawking particles continuously emitted from the black hole horizon according to the 't Hooft model and the original atom word to refer to the fact that any black hole would be some kind of an elementary micro or macro-scopic (depending on its varying mass) quantum object in a non-stationnary state, without any tangible inside but a frontier with some kind of non-local properties.


If we could do experiments with radiating black holes, this entanglement would have been detectable: if at one side of the black hole a particle emerges that happens to be strongly suppressed by a Boltzmann factor, then at the other side, the same particle will be seen to emerge with probability one – not suppressed at all! 
Indeed, the heat bath mentioned earlier, is a strangely unphysical one: antipodes in 3-space are 100 % entangled. In practice, this means that the stationary Hartle-Hawking state is extremely improbable in describing the universe far from the location of the black hole.
Gerard t Hooft (Submitted on 17 May 2016)




An already observed entangled Hawking radiation... but from an atomic Bose-Einstein condensate sonic "analogue" black hole!
Waiting for an hypothetical observation of quantum gravity as soon as 2017 whatever it will look like, let's landing smoothly down on earth for a while and contemplating the amazing ingenuity of physicists to make their dreams come true in the lab before heavens:

The analogy between sound propagation in nonhomogeneous media and light propagation in curved spacetimes has opened the possibility to detect the analogue of black hole radiation in the lab [1 {*}]. Indeed, when sound waves propagate in a flowing medium whose velocity crosses at some point the speed of sound, they experience the analogue of an event horizon. If the phonon state is stationary and regular across this sonic horizon, one expects to obtain a thermal flux of phonons with a temperature kBT=κ/2π, where κ is the gradient of the flow velocity evaluated at the sonic horizon. Since the analogy works perfectly in the hydrodynamical limit, the above result should be valid at least when κ is much smaller than the critical wave-vector characterizing the dispersion [2345]. 
Following the original work of Unruh, various setups were proposed, see [6] for a review. In Refs. [...] the particular case of sound waves in dilute Bose-Einstein condensates (BEC) was considered. These condensates have nice properties both from an experimental and a theoretical point of view. From the first, progress in the manipulation and control of their physical properties is rapid, and from the second, the equations for the condensate and the phonons are well understood.
(Submitted on 22 May 2009 (v1), last revised 2 Oct 2009 (this version, v4))

We observe spontaneous Hawking radiation, stimulated by quantum vacuum fluctuations, emanating from an analogue black hole in an atomic Bose–Einstein condensate. Correlations are observed between the Hawking particles outside the black hole and the partner particles inside. These correlations indicate an approximately thermal distribution of Hawking radiation. We find that the high-energy pairs are entangled, while the low-energy pairs are not, within the reasonable assumption that excitations with different frequencies are not correlated. The entanglement verifies the quantum nature of the Hawking radiation. The results are consistent with a driven oscillation experiment and a numerical simulation.
Observation of quantum Hawking radiation and its entanglement in an analogue black hole  Jeff Steinhauer Nature Physics (Received 23 November 2015 Accepted 15 July 2016 Published online 15 August)

Jeff Steinhauer - Observation of thermal Hawking radiation...


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