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A Can an event horizon telescope observe quantum gravity effects

  1. Jul 18, 2016 #1
    its commonly stated that no particle accelerator will ever be built that can probe the planck scale.

    what about an can an event horizon telescope observe quantum gravity effects near a black hole, such as Sagittarius A*

    do the various candidates of QG such as string theory LQG asf gravity, QFT on curved spacetime, sugra etc offer predictions as to what to expect around a event horizon telescope?

    this paper makes claims LQG can observe it,

    Quantum Gravity Effects around Sagittarius A*
    Hal M. Haggard, Carlo Rovelli
    (Submitted on 1 Jul 2016 (v1), last revised 5 Jul 2016 (this version, v2))
    Recent VLBI observations have resolved Sagittarius A* at horizon scales. The Event Horizon Telescope is expected to provide increasingly good images of the region around the Schwarzschild radius rS of Sgr A* soon. A number of authors have recently pointed out the possibility that non-perturbative quantum gravitational phenomena could affect the space surrounding a black hole. Here we point out that the existence of a region around 76rS where these effects should be maximal.
    Comments: 5 pages; Received honorable mention in the Gravity Research Foundation 2016 Awards for Essays on Gravitation
    Subjects: General Relativity and Quantum Cosmology (gr-qc)
    Cite as: arXiv:1607.00364 [gr-qc]

    Realistic Observable in Background-Free Quantum Gravity: the Planck-Star Tunnelling-Time
    Marios Christodoulou, Carlo Rovelli, Simone Speziale, Ilya Vilensky
    (Submitted on 17 May 2016)
    A gravitationally collapsed object can bounce-out from its horizon via a tunnelling process that violates the classical equations in a finite region. Since tunnelling is a non-perturbative phenomenon, it cannot be described in terms of quantum fluctuations around a classical solution and a background-free formulation of quantum gravity is needed to analyze it. Here we use Loop Quantum Gravity to compute the amplitude for this process, in a first approximation. The amplitude determines the tunnelling time as a function of the mass. This is the key information to evaluate the astrophysical relevance of this process. The calculation offers a template and a concrete example of how a background-free quantum theory of gravity can be used to compute a realistical observable quantity.
    Comments: We consider this paper important. 16 pages, 12 figures
    Subjects: General Relativity and Quantum Cosmology (gr-qc); Cosmology and Nongalactic Astrophysics (astro-ph.CO); High Energy Astrophysical Phenomena (astro-ph.HE); High Energy Physics - Theory (hep-th)
    Cite as: arXiv:1605.05268 [gr-qc]
    (or arXiv:1605.05268v1 [gr-qc] for this version)

    are these claims credible, that a telescope can help distinguish various QG theories of astrophysical objects?

    does string/m theory as a theory of QG offer predictions and if so, how does it compare to Rovelli et al's papers as to what the event horizon telescope observing Sagittarius A should observe?

    i.e Isaac Newton, Albert Einstein, and Gunner Nordstrom all offer predictions as to what gravity would do in bending star light during an eclipse. Nordstrom's theory predicted no bending of star light, Newton one value and Einstein a value twice as large as Newton's. Einstein's prediction was confirmed and general relativity is the theory of classical gravity.

    What Rovelli is claiming is something similar. the event horizon telescope can observe the event horizon surrounding Sagittarius A* at horizon scales and LQG offers predictions that differ from what classical GR provides. Does string theory/m theory also offer predictions on what event horizon telescope can observe the event horizon surrounding Sagittarius A* at horizon scales should observe and how do they compare with Rovelli's LQG?

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  3. Jul 18, 2016 #2


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    The Sag A* event horizon is a very sensible place to look for QG effects because it is the closest available supermassive black hole by a large margin.

    I suspect that the precision of the observations will be barely enough to even see if anything remotely resembling classical GR predictions are present for decades before our experimentally refined understanding of that background and our instrumentation is good enough to make meaningful distinctions between GR and QG predictions that differ from GR in the strong field limit. (Only early hint are the LIGO gravity wave results that match GR predictions to very high precision suggesting that any discrepancy between GR and QG in the strong field is a pretty small percentage difference from the GR prediction and that QG effect in the strong field, if any, are small, and small effects take more precision to see.)

    Rovelli's argument that there might be something interesting there at about 7/6th of the event horizon involves all sorts of conjecture and surmise and even the GR predicted phenomenology of that region which would be the background against which QG effects are evaluated (e.g. photon rings) is itself pretty speculative, because GR mostly uses some very idealized models of different kinds of possible black holes that may not be exactly replicated physically. Still, as a heuristic argument for where it makes most sense to start testing hypotheses, it is as good a guess as anybody's. However, his reasoning suggests that there is more room for error with interesting stuff closer to the event horizon than there is for interesting stuff further away (which, unfortunately, would probably makes it harder to see with a telescope.)

    If we ever do observe strong field phenomena with enough precision to distinguish GR and QG, whether that is in 5 years or 50 years or 500 years, it is very likely that it will be in the close vicinity of the Sag A* event horizon, simply because seeing it anywhere else with comparably strong gravitational fields is likely to be harder.

    I recently corresponded with Sabine at Backreaction about the predictions of various QG theories and the gist of her response was that very little QG phenomenology work that would produce the kind of testable predictions one would like to have has been done.
  4. Jul 18, 2016 #3
    very interesting what you say. since Sabine bee is a QG phemon can u ask her to review Rovelli's paper and get back with us?

    string theory does offer some higher-order quantum gravity corrections to GR, not sure how this applies in phenom.
  5. Jul 19, 2016 #4


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