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4

About evidence supporting the existence of Event Horizons in these very compact objects, here are some news from the well known Cygnus X-1, one of the most studied compact objects and the most promising candidate for a stellar collapse black hole: ... evidence of just such an event horizon may have been detected in 1992 using ultraviolet (UV) ...


15

...why do we trust black hole physics? ... (physics which is derived by combining quantum mechanics and GR such as Hawking Radiation, things relating to the Information Paradox, etc. ) Formally, there isn't quite a reason to because we've not observed these things yet. But that's also perfectly okay as well because that is how science sometimes works: ...


3

General relativity (GR) turned out to be a great mathematically beautiful theory with amazingly accurate experimental predictions/observations (e.g, bending of light, precession of Mercury, etc). This theory naturally provides some simple solutions which are called black holes. In that sense one should take them seriously as they come from a firmly ...


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At first many people didn't care much for black holes. But later people showed that they were pretty unavoidable features of the theory of general relativity and that theory made other quite precise predictions that were tested and found good. So when you are told that black holes are required if you have GR and GR looks like the best game in town then it ...


2

Nobody knows for sure. If you take a look at the mathspages Formation and Growth of Black Holes you can read about two different interpretations of general relativity: "Historically the two most common conceptual models for general relativity have been the "geometric interpretation" (as originially conceived by Einstein) and the "field interpretation" ...


1

Although General Relativity is used to calculate effects of strong gravity fields, and has passed all tests we can do so far, we don't know about black holes as GR cannot deal with quantum level events, which a black hole may turn out to be. We have lots of theories based on GR: (wormholes, other dimensions in this universe, other universes, etc...) but no ...


2

So, "phenomenological quantum gravity" is the particular field where we consider experimental tests of quantum gravity (with the hopes of winnowing the 3 dozen competing theories down to a handfull). To the best of my knowledge, Giovanni Amelino-Camelia pioneered the field (c.f., Amelino-Camelia's review article). At any rate, when focusing on loop quantum ...


2

Please correct me if I'm wrong, but I thought that no one had been able to show how General Relativity (GR) emerged from LQG in the semi classical limit ? In this context, it may make sense to ask for experimental manifestations but to me it would seem more important to make sure first that LQG gives in some limit a classical theory of gravitation before ...


0

I just want to add to Lumo's answer: The paper by vafa and Strominger instigated a lot of work in determining the statistical formulation of entropy in black holes. Although it must be pointed out that most of these are for cases with supersymmtry and (near) extremal conditions at small couplings. There has also been work in trying to address the microscopic ...


0

This is a very profound question in physics. Given that a black hole has an entropy which scales as $$S_{BH} \sim \frac{A}{4}, $$ the question is how does this relate to $S_{Boltzmann} = K_B \ln W$. As in, what are the microstates of the theory which hold the information in the black hole. This was answered in part by a series of papers by Vafa, Strominger, ...


0

I'm not sure anyone has tried (except possibly Smolin in some of his older papers). But it wouldn't be that hard: take a simple loop-quantum-gravity spin-foam-analog model in 25+1 dimensions (which is going to be rather more complex than the usual 3+1 dimensional spin foam), pick a ground-state-like solution for it that looks something like an extended ...


1

Sitting at infinity, you will see something more and more red-shifted - but never actually stop radiating as you would expect it from a black hole. The reason being that your coordinates (the asymptotically flat ones) diverge at the radius of the event horizon. Specifically, in your coordinates the metric of a Schwarzschild black hole (which is not entirely ...


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On the contrary Deser and others [1, and refs therein] have argued that trying to construct a theory of a graviton, that is, a massless spin-2 field in a flat background, consistent with special relativity, then "[c]onsistency [leads] us to universal coupling, which implies the equivalence principle" [1]. The argument is summarized by MTW [2, Box 17.2.5, see ...


0

A key point to make is that people think that a solution to Einstein's equations is a particular space-time geometry, a particular background geometry, when really a solution is an equivalence class of distinct geometries related to each other through (what mathematicians call) diffeomorphisms. And if you want to talk about observables and hence physics, you ...


0

Maxwell's theory in Minkowski spacetime is background-dependent because the Minkowski metric - a fixed geometric structure - is PART of the FORMULATION of the theory. The Minkowski metric appears in the action principle for example. General relativity is profoundly different because there is no fixed background geometric structure in the Einstein-Hilbert ...


3

Anything is possible with enough persistence. The probability of correctly learning quantum physics and relativity without any help or feedback from a professor, or even watching a blackboard presentation, is nonzero. But the chances of giving up or arriving at misconceptions are a lot higher. To learn any subject, you must focus on it. You won't learn to ...


1

There is a tiny bit more going on than the otherwise excellent answer by zeldrege suggests. Imagine that you wish to probe an unspecified object to examine its structure. If we use light to look at the structure of an object, we need to have its wavelength smaller than the size of the details we wish to look at. Probing an object that has a (linear) size ...


3

You need to distinguish between the virtual gravitons that appear in a quantum field theory calculation of a gravitational interaction and real gravitons that form a gravitational wave. The sort of experiment you're describing requires the emission of real graviton i.e. the emission of a gravitational wave. The trouble is that the coupling constant for the ...


-1

Why can't gravitons be created in traditional particle accelerators? Because gravitons are hypothetical virtual particles, proposed by particle phyicists advocating an "exchange mechanism" for gravity. This exchange mechanism is modelled using virtual particles, and it's perfectly valid for electromagnetism: when the electron and the proton attract each ...


0

General relativity and quantum fields evolution in curved space There's a problem with this I'm afraid. Gravity is all to do with curved spacetime, but that isn't curved space and curved time, it's a curvature of "the metric", or more simply, a curvature in your plot of measurements. See Baez: "Note: not the curvature of space, but of spacetime. The ...


4

The large scale structure of the universe can be described with general relativity. Whether the structure of general relativity can be described with quantum mechanics is an unsolved problem. Many people certainly think so and have spent a considerable amount of effort on trying to formulate such a description. For the second part of your question, I would ...



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