3 ms·

From https://journals.aps.org/prd/abstract/10.1103/PhysRevD.107.084042 https://journals.aps.org/prd/abstract/10.1103/PhysRevD.107.0... : > We show that topolog

From https://journals.aps.org/prd/abstract/10.1103/PhysRevD.107.084042 https://journals.aps.org/prd/abstract/10.1103/PhysRevD.107.0... : > We show that topological solitons are remarkably similar to black holes in apparent size and scattering properties, while being smooth and horizonless. Incoming photons experience very high redshift, inducing phenomenological horizonlike behaviors from the point of view of photon scattering. Thus, they provide a compelling case for real-world gravitational solitons and topological alternatives to black holes from string theory. This is certainly interesting but so far they have only looked into Schwarzschild black holes (the simplest kind of black holes). Their case would be much more compelling if they also found soliton solutions that looked like non-static (Kerr) or even non-stationary/dynamic black hole solutions (<-> black hole formation). I know far too little about solitons, unfortunately, to estimate how easy/difficult it would be to find/construct those.

raattgift 3y ago
> only looked into Schwarzschild black holes What plane is the authors' "equatorial plane"? Shortly after their eqn (31), "One should a priori do a similar computation out of the equatorial plane to obtain the total size of the solitons. Due to the complexity of the geodesic equations, this is only possible numerically. However, we will see in the next section that the outer photon shell is actually almost spherically symmetric and slightly flattened at its poles". (ETA: "... the Schwarzschild topological solution is not spherically symmetric ...", just before section A. Methodology). Central SMBHs' spin appears to be largely far from parallel to their host galaxies' angular momentum[1]. Spin gives us an equatorial plane for the SMBH. Is there physical motivation for picking an equatorial plane if we found an SMBH with negligible J (and no jets)? I don't doubt that people would land on some convention (e.g. equatorial plane parallel to host galaxy's north if available, our galaxy's north if not), but do doubt that the convention would reflect physics (i.e., that there is such a plane where J = 0 or even that such a plane is relevant where J <<< 1, even if the plane is thereby picked out) rather than useful coordinates. I scanned the paper only briefly (I'm not really interested in black holes out of a cosmological or at least astrophysical context and am in no hurry to jump ahead of further EHT results) but found myself curious about the authors' "these solutions can be embedded in string theory" as their ref [17] at first glance seems to be about AdS, an acronym which is found only once in this paper. What provokes my wondering is in particular, "and so an asymptotic observer will barely notice the difference between the parallax angles" (among other references to an asymptotic observer), drawing a comparison with a distant Schwarzschild observer, again around eqn (31). "Barely notice" is, at least qualitatively, interesting wording. Can an observer at infinity more than barely notice the difference between a cold noncompact spherically symmetric uncharged nonspinning central mass and a Schwarzschild BH? [1] This is especially true for Sgr A*, see EHT paper https://iopscience.iop.org/article/10.3847/2041-8213/ac6674 https://iopscience.iop.org/article/10.3847/2041-8213/ac6674 which is wildly "tilt"ed (to use authors' expression).

raattgift 3y ago

> only looked into Schwarzschild black holes What plane is the authors' "equatorial plane"? Shortly after their eqn (31), "One should a priori do a similar computation out of the equatorial plane to obtain the total size of the solitons. Due to the complexity of the geodesic equations, this is only possible numerically. However, we will see in the next section that the outer photon shell is actually almost spherically symmetric and slightly flattened at its poles". (ETA: "... the Schwarzschild topological solution is not spherically symmetric ...", just before section A. Methodology). Central SMBHs' spin appears to be largely far from parallel to their host galaxies' angular momentum[1]. Spin gives us an equatorial plane for the SMBH. Is there physical motivation for picking an equatorial plane if we found an SMBH with negligible J (and no jets)? I don't doubt that people would land on some convention (e.g. equatorial plane parallel to host galaxy's north if available, our galaxy's north if not), but do doubt that the convention would reflect physics (i.e., that there is such a plane where J = 0 or even that such a plane is relevant where J <<< 1, even if the plane is thereby picked out) rather than useful coordinates. I scanned the paper only briefly (I'm not really interested in black holes out of a cosmological or at least astrophysical context and am in no hurry to jump ahead of further EHT results) but found myself curious about the authors' "these solutions can be embedded in string theory" as their ref [17] at first glance seems to be about AdS, an acronym which is found only once in this paper. What provokes my wondering is in particular, "and so an asymptotic observer will barely notice the difference between the parallax angles" (among other references to an asymptotic observer), drawing a comparison with a distant Schwarzschild observer, again around eqn (31). "Barely notice" is, at least qualitatively, interesting wording. Can an observer at infinity more than barely notice the difference between a cold noncompact spherically symmetric uncharged nonspinning central mass and a Schwarzschild BH? [1] This is especially true for Sgr A*, see EHT paper https://iopscience.iop.org/article/10.3847/2041-8213/ac6674 https://iopscience.iop.org/article/10.3847/2041-8213/ac6674 which is wildly "tilt"ed (to use authors' expression).