6 ms·

To Test Einstein’s Equations, Poke a Black Hole

52 points by abhishekjha 9y ago

DiabloD3 9y ago
Problem is, where do we get a black hole to poke? UCLA GCG and other efforts have failed to image occlusion of orbiting stars or gravitational lensing effects while viewing Sagittarius A* (the supermassive black hole at the center of our galaxy), thus it may not actually be a black hole (instead, it may be even stranger, like an absolutely gigantic plasmoid that somehow ties the room together). I know they meant theoretically poke it, but we've been having trouble finding nearby ones.
- qubex 9y ago
  This is just about the properties of the dynamic system defined by the Einstein field equations, and proof that perturbations do not self-sustain and increase in intensity.
- raattgift 9y ago
  > where do we get a black hole to poke As qubex suggests, this is really about confirming that asymptotically Schwarzschild spacetime (and its close relatives) is mathematically robust [1]. Asymptotically here means that if we perturb (gravitationally or electromagnetically) a Schwarzschild black hole the resulting radiation from "balding" will decay exponentially fast and die off to ground state well before spatial infinity. This is also a useful check on practices like (in cosmology) taking a boundary at some reasonable radius from a galaxy as if it were asymptotically Schwarzschild space, and stitching it into the expanding Robertson-Walker spacetime. There is no poking of astrophysical black hole candidates involved. That said, signals from LIGO and (eventually) LISA will provide experimental evidence to check some "balding" conjectures for various types of extremely strong perturbations (NSes falling into BHs, for example). Electromagnetic astronomy isn't really directly helpful, other than double-checking that one of the colliding compact objects is almost certainly not a black hole. Finally, "other efforts" will soon include the Event Horizon Telescope, which was already the only practical platform for direct observation of a candidate BH. For Sgr Astar, observations from the South Pole Telescope have to be captured in Antarctic autumn and this time around the recorded data had to wait in Antarctica until Antarctic summer (when the physical media could be flown out in reasonable weather; there is insufficient bandwidth from Antarctica to send it over a network), so the first tranche of data are still being crunched. It's very premature to do wild speculation about the nature of Sgr Astar (and your speculation is really wild). - -- [1] Hintz works on the stability of several black hole solutions, for example https://arxiv.org/abs/1612.04489 https://arxiv.org/abs/1612.04489 -- he here he deliberately breaks the symmetries of the Schwarzschild solution to see if removing tiny angular momentum by hand poses mathematical problems. I haven't looked at the Klainerman and Szeftel paper [ https://arxiv.org/abs/1711.07597 https://arxiv.org/abs/1711.07597 ] yet.
  - beobab 9y ago
    Good old Randall Monroe has come up with a theory in his What If series (it's in the book) whereby it's almost safe to touch one with your bare hands if it's immersed in a liquid which your hand is more buoyant than, but he counsels that it's probably not a great idea...
    - raattgift 9y ago
      Touch one what? If you put your hand, whatever it's covered in, through a black hole horizon, you're not going to get your hands back (unless you let the rest of you fall through too). You can have a very mild ("no drama" conjecture) black hole event horizon for ultra-massive black holes. The curvature at the horizon gets gentler with higher-mass, so you can envisage an indistinguishability of the tidal/shear stresses at the horizon versus the tidal/shear stresses in empty space many many light years outside the horizon. You still won't get your hands back. The horizon isn't actually any sort of surface (nothing bounces off it) but you should sure feel the lack of nerve impulses returning from whatever bits of your anatomy are on the inside if your brain is on the outside. Ouch. Also, the horizon is likely to be sufficiently "sharp" that on lots of reasonable orbits above it that let you dip your hands in (without all of you falling in too), your hands will be fairly neatly and very rapidly torn off. (It would be fairly easy to misjudge the exact location of a horizon though.) I suppose if you have a sufficiently small black hole (and ignore the extreme hostility of the environment near it) then if you have lots of "shielding" it is plausible that you can move the location of the horizon (from your perspective) such that your hand can go to where the horizon would be without the shielding. I don't think that would really count as "touching", though. It's a little like blowing on the surface of a soap bubble then waving a pin around where the film was (and returns to) in the absence of the blowing air. "Look, I didn't pop it!" It's not like putting a bit of scotch tape on a balloon and putting a pin into the tape ("look, no pop!").
      - saagarjha 9y ago
        > The horizon isn't actually any sort of surface (nothing bounces off it) but you should sure feel the lack of nerve impulses returning from whatever bits of your anatomy are on the inside if your brain is on the outside. Would your hand even be connected to you at that point? The bonds holding the molecules together wouldn't work, right?
- imglorp 9y ago
  How likely are we to witness an occlusion of Sgr A, if one were to happen? Its diameter is smaller than the Sun-Mercury orbit[1], plus the stars in close orbit have a period of 11 years at the shortest. So they're far away from A* plus they don't transit very often. 1. https://en.wikipedia.org/wiki/Sagittarius_A* https://en.wikipedia.org/wiki/Sagittarius_A*

52 points by abhishekjha 9y ago

Problem is, where do we get a black hole to poke? UCLA GCG and other efforts have failed to image occlusion of orbiting stars or gravitational lensing effects while viewing Sagittarius A* (the supermassive black hole at the center of our galaxy), thus it may not actually be a black hole (instead, it may be even stranger, like an absolutely gigantic plasmoid that somehow ties the room together). I know they meant theoretically poke it, but we've been having trouble finding nearby ones.

qubex 9y ago
This is just about the properties of the dynamic system defined by the Einstein field equations, and proof that perturbations do not self-sustain and increase in intensity.
raattgift 9y ago
> where do we get a black hole to poke As qubex suggests, this is really about confirming that asymptotically Schwarzschild spacetime (and its close relatives) is mathematically robust [1]. Asymptotically here means that if we perturb (gravitationally or electromagnetically) a Schwarzschild black hole the resulting radiation from "balding" will decay exponentially fast and die off to ground state well before spatial infinity. This is also a useful check on practices like (in cosmology) taking a boundary at some reasonable radius from a galaxy as if it were asymptotically Schwarzschild space, and stitching it into the expanding Robertson-Walker spacetime. There is no poking of astrophysical black hole candidates involved. That said, signals from LIGO and (eventually) LISA will provide experimental evidence to check some "balding" conjectures for various types of extremely strong perturbations (NSes falling into BHs, for example). Electromagnetic astronomy isn't really directly helpful, other than double-checking that one of the colliding compact objects is almost certainly not a black hole. Finally, "other efforts" will soon include the Event Horizon Telescope, which was already the only practical platform for direct observation of a candidate BH. For Sgr Astar, observations from the South Pole Telescope have to be captured in Antarctic autumn and this time around the recorded data had to wait in Antarctica until Antarctic summer (when the physical media could be flown out in reasonable weather; there is insufficient bandwidth from Antarctica to send it over a network), so the first tranche of data are still being crunched. It's very premature to do wild speculation about the nature of Sgr Astar (and your speculation is really wild). - -- [1] Hintz works on the stability of several black hole solutions, for example https://arxiv.org/abs/1612.04489 https://arxiv.org/abs/1612.04489 -- he here he deliberately breaks the symmetries of the Schwarzschild solution to see if removing tiny angular momentum by hand poses mathematical problems. I haven't looked at the Klainerman and Szeftel paper [ https://arxiv.org/abs/1711.07597 https://arxiv.org/abs/1711.07597 ] yet.
- beobab 9y ago
  Good old Randall Monroe has come up with a theory in his What If series (it's in the book) whereby it's almost safe to touch one with your bare hands if it's immersed in a liquid which your hand is more buoyant than, but he counsels that it's probably not a great idea...
  - raattgift 9y ago
    Touch one what? If you put your hand, whatever it's covered in, through a black hole horizon, you're not going to get your hands back (unless you let the rest of you fall through too). You can have a very mild ("no drama" conjecture) black hole event horizon for ultra-massive black holes. The curvature at the horizon gets gentler with higher-mass, so you can envisage an indistinguishability of the tidal/shear stresses at the horizon versus the tidal/shear stresses in empty space many many light years outside the horizon. You still won't get your hands back. The horizon isn't actually any sort of surface (nothing bounces off it) but you should sure feel the lack of nerve impulses returning from whatever bits of your anatomy are on the inside if your brain is on the outside. Ouch. Also, the horizon is likely to be sufficiently "sharp" that on lots of reasonable orbits above it that let you dip your hands in (without all of you falling in too), your hands will be fairly neatly and very rapidly torn off. (It would be fairly easy to misjudge the exact location of a horizon though.) I suppose if you have a sufficiently small black hole (and ignore the extreme hostility of the environment near it) then if you have lots of "shielding" it is plausible that you can move the location of the horizon (from your perspective) such that your hand can go to where the horizon would be without the shielding. I don't think that would really count as "touching", though. It's a little like blowing on the surface of a soap bubble then waving a pin around where the film was (and returns to) in the absence of the blowing air. "Look, I didn't pop it!" It's not like putting a bit of scotch tape on a balloon and putting a pin into the tape ("look, no pop!").
    - saagarjha 9y ago
      > The horizon isn't actually any sort of surface (nothing bounces off it) but you should sure feel the lack of nerve impulses returning from whatever bits of your anatomy are on the inside if your brain is on the outside. Would your hand even be connected to you at that point? The bonds holding the molecules together wouldn't work, right?
imglorp 9y ago
How likely are we to witness an occlusion of Sgr A, if one were to happen? Its diameter is smaller than the Sun-Mercury orbit[1], plus the stars in close orbit have a period of 11 years at the shortest. So they're far away from A* plus they don't transit very often. 1. https://en.wikipedia.org/wiki/Sagittarius_A* https://en.wikipedia.org/wiki/Sagittarius_A*