3 ms·

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 im

by 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?
  - raattgift 9y ago
    These are two excellent questions that won't find justice done to them in a comment on a discussion board like this. :-( I won't try sketch a semiclassical gravity attack on the second question, which is the harder of the two, and for the first, I'll mainly feint and point you to Greg Egan at http://www.gregegan.net/SCIENCE/Rindler/RindlerHorizon.html http://www.gregegan.net/SCIENCE/Rindler/RindlerHorizon.html as a starting point. On the one hand, for a sufficiently massive black hole, the Rindler solution he explores is an excellent approximation. On the other hand, his exact solutions are (a) not fully applicable in the dynamical spacetime of a sufficiently massive black hole (even one with lots of symmetries and otherwise in isolation) because the Rindler horizon is a local structure while the BH horizon is a global one [1]; there is a local boundary that forms a point of no return for objects near enough a black hole though you have to calculate where that is; (b) Egan's rope is a classical object, whereas when we're at the level of cellular signal transduction, molecular bonds, and chemical bonds, classical simplifications are already probably cheats or at least misleading. On the other hand, there are lots and lots of particles involved, and tracing the evolution of each of them in some suitable coordinates would be an enormous amount of work. So I trust my own intuition only to the extent that (as Egan notes) there are some decent mathematical similarities between the Rindler horizon and a BH trapping surface. Sadly, there will be no contact with observation in our lifetimes, but we might make quick and dirty numerical solutions that will give a strong theoretical prediction. Additionally, it is at least plausible that we will be able to do small-scale tests of Rindler space in a few decades. Until then be wary of people offering glib responses, especially really wrong glib responses like "the hands will never pass through the horizon because of time dilation" etc., which are sadly commonplace in online forums. Finally, my own thinking was that a powered hyperbolic orbiter (rocketman!) with sufficient momentum dropping his hands past the local point of no return would end up with stumps, and what does the actual ripping is rocketman's momentum [2]. In my head is a picture of Wile E Coyote running into a quicksand (or cement or tar etc) trap and either being tripped up and pulled into the quicksand or being unlucky enough to have enough forward momentum that he leaves his feet behind on the first step. Roadrunner corretctly judged exactly where the local trapping surface was and so skimmed right over the trap; Coyote miscalculated where Roadrunner's trapping surface would be (given more global knowledge of the configuration) and then his own. - -- [1] In particular, BHs (i.e., compact masses sourcing a trapping surface) can last extremely long times compared to the age of the universe and can in principle be observed by anyone in the BH's Hubble Volume, while real objects cannot be Rindler observers for very long (what fuels the acceleration?), and the Rindler horizon is peculiar to the Rindler observer rather than a trapping surface. Nevertheless, one can take the analogy seriously [ https://arxiv.org/abs/1305.4986 https://arxiv.org/abs/1305.4986 for instance ] [2] So I can "cheat" here by having the ripping happen between bits of tissue that are all outside the horizon at the time of rip; the tissue that is already inside the horizon is then irrelevant, and it's mainly whole cells that fall in. The centres of momentum and mass of the unfortunate astronaut are highly dynamical during this, so it's not so shocking a cheat.
rcthompson 9y ago
Wouldn't the downward current generated by the draining of the fluid into the black hole drastically offset any buoyancy effect?
yaks_hairbrush 9y ago
I believe Randall Munroe was talking about touching neutron star material, if hypothetically we could stably put it on earth.

by beobab 9y ago

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?
- raattgift 9y ago
  These are two excellent questions that won't find justice done to them in a comment on a discussion board like this. :-( I won't try sketch a semiclassical gravity attack on the second question, which is the harder of the two, and for the first, I'll mainly feint and point you to Greg Egan at http://www.gregegan.net/SCIENCE/Rindler/RindlerHorizon.html http://www.gregegan.net/SCIENCE/Rindler/RindlerHorizon.html as a starting point. On the one hand, for a sufficiently massive black hole, the Rindler solution he explores is an excellent approximation. On the other hand, his exact solutions are (a) not fully applicable in the dynamical spacetime of a sufficiently massive black hole (even one with lots of symmetries and otherwise in isolation) because the Rindler horizon is a local structure while the BH horizon is a global one [1]; there is a local boundary that forms a point of no return for objects near enough a black hole though you have to calculate where that is; (b) Egan's rope is a classical object, whereas when we're at the level of cellular signal transduction, molecular bonds, and chemical bonds, classical simplifications are already probably cheats or at least misleading. On the other hand, there are lots and lots of particles involved, and tracing the evolution of each of them in some suitable coordinates would be an enormous amount of work. So I trust my own intuition only to the extent that (as Egan notes) there are some decent mathematical similarities between the Rindler horizon and a BH trapping surface. Sadly, there will be no contact with observation in our lifetimes, but we might make quick and dirty numerical solutions that will give a strong theoretical prediction. Additionally, it is at least plausible that we will be able to do small-scale tests of Rindler space in a few decades. Until then be wary of people offering glib responses, especially really wrong glib responses like "the hands will never pass through the horizon because of time dilation" etc., which are sadly commonplace in online forums. Finally, my own thinking was that a powered hyperbolic orbiter (rocketman!) with sufficient momentum dropping his hands past the local point of no return would end up with stumps, and what does the actual ripping is rocketman's momentum [2]. In my head is a picture of Wile E Coyote running into a quicksand (or cement or tar etc) trap and either being tripped up and pulled into the quicksand or being unlucky enough to have enough forward momentum that he leaves his feet behind on the first step. Roadrunner corretctly judged exactly where the local trapping surface was and so skimmed right over the trap; Coyote miscalculated where Roadrunner's trapping surface would be (given more global knowledge of the configuration) and then his own. - -- [1] In particular, BHs (i.e., compact masses sourcing a trapping surface) can last extremely long times compared to the age of the universe and can in principle be observed by anyone in the BH's Hubble Volume, while real objects cannot be Rindler observers for very long (what fuels the acceleration?), and the Rindler horizon is peculiar to the Rindler observer rather than a trapping surface. Nevertheless, one can take the analogy seriously [ https://arxiv.org/abs/1305.4986 https://arxiv.org/abs/1305.4986 for instance ] [2] So I can "cheat" here by having the ripping happen between bits of tissue that are all outside the horizon at the time of rip; the tissue that is already inside the horizon is then irrelevant, and it's mainly whole cells that fall in. The centres of momentum and mass of the unfortunate astronaut are highly dynamical during this, so it's not so shocking a cheat.

rcthompson 9y ago

Wouldn't the downward current generated by the draining of the fluid into the black hole drastically offset any buoyancy effect?

yaks_hairbrush 9y ago

I believe Randall Munroe was talking about touching neutron star material, if hypothetically we could stably put it on earth.