
To Test Einstein’s Equations, Poke a Black Hole - abhishekjha
https://www.quantamagazine.org/to-test-einsteins-equations-poke-a-black-hole-20180308/
======
DiabloD3
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.

~~~
raattgift
> 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
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
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
> 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
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.

~~~
raattgift
Thinking about my [2] cheat a little further, with a small black hole the time
dilation gradient near the black hole is large, so the inner bits of the
hyperbolically-orbiting experimenter will feel a drag compared to the outer
bits (relative to the horizon). The "drag" is because the outer parts are
winning a race with the inner parts thanks to gravitational time dilation,
against a suitable set of _local_ coordinates on the astronaut. If the
astronaut is facing inwards with a rocket on his back, then inner bits are
liable to be torn off and left behind. [1]

The gradient is much lower for a much larger black hole, so this sort of drag
becomes apparent ever closer -- or even right at -- the outside of the
horizon. For a sufficiently large black hole, dipping one's hands towards the
black hole will probably not feel any different compared to deep space, even
very close to the horizon. However, one would then expect a short sharp shock
dipping one's hand _into_ the horizon. Bye-bye hand.

\- --

[1] Put the rocket pack on the closer-in side, and the intermolecular forces
etc that hold the astronaut into familiar shape will decelerate the whole
astronaut along a suitable choice of coordinates. The coordinates you want are
probably integral over Fermi normal ones for ever-smaller components, lots of
fun to calculate.

