
Gravitational waves eject black hole from galaxy - anigbrowl
https://www.nasa.gov/feature/goddard/2017/gravitational-wave-kicks-monster-black-hole-out-of-galactic-core/
======
strainer
This has curious similarity with errors which can appear in basic n-body
simulations, where excessively close interactions can break orbits by throwing
bodies too far from circumstances in one timestep to the next.

In n-body simulation the 'point gravity' objects are kind of mathematical
singularities which never collide because they have no size. The integration
time-step gives them a kind of radius or gradient of proximity where very
large errors occur, where virtual energy can be added or lost.

I read in these cases of true black hole collision, the shedding of energy
through gravitational waves is the main cause of their orbits decay - without
making the waves, the holes could circle each other for a very long time.
Presumably energy&momentum is maintained over the collision of the two
astrophysical non-point singularities, unlike the basic n-body excessive
gravitation phenomenon.

The full details of what actually happens during the collision/overlap of
black-hole 'calculation radii' are surely mysterious.

~~~
z3t4
this is also my experience with simulated gravity and galaxy systems. i guess
newton physics alone would be enough to make a body flung out if it comes
close enough. and i guess a merge _is_ close enough.

i was working on a space mmo and after leaving the server running for weeks
some body would always be flyng away with crazy speed.

~~~
strainer
The newtonian model itself doesn't produce this behaviour as it is continuous,
so however much bodies accelerate towards each other they can decelerate just
as much once they have passed. In n-body simulation the continuous model is
'squeezed' into time-slices, where bodies can accelerate towards each other in
one time-slice and in the next be too far past each other to slow each other
down again.

Seemingly in the real astrophysical world, blackholes will free-fall towards
each other by their masses 'dragging' on spacetime, causing deep perturbations
in spacetime (as gravity waves). Eventually there is a 'merge' as you called
it, an operation during which the resulting merged body can get a directed
kick from the persisting spacetime perturbations, one powerful enough to eject
it from a galaxy.

In the real world, overall it is expected energy and momentum is _not_ defied
during this merge operation, it seems remarkable considering how exceptional
and energetic these events appear.

edit ...on second thoughts, this 'merge event' produces GRAVITATIONAL
PROPULSION - sorry for the caps but I think they might be justified.

~~~
kristianp
Finally we have reactionless space travel! All we have to do is merge two
black holes!

~~~
AnimalMuppet
It's not reactionless. It's essentially a "gravitational wave rocket".

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dogma1138
This make me wonder if the galactic void is full of ejected black holes which
will be nearly undetectable since the X-ray radiation comes from the accretion
disk and the matter falling into the black hole. This mechanism could be
potentially be another candidate for "dark matter".

~~~
jcarreiro
This is a good observation, but gravitational lensing surveys have all but
eliminated massive compact objects as a potential source of dark matter; see
for example
[https://www.ncbi.nlm.nih.gov/pubmed/17359015](https://www.ncbi.nlm.nih.gov/pubmed/17359015).

~~~
saagarjha
Unrelated, but why is a paper on astrophysics on the _NIH_ website?

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semi-extrinsic
The NIH (specifically PubMed) just indexes most scientific journals. This
paper is actually in Phys. Rev. Letters (which the NIH site links to, if you
click the DOI link below the abstract):

[http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.98....](http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.98.071302)

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yodon
Amazing - the calculations in the article suggest gravitational waves emitted
by a pair of colliding black holes can have sufficient energy to eject the
combined object from the potential well of the galaxy.

~~~
M_Grey
It's a scale that's hard to grapple with, when the energy being carried away
in those waves is far more than the mass of the Sun.

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gjm11
Actual headline has "... out of galactic core". The black hole is still in the
galaxy, though perhaps in 20 million years it will have escaped.

The involvement of gravitational waves is conjecture and the evidence for it
is highly indirect (it all sounds pretty plausible to me, but what do I
know?).

~~~
yodon
Gravitational waves are among the most difficult physical phenomena to detect
directly, because of the very small size of the effects involved and the huge
masses required to cause those small effects (one commonly encounters numbers
like 10^-51 when doing gravitational wave calculations). The original
experimental demonstration of gravitational waves was from analyzing the
change in rotational frequency over time of an orbiting binary pulsar and
computing that the energy loss in the system was precisely matched by the
general relativistic prediction of energy radiated away in gravitational
waves. The importance of this work was recognized with the Nobel Prize in
Physics [0].

[0]
[http://www.astro.cardiff.ac.uk/research/gravity/tutorial/?pa...](http://www.astro.cardiff.ac.uk/research/gravity/tutorial/?page=3thehulsetaylor)

~~~
ab5tract
Can someone please explain why such waves that supposedly have minimal/hard-
to-detect effects are somehow valid explanations for pushing around black
holes?

Meanwhile, electrical forces are clearly powerful and yet still neglected in
the mainstream.

~~~
grigjd3
1\. Electromagnetic radiation has been the primary tool of astronomers and
astrophysicist since before the first telescope. It is not "neglected" by any
stretch of the imagination.

2\. At large scales, the total number of electrons and protons tend to balance
and thus result in electrically neutral objects.

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dgudkov
The whole idea of it reminds me the gliders in Conway's Game of Life.

~~~
cglider
For those interested in seeing the metaphor
[http://jsfiddle.net/ankr/tgjLA](http://jsfiddle.net/ankr/tgjLA)

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alnitak
Imagine you are an Earth-level (0.7 on Kardashev's scale) civilisation on the
path of this monster.

In fact, let's just imagine it were to happen in the Milky Way. What happens
to us then?

~~~
flashman
(Napkin math time.) A black hole with 1 billion solar masses would pull Earth
with a force equal to the Sun from half a light year away. It does the same
thing to Uranus from a distance of 9.6 light years. It would have enough
influence to disturb those orbits from much further away.

So if something similar entered our galactic neighbourhood, we would notice
changes in the proper motion of stars closer to it, discrepancies between the
planets' predicted and actual positions, and (possibly before anything else)
predictable errors in GPS timing (since this is probably the most commonly-
encountered system depending on accurate knowledge of speed, time and
gravity).

~~~
grigjd3
While the point you are aiming for is correct, your "napkin math" is wrong and
ignores the equivalence principle.

~~~
AnimalMuppet
A correction is more informative than just a bare statement that something is
wrong...

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Sniffnoy
What I'm wondering is, if this is the explanation, why don't we see this more
often? Plenty of galaxies have formed from mergers, right? So the question is,
how much of a difference between the black holes does there need to be for
this to occur? The video seems to imply only a small one, in which case one
might expect this to be a common occurrence. So why haven't we seen this
before? Or is that not the case?

~~~
jschwartzi
Because we would need to be looking. The sky is so very vast, and our imaging
equipment needs to look at such a small part in order to resolve the galaxies
beyond our own.

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jarmitage
What happens to a black hole-less galaxy?

~~~
gliese1337
Not much. Relative to a star, central supermassive black holes are, well,
super-massive. But relative to the total mass of the galaxy, or even just the
core region, they aren't _that_ big. The rest of the galaxy will just keep
orbiting around its center of gravity, regardless of whether or not there
happens to be a black hole there.

~~~
saagarjha
In what kind of galaxy is a 1 billion solar mass black hole not a significant
portion of the mass?

~~~
idlewords
Pretty much any galaxy. The Milky Way has something like 300 billion stars,
with the "average" star weighing 1/4 as much as the sun. So even the stellar
mass of a galaxy like ours outweighs such a black hole by two orders of
magnitude, and most of a galaxy's mass is dark matter.

~~~
saagarjha
For whatever reason I had the number "300 million" in my head for the size of
the Milky Way; I see your point.

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JumpCrisscross
What is the probability that the ejected black hole (if it is indeed such a
thing) goes on to spawn a new galaxy?

Black holes are likely responsible for galaxy formation and evolution [1].
Since new black holes don't spin up from the void, I had assumed this meant
all the galaxies to ever be formed have been formed (excluding effects of
mergers). This, however, produces a new origin method (even if extraordinarily
low-probability).

[1]
[http://www.roe.ac.uk/~pnb/papers/bh_nature.pdf](http://www.roe.ac.uk/~pnb/papers/bh_nature.pdf)

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ProAm
The narrator of that video was really annoying. NASA needs to step up its PR
game if they want these videos to have impact or go remotely 'viral'

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AlexCoventry
How does a gravitational wave impart momentum to a mass it passes through?

~~~
jschwartzi
By warping spacetime. It's the same thing that causes us to fall toward Earth.
The presence of Earth's mass warps spacetime, and thus the Earth imparts
momentum on us.

~~~
AlexCoventry
Anywhere I can look for a more detailed explanation of the process by which a
gravitational wave imparts momentum, though? It seems like the mechanism must
be quite different from the geodesics in a constant gravitational field.

~~~
raattgift
The nasa.gov link summarizes the speculation about a large anisotropic burst
of gravitational radiation from the merging smaller precursors to the runaway
supermassive black hole. One is free in GR to consider this as uniformly
radiated GWs carrying away residual momentum along the x^1 and x^2 spacelike
axes and most of the momentum from the spacelike x^3 axis as the merging black
holes "bald" their asymmetries in line with the no-hair hypothesis. (The
^1..^3 are indices in the Einstein convention).

We can generalize somewhat, to answer your more broadly worded question.

First let's start with a system losing energy-momentum via gravitational
radiation (e.g. PSR1913+16, but isolated in vacuum asymptotically flat
spacetime) and under time-reversal[1], so that the system is gaining energy-
momentum from gravitational radiation arriving from infinity in such a way
that the orbit _outspirals_. (This requires a highly improbable spacetime
towards (time-reversed) past infinity).

In this time-reversed picture, if you alter the incoming gravitational
radiation, the energy-momentum imparted to the binary system will necessarily
perturb their orbit differently; instead of causing the line segment through
the barycentre between the pulsars' centres-of-mass to grow along its length
with carefully timed + linearly polarized GWs[2], you can cause the entire
system to move in a particular direction.

Think of incoming gravitational radiation that is highly anisotropic but
arriving with lucky enough timing that the GW (in a gauge in which it is a
plane wave with x linear polarization seen from "above" or "below" the plane
of the binary orbit[2]) is always extended along this axis by stretch-
squashing the advancing and receding bodies differently. An inertial observer
in this asymptotically flat spacetime at a great distance and not feeling the
GWs will see the system precess.[3]

Chapter 7 (Perturbation theory) in Carroll's _Spacetime and Geometry_ in
subchapters 7.4-7.6 has a reasonably good inductive view of the mathematics in
the limit of weak gravity, and the time-reversal trick should be workable with
the contents of the chapter.

You could introduce anisotropy in a physically plausible (barely; Hulse-
Taylor's orbital period is just a few hours) way by putting the binary pulsars
near a pair of carefully arranged inspiralling supermassive black holes.

Another approach would be to consider Gravitational Bremsstrahlung ("GB") (see
e.g. (d) at Kovacs & Thorne [1978] at [http://articles.adsabs.harvard.edu/cgi-
bin/nph-iarticle_quer...](http://articles.adsabs.harvard.edu/cgi-bin/nph-
iarticle_query?bibcode=1978ApJ...224...62K&db_key=AST&page_ind=19&data_type=GIF&type=SCREEN_VIEW&classic=YES)
which has an extremely priceless admission in its final sentence.). The tl;dr
of "GB" is that just as electromagnetic bremsstrahlung changes the momentum
and direction of a small charge moving past a large charge with the emission
of electromagnetic radiation, a small mass moving past a large mass will emit
gravitional waves. One can again time-reverse in each case.

The usual case of electrons decelerating in a medium and throwing off photons
under time reversal is electrons absorbing photons and accelerating in a
medium. Likewise (given a suitable frame of reference and gauge, and in
analogy with cyclotron radiation as a form of electromagnetic bremsstrahlung)
a small mass body deflecting around a much larger mass decelerates and throws
off gravitons, but under time reversal absorbs gravitons and accelerates
instead.

Finally, choosing particular (families of) observers and considering things in
a frame of reference constructed on their attributes, choosing different sets
of coordinates and doing gauge-fixing is normal in GR; some people hate it
because relating one set of observations under particular choices like these
to the observations of the same system under different choices is either hard
to intuit or conversely hard to solve the equations for (and sometimes both!).
I hope the above isn't a total confusing mess.

[1] in case time-reversal in this context seems crazy on its face, consider
this much simpler Q&A:
[http://van.physics.illinois.edu/QA/listing.php?id=31314&t=is...](http://van.physics.illinois.edu/QA/listing.php?id=31314&t=is-
gravity-time-reversible)

[2] this is seriously (overly) simplified :) in the ordinary non-time-reversed
picture the orbiting system is always shedding off a spectrum of gravitational
waves and the plane of polarization changes twice per orbit; I instead focus
on the instantaneous state where an observer above the plane of rotation only
looks twice per orbit, seeing the system with one star at 12 o'clock and the
other at 6 o'clock, and [3] the whole "clock" precesses against the distant
observer's cartesian coordinates with an origin constantly on herself.

P.S: there are some more article-relevant gory details at
[http://iopscience.iop.org/article/10.1086/421552](http://iopscience.iop.org/article/10.1086/421552)
("How Black Holes Get Their Kicks"). Note the authors raise the analogy with
bremsstrahlung (where the masses are very different) at the bottom of page L6
- page L7.

~~~
AlexCoventry
It was very generous of you to give such a detailed explanation. Thank you.

> I hope the above isn't a total confusing mess.

Not at all. Pretty confident that I understand the explanation in [3], though
not sure how that applies to a single mass but you've given me plenty of
places to look for an explanation.

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tlogan
I wonder whether this even also displaced dark matter.

And if both dark matter and black hole are displaced will galaxy just
disintegrate?

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narag
Is it possible to create a "sail" or a "surf board" to get propulsion from
this waves?

~~~
AnimalMuppet
If the _black hole_ got propulsion from this wave, I'm pretty sure it's not
hard at all. You just have to be in the vicinity - no special shape or
structure required.

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yiyus
It's pretty cool.

