
Simulating Extreme Spacetimes - MichaelAO
http://www.black-holes.org/explore/movies
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verytrivial
Somewhat off-topic, but regarding visualization, we're told that Andromeda is
on a collision course with the Milk Way, but also that it is very unlikely
that e.g. the Sun will actually collide with any other stars -- galaxies are
approximately empty over all. I have always want to know what it would look
like from Earth to see the stars of the two galaxies 'sliding across' the
celestial sphere during the merge. Has anyone seen something like that? I
search online for it every few years.

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scatters
Even at closest approach, there won't be any appreciable change in the night
sky over a human lifetime. Note that in the gallery at the bottom of
[http://www.nasa.gov/mission_pages/hubble/science/milky-
way-c...](http://www.nasa.gov/mission_pages/hubble/science/milky-way-
collide.html) the shortest time between frames is still 50 million years.

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lisper
From the original article:

" it will take 4 billion years before the strike. ... it will take an
additional two billion years after the encounter for the interacting galaxies
to completely merge"

Also, it's important to keep in mind that galaxies have a pretty low density.
If you shrank the sun down to the size of a ping pong ball, earth would be a
poppy seed about 10 feet away. Proxima centauri, the nearest star, would be
500 miles away.

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ISL
Something the general reader may not know: As late as ~2004-7, calculations of
this type were impossible. It was a major achievement when numerical
relativists were able to achieve a single orbit of a black-hole black-hole
binary system.

Figuring out how to make it work was the culmination of decades of attempts at
the problem. Improved computational power didn't hurt, but the major
improvement was finding the right metrics in which to do the simulations.

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openasocket
From what I remember about numerically solving differential equations, your
approximations will completely diverge from the actual solution near singular
points in the equation, so you have to transform the problem to come up with a
singular solution. I imagine the interaction of two singularities would result
in some interesting singular points. Is that what you are alluding to when you
say "finding the right metrics" or am I completely off base?

In either case, could you link to one of those 2004-7 papers? It sounds like a
fascinating read.

~~~
ISL
A tricky part was making sure that numerical instabilities didn't propagate
outward away from the event horizon. I believe there are now several solutions
to the problem, but the one I find most intuitive is simply extending
spacetime ad infinitum inside the black hole in order to allow gravitational
waves to propagate inward forever.

More here:
[https://en.wikipedia.org/wiki/Numerical_relativity](https://en.wikipedia.org/wiki/Numerical_relativity)

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MichaelAO
Super cool research going on here. I came across their website after seeing,
"What the first LIGO detection would look like up close"
[https://www.youtube.com/watch?v=Zt8Z_uzG71o](https://www.youtube.com/watch?v=Zt8Z_uzG71o)

[edit] this might be a better link for visuals of the simulations:
[https://www.youtube.com/user/SXSCollaboration/videos](https://www.youtube.com/user/SXSCollaboration/videos)

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Aelinsaar
This almost feels pornographic, it's so good. In the particular the extreme
counter-rotating example is so interesting to consider, but really exciting to
see simulated. I just imagine the ergoregions of each body going absolutely
nuts in those last few moments of merger.

~~~
sevenless
It would be interesting to do the ray tracing and see how the mergers would
actually appear to an observer.

~~~
Aelinsaar
You could probably do it with a handful of test photons, without breaking the
bank.

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whiloop
There's a visible-wavelength simulation of a gravitational singularity in free
space running through Apache on an old Sun machine at Dal:

[https://web.cs.dal.ca/~hannon/](https://web.cs.dal.ca/~hannon/)

;)

