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Cheap Astronomy
2008-Oct-26, 02:47 AM
If you had a two black holes (orbiting a common centre of mass yada, yada), is it conceivable that stuff could get sucked out of one (or the other) such that an external observer could see black hole contents that has been drawn back out past the event horizon?

spratleyj
2008-Oct-26, 02:51 AM
If you had a two black holes (orbiting a common centre of mass yada, yada), is it conceivable that stuff could get sucked out of one (or the other) such that an external observer could see black hole contents that has been drawn back out past the event horizon?

Well, the only way would be via Hawking Radiation, because it's impossible for something that's passed the event horizon to escape, the pull of gravity from black hole (a) couldn't "pull" anything out of black hole (b)...

Neverfly
2008-Oct-26, 02:54 AM
Yes, if a was close enough to b that a could pull matter out of b, then they would already be colliding and the event horizon would spread around them.

Celestial Mechanic
2008-Oct-26, 04:08 AM
...
"And at these prices you won't see many more!"
:lol:

mugaliens
2008-Oct-26, 12:54 PM
I've often wondered about a glancing collision between two black holes, if the relativistic dynamics would allow for changes in the location of the event horizon which may expose matter inside the black hole.

The analogy would be this: two droplets touch mid-air, and become one. But two droplets collide mid-air, and tear one another asunder, such that many tiny droplets are formed.

The details are different (event horizon vs surface tension), but the question remains valid - could such a collision cause the event horizon, in spots, to change location faster than the matter inside, such that the internal matter is exposed, even if only briefly?

Neverfly
2008-Oct-26, 10:20 PM
I've often wondered about a glancing collision between two black holes, if the relativistic dynamics would allow for changes in the location of the event horizon which may expose matter inside the black hole.

The analogy would be this: two droplets touch mid-air, and become one. But two droplets collide mid-air, and tear one another asunder, such that many tiny droplets are formed.

The details are different (event horizon vs surface tension), but the question remains valid - could such a collision cause the event horizon, in spots, to change location faster than the matter inside, such that the internal matter is exposed, even if only briefly?

Wouldn't that require that mass travels faster than light to expose them?

And anything that was ejected far enough to be observed would suffer <chuckle> explosive decompression upon leaving such intense gravity.

grant hutchison
2008-Oct-26, 10:40 PM
In another thread, I posted links (http://www.bautforum.com/1315882-post25.html) showing what happens to the event horizons as two black holes approach each other. The horizons reach outwards and merge, and then the linked pair of black holes "rings down" to reach a stable configuration as a single black hole.

Grant Hutchison

mugaliens
2008-Oct-26, 11:24 PM
In another thread, I posted links (http://www.bautforum.com/1315882-post25.html) showing what happens to the event horizons as two black holes approach each other. The horizons reach outwards and merge, and then the linked pair of black holes "rings down" to reach a stable configuration as a single black hole.

Grant Hutchison

From which frame of reference?

What would it look like from a spacecraft approaching two tightly co-orbiting black holes of equal mass along their common orbital axis?

Edited to add: I followed the links, and am pondering the toroidal phase...

grant hutchison
2008-Oct-27, 12:29 AM
From which frame of reference?This shouldn't make a difference, since the event horizon is a global property of spacetime: it's a hypersurface separating signals that can get out from signals that can't get out. So choosing a new set of coordinates can't tweak a spacetime event from one side of the horizon to the other.

Grant Hutchison

mugaliens
2008-Oct-27, 08:38 PM
Gotcha.

But "What would it look like from a spacecraft approaching two tightly co-orbiting black holes of equal mass along their common orbital axis?"

grant hutchison
2008-Oct-27, 08:40 PM
Gotcha.

But "What would it look like from a spacecraft approaching two tightly co-orbiting black holes of equal mass along their common orbital axis?"Complicated, I imagine. :)

Grant Hutchison

Jeff Root
2008-Oct-27, 08:58 PM
What would it look like from a spacecraft approaching two tightly
co-orbiting black holes of equal mass along their common orbital axis?
Approaching??? If the spacecraft is *approaching* from a distance,
it will not be able to keep up with the motion of the black holes about
one another, so will not remain on their common orbital axis.

If the spacecraft is midway between the black holes, it looks like
spaghetti!

-- Jeff, in Minneapolis

grant hutchison
2008-Oct-27, 10:08 PM
Approaching??? If the spacecraft is *approaching* from a distance,
it will not be able to keep up with the motion of the black holes about
one another, so will not remain on their common orbital axis.It's not evident to me why this should be the case.


If the spacecraft is midway between the black holes, it looks like
spaghetti!Are you assuming stellar mass black holes, here? We can make the tidal forces arbitrarily low by increasing the black hole masses.

Grant Hutchison

Jeff Root
2008-Oct-27, 10:57 PM
Approaching??? If the spacecraft is *approaching* from a distance,
it will not be able to keep up with the motion of the black holes about
one another, so will not remain on their common orbital axis.
It's not evident to me why this should be the case.
There might possibly be a point like Lagrange point 2 in which the
spacecraft's speed in orbit around both black holes matches the
speed of the black holes orbiting each other, but with frame dragging
and gravitational radiation, I kind of doubt it. In any case, that would
only be at a point, not along a line of approach. Not that I meant this
very seriously. I think it's probably right, but not particularly important.




If the spacecraft is midway between the black holes, it looks like
spaghetti!
Are you assuming stellar mass black holes, here? We can make the
tidal forces arbitrarily low by increasing the black hole masses.
I'm pretty sure that the tidal force midway between two "tightly
co-orbiting" black holes will be much greater than the tidal force
at the event horizon of one black hole of the same total mass.
But yes, I generally have stellar-mass black holes in mind when the
mass is otherwise unspecified.

-- Jeff, in Minneapolis

grant hutchison
2008-Oct-27, 11:20 PM
There might possibly be a point like Lagrange point 2 in which the
spacecraft's speed in orbit around both black holes matches the
speed of the black holes orbiting each other, but with frame dragging
and gravitational radiation, I kind of doubt it. In any case, that would
only be at a point, not along a line of approach. Not that I meant this
very seriously. I think it's probably right, but not particularly important.In the Newtonian case, with two equal masses, there's a stable line at right angles to the orbital plane and threading L2: objects can resonant happily back and forth along the line.


I'm pretty sure that the tidal force midway between two "tightly
co-orbiting" black holes will be much greater than the tidal force
at the event horizon of one black hole of the same total mass.Why "much"? Aren't the tidal forces merely additive?

Grant Hutchison

mugaliens
2008-Oct-28, 05:59 PM
There might possibly be a point like Lagrange point 2 in which the
spacecraft's speed in orbit around both black holes...

Perhaps I didn't paint the picture well enough.

Two black holes, their centroids in an x-y plane, are orbiting one another around the z axis. At all points along the z axis, the net gravitational attraction orthoganal to the z axis is zero. The z axis is dynamically unstable, but it takes only minute corrections to remain on the z axis.

Thus, an object placed on the z axis and shot along it, between the black holes, would see...

What?