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When 2 black holes approach each other, they both bend space in an opposite direction. There must always be a flat space between 2 colliding black holes.

However, I heard that they actually merge, becoming one...

I wondered, how can this happen?

Also, is it a good way to get very-very close to the center of a black hole by pushing another one close to it, while our probing spacecraft occupies the flat space in between?

EDIT: Obviously, the flat space between the black holes only exists at a single point (L1). But around that point there is a relatively flat space, where a small enough spacecraft can have enough structural strength to survive. Also, around L1, time dilatation is small, so a probe can get out within reasonable time, or can send signals.

EDIT2: Ok, maybe space at L1 is not flat... Let me try to explain what I was thinking:

B1~X~L1~Y~B2

B1,2: black hole1,2
L1: Lagrange 1
X,Y: 2 extreme points on the spacecraft

Forces pulling the spacecraft apart:

F_B1<--X~L1~Y-->F_B2

Forces crushing the spacecraft:

X-->F_B2 ~L1~ F_B2<--Y

Since the forces are equal at L1, pulling apart and crushing should cancel out each other, the closer to L1, the more they cancel.

I wonder if this thinking is wrong because gravity is a pseudo-force, and they cannot simply be added together.

If this is the case, do the tidal forces strengthen or weaken between 2 massive objects?

rob
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Zoltan K.
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1 Answers1

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You are mixing up the closely-related ideas of gravitational force versus spacetime curvature.

Spacetime curvature is associated with a gravitational field which changes as you move through space. For example, the gravitational field near the surface of the Earth is stronger than the gravitational field at the altitude of geostationary satellites. This variation in the gravitational field strength as you move nearer and farther from a massive object means that extended objects are subject to a gravitational stretching force. If you had a rock near the Earth whose diameter were roughly the width of the United States (which we actually do have), the side of that rock nearer to the Earth would be attracted to Earth more strongly than the far side. The different accelerations of the different pieces of the effect of elongating the object. This stretching is called a tidal force.

You are correct that a point exists on the centerline between two black holes where their gravitational attractions cancel each other out. However, the tidal interaction does not cancel out. An extended object at this zero-field point would have its two ends attracted in opposite directions, towards the two black holes. This tidal force demonstrates that the spacetime at this location is still curved.

rob
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