# Gravity working on BH

## Main Question or Discussion Point

Hi.

BHs receive gravity of other stars and then move. Does this mean that information of gravity of other stars go beyond event horizon around BHs to let them move?

## Answers and Replies

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bcrowell
Staff Emeritus
Gold Member
Yes. An event horizon is like a one-way membrane. It lets information in, but not out.

Although you didn't ask about it, we could also ask about the transmission of information in the opposite direction by the same mechanism. The black hole does make gravitational forces on other stars. But this isn't really what we mean by transmission of information out through the event horizon. What we mean by that is really that a particle's world-line escapes -- and that's what doesn't happen.

Thanks.

Signal of gravity from other stars lose speed approaching the event horizon thus does not it take infinite time to go through the event horizon ?

Dale
Mentor
Signal of gravity from other stars lose speed approaching the event horizon thus does not it take infinite time to go through the event horizon ?
If by "signal of gravity" you mean gravitational waves, then they travel at c locally. They don't slow down in a coordinate independent sense.

I suspect you are thinking of a coordinate dependent effect of the Schwarzschild coordinates.

bcrowell
Staff Emeritus
Gold Member
If by "signal of gravity" you mean gravitational waves, then they travel at c locally. They don't slow down in a coordinate independent sense.

I suspect you are thinking of a coordinate dependent effect of the Schwarzschild coordinates.
I think sweet_springs was referring to the black hole's static gravitational force acting on infalling matter, not gravitational waves approaching the black hole.

Signal of gravity from other stars lose speed approaching the event horizon thus does not it take infinite time to go through the event horizon ?
Assuming I'm right, and you're talking about infalling matter, then your statement about taking infinite time to go through the event horizon is only for a distant observer.

Yes. An event horizon is like a one-way membrane. It lets information in, but not out.
Don't people believe that information is not lost but preserved in the hawking cloud?

I read sweet springs question to be this:
If two stars are on opposite sides of a BH, is the BH "absorbing" or "blocking" the gravitational influence of one star's "gravitational message" to the other star?
Sometimes the distance "line of sight" between two object on each side of a BH is characterized as being infinite.

Yes. An event horizon is like a one-way membrane. It lets information in, but not out.

Although you didn't ask about it, we could also ask about the transmission of information in the opposite direction by the same mechanism. The black hole does make gravitational forces on other stars. But this isn't really what we mean by transmission of information out through the event horizon. What we mean by that is really that a particle's world-line escapes -- and that's what doesn't happen.
Wouldn't changes of position be information?
If we imagine a binary BH system orbiting a barycenter wouldn't the orbits require constant exchange of information or waves,gravitons or something that updated the geometry relative to their changing locations?
Or is such a system impossible in principle because of the barrier imposed by the horizon??

Hi.

Assuming I'm right, and you're talking about infalling matter, then your statement about taking infinite time to go through the event horizon is only for a distant observer.
Thanks. Let me state my original question more clearly.

There is a system composed of a star and a BH. The BH moves receiving gravity originated from the star. May I understand right that the BH does not move for a distant observer because gravity caused by the star cannot enter into the even horizon in finite time ?

Regards.