How Does Gravity from Nearby Stars Affect Black Hole Movement?

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Discussion Overview

The discussion centers on the influence of nearby stars on the movement of black holes (BHs) and the implications of event horizons in terms of gravitational information transmission. Participants explore concepts related to gravitational waves, the nature of event horizons, and the interaction between black holes and surrounding stars.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that black holes receive gravitational influence from nearby stars, which may affect their movement.
  • Others argue that the event horizon acts as a one-way membrane, allowing information to enter but not exit, raising questions about the nature of information transmission.
  • A participant questions whether gravitational signals from stars slow down as they approach the event horizon, suggesting that this could imply infinite time to cross it.
  • Another participant clarifies that gravitational waves travel at the speed of light locally and do not slow down in a coordinate-independent sense, indicating a potential misunderstanding of coordinate effects.
  • Some participants discuss the idea of whether a black hole absorbs or blocks gravitational influence from stars on opposite sides, considering the implications of "line of sight" in gravitational interactions.
  • There is a suggestion that changes in position of binary black holes would require constant information exchange, raising questions about the feasibility of such systems due to event horizon constraints.
  • A later reply seeks to clarify the original question regarding the movement of a black hole in response to a star's gravity, emphasizing the perspective of distant observers regarding the time taken for gravitational influence to be felt.

Areas of Agreement / Disagreement

Participants express differing views on the nature of information transmission through event horizons and the implications for black hole movement. There is no consensus on the effects of gravitational signals and the interpretation of event horizons.

Contextual Notes

Participants reference coordinate-dependent effects and the implications of Schwarzschild coordinates, indicating potential limitations in understanding the dynamics involved.

sweet springs
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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?

Thanks for your teachings in advance.
 
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Yes. An event horizon is like a one-way membrane. It let's 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 ?
 
sweet springs said:
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.
 
DaleSpam said:
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.

sweet springs said:
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.
 
bcrowell said:
Yes. An event horizon is like a one-way membrane. It let's 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.
 
bcrowell said:
Yes. An event horizon is like a one-way membrane. It let's 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.

bcrowell said:
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.
 

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