shminux comments on Questions for shminux - Less Wrong

-4 Post author: Thomas 22 June 2012 07:35PM

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Comment author: shminux 23 June 2012 05:42:13PM 1 point [-]

In a real world, the escape speed from the system (Earth)-(any black hole) is heavily dependent from the direction you choose to escape. In the direction from Earth to the black hole, it is greater then c. While in the opposite direction it is only the well known 11+ km per second.

That cannot be right. For example, in the Earth-Sun system the escape velocity from the Earth's surface is about 11.2km/s (to escape the Earth), but this only gets you to an orbit around the Sun. You need to accelerate to about 42.1 km/s to escape the solar system (neglecting the effects from other planets), regardless of the direction of travel.

For an observer, far away on the other side of the super massive black hole in the Center, we are sometimes behind the event horizon, sometimes we are not. True?

No, not true. Once you are behind the event horizon, you only have moments to live until you run into the singularity, and you can certainly never get out (barring FTL travel). I suspect that I misunderstand your setup, that's why we are having difficulties.

Comment author: Thomas 23 June 2012 08:05:51PM *  -1 points [-]

You need to accelerate to about 42.1 km/s to escape the solar system

Sure, but I said "the Earth". Never the less you may include the Sun, okay. It is 40+ km per second then, to escape the system: a black hole -- our planet, in one direction. You can't in the other, you will stumble into a black hole in other direction.

My point was, we have different escape velocities for different directions, from one point. Don't we?

Comment author: shminux 23 June 2012 08:42:48PM -2 points [-]

No, it's the same velocity regardless of direction, because the escape velocity is determined by the potential energy, which is just a number for each point and is direction-independent.

Comment author: Thomas 23 June 2012 09:22:46PM 0 points [-]

From here, you can escape the system planet Earth-SupermassiveBlackHole in almost every direction easy. But not even the light will escape this system if it goes from here toward the SMBH.

From the same point, much different escape velocities, dependent of the escape direction.

What do I miss?

Comment author: shminux 24 June 2012 01:46:09AM -1 points [-]

Just like classically light gets consumed by the ground if you aim it wrong, in GR light gets consumed by the black hole if it gets close enough to the horizon (1.5x the horizon radius for a non-rotating black hole). If you aim it better, it misses the black hole and escapes to infinity.

Comment author: Thomas 24 June 2012 07:02:45AM 0 points [-]

Yes. And a rock flown 1000 km per second will not escape in one direction, it will escape in other.

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