Since we know that close to a black hole event horizon time slows much so that an object falling into a black hole appears to slow as it approaches the event horizon, taking an infinite time to reach it, and so how can we define the black hole speed with time frozen as it appears to be?
As black hole speed data we can refer to NASA's Chandra X-ray Observatory that has shown a massive black hole speeding away from its host galaxy at a speed of several million miles per hour.
How can we understand all that?
As black hole speed data we can refer to NASA's Chandra X-ray Observatory that has shown a massive black hole speeding away from its host galaxy at a speed of several million miles per hour.
How can we understand all that?
A black hole is an object in outer space. Its location is limited. So there is a domain around it where gravity is moderate. We can discuss if at the centre there is really that singularity as GRT describes it, but for defining at what location the BH is only the domain around it needs to be defined and then inside that domain there is something with weird properties.
Now, at what speed does this object move? It moves with a speed as seen by the external observer. When we describe the speed as seen from earth then that speed is meant. If an observer close to the event horizon has a different notion of speed makes no difference for us.
Regards,
Paul Gradenwitz
Dear Alter,
When you write:
"an object falling into a black hole appears to slow as it approaches the event horizon, taking an infinite time to reach it"
It seems that you are considering Schwarzschild's metric to describe what happens to the object as it approaches the horizon. However, it is an external metric intended to describe non-rotating and motionless objetcs.
Moreover, it is an approximation to weak fields. Indeed, even if the components of the external metric are correct, its coordinate system is less and less adapted as the object approaches the horizon. The curvature of space-time on the horizon is actually not infinite, it suffices to adapt the coordinate system. Thus, an object will end up crossing the horizon. It is not a "pathology of space-time", the curvature is not really infinite and space and time are not reversing their signs in the metric beyond the horizon. But this is a fairly common mistake in the community of physicists.
Hoping that my answer could help you,
Alexandre.
Dear Altair,
You can think of the region, as described in the coordinates of a remote observer as looking like a frozen shell, sometimes described as an "eternally collapsing object" or "ECO". You could fly past that object in a spaceship, the appearance of slowing is only local. Relative to you then, the slowed region is moving and can be assigned a speed by the remote observer.
The thing is if you talk about speed of the whole black hole, it is more a question of the outside of the hole, with its own normal time. So there is no problem.
For sure.
However, I find it a misleading story! What does it really means an "outside" of a hole? Where is indeed its boundary? For sure, one cannot observe the "velocity" of its "surface" em relation to the external world, since Ithere is no time reference for defining that! At the very surface, the time is frozen, everything outside is not part of the black hole of course. What we could maybe measure is only the velocity of the matter that might be eaten by it! Before those matters cross its surface! What I would spect is that any matter eaten by the hole would not go inside it, but would be frozen forever at its surface, and this because the time is frozen at the very surface. The only possibility for mass to enter the hole is if time is not frozen afterall!
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