B How Can We Observe Black Holes Growing?

[Grinkle]

Can one observe expansion of the EH even if one cannot ever get a light signal from whatever fell in confirming that it fell in?

 

[PAllen]

Can one observe expansion of the EH even if one cannot ever get a light signal from whatever fell in confirming that it fell in?

In a sense, yes. There are computer generated images of what you would see against background stars as a neutron star or small BH merges with a bigger one, resulting in a single larger BH visible against background stars. However none of what see comes from at or inside a horizon.

 

[Ibix]

Can one observe expansion of the EH even if one cannot ever get a light signal from whatever fell in confirming that it fell in?

Depends what you mean by "observe". You could measure a black hole's mass by descending to some $r$ coordinate slightly above the horizon and hovering and measuring the acceleration needed to do so. Then dump in a load of mass and repeat. You'll find the same acceleration at a larger $r$. Or you could look at gravitational lensing.

But none of that is really observing the event horizon - it's just measuring the mass. You can't really observe the horizon because it isn't a thing, just a null surface.

 

[Grinkle]

In a sense, yes.

I am very likely missing something deep about curved space-time, but to me, that is the same thing as grossly / coarsely seeing something cross the EH.

The fact that I can't ever get photons from at or beyond the EH and the fact that photons redshift asymptotically as they are emitted from closer and closer to the EH are measurement limitations.

If I can see the EH expand by watching what it obscures or lenses, I observed something cross it with a measurement that doesn't depend on photons from the in-falling object, no? I guess its not really a claim that I saw that "thing" cross the EH - but in principle one can measure the total in-fall over time for a BH and if one has enough information on whatever is falling in one can construct a sequence of the in-falling events.

 

[Grinkle]

@Ibix If I read your post first I wouldn't have bothered with mine - thanks!

 

[Orodruin]

I mean, one issue here is using the Schwarzschild metric and test particles falling into the black hole of that. This is not really a physical scenario because it does not actually describe the spacetime of anything with appreciable mass falling into a black hole. It just happens to be a relatively good approximation for many situations when the contribution of the infalling object to the spacetime geometry is negligible. This is certainly not the situation in the merger of two similar sized black holes.

 

[PAllen]

I am very likely missing something deep about curved space-time, but to me, that is the same thing as grossly / coarsely seeing something cross the EH.

The fact that I can't ever get photons from at or beyond the EH and the fact that photons redshift asymptotically as they are emitted from closer and closer to the EH are measurement limitations.

If I can see the EH expand by watching what it obscures or lenses, I observed something cross it with a measurement that doesn't depend on photons from the in-falling object, no? I guess its not really a claim that I saw that "thing" cross the EH - but in principle one can measure the total in-fall over time for a BH and if one has enough information on whatever is falling in one can construct a sequence of the in-falling events.

Here is a dertailed, accurate, simulation of what would be seen against a stellar background for two equal size BH. It is easy to imagine how it would look for other cases: