Say humans in the future could detect super small changes in gravity (like literally the difference between an earth with one extra atom and another earth).

Could we send a machine into the black hole which can shift its weight away or closer to the black hole so that it can choose how much mass it wants to add to the singularity at a time. Would the change have an effect outside the event horizon for our incredible machine to detect.

So if it works than if first bit of the information is 0 it shifts it's weight forward so our machine detects it and if it is it shifts it back so we find out a split second later?.

Of course, there is the problem that from our perspective it would take ages (maybe forever if I remember correctly) from our perspective for the object to reach the singularity. So I propose a different version.

The object falling to the black hole is horizontally entered and it can decide if it wants to move a heavy ball inside it to the left or to the right. If it is to the left it is a one or to the right it is a zero. this way we don't have to wait until it hits the singularity, just until it passes the event horizon.

Would our machine detect it?

(Note, this is not about the limits of the machine or us, more on whether or not the gravitational effect will carry on outside the event horizon)


The problem with this scheme is that gravitational waves (which is essentially what the device would be making) move at the speed of light through spacetime. But the black hole event horizon is a trapped surface: light or gravitational wave trajectories will not be able to cross it from the inside. In fact, they will also end up in the singularity. So the signals from the probe will never reach the outside.

One can see this by using a Penrose diagram.

  • $\begingroup$ So why does the black holes own gravity have an effect outside the EH? $\endgroup$
    – yolo
    Sep 4 '18 at 15:59
  • $\begingroup$ Because black holes are not objects hidden by a horizon, but the entire space-time curvature structure: in a sense they exist wherever their gravity is felt. $\endgroup$ Sep 4 '18 at 23:32
  • $\begingroup$ Ahh... OK that's interesting $\endgroup$
    – yolo
    Sep 5 '18 at 15:56

Limiting to smaller mass change is not conceptually different from large mass change.

You measure gravity of a black hole and note it down.

This black hole is on a path to merge with another one in say a year.

You do not observe anything for an year.

After a year, you measure gravity of the merged black hole and you will find it increased.

This is kind of an information that tells you how much mass/energy has changed behind the event horizon.

The only difference is that what was added was another black hole rather than a single proton or a small mass.

So, conceptually, you can figure that the mass/energy has changed behind the event horizon.

How that information comes to you - from behind event horizon, or some other way that safe guards the event horizon boundary concept, is a different question. The fact is that you have information about how much mass/energy has changed behind the event horizon and you do not have to wait for eternity to measure this change.


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