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I'm holding two very massive, dense objects, one in each hand at arms length. I fall through the event horizon of a very large black hole. The tidal forces at the event horizon are not large, so I survive.

Now inside the event horizon, I bring the masses that I hold together. This should affect the shape of the event horizon. Using this method, I will communicate information from inside the event horizon, to an observer outside the event horizon, who is closely monitoring the geometry of the event horizon.

This seems natural, and also seems to contradict the idea that information cannot be communicated outwards across an event horizon. Is the description right, and information can be communicated outwards from within an event horizon? Or is the description wrong? In which case, what have I missed?

Thanks for taking a look.

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  • $\begingroup$ Assuming for the sake of argument that you make it into the black hole without encountering a firewall (which is still up for debate), what makes you think you affect the event horizon by moving these masses in the interior? $\endgroup$
    – d_b
    Aug 16, 2020 at 0:47
  • $\begingroup$ The nature of mass inside the event horizon determines the event horizon. Two examples: 1. The angular momentum of the mass affects how spherical/ellipsoid the event horizon is. 2. The quantity of the mass affects the surface area of the event horizon. $\endgroup$
    – tobuslieven
    Aug 16, 2020 at 0:51

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The description is wrong. After you fall past the event horizon no part of the horizon is in your future light cone. You can no more affect the horizon than you can affect yesterday, and for the same reason.

No matter how you wiggle those masses outside of the horizon you cannot send a signal to yesterday, therefore no matter how you wiggle those masses inside the horizon you cannot send a signal to the horizon. It is easiest to see this using Kruskal–Szekeres coordinates where light cones form standard 45 degree lines.

EDIT: from some of the comments below there is a bit of confusion about the relationship between the horizon and the singularity. The singularity is not in the past of any portion of the horizon, so the singularity does not cause the horizon. Also, for technical reasons the singularity is not part of the spacetime manifold.

The standard Schwarzschild black hole is a vacuum spacetime, meaning it does not contain mass anywhere. A more realistic spacetime is the Oppenheimer Snyder spacetime which models the gravitational collapse of a spherically symmetric cloud of dust. This is not a vacuum solution and has mass, but note that in this spacetime the event horizon forms prior to the formation of the singularity. So again it is incorrect to think that the horizon is caused by the singularity or that gravitational waves emitted inside the horizon could reach the horizon.

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  • $\begingroup$ I thought spacetime itself can travel faster than light. Otherwise, how does the singularity cause the event horizon? The nature of the singularity is determining the nature of the event horizon. $\endgroup$
    – tobuslieven
    Aug 16, 2020 at 0:56
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    $\begingroup$ The singularity doesn’t convey anything to the event horizon. The singularity is a moment in time in the causal future of the interior of the event horizon. It is not an object sending signals to the horizon. The horizon is a global feature of the spacetime, and the singularity is a different geometrical feature of the spacetime. One does not cause the other any more than the equator causes the poles. $\endgroup$
    – Dale
    Aug 16, 2020 at 1:11
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    $\begingroup$ That's a one word answer. Would have been shorter than your comment. But I'll do as you say. $\endgroup$
    – tobuslieven
    Aug 16, 2020 at 1:17
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    $\begingroup$ The horizon isn't caused by anything inside the horizon. It's caused by collapsing matter, and for a black hole it necessarily forms before the spacetime singularity. $\endgroup$
    – d_b
    Aug 16, 2020 at 1:33
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    $\begingroup$ @tobuslieven you cannot remove anything from within an event horizon by the mathematical definition within GR of what the horizon is $\endgroup$
    – anna v
    Aug 16, 2020 at 6:36
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From the point of view of a distant observer, you never reach the event horizon. Gravitational time dilation becomes extreme. You slow down as you approach the event horizon, and essentially become frozen in time. Even after an infinite time in the distant observer's coordinates, you still haven't reached the event horizon.

Thus the observer never sees effects of what you do after passing through the event horizon.

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  • $\begingroup$ It sounds like from the perspective of an outside observer, the mass of a black hole isn't inside the event horizon either, but is slowly approaching it. $\endgroup$
    – tobuslieven
    Aug 16, 2020 at 18:41

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