Is the following a possible scenario? If not, why not?

Assume there is a supermassive black hole $Z$ isolated in inter-galactic space. Nearby and stationary relative to $Z$ is observer $A$. A number of light years away spaceship $X$ is travelling at, say, $0.25c$ directly towards the black hole and (eventually) $X$ falls directly into $Z$. $X$'s mass as observed by $A$ will increase as $X$'s velocity approaches $c$ as $X$ approaches $Z$'s event horizon. It seems that $X$'s velocity should be so close to $c$ as it encounters the event horizon of $Z$ that $X$'s apparent mass should cause it to have its own event horizon.

I am assuming that $A$ will observe that $X$ becomes a black hole just prior to its event horizon merging with $Z$'s.

Now assume that just prior to the merging of the event horizons a spaceship $Y$ is travelling a few light seconds behind $Z$ on the same path, but at about $0.5c$. Just prior to the merging of event horizons $Z$ radios $Y$ (to whom $Z$ will NOT appear to have an event horizon) and $Y$ relays this message to $A$. (Note: This could also happen in reverse, so a fast conversation may be possible.)

What is wrong with this, as it appears to anyone stationary with respect to $Z$, that $A$ and $Z$ are communicating, with information being exchanged from within a black hole?


1 Answer 1


What you describe is not what observer $A$ would see. Observer $A$ will certainly see the spaceship $X$ accelerating towards the black hole, but as $X$ approaches the black hole $A$ will see $X$ start to slow down and come to a near halt at the event horizon. In fact $A$ will never see $X$ cross the horizon, or more precisely in $A$'s co-ordinate system it takes infinite co-ordinate time for $X$ to reach the horizon.

The other problem is that fast moving objects do not become black holes as they approach the speed of light. This is obviously the case because movement is relative i.e. it is impossible to say which object is moving and which is staying still. If you saw a fast moving rocket become a black hole the principle of relativity would be violated unless the rocket could see you becoming a black hole. This is dicussed in more detail in Relativistic mass as seen by different observers.


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