When two neutron stars collide they may form a black hole which is not a supermassive black hole but it should provoke that outside its newly formed event horizon photons are emitted as a consequence of the titanic collision and this EM-radiation is highly redshifted but also slowed down by black hole gravitational time delay. As it could be some speculation about the possibility that the photons are slower as they are closer to the event horizon and increase their velocity while they move away from the event horizon to reach the full speed when they move to the regions of spacetime more distant from the BH center where spacetime is inert and doesn't have influence to the resulting photon speed (intrinsic photon speed minus inward spacetime motion) the time needed for that kind of signals to reach regions near the BH where gravitational time delay may be neglected could be extremely long and by this reason I have a question... Can the signal of collision of the two neutron stars that is emitted towards Earth at the very proximity of the newly formed event horizon be so time delayed that it could be observed over an astronomically very long period of time?


1 Answer 1


The signal of the collision can in principle be detected forever. It will continue to be emitted until the end of time, although it will get ridiculously faint and eventually be too subtle for a physical detector to notice it. It is similar to how the oscillation of a pendulum dies off, but technically just gets fainter and fainter until it is undetectable but non-vanishing.

This is due to the fact that, for an external observer, the event horizon never actually forms. When you look at a black hole formed by stellar collapse (or collision), you won't really see a black hole. What you actually see is a star (or stars) undergoing a gargantuan redshift. While it is, in practice, indistinguishable from a black hole, you never really see the black hole forming. For the outside observer, it takes infinite time for the black hole to form.

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    $\begingroup$ It might be worth noting that "eventually" is actually "in a fraction of a second". $\endgroup$
    – TimRias
    Mar 22 at 16:13
  • $\begingroup$ @safesphere Thanks for the comments and correction. I removed the bit about the speed of light, since it is indeed variable in GR. $\endgroup$ Mar 26 at 3:52

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