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I apologize if this is a silly question, but...

From what I've been told, new black holes cannot form, because time slows to nearly stopped as matter approaches the event horizon. Instead, a new black hole is really an approximation of a black hole, with matter so close to the event horizon that it behaves like a black hole. Nevertheless, true black can exist, provided they have existed since sufficiently early in the universe. (I hope that's somewhat close to accurate.)

My question is: is it known (or could it even be determined) which of these scenarios describes the black hole at the center of M87? Is it a true black hole that has always existed, or an approximate black hole that formed at some point?


marked as duplicate by GiorgioP, Jon Custer, Buzz, A.V.S., Kyle Kanos Apr 15 at 11:26

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    $\begingroup$ Possible duplicate of How long does it take for a black hole to form? $\endgroup$ – Bones Apr 12 at 4:50
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    $\begingroup$ in the link you give , I do not see where the age of the black hole comes in "sufficiently early universe". It is the mathematical description using GR that changes the perspective of observer of black hole. The M87 is a black hole because it can be described by GR equations for the given framework.:telescopes detecting inflalling matter . $\endgroup$ – anna v Apr 12 at 4:50
  • $\begingroup$ @annav Sorry for the "sufficiently early" language, that's just my attempt to avoid getting into a debate on semantics. $\endgroup$ – Owen Apr 12 at 5:23
  • $\begingroup$ All black holes are "approximate". A "true" eternal Schwarzschild black hole is a wide spread misconception, because the Schwarzschild solution is valid only outside the horizon where "true" and "approximate" black holes are the same. $\endgroup$ – safesphere Apr 12 at 5:49
  • $\begingroup$ Good question, though I will comment on a somewhat similar but still different aspect. We can't say with 100% certainty that it's a regular Schwarzschild (or rather, Kerr) black hole as a solution to Einstein's equations. There are various proposals for black hole-like compact objects that mimic black holes. With more, higher precision measurements of shadows and gravitational waves, one hopes to detect deviations from GR. You can have a look at a review that came online today that discusses exactly this arxiv.org/abs/1904.05363 $\endgroup$ – Avantgarde Apr 12 at 6:50

The notion that black holes "never" form because time "freezes" is somewhat of a misconception. It arises from trying to interpret the results of general relativity from a very traditional (Newtonian) point of view.

What is true is that an outside observer will see the clocks of objects approaching the event horizon slow down. However, it is more accurate to think of this as a consequence of the fact that it becomes harder and harder for a signal (light) to reach us from the object. As the object comes closer to the event horizon it takes longer for any signal from the object to reach us.

But this should not be confused with the idea that the object is outside the horizon "now". We normally do not interpret distant events (e.g. a supernova) that we observe "now" as happening "now", instead we usually interpret them as having happened a long time ago. The same should be done with signals observed from an object falling into a black hole.

It is important to realize that while (in principle) signals from the object we continue to reach us until eternity (making it look like it never crosses the horizon), it is impossible for us to send any signal to the object, because by the time the signal reaches the object it will have long crossed the event horizon.

This leads to the following thought experiment: Suppose you drop a mirror into a black hole, and this mirror reflects the image of a clock that you (the outside observer) hold in your hand. What you will "see" is the reflected clock (your clock) come to a halt. In some sense the indicated time, can be interpreted as the time (on your clock) at which the object crossed the event horizon, which will be in the past.

Getting back to M87*. I have dismissed the notion that it has "never" formed a black hole. At same time, I have mentioned that in principle signals from all things that went in to make the black hole should still be able to reach us. So why aren't we seeing all these things? The reality is the signals from an object approaching the event horizon rapidly become fainter and distorted (due to time dilation and how signals propagate near a black hole). In fact the intensity of any signal will decay exponential on a time scale set by the size (mass) of the black hole. In case of M87 any signal from an object falling in will disappear in a matter of days.


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