4
$\begingroup$

I am trying to understand the concept of event horizon and what really the word "event" means and have some questions. I apologize if the questions seem stupid or vague as I am not a physics student or professional: just someone who is curious.

As the black-hole sucks more and more mass, the gravity of the black-hole increases. Are things affected by this increased gravity not examples of events occurring due to changes (to the mass of the singularity) inside the (event horizon of the) black-hole ? In that case, isn't this an example of an event inside the black-hole influencing something outside it ?

Building on the same question: Couldn't gravity wave patterns during black hole collisions give us some insight into the geometry of the singularities inside the horizon ?

(Also, observing how the gravity envelope changes as mass (stars, planets, etc.) enter the horizon might provide clues on what is happening inside as the mass falls to the center. Also, If we are ever able to really observe closely and interpret BH collisions (probably even rarer), looking for changes to the gravity envelope past the moment when the two event horizons start merging could present interesting insights. However, I am not sure if we have such an advantage of observing (the changes in effects of gravity around) a BH from that many angles and at that precision today or not.)

Or is mass, somehow, also considered to be a property of the surface/event-horizon ?

$\endgroup$
  • $\begingroup$ just a thought - the information that something has fallen into a black hole is known to the outside universe, so the information that mass of black hole increased need not come from inside of a black hole. Then the question could be, wheter event horizon reacts to what is happening to the object inside the black hole as it falls to singularity. If not, then the only informations you can get from event horizon are those that you new already by observing the object outside - like its mass, charge, angular momentum etc... $\endgroup$ – Umaxo Nov 15 '19 at 11:50
  • 2
    $\begingroup$ Related, possible duplicate: physics.stackexchange.com/q/937/123208 "in a certain sense gravity can't cross the horizon, and it is that very property that forces gravity outside of it to remain the same." Also see math.ucr.edu/home/baez/physics/Relativity/BlackHoles/… $\endgroup$ – PM 2Ring Nov 16 '19 at 16:29
  • 1
    $\begingroup$ Possible duplicate of How does gravity escape a black hole? $\endgroup$ – PM 2Ring Nov 16 '19 at 16:40
0
$\begingroup$

According to what I have read in the case of black holes the radius doubles as the mass doubles. So if the black hole acquires more mass it's event horizon would get bigger. This "event" would cause a change in the black hole's capacity to affect material outside itself in a relative manner. I think your second question is excellent - it is likely we could gain increased information as we learn to interpret gravitational waves but someone else would have to answer that. Like you I'm just a curious (non-physicist)person. Thanks

$\endgroup$
  • 1
    $\begingroup$ No, we cannot get any information from inside the event horizon, by any mechanism. Roughly speaking, that'd be equivalent to sending information backwards in time. $\endgroup$ – PM 2Ring Nov 19 '19 at 10:08
  • $\begingroup$ @PM 2Ring. I wonder if you could clarify for me ... I understand the escape velocity of a black hole exceeds the speed of light (and therefore no information ever leaves as you say) but does space itself also move inwards with time? (as opposed to it just being a stationary path along which matter travels) $\endgroup$ – Wookie Nov 19 '19 at 13:45
  • 1
    $\begingroup$ Sort of, but it's probably better to think of it as something happening with your coordinates, rather than space itself flowing. OTOH, the "river" model is probably a better mental model than the "rubber sheet" model. See physics.stackexchange.com/a/106519/123208 physics.stackexchange.com/a/183937/123208 and jila.colorado.edu/~ajsh/insidebh/waterfall.html $\endgroup$ – PM 2Ring Nov 19 '19 at 13:59
  • $\begingroup$ That's really helpful. I'm just going to quote a couple of lines from those pages in case anyone reads down through this: "the concept of space falling into a black hole is mathematically correct" ---- "in-fall velocity of space passes the speed of light at the outer horizon but slows down to less than the speed of light at the inner horizon. The velocity slows all the way to zero at the turnaround point inside the inner horizon. At this point space turns around and accelerates back out hitting the speed of light again" $\endgroup$ – Wookie Nov 20 '19 at 12:18

Not the answer you're looking for? Browse other questions tagged or ask your own question.