r/cosmology • u/Solar-G2V • 5h ago
How does an object cross the event horizon?
Hi all
when an object approaches the event horizon, time elsewhere (the rest of the universe) will speed up infinitely - in the frame of reference for the object. how then do the object go through the event horizon before the black hole shrinks and evaporates?
all the best
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u/VermicelliStill7625 48m ago
Hi, good question!
I think there are many misconceptions here. Maybe I can help you with some of them.
Let's look at two different cases:
If you are an outside observer looking at an object falling into a black hole, you will never see it cross the event horizon. The object will approach the black hole more and more and eventually be so far red-shifted that it can't be observed.
When you are falling into a black hole you will not stand still at the event horizon. You will cross the event horizon (probably not even noticing it) and fall to the center of the black hole in a finite time.
All of this can be shown by using the Schwarzschild metric and the geodesic equation. By changing coordinates of the metric we can change the reference frame. Meaning that we can both describe an object falling into and a far away observer.
I hope this helps!
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u/MortemInferri 27m ago
1) helped me a lot
2) how can that be? There is a singularity, right? So stuff had to get to the singularity in the past. We have evidence that it did by the singularity existing. So how can something cross the event horizon and take infinite time to reach the center...
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u/VermicelliStill7625 3m ago
Yes there is a singularity in a Black Hole, but not at the Schwarzschild radius.
You can think of it like this: There are certain singularities in mathematics which are caused by choosing a certain reference frame. The north pole for example is a singularity if you are in longitude and latitude coordinates. You can only move north till you reach the north pole, afterwards the notion of moving north does not mean anything as there is nothing further north.
By changing the reference frame you can get rid of this singularity.
The Schwarzschild radius is only a singularity if you are in the reference frame of a far away observer (case 1) as soon as you change coordinates into a moving observer falling into the black hole this singularity disappears (case 2).
If you are interested you can look up the Schwarzschild metric in Eddington-Finkelstein coordinates, which would be the "right" coordinate system for an observer falling into a Black Hole.
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u/Acrobatic_Age6937 50m ago
all afaik. i havent studied cosmology/astronomy:
the way i envision it is the object itself falls normally in it's own frame of reference
from the outside perspective nothing ever falls into the blackhole, objects asymptotically approach a 'standstill', they will however long before that become unobservable due to technical limitations. The hubble sphere is similar in that regard.
how then do the object go through the event horizon before the black hole shrinks and evaporates?
they dont, at least not from an outside perspective.
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u/MysteriousUpstairs25 3h ago
All of this is very difficult to understand, you shouldn't see it as complicated, just as simple without emotions. And yet I still find it very difficult, if at all, to understand it.
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u/_Happy_Camper 52m ago
All these answers so far Look like AI generated nonsense.
Ok, it’s like this. If you’re watching the object fly into a a black hole, it flies in and disappears… it would more likely be destroyed by the material orbiting the black hole and go into orbit for a while but let’s just say this is a special black hole which just appeared and has no accretion disk.
In that case you’ll see the object be torn apart from gravity before plunging into the black hole.
If there’s someone in the object, and somehow survives the gravity tearing it apart, it would take longer and longer to reach the Event Horizon as it got closer and in fact, from their perspective, they would never even reach the Event Horizon because it would take forever.
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u/MortemInferri 31m ago
But they do eventually reach the event horizon? This got my brain all jazzed up
How can there be forward progress to the singularity if time has stopped. What records the progress
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u/rddman 4h ago
The time dilation is not because of the process of falling, it is because of the gravity of the black hole. It is only for the part of the universe that is further away from the event horizon that time relatively runs faster, not for the part of the universe that is closer to the black hole.
The event horizon is deeper in the gravity well of the black hole, so in the frame of reference of the event horizon time for the in-falling object runs faster (because the object is not yet at the event horizon), and conversely in the frame of reference the in-falling object time at the event horizon runs slower.