Comment by maffydub | Hacker News Reader

Comment by maffydub | Hacker News Reader

On "Black Holes have a strong gravitational pull.”...

> For a black hole you keep falling towards the center, cross the horizon, and the gravitational pull continues to increase.

I've always struggled with "crossing the horizon".

- From the perspective of an observer falling into the black hole, I understood that the event horizon falls away from you, so you never reach it (maybe you get "closer", though - not sure).

- From the perspective of an observer away from the black hole, time appears to slow down for the object falling into the black hole, so it never crosses the event horizon.

Am I right? What does it mean to talk about crossing the event horizon if this never happens from either of the observers' perspectives?

There's a difference between what observers see, and what happens.

In the abstract, in a 2d spacetime diagram (ie ignoring angular degrees of freedom), the event horizon is just a line, and an infalling timelike worldline will cross it.

Cf [1], a black hole spacetime diagram in Kruskal-Szekeres coordinates, via [2].

[1] https://i.stack.imgur.com/XUokp.gif

[2] https://physics.stackexchange.com/a/82711

It would have to cross the event horizon eventually, or the black hole would be this black dot that never grows and is surrounded by the worst traffic jam in galactic history.

Yet, GR is adamant on giving us no possibility of ever seeing anything fall into a black hole. So, how exactly does that work?

And yeah, I am aware we've watched things falling into black holes, although I'm not sure how much different the "it's frozen just before the horizon" scenario would look like.

The light emitted by said object would get more and more red-shifted. That's a consequence of "time would appear to pass slower and slower".

It's not like we can observe light from near the horizon anyway. An active black hole is a hellish place to observe anything.

What observations we have come from gravitational waves.

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