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Comment by ganzuul | Hacker News Reader

Why is it that some experts say stuff falls into the center of a black hole while others say it slows down and never passes the event horizon? Is it just semantics?

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From the external view you never enter, just get dimmer as you approach the speed of light and the time dilation keeps increasing.

From the viewpoint of the one falling in, you just fall in and cross the horizon without noticing.

Does make you wonder if time dilation gets so extreme, could another black hole wonder by, offset the gravity of the black hole, and let you escape. Even if it takes a billion years.

The time dilation does not act in that way. You will not experience the outside universe speeding up, at least not by very much; the reason for this “paradox” is primarily that photons coming off you take longer and longer to climb out, and the ones from right when you crossed the event horizon never will.

> Does make you wonder if time dilation gets so extreme, could another black hole wonder by, ... Even if it takes a billion years.

This could even be the most likley scenario, since ultra massive black holes eats vast amounts of smaller over the course of billion years.

I suspect that would fall under the rule that if two black holes’ respective event horizons ever cross, they merge and initiate the eventual merger of the two respective black holes.

I've seen this vaguely referenced, but when I dig in I don't find support for it.

It's far from obvious that just because the event horizons (which is just a mathematical concept and a 3D area of space) should control the trajectory of the singularity.

With two identical blackholes, with a event horizon of radius R, why should a singularity 2R away be unable to escape?

Once you've crossed the horizon from your point of view, there is no escape. If another, larger black hole passes by, at best you'd be within a larger event horizon comprised of both, I think.

Right, but say you end up half way between two equal mass blackholes, under zero gravitational force. Are those two black holes definitely going to merge? If they were going near light speed in opposite directions could they not merge and free you as you stay equidistant from both?

Well, if you're halfway and outside of hte event horizon... I don't know. But you definitely can't escape an event horizon.

I don't know what would happen if two event horizons overlapped briefly, as two black holes passed by each other at significant speeds. That's definitely above my paygrade :P

Well imagine you are X distance from the singularity, and time dilation is hugely slowing down time from an external observer. As blackholes approach each other, the distance between the singularity and the event horizon significantly decreases. They become kidney shaped before the blackholes merge.

The trick is can a blackhole approach quickly enough to catch you as you keep dropping as you past horizon. Normally I'd say no, but the time dilation significantly complicates things.

IIRC, it depends on the frame of the observer.

An outsider will see in-falling objects slow down, redshift, and never cross the horizon; but if you're falling in, this happens in finite time, you don't get to see the universe rapidly age, the black hole doesn't evaporate from Hawking radiation before you reach it.

PBS SpaceTime had a recent episode that helps with this apparent paradox.

Really their entire playlist on black holes (and entire catalog for that matter) is worth a watch.

https://youtu.be/Rogm_lpVZYU

I think only the last photons at the event horizon of you never escape the event horizon. So as you're falling toward the horizon you're colour shifting as the photons move slower and slower until you cross the event horizon at which point your last reflected photons outward speed matches their inward gravity and they freeze, balanced on the edge. You actually keep falling toward the singularity but no one will ever see you after the horizon because any reflected photons can't escape the gravity well.

You won't actually see the frozen photons though, just the ones before it. Once you see them, though, the image on the horizon is gone.

I believe there is nothing actually inside a black hole -- to us, it appears as a giant sphere, where all of the matter is at the surface, but there is no "inside." It' s like a giant shell with no interior because spacetime breaks down. All matter on the shell is at zero distance from all the other matter on the shell.

There is also a theory that as you approach the horizon you'll be incinerated by a kind of hidden firewall. This apparently contradicts GR - but on the other hand if you're allowed to fall in it seemingly contradicts QM.

Frame of reference. The object falling in experiences time as normal and actually falls in.

From an outside observer they never cross the event horizon, due to the time dilation mentioned by the other comment.

> From an outside observer they never cross the event horizon, due to the time dilation mentioned by the other comment.

Doesn't that imply that if we look at a black hole from a safe distance, the event horizon will appear to be a cluttered frozen motionless ring, full of all the stuff that is in the process of falling in but which from out point of view, never will?

Almost, another detail is the closer the object gets to the event horizon, the more it's light redshifts - until it's no longer in the visible spectrum at all. You can think of the black hole's gravity as stealing energy from the light, shifting it into a lower energy spectrum.

Sort of, the red shift continues towards infinity, but since photos are quantitized there is a last photon.

Also don't forget even if you freeze forever as viewed by an external observer, the blackhole is growing, and will at some point consume everything that had previously fallen in.

The closer you are to the black hole the slower the time passes.

Why is that again? Falling in does not seem to imply accellerating towards the speed of light. Accellerating surely, but speed of light? Why would it?

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