It has always bugged me that the model is that space time around a black hole is non Euclidean space that light cannot escape from. How does anything approach a sphere that is surrounded by space that is so distorted? Is not everything suspended in the last few (Euclidean) meters before the event horizon?
> Is not everything suspended in the last few (Euclidean) meters before the event horizon?
No. The "distortion" doesn't prevent things from falling in; it helps things to fall in. The event horizon, heuristically, is the point at which the "helping things to fall in" becomes strong enough that even outgoing light can't overcome it.
To expand on this explanation: this is why many physicists use the analogy of a stretchy fabric to describe gravity and warping of spacetime. Such as placing a large mass on the fabric helps draw all other masses towards it. But it is also important to remember that this is an analogy and loses a lot of (important) nuance because it's meant as a simplification to laymen.
There are many complexities that this type of model will not account for and it's worth noting that higher dimensional geometries don't "play nice", in that your visual intuitions will mislead you. Such as the stretchy fabric model is a 2D spacetime fabric stretching into a 3rd dimension that is otherwise not accessible by those flatlanders who experience a pulling force but do not see where that fabric is stretching into. Similarly we can not think of a black hole (or any mass) as a 3 dimensional version of a hold because it is "stretching" into higher dimensions, but the result of this distortion is the gravitational force we feel. Even this is a great oversimplification. If it helps you can think instead think of gravity as an inter-dimensional spaghetti monster that invisibly grabs everything and the closer you are to it the harder it is able to pull (just like you!). That might help remind you that the analogy is incomplete and unrealistic at some level.
I'd generalize the abstracted phenomena here and make sure that when anyone is listening to a scientific explanation they understand that it is incomplete (compared to the level of current understanding or prevailing hypotheses/theories). Truth has a lower limit in complexity and often the complexity is rather high. Physicists __love__ simple theories (they call it "elegant") and the whole goal is to make things as simple as possible. But obviously that threshold is rather high considering the level of math physicists do. So keep that in mind when hearing explanations because if it can be explained to your grandma, you're losing a lot of information. Even if as distilled as possible. (Also, Einstein never said if you understand something you can explain it to a layman. Not sure where that came from but it is a silly notion without addendums akin to what has been said here)
So take analogies for what they are, but do not rely on them as complete or even "good enough" models. They're explanatory aids, not explanations. Maybe scientists can do better on this, but it's a tough job already and requiring being a scientific communicator (to the masses) is too high of an expectation. Scientists are still people too and our brains are small and time is limited. We can only do so much lol
Because gravity distorts _space-time_ - not just 3-D space - then the event horizon is the boundary at which gravity means that all future paths (technically everything within your future light cone) lie either on or within the event horizon, leading to a stable orbit at the horizon or spiral into the centre of the black hole respectively.
Space isn't distorted at the event horizon and you wouldn't notice any effects from crossing it - other than no longer being able to see the outside universe.
Technically, in GR gravity is the distortion of spacetime, it does not have independent existence that acts on spacetime. It's mass that distorts spacetime.
Yes anything falling in approaches the speed of light as it approaches the event horizon. So to external observers they slow down. However keep in mind that all black holes are growing (and will continue until the cosmic backround radiation gets lower than the rate of hawking radiation. So I think of black holes as onions, everything approaches the event horizon and then (to external observers) get frozen at the edge, until the black hole gets bigger.
Also keep in mind that from the perspective of someone falling in, there's nothing particularly noticeable as you cross the event horizon.
> anything falling in approaches the speed of light as it approaches the event horizon
Only relative to static observers, i.e., observers who are hovering at a constant altitude above the horizon. But there is no such observer at the horizon, so no observer ever sees the infalling object going past them at the speed of light.
> So to external observers they slow down.
No, that isn't the reason they appear to slow down to external observers. This is curved spacetime and you can't use your intuitions from Special Relativity, since that is only valid in flat spacetime.
The reason the infalling object appears to slow down to an external observer is that the light it emits takes longer and longer to get back out to the external observer, because of the way spacetime is curved.
> I think of black holes as onions, everything approaches the event horizon and then (to external observers) get frozen at the edge, until the black hole gets bigger.
This view is wrong. Things fall into the hole. They don't pile up at the horizon. They appear to slow down as they approach the horizon, but that is an optical illusion created by the curvature of spacetime.
Objects falling towards the event horizon accelerate ever closer to light speed, which causes time dilation. So it's not just that light takes longer to escape, right?
The dilation gets pretty extreme, you could watch suns be born and die as you fall in.
>Also keep in mind that from the perspective of someone falling in, there's nothing particularly noticeable as you cross the event horizon.
So I've heard that before, but let's say you are slowly spiraling into a supermassive black hole with very low tidal forces near the event horizon. You get close to the event horizon. Light can't escape. You stick your hand into / past the event horizon. Seems like a pretty definite discontinuity in "noticeability", if one second you can see your hand and move it around, and the next you can't see it and can't yank it back.
> You stick your hand into / past the event horizon.
That wouldn't really happen. When people say that light cannot escape, they don't mean that a beam of light can't pass the horizon heading outwards, they mean that the curvature of spacetime is such that the photons will always curve back around and fall back into the hole.
Locally, the horizon will look more or less the same to you, except at high curvature you'll start to see Doppler shift of light from outside vs. inside. At extreme curvature, an arm pointed towards the singularity will look red shifted and an arm pointed away from the singularity will look blue shifted. Although at this level of curvature, you'd already be ripped apart so you wouldn't really notice anything at all.
Nothing anywhere close to event horizon can "slowly spiral in". There are no stable orbits within 1.5x the diameter of the event horizon. To get anywhere close without falling in would require a very powerful rocket, one that could accelerate to near light speed just to stay there.
Now if you are falling in your hand never disappears, it's just that your body would accelerate into the black hole a little faster than your hand is accelerating because you are pulling back on your hand. But your hand never could have a negative velocity (moving away from the blackhole) no matter what you do.
