Matter falling into a black hole

[MikeeMiracle]

TL;DR Summary

Matter falling into a black hole

As I understand it, as you get closer to a black hole no matter what trajectory you were on approaching it, you will get pulled into the disk of spinning matter around the black hole which I assume is around it's equator?

I am just curious as to what is happening to the "space" in the disk, and above/below the disk why anything with any mass is always pulled into the disk itself. I guess, why does the "disk" exist and not a "sphere" of matter falling in from all directions.

I am a keen enthusiast but have not studied the topic so please no responces with equations or anything like that, just a no doubt over-simplification.

Thanks

 

[anorlunda]

You can ask the same question about the rings of Saturn. Or why many galaxies are disc shaped rather than spheres. You can find clues here.

The Potato Radius: a Lower Minimum Size for Dwarf Planets

 

[phinds]

As I understand it, as you get closer to a black hole no matter what trajectory you were on approaching it, you will get pulled into the disk of spinning matter around the black hole which I assume is around it's [sic] equator?

Keep in mind, a BH does not necessarily even HAVE an accretion disk, although in practice most still do (and those discs will be eaten eventually).

 

[PeroK]

Summary:: Matter falling into a black hole

As I understand it, as you get closer to a black hole no matter what trajectory you were on approaching it, you will get pulled into the disk of spinning matter around the black hole

This is not true at all. A black hole, outside the event horizon, is just a regular gravitational field.

To be clear: if you were on a trajectory that would not have impacted the star before it became a black hole, then you should avoid the black hole as well. If your trajectory would have impacted the star, then you might still avoid the black hole. It depends on the "impact parameter" for the trajectory.

The critical radius is 3/2 times the Schwarzschild radius. That's as close as you can get without definitely getting trapped.

I suppose it depends what you mean by "close enough" and "any trajectory".

One difference between GR and Newtonian gravity is:

Theoretically, in Newtonian gravity, you would always escape a point mass, unless you were headed directly for it. But, in GR below a certain impact parameter (which approximately translates to initial angular momentum) you do not escape.

 

[MikeeMiracle]

So the disk is all about the angular momentum of the combined system once something starts to fall in? I suspected angular momentum would play a part.

 

[MikeeMiracle]

@PeroK Yes your right of course when you think about it, it should be just a regular gravitaional field outside the event horizon...

...You see this is the problem with believing you have a good understanding of something and then watch pop sci-fi videos as your bored, they just confuse and contradict your common sense and give you limited info to back up their claims.

Thanks for putting me back on the right path :)

 

[PeroK]

@PeroK Yes your right of course when you think about it, it should be just a regular gravitaional field outside the event horizon...

...You see this is the problem with believing you have a good understanding of something and then watch pop sci-fi videos as your bored, they just confuse and contradict your common sense and give you limited info to back up their claims.

Thanks for putting me back on the right path :)

I've updated my response.

 

[stefan r]

...

The critical radius is 3/2 times the Schwarzschild radius. That's as close as you can get without definitely getting trapped.

...

If this statement is true then I misunderstood the Penrose process. When 2 particles fall into the ergosphere one of them can not only escape but under specific circumstances one of them can leave with extra energy. Some part of the mass falling in always gets trapped.

 

[PeroK]

If this statement is true then I misunderstood the Penrose process. When 2 particles fall into the ergosphere one of them can not only escape but under specific circumstances one of them can leave with extra energy. Some part of the mass falling in always gets trapped.

I was thinking about single particle free-fall only.