Question concerning black holes. Are they 2-D and flat, like a hole in the “fabric” of space, or are they 3-D and spherical, like a superdense star (I know they’re not stars; just drawing a comparison of size, shape, and gravitation)?

Well that’s an interesting question. I was under the impression that in terms of the physical space they occupy, black holes are 0 dimensional.

Also, being 3-D does not necessarily equate to being spherical.

The black hole may have zero size, but it’s gravitational effects are the same as any other object - the event horizon is a sphere. If the black hole is rotating, there will be a deviation from that, and an accretion disk will form outside the event horizon in the plane of rotation.

Well, they were stars. They are collapsed stars. 3-D objects. If they had 2 dimensions, they would be planes. If they had 0 dimensions, they wouldn’t have any gravity.

IANA Physicist, but that’s not necessarily the case, is it?

Black holes are considered a singularity, which is just a point (0 dimensions) in space. Even sub-atomic particles are thought to be point-particles of energy with 0 dimensions.

Whether or not this is actually the case is still theoretical.

Would someone please explain what they mean by 0-dimensionality? Anything with dimensions less than Planck length would be meaningless, no?

In physical 3D space a point has 0 dimensions. It is a specific location, but occupies no space. I can’t tell you what happens when you include other than the 3 spatial dimensions.

0 dimensions is a point in 3 dimensional space. So, it has no width, no height, no length.

As far as I know, the Planck length—the theoretical minimal limit to the granularity of spacetime—wouldn’t necessarily preclude the possibility of a 0 dimensional point.

If it had zero dimensions, it would have infinite density.

They can put an upper bound on size but for all intents and purposes electrons and quarks act as if they were point particles.

Black holes of course do have infinite density,

Quantum gravity hasn’t quite been worked out yet, so there are several theories and hypothesis that attempt to explain the mathematical indications of infinities in GR.

Blackholes form accreation disks of stuff orbiting them as it slowly falls into the center. This disk is relatively flat, and since the actual hole is invisible, its what artists often draw when they need to churn out a “blackhole” picture.

I suspect that image is at least partly why people often picture blackholes as 2D.

Tangent alert

What I know about cosmology and physics could fit into a thimble with room to spare, so please bear that in mind for this question:

Stipulating my presumption that many, if not most galaxies have black holes at their centers (correct me if there’s no evidence or math to support this), if a black hole has infinite density, doesn’t it, to some extent, have a gravitational effect on everything in the universe to a somewhat greater extent than all other objects and, if so, why aren’t all the galaxies collapsing, however slowly, into their local black holes?

Yes, and to my understanding is that that is the defining characteristic of a black hole. That said, the laws of physics kind of break down at that point, and since there is no useful information that can escape a black holes event horizon we really have no idea what is going on down there. From that point of view you may as well define the black hole in terms of its event horizon.

A black hole has a finite mass but an infinite density. A super sized star that collapses into a black hole has exactly the same mass and gravitational attraction as it had when it was a sun. as it sucks in more matter it will increase in mass but it is never infinite in mass.

So no.

They have infinite density but finite mass, and the later is what counts. Relatively far away, the gravity from a black hole and that from a star of the same weight aren’t different. Its just that with a black hole you can get really close.

Remember the effect of gravity proportional to M/R^2 so as R goes to 0, the effect gets infinitely large.

With a star as you get close you enter the interior of the star and so some of the gravitation from the mass of the star pulls you in the opposite direction. With a black hole you can get infinitely close until the force of gravity is so large you can never escape.

Despite the notion that a singularity has infinite density, wouldn’t mean it has infinite mass. Also, gravity, as a force, is an incredibly weak one, and its strength falls off by the square of it’s distance (just like light/electromagnetism).

So, a singularity’s point of intensity is it’s strongest (in regards to its mass) right at the source, and would fall off with an inversely squared curve. Which, as you can see is rather steep, then gradates indefinitely at a rather weak and slow decline (on top of its already, relatively weak force).

Isn’t the infinite density simply the result of being 0 dimensional and having 0 volume? I feel like it should actually turn into a divide by 0.