[Physics] What does spacetime look like near a black hole

black-holesgeneral-relativitygravitysingularitiesspacetime

I saw three different depictions:

Never ending 'hole':

hole

Point:

point

Paraboloid:

paraboloid

Also, do black holes rip space-time, if not, is there any other way to?

P.S. I'm not sure if the last one is a point as well.

Best Answer

These diagrams try to depict a slice of spacetime, but likely confuse it with a gravity well. A gravity well diagram depicts with an altitude for each location how much potential energy a particle in that location would have. There is also likely some confusion with the classic rubber sheet analogy used to describe gravity (plus, in many media, graphical designers not using the right shape to illustrate an article). A spacetime diagram instead tries to depict how spacetime is curved, not how much energy you would have in a particular spot.

The closest we can get to that for a non-rotating black hole is Flamm's paraboloid: $$z(r)=2\sqrt{r_s(r-r_s)}$$ where $r_s$ is the Schwarzschild radius. This is a surface in 3D space that corresponds to a 2D slice of the black hole spacetime in the equatorial plane for a given time. Distances between points measured along the surface correspond to distances measured between points in the black hole spacetime.

Flamm's paraboloid by AllenMcC, CC BY-SA3.0 via Wikimedia (By AllenMcC., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=3871398)

However, the paraboloid only represents the spatial curvature and not the temporal curvature. It represents a single "moment in time" and a particle moving in spacetime will not stay on the paraboloid. One could imagine other paraboloids stacked on top of the first one with a distance corresponding to the time distance $dt^2$, but since this varies with radius the picture becomes pretty unclear immediately.

Also, note that this parabolid ends at the event horizon $r_s$. It is however entirely valid to consider the full parabolid $$z(r)^2=4r_s(r-r_s),$$ but the meaning is subtle: the lower half does not correspond to the interior of the black hole, but a so-called Einstein-Rosen bridge which is a form of wormhole.

To really understand the structure of spacetime Penrose diagrams are useful because they actually try to show the topology of (a part of) spacetime. For a black hole they make it clear that the singularity is not pointlike, neither a circle or some other space-like shape, but a space-like thing similar to a particular moment in time. This is why it is so unavoidable and hard to illustrate.

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