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I am interested in finding out if a black hole or mass on the verge of forming a black hole with enough spin could generate enough centrifugal force to change shape into a torus?

Specifically I am interested in creating a stellar object that a small ship or pod specially designed could travel through the center of the torus shaped black hole. Using the gravity of the black hole to speed up the ship, so the ship could travel at near light speeds but could also drop mass or accelerate or push off magnetic fields created by the black hole when leaving the center and maintain the near light speed for longer or possibly exceed light speed.

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    $\begingroup$ No $\endgroup$
    – John Rennie
    Aug 14, 2017 at 11:42
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    $\begingroup$ Welcome to WorldBuilding! If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! $\endgroup$
    – Secespitus
    Aug 14, 2017 at 11:49
  • $\begingroup$ The centre of a black hole is a gravitational singularity, essentially a single point of infinite gravity, and to drastically over simplify it the rest of the black hole is essentially the area where you can't escape the pull of this point. This means it has to be spherical in order to be the shortest possible distance from the singularity. $\endgroup$
    – Static
    Aug 14, 2017 at 11:52
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    $\begingroup$ @Static That last part isn't true; ring singularities may exist in rotating black holes. $\endgroup$
    – HDE 226868 ♦
    Aug 14, 2017 at 12:18
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    $\begingroup$ Positing New Physics, there is a remote possibility for toroidal black holes, if said black holes are actually "Dark Energy Stars" (i.e. not Einstein Black Holes): Are Black Holes Actually Dark Energy Stars?. An appealing idea, but no evidence so far. $\endgroup$
    – David Tonhofer
    May 4, 2020 at 21:30

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My Answer is: Yes, torus black holes don't contradict known physics and can therefore exist.

Assume, you take our sun and spread it's mass evenly on a circle with radius $1 ly$ around you. Obviously it won't affect you too much. The mass is too far away. But now consider you let some object fall towards the circle. It turns out that the gravitation is strong enough to speed it up to light speed.(I leave the math out if you don't mind.) So yeah, you've got a black hole. And since it forms around the circle, it is a torus. A slow rotation would already be enough to stabilise it.

Technically you can construct black holes in any form you want, but most forms are highly unstable. A torus should be stable as long as the spin is high enough. However the speed you need is proportional to $\frac{1}{r}$. That means if you make your radius big enough, it doesn't needs to spin faster than light.

The exact values of the rotation speed, the thickness of the event horizon depending on the mass and the radius $r$ are difficult to calculate. (At least for me.) So I cannot give you a formula.

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  • $\begingroup$ Actually no. Your entire logic completely fails apart for one simple reason: you are analysing effects from centre of radius of torus, but you completely ignored one important thing: you are already inside the black hole. From the outside it's just another Kerr metric event horizon, and it's NOT toroidal. $\endgroup$
    – M i ech
    Aug 17, 2017 at 1:27
  • $\begingroup$ Why am I inside a black hole? In the described situation the gravitation affects me less than the sun does right now. Wouldn't that mean that we are inside the event horizon of the sun? $\endgroup$
    – lurch
    Aug 17, 2017 at 10:26
  • $\begingroup$ In the kerr metric it is assumed that the singularity is relatively small (at maximum it has about the radius of the schwarzschild radius of a non rotating black hole, if I get it right) since we don't have that in our situation, your argument breaks down $\endgroup$
    – lurch
    Aug 17, 2017 at 10:55
  • $\begingroup$ You are in centre of mass. For external observers you may as well sit in the middle of BH of same mass as entire toroidal assembly. $\endgroup$
    – M i ech
    Aug 17, 2017 at 14:34
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    $\begingroup$ @lurch The event horizon of a blackhole (the area in which gravity is high enough to capture light by capturing photons) is spherical, period, it's centre is the centre of mass of the blackhole, period, so if you are at the centre of mass of a blackhole of any mass you are within the spherical event horizon, period. A solar mass black hole has a volume of only 107.9 cubic kilometers or it lacks the density to behave as any kind of blackhole. $\endgroup$
    – Ash
    Nov 1, 2017 at 16:31
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Possibly but we just don't know what's past the event horizon where light emission ceases, which is almost certainly going to be pretty well spherical regardless of the underlying topography of the actual singularity inside. You might get something like a gravitational torus in a Quasar if you had multiple singularities orbiting a mutual centre, but again you won't know it's there because the event horizon is going to be outside and around it.

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A device that does something similar appears in the Colin Kapp novel The Chaos Weapon. It's built like this:

  1. First, find 6-8 stellar mass black holes whose axes of rotation lie approximately in the same plane.
  2. Adjust their mass, charge and their speed and axis of rotation by carefully feeding them until they have pretty much the same mass, charge and rotation rate, and their axes lie in the same plane.
  3. Set them orbiting in a Niven-style Klemperer rosette arranged so that once each orbit, all the black holes' axes of rotation are tangent to the orbit simultaneously, so that the frame-dragging effects of all their rotations provide acceleration to something passing through the ring of black holes.

In the book, this is used to build a gun that fires stars as projectiles. The really dedicated engineer can set up a series of them, to increase the total acceleration supplied.

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As HDE 226868 already pointed out, singularities of rotating black holes can indeed be ring-shaped and not point-shaped. However, event horizon of any black hole would envelop central singularity of any shape. Any object attempting to travel through such ring would have to cross into event horizon, and, as science states, it can never go back.

It is also speculated that rotating black hole can act as a Wormhole and indeed allow traveling object back into the universe. However, this theory is by no means a proved science.

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As pointed out in other comments & answers, the black hole's event horizon would be your primary obstacle.

However, toroidal planets should be able to exist. Perhaps you could combine a planet's gravitational energy with some other giant device to create the kind of acceleration you seek. Of course, at such speeds, assuming the ship remains intact, interstellar hydrogen becomes intense radiation that would kill everyone within a couple minutes.

It seems that spacetime folding is the best solution for FTL travel.

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