A black hole of planetary mass located at the centre of a planet-sized balloon.

Imagine landing on it!

Presumably it would have to be ribbed as you can't actually inflate it.

2. Ribbed with active Loftstrom loops, maybe? I can't think of a material that would hold it up, but a dynamic support system might work. It would also need some kind of stabilization system.

3. As you've mentioned, keeping it supported would be very difficult, and you'd need an active stationkeeping system to keep the shell centered on the black hole. But assuming that you can manage those problems, landing on it or walking around on it would be the same as for an ordinary planet. Don't try to dig too far down, though.

4. Look up "supramundane planet" by Paul Birch. It's a shell kept at the 1 G point on a gas giant or other object (a black hole would do) by dynamic support (rings about the planet or object spinning at greater than orbital velocity, supporting the shell). Yes, similar to the Loftstrom loop.

5. Originally Posted by Paul Beardsley
A black hole of planetary mass located at the centre of a planet-sized balloon.
[...]
Presumably it would have to be ribbed as you can't actually inflate it.
Or maybe you could inflate it? The black hole would only have a radius of around a centimeter, so the rate of gas loss might not be so bad.

6. The World is Round by Tony Rothman. A HUGE world but with (roughly) Earth-normal gravity, and a day/night cycle pattern that will make you go spare.

Fred

7. Originally Posted by IsaacKuo
Or maybe you could inflate it? The black hole would only have a radius of around a centimeter, so the rate of gas loss might not be so bad.
So you're assuming it would have an atmosphere?

Would a black hole have an atmosphere? Wouldn't the air particles end up being either swallowed up or ejected from the system?

8. Originally Posted by Van Rijn
Look up "supramundane planet" by Paul Birch. It's a shell kept at the 1 G point on a gas giant or other object (a black hole would do) by dynamic support (rings about the planet or object spinning at greater than orbital velocity, supporting the shell). Yes, similar to the Loftstrom loop.
I can understand how that would help at the "equator" of the shell, but what about the poles? They can't be rotating at all, so you couldn't make them rotate faster than 1 G, right?

9. Originally Posted by Jens
I can understand how that would help at the "equator" of the shell, but what about the poles? They can't be rotating at all, so you couldn't make them rotate faster than 1 G, right?
The shell is not what is spun above orbital speed. Rather, the idea is to have rings of material moving faster than orbital velocity holding up the shell. It could be done with pellets moving through tubes within the shell coupled electromagnetically (like magnetic levitation trains), and there would be polar tubes as well as equatorial ones. (And actually, he proposes things a bit more complex than that, but that gives you an idea. If you want to see more details, start here - Paul Birch's pdf on his supramundane planet idea)

10. Originally Posted by Jens
I can understand how that would help at the "equator" of the shell, but what about the poles? They can't be rotating at all, so you couldn't make them rotate faster than 1 G, right?
To support a shell dynamically, the "rotation" happens inside the support tubes. Imagine a metal hollow loop full of magnetised moving bullets. The magnetic fields prevent contact with, and transfer momentum to, the loop. Now imagine a spherical basket "woven" from these loops, providing outward thrust in all directions.

11. Originally Posted by Jens
So you're assuming it would have an atmosphere?
No, I'm pondering the possibility that the large planet-sized balloon is inflated with gas. The OP considered this impossible because this gas would surely get sucked up by the black hole.

But I'm noting that the black hole is pretty small, so the rate of gas loss into the black hole might not be so bad.

12. Perhaps you could put an airtight shell around the black hole to further reduce the loss of gas?

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Originally Posted by IsaacKuo
I'm noting that the black hole is pretty small, so the
rate of gas loss into the black hole might not be so bad.
It would be very bad. If you started with the whole
shebang magically in place, gravity would collapse the
atmosphere to the center in about 22 minutes.

Even if the air doesn't swirl around but just falls straight
down, it will probably be compressed in the horizontal
direction enough to glow as plasma. It will be pulled
down so fast, though, that decompression in the vertical
direction might prevent it from heating that much. At
the event horizon it will be moving downward at c.

-- Jeff, in Minneapolis

14. Originally Posted by Paul Beardsley
Perhaps you could put an airtight shell around the black hole to further reduce the loss of gas?
The inner shell, being closer to the source of gravity, would need even more support.

15. Originally Posted by Jeff Root
It would be very bad. If you started with the whole
shebang magically in place, gravity would collapse the
atmosphere to the center in about 22 minutes.
Nonsense. Magic doesn't hold it in place, outward pressure does.

Even if the air doesn't swirl around but just falls straight
down, it will probably be compressed in the horizontal
direction enough to glow as plasma. It will be pulled
down so fast, though, that decompression in the vertical
direction might prevent it from heating that much.
This is physically impossible. Adiabatic compression heating takes place regardless of how quickly or slowly the compression takes. It even works in reverse (as in negative compression speed).

At the event horizon it will be moving downward at c.
The event horizon is only about 1cm in diameter! That's the size of the "drain hole". Immediately around it, extreme adiabatic heating and swirling motion will cause extreme amounts of outward photon pressure.

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Originally Posted by IsaacKuo
Originally Posted by Jeff Root
It would be very bad. If you started with the whole
shebang magically in place, gravity would collapse the
atmosphere to the center in about 22 minutes.
Nonsense. Magic doesn't hold it in place, outward
pressure does.
No, *NOTHING* is holding it in place. We start by
magically setting the situation up, then instantaneously
let nature take over, and the air starts falling into the
black hole.

However, I now realizer that the magic needs to be more
magical than I previously thought. The downward pressure
of all that air would be too great to be held up by anything
real. It is a situation which fundamentally cannot exist. It
requires the presence of the black hole in order to get the
initial conditions right, yet the black hole cannot be present
because the air would be falling into it.

The outward pressure at the surface of the black hole of
course is zero.

Originally Posted by IsaacKuo
Originally Posted by Jeff Root
Even if the air doesn't swirl around but just falls straight
down, it will probably be compressed in the horizontal
direction enough to glow as plasma. It will be pulled
down so fast, though, that decompression in the vertical
direction might prevent it from heating that much.
This is physically impossible. Adiabatic compression
heating takes place regardless of how quickly or slowly
the compression takes. It even works in reverse (as in
negative compression speed).
I wasn't saying anything about the speed of compression.
I was talking about the amount of compression. The air
is compressed horizontally but decompressed vertically.
The decompression counteracts at least some of the
compression.

It is decompressed because gravity sucks the air down
fastest at the bottom, and less fast toward the outside
of the sphere. At the very bottom it is sucked down at
the speed of light.

Originally Posted by IsaacKuo
Originally Posted by Jeff Root
At the event horizon it will be moving downward at c.
The event horizon is only about 1cm in diameter!
That's the size of the "drain hole". Immediately around it,
extreme adiabatic heating and swirling motion will cause
extreme amounts of outward photon pressure.
Oh. You're depending on swirling motion and photon
pressure, not just air pressure. Well, in that case, maybe.

I was assuming no significant swirling, in which case the
air goes straight down, even with a teensy 1 cm drain,
and since it is all accelerating downward under gravity,
I figured it wouldn't get compressed all that much. Not
nearly enough for photon pressure to be a major factor.

-- Jeff, in Minneapolis

.
Last edited by Jeff Root; 2011-Nov-15 at 12:26 AM. Reason: fix broken quote

17. Originally Posted by Jeff Root
No, *NOTHING* is holding it in place. We start by
magically setting the situation up, then instantaneously
let nature take over, and the air starts falling into the
black hole.
Okay, but not very quickly. The situation starts off with a pressure inflated balloon, so the starting assumption is that pressure is holding up the balloon.

It's like putting a cm sized drain hole in the bottom of the ocean. Sure, water will start leaking out...but it will take a long time for it all to drain out.

However, I now realizer that the magic needs to be more
magical than I previously thought. The downward pressure
of all that air would be too great to be held up by anything
real. It is a situation which fundamentally cannot exist.
Ridiculous. Stars exist. This is a fact, and it is not magical. Stars are held up by things that are real.

What holds up a typical star? Photon pressure. This photon pressure is ultimately powered by heat input from fusion reactions. But energy input can come from other sources also. In the case, heat input comes from the extreme energy radiated by a black hole taking in surrounding matter.

The outward pressure at the surface of the black hole of
course is zero.
Which is fine, because the outward pressure is caused by things around the black hole. This applies to stellar mass black holes and supermassive black holes, both of which we can observe in real life. Even though these black holes are much larger than the hypothetical planetary mass black hole we're talking about, the same principle applies--most of the incoming mass doesn't just plummet into the black hole. In fact, most of the mass gets pushed back outward.

Mother nature just doesn't like cramming large volumes of mass into tiny volumes, even when the center of that tiny volume is an inescapable black hole.

Oh. You're depending on swirling motion and photon
pressure, not just air pressure. Well, in that case, maybe.

I was assuming no significant swirling, in which case the
air goes straight down, even with a teensy 1 cm drain,
and since it is all accelerating downward under gravity,
I figured it wouldn't get compressed all that much. Not
nearly enough for photon pressure to be a major factor.
It is impossible to avoid swirling, and it is impossible to avoid extreme amounts of compression and adiabatic heating.

IF the initial conditions assumed a long thin 1cm diameter rod of material, then sure. The cylinder of material would more or less just fall straight down into the 1cm hole. But we're talking about a sphere of material, not a long thin rod. Even if it moved "straight" down, you're looking at millions of square kilometers compressing onto 3 square centimeters. That implies extreme compression and adiabatic heating--no way around it!

18. Originally Posted by Paul Beardsley

A black hole of planetary mass located at the centre of a planet-sized balloon.

Imagine landing on it!

Presumably it would have to be ribbed as you can't actually inflate it.