# Artifical Onboard Gravity in a black hole's orbit

So this is a fictional question since I am dumb to science yet trying to write up a science fiction playing in the far future...but still would need the understanding of some deeper physics to be able to set things up.

I have a scenario where a space station tested an onboard jumpdrive. The test failed and it sent the station to the wrong coordinates. It was appearing right near a black hole, and got into an outer orbit of it. It's systems were immediately deactivated, and the crew evacuated.

Now later, some people would come to it to salvage something from it that is very important to them. My question is, how would the reactivation of a stationwide artificial gravity, made by a core that creates a "fake mass" change the orbit of the station around the black hole? Would it be pushed outwards by having more gravity, or would it be tossed deeper inside, thus risking to reach the event horizon and never return? Which is more possible, or is it possible that there would be a constant change in balance where each could happen at any second? What do you think?

Like with any poorly defined physics breaking handwavium it depends.

If you assume that any virtual mass is created with the same velocity as the ship itself then, (assuming that the mass of the spaceship is negligible compared to the mass of the blackhole), the orbital characteristics will be preserved.

However you could also argue that creating a virtual mass doesn't impart it with any velocity, and the net velocity of the ship as a whole remains constant when the gravity turns on, causing the orbit to degrade as the effective mass of the system greatly increases.

Since you're making virtual mass up, you can have it do whatever you want it to. It's possible that it only affects objects within a small finite range and that turning on the gravity allows humans to walk the decks of the ship without affecting the mass of the ship for the sake of orbital calculations.

I'd strongly recommend asking yourself what complications you want to see happen when the gravity is turned back on rather than going into detail about the physics of it. We accept pretty crazy things in the hardest of our sci-fi. If you write it well you'll be able to get away with whatever you want.

• Thanks, your response is encouraging! That was what I was going for, considering the words“Any sufficiently advanced technology is indistinguishable from magic”. I just did not want to write something that is utterly stupid, impossible. I imagine extradimensional energies and unstable plasma and things alike that can explain away certain things, like how melange explains space travel in Dune. Feb 4, 2022 at 21:08
• Another question tho is, if I variate how big the black hole is, can I also variate on how long would it take for the changing trajectory to get to the point of no return, right? So I can give any long or short time period for the crew to do their salvage operation? Feb 5, 2022 at 9:18

By your description, activating the "fake mass" generator to produce artificial gravity ought to tighten the orbit around any object, never mind a black hole.

Why? Orbital period at a given distance and eccentricity is determined by the combined mass of the orbiting objects -- that is, in the Earth-Moon system, if the Moon were much lighter or heavier, but distances were kept the same and the shape of the orbit not allowed to vary, a lighter Moon would lengthen the month, while a heavier one would shorten it. Moreover, since these are the only objects in system whose masses are significant (spacecraft and small asteroids may as well not exist, they're so small/light), all other orbiting objects would have faster or slower orbits at the same altitude and eccentricity.

However, adding mass without changing velocity involves creating a lot of kinetic energy (especially if the mass added is enough to produce a useful gravitational field), which is forbidden by conservation laws. Therefore, instead of holding the same orbit after a sudden mass change, the kinetic energy would remain constant, and the increased mass would travel slower -- hence losing orbital height, at least over part of the orbit (the effect would be the same as a large braking maneuver for a spacecraft; the maneuver point would be the new apoapsis and a new periapsis would be established at a lower altitude).

The combined mass of the system (black hole and starship/station) can be treated as having not changed, since the "fake mass" can't be a significant fraction of the black hole's mass (else anyone within considerable distance would be squashed by the gravity) -- therefore the lower orbit would result in a decrease in orbital period and a (possibly quite exciting) excursion closer to the event horizon.

• Thank you, that is a very through explaination, and helps me in the sense of that I was not thinking of something utterly stupid so I can work it into my story. It is not a massive spacestation by the way just a small science vessel. 5 story in the middle section in "height". Feb 4, 2022 at 21:05

If mass comes out of nowhere, then pedantically speaking the attraction between the station and the black hole will be stronger. In practice, though, a black hole massive enough for you to orbit will ve either so small that the tidal forces would have already torn the station apart anyway, and/or are so much more massive than our solar system that a few extra trillions of trillions of kilograms in the station won't make a difference.

By the way: that extra mass would really have to be teleported or magicked into the station. If you've got enough energy to convert into the 'artifical mass', that stored energy would itself have the same gravitational pull as the mass resulting from its conversion.

There are also two neat ways to generate "gravity" in a ship that do not require adding mass to it: accelerate linearly (which usually reduces mass, since you usually spend fuel for this) or rotate around the ship's axis, which keeps your mass constant but makes you centrifugally stick to a wall (which then becomes your floor).

• I am going with "magic" since this is a far-in-the-future sci-fi. Extradimensions, dark-energy, negative-energy whatever explaination could work to create "fake mass". I wanted to create a danger that has the crew in constant worry wether they can get in and out before they get too close to event horizon. Feb 4, 2022 at 21:01

Any mass added will lower the orbit, in this case unsafe

https://en.wikipedia.org/wiki/Standard_gravitational_parameter

Circumvent the issue ?

Maybe superfluous (there is an acknowledged answer) but I may add the option to avoid the risk alltogether.

option 1) implement your artificial gravity differently. Suppose your gravitational field could be invoked in a unidirectional way, that is outward only. It will serve the crew and not add up to the mass of the ship, which pulls the ship inward.

option 2) is to not switch artificial gravity on. They won't need it to salvage the goods.

Switch polarity

Edit: If they have a technician in the crew, maybe the artificial-gravity device could be switched to reverse ? just in time.. a negative mass would make the ship move away from the black hole!

Danger: velocity decreases when mass increases

Thinking about this question, I run into an issue, when the artificial gravity is "automatically switched on" when the crew enters the ship in orbit. Any abrupt change in gravity cannot be done safely, when impulse is conserved. If impulse is to be preserved, the velocity of the ship would have to be lowered drastically, to compensate for the huge change in mass. An abrupt change of mass at high velocity will result in a deadly blow for the ship and for any crew inside ! There could be a solution for this type of artificial gravity.. when it is switched on, it should gradually build up its gravitational field. Bad news is: much of the initial orbit velocity will get lost, when impulse is preserved and mass increased. Without orbit velocity, the ship will fall into the black hole.

• The point is I need there to be a danger. The artifical gravity is automatically switching on, curtesy of the onboard VI, sensing that they stepped on the station. They would not need it indeed, but they cannot circumvent it turning on. I could write the story without the danger yes, but than it would be less exciting, would not it? :D Much less exciting. Feb 5, 2022 at 9:13
• @RichardKutsera thx, it was not my intent to kill your stpry !! Is anyone anticipating this gravity device switch to on ? If so, they could bring in an expert.. I've added an option. Feb 5, 2022 at 10:16
• THey actually could do that however: 1: I wanted the tension of them not knowing how long time they have. If I variate the size of the black hole I can change that to my liking right? How long would it take for the station to fall into the event horizon? 2: I wanted the station to have some secrets on board which I wanted to be lost forever...therefore, if they make it so it escapes falling into the black hole, the secrets are avaible for them to look for again. That does not serve the story well, unless I change it drastically. Feb 5, 2022 at 11:29
• It also depends on the initial orbit. When the ship is falling right into the black hole, you won't change anything adding more mass. The fall will proceed with the same speed. But when there is an orbit (any orbit) and you switch on artificial gravity, the ship will make an abrupt turn in the direction of the black hole, toward a much lower orbit, or.. spiral in. There is a threshold, that would bring you around and escape. You'd have to model a new orbit.. A bending move inward will be quite difficult to calculate, a simulation could be easier, that would require some programming ! Feb 5, 2022 at 11:47
• The station has an onboard VI (VIrtual Intelligence) It's computing power would be sufficient to calculate and intiate any movement, so I'll have to circumvent it saying maybe that the station has left no more fuel for the manouver required, OR the VI was overriden so it has no permission anymore to switch itself off to the AI, and switch out of power reservation mode, therefore cannot initiate the manouver. Feb 5, 2022 at 12:06