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There is an advanced civilization that need to move the supermassive blackholes (SMBH) of their galaxies.

I have seen some similar questions on here but the ones I have read are for moving much smaller black holes. I am not sure if you can charge a supermassive black hole enough to move it that way or use gravity to move it as it would take millions to billions of stars and would likely not be accurate.

Their main method of pushing bodies to high speeds is using Nicoll-Dyson beams, could the Dyson beams if using blue giants or longer lived stars push the SMBH? multiple stars can be used to push and direct the black hole or could they use the beams to propel large masses at the blackhole to impart momentum?

Could the Dyson beams by either strong lasers or propelling large masses push an SMBH of any mass, up to the largest of tens of billion solar masses? or can any other method be used to move them? And what would be the highest speeds they could be propelled to, with a large number of stars to power the push?

Edit- My universe has no FTL, warp drives, wormholes, quantum entanglement, vacuum energy and other similar things. The question is about moving the large SMBH mass by pushing or pulling by standard physics methods (if that is the correct term) but more theoretical answers, like those that came before this edit are also welcome.

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  • $\begingroup$ Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. $\endgroup$
    – Community Bot
    Jan 31, 2023 at 19:57
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    $\begingroup$ "Excuse me Good Sir, but could I ask that you move over a bit? By a couple of lightyears? That would be awfully kind and graciously appreciated" $\endgroup$ Jan 31, 2023 at 20:42
  • $\begingroup$ Without FTL (and such fast travel methods) I would think that travel times involved in doing anything this massive would be so large your civilization would fall apart or change drastically long before completing the project. $\endgroup$ Jan 31, 2023 at 23:37
  • $\begingroup$ @TheUndeadFish The story explains the unified and unchanging goals of the empire, the issue is making intergalactic journeys outside of galaxy groups within expansion times of 10-20 billion years and velocities at minimum over the galaxy groups escape velocity, so over 10% light speed should be fine. $\endgroup$
    – user99106
    Jan 31, 2023 at 23:50
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    $\begingroup$ Not a duplicate, but will likely have some good ideas: worldbuilding.stackexchange.com/q/44910/40609 $\endgroup$
    – JBH
    Feb 1, 2023 at 2:21

4 Answers 4

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Shkadov thruster gravity tug

We'll pull the SMBH with an array of Shkadov thrusters.

Finding star mass & Shkadov acceleration

Borrowing from HDE 226868's answer to Which type of star would be best used for a Shkadov thruster to reach Andromeda as soon as possible?, which asks what type(s) of stars might be best suited for intergalactic travel. It is determined that stars above ~2 solar masses have a terminal velocity of 0.045 c. This is because gains in stellar luminosity and Shkadov thrust are bogged down by gains in stellar mass and limited by stellar lifetime. Making stars bigger has little effect on top speed, however, as we'll later see, it has a big effect on travel time.

In the same question, L.Dutch's answer extracts acceleration proportionality coefficients for various star types, based on the idea that acceleration is proportional to luminosity. A one-solar-mass Shkadov thruster can reach ~20 m/s delta-v inside 1 Myr, for an acceleration of $6.35 \cdot 10^{-13}$ $\text{m/s}$. We can multiply L.Dutch's coefficients by this baseline acceleration to infer accelerations for higher-mass stars. A 2-3 solar mass star might seem a reasonable place to start; however, cranking the numbers, the time it takes one to reach its top speed of 0.045 c is greater than the age of the universe by about 2x, and stellar lifetime of such stars is only around ~400 Myr.

From this information alone, it becomes impossible to make a Shkadov thruster perform on reasonable timescales without the assumption that stars can have their lifetimes artificially extended by technological means, i.e., removing the "rot" of heavy metals from their cores. (Suppose a powerful beam (perhaps redirected from the Shkadov thruster) can drill a hole down to those inner regions, clearing a channel for the heavy element plasma to "leak" out, in part due to great pressure.) I will proceed under that assumption.

(Note: increasing the star's lifetime likely changes our initial assumptions about the top speed. For reasons discussed later though, it might not matter much.)

Now that we have the ability to prolong star lifetimes, it's perhaps reasonable to jump to the most massive star on L.Dutch's table. A 60 solar mass star can reach the top speed of 0.045 c in ~21 Myr, with an acceleration of $2.03 \cdot 10^{-8}$ $\text{m/s}$. (Note: this will NOT be the SMBH's rate of acceleration, too.) The star will die in a spectacular supernova in only 3 million years, but by removing the heavy elements and keeping the star properly fueled, it will burn for much longer.

Initial look at Shkadov tug performance

The next step is figuring out how to arrange the Shkadov thruster to tug the SMBH. I do this using Newton's law of universal gravitation:

$$F=G\frac{Mm}{r^{2}};$$

Where, $M$ is the SMBH mass and $m$ is the star's mass (the rest should be familiar). We want to find the distance, $r$, to position the 60 solar mass star in such a way that the gravitational acceleration of the star is balanced by its own Shkadov acceleration, such that it neither falls into nor pulls away from the SMBH. To do this, we simply set $\frac{F}{m}$ (acceleration of the star towards SMBH) to the Shkadov acceleration found earlier, $a=2.03 \cdot 10^{-8}$ $\text{m/s}$, and plug, say, 1 million solar masses for $M$.

Rearranging the equation, I get a general formula,

$$r=\sqrt{G\frac{M}{a}},$$

And a distance of around 540,000 astronomical units (AU), or 8.5 light-years. The gravitational pull of the star on the SMBH is 4 orders of magnitude weaker, an acceleration of $1.22 \cdot 10^{-12}$ $\text{m/s}$. This is the acceleration of the SMBH towards the destination, and at that rate it'll take infeasibly long, over 350 billion years, to reach 0.045 c.

What we need is more mass pulling on the SMBH. To get the full Shkadov acceleration, we'd need another SMBH's worth of these thrusters stationed at the 8.5 light-year mark, approximately 17,000 stars of 60 solar masses. If you could command that many stars, you might as well forego the whole "tote the SMBH across a sliver of the universe" and just travel to the other galaxy and make another SMBH.

Building a proper gravity tug

Let's assume you're able to gather 1/50th of the previously mentioned number of stars. 340 in total. Such a mass would impart $4.14 \cdot 10^{-10}$ $\text{m/s}$ acceleration on the SMBH and reduce the 0.045 c delta-v expenditure time to ~1 billion years. If half the delta-v is spent on acceleration and half on deceleration, the SMBH could travel 11.6 million light-years (Mly). If the SMBH never performed deceleration, it could travel over 23 Mly.

1 billion years to reposition over 1,021,600 solar masses nearly 12 Mly is really not that bad. Keep in mind this is using several hundred 60 solar mass stars, each accelerating harder than any other Shkadov thruster in the entire universe.

But the question still remains, How would you position all these enormous stars so closely together?

You could be erratic about it and build something like a mini globular cluster, hope none of the stars crash into each other in 1 billion years. And even if some do, it's not a big deal. Maybe you can handwave something like the Shkadov thrusters redirecting some of their thrust to prevent collisions or ejections.

Or, you could assemble them into a semi-stable orbital configuration, such as Jenkin's toroidal swarm configuration.

jenkin's config

The configuration consists of objects on slightly eccentric, slightly tilted, and slightly offset trajectories. This arrangement has the benefit of keeping "neighbors next to neighbors", so no high-velocity near-misses, and is relatively stable.

Due to the mass of the torus itself, the elliptical orbits will precess over time. ... That will happen after thousands of orbits, give or take a few orders of magnitude depending on the total mass of the torus relative to the inner sun. It can also be caused by an oblate sun (which happens if the sun is spinning rapidly). However some well-placed masses inside or outside of the torus can prevent the near point from precessing out of the central plane, eliminating this problem. There is also precession of the whole torus around the central axis, but the torus is already symmetric about the central axis so this is not a problem.

Those "well-placed masses" are not depicted in the above diagram, but are in this one:

jenkin's config 2

A ring of objects orbiting within the torus helps to keep the configuration stable and adds to the coolness of it all.

The swarm configuration could be light-years across, taking many thousands of years for a single star to complete a single orbit, and the central mass perhaps being a more tightly bound cluster of thrusters. Intersections of a star's orbital track with the drive plume of another Shkadov thruster are unlikely due to the vast distances, and are probably harmless anyway. Over 1 billion years, the thrusters could complete a million orbits and so orbital corrections, possibly by redirecting some thrust, would be required for each thruster.

All in all, roughly to-scale the whole thing would perhaps look like:

diagram


Afterword

I'd like to point out the ludicrous amount of energy we're looking at here.

The SMBH of 1 million solar masses has a mass-energy content of 1.8E53 Joules. At top speed, the SMBH has a kinetic energy of 1.9E50 Joules, only 3 orders of magnitude less. It requires twice that to decelerate again. That's nearly 4,000 solar masses worth of antimatter annihilation energy packed into sheer speed. Four. Thousand. Solar masses.

Put a different way, that's the complete energy output of over 30 million Sun-like stars for over 1 billion years. To get up to 10% c like you initially wanted would require nearly 200 million suns' worth of combined energy for 1 billion years. That's basically a galaxy's worth. It's also nearly 11,000 solar masses worth of antimatter annihilation energy...

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  • $\begingroup$ Good answer for gravity pull option. We can slightly bulk up stellar engine speeds either using a Caplan thruster or shkadov with star lifting particle propulsion added but it will still be far too slow but the toroidal swarm is interesting. $\endgroup$
    – user99106
    Feb 1, 2023 at 18:42
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Gravity tractor.

The basic concept is that you have a space vehicle orbiting a body, thrusting perpendicular to that orbit. The body's gravity pulls it back into orbit, which requires that the vehicle's gravity pulls the body in the direction of its thrust. It's slow, but very predictable.

The benefit of gravity tractoring a black hole is, you don't have to touch it. You don't have to get near the accretion disk. You don't have to worry about its charge or rotation. As long as you keep providing thrust, it keeps moving.

Now, there is one problem, and it's the sheer number of zeros involved. Gravity tractors are slow when they're being considered for moving asteroids. But then again, they're slow because they're being considered with unmanned probes launched by current technology. If you had the right vehicle (a star?) with enough mass and thrust, and enough time, you've got nothing to worry about.

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  • $\begingroup$ Interesting but it is stated that they don't know if it will work on larger asteroids, even though the image of countless high powered ships doing this to a supermassive black hole is cool, I don't know if it is possible? $\endgroup$
    – user99106
    Feb 1, 2023 at 0:34
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    $\begingroup$ I don't know why it wouldn't work on a larger body. The physics is simple enough, and even black holes obey the law of gravity. $\endgroup$
    – Cadence
    Feb 1, 2023 at 1:11
  • $\begingroup$ It will work, and it can work fast, as long as you have a super-supermassive black hole several orders of magnitude greater than the one you want to move as a tractor. The problem then is how do you move a super-supermassive black hole to use it as a tractor for sepharding other smaller black holes. Moving a heavy object is going to need a lot of energy, and no matter the method used, trying to move supermassive black holes is going to use up a considerable fraction of the energy available in the whole galaxy. $\endgroup$
    – Rekesoft
    Feb 1, 2023 at 9:15
  • $\begingroup$ Do you have any maths for top acceleration by this method? $\endgroup$
    – user99106
    Feb 1, 2023 at 18:44
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Wormhole

You know Dirac tensors? Well, that's OK. Basically your folks produce a wormhole between the black hole to be moved and another distant and very massive wormhole which for this use is tractable in that it is so intractable, and it does not move.

Once the wormhole is open, the black hole to be moved is tipped into its own wormhole. Before it can meet and merge with its massive partner, the far end of the wormhole is switched to end at a destination which is somewhere else and something less massive because everything else is something less massive and also somewhere else. That wormhole situation does not last for long but they do not need long. The moving black hole exits the worm hole and replaces the less massive anchor.

These wormholes are not good for moving anything else besides black holes because the wormholes are too small for even photons to traverse. Only black holes are small enough to travel these wormholes because black holes are dimensionless points and immeasurably small.

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    $\begingroup$ I'm not sure I get it. You're saying drop a wormhole into a black hole so it may transport the singularity somewhere else? From what I've heard, a wormhole falling into the event horizon of a BH invariably creates a CTC/time machine, similar to a wormhole on a relativistic rocket. But besides that, +1 interesting premise. $\endgroup$
    – BMF
    Jan 31, 2023 at 21:07
  • $\begingroup$ Sorry I forgot to say no wormholes but an interesting idea for if I do go down the wormhole route to solve no ftl issues. $\endgroup$
    – user99106
    Jan 31, 2023 at 21:17
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I'm going to borrow a bit from Q in Star Trek. Just lower the gravitational constant of the universe, but only on one side of the black hole. Alternately, you could create a gravity-blocking field on one side of the black hole, allowing the gravity from the other half of the universe to pull the black hole in the direction you want it to go.

One of the problems you would have with this IRL is that time is frozen around a black hole. This doesn't just mean that light can't get out, it means that light can't get in. Changes in gravitational potential also propagate at the speed of light, so it would take forever (literally) for a black hole to even notice that something around it has changed. Check your local laws and ordinances to see if this applies. :)

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  • $\begingroup$ Another interesting answer but I should have updated my question after the first answer, no changing of constants etc but on that note, that is a level of power I have been thinking about, how would gravitational constants be changed? it sounds to overpowered to achieve. $\endgroup$
    – user99106
    Jan 31, 2023 at 22:16
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    $\begingroup$ You're talking about moving a billion solar mass black hole, and you're worried about something being overpowered? This is all well into the realm of "sufficiently advanced technology," so you can invent your own technobabble. $\endgroup$ Jan 31, 2023 at 22:45
  • $\begingroup$ I mean, to be fair, changing the gravitational constant in particular is pretty OP. (Pretty sure energy conservation is in jeopardy there.) $\endgroup$
    – BMF
    Jan 31, 2023 at 23:04
  • $\begingroup$ @BMF, and yet that was the answer provided in "Deja Q". It was only done on a small scale, keeping a moon from crashing into a planet for long enough for the inhabitants to escape. en.wikipedia.org/wiki/Deja_Q $\endgroup$ Jan 31, 2023 at 23:17
  • $\begingroup$ Having a lot of mirrors around a star focusing the light into a beam, even on many stars is still very tame compared to the idea of changing constants. Although to be fair by the time we start building Dyson swarms we could have a much greater understanding of the universe for OP techniques. But I am staying away from things that can't be explained fully with known science and maths. $\endgroup$
    – user99106
    Jan 31, 2023 at 23:43

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