The Background:

It's the year 4000 (or whatever), and mankind has cheated FTL travel. As a result of this 'cheating' rather than solving, however, they can't easily go beyond their borders.

However, the earthlings have managed to spread out and the newly minted intergalactic race has, over a couple thousand years, managed to inhabit a good 500-2000 nearby planets. There are no aliens to worry about, and mankind has space all to themselves.

But all cannot be so peaceful in the little corner in which mankind has settled; and suddenly the planets upon which our nomadic earthlings have settled start to feel a disturbing little pull. The source of this pull begins on the outer planets near one of the edges, but slowly moves through human territory, enveloping and destroying anything its reach extends to.

The Question:

Originally, I cited the source of this pulling sensation as the 'Big Crunch' - which has, admittedly, been given the scientific toss. It also doesn't give quite the creeping sensation since it would likely affect all planets simultaneously, rather than starting with the outer ones first. https://en.wikipedia.org/wiki/Big_Crunch

Lately, I've moved to the idea of a supermassive black hole. However, even doubling the number of semi-inhabitable planets within a reasonable range, this black hole would have to be ubermassive. I've done some searching, and found black holes at the center of solar systems/galaxies/you name it. But I can't find the gravitational reach of such a ubermassive black hole. So I can't check this for plausibility. I'm left with the consolation that it doesn't have to suck in planets, just pull them off their orbit (a planet without a sun isn't very good at sustaining life), but I don't know if this is within the realm of physics or will get me laughed out of the publishing industry. https://en.wikipedia.org/wiki/Supermassive_black_hole

So, the question then, is could an ubermassive black hole exist (through natural means or through unnatrual means - it doesn't have to be natural, just self sustaining). And how big would such a one have to be in order to eat/destabilize a galaxy? If a black hole won't work, is there any other source of gravity that could believably cause this much damage while still traveling through space?

Sorry for the Wikipedia article links, they were, surprisingly, the best articles I could find. Most of the others were too advanced or meant for children.

Source for currently known inhabitable planets and their distance from Earth: https://en.wikipedia.org/wiki/List_of_potentially_habitable_exoplanets

Also, this probably goes without saying, but scientific is best. That said, I'll make due with plausible. I just don't want to get laughed out of civilized society.

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    $\begingroup$ Hi Mackenzie, would you mind clarifying what your question is? I see at least two candidates but I'm not sure which one. Also, if your question is purely about blackholes then the first three paragraphs, while nice background, kind of distract from the question you want to ask. $\endgroup$ – Green Dec 21 '17 at 20:29
  • $\begingroup$ And welcome to Worldbuilding! :) $\endgroup$ – Green Dec 21 '17 at 20:30
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    $\begingroup$ Two things: intergalactic means they live in more than once galaxy. Interstellar is the word your looking for. Secondly, gravity has no maximum range. The force falls off with the inverse-square law. $\endgroup$ – Stephan Dec 21 '17 at 23:04
  • $\begingroup$ I'm thinking along the line of Star Trek - Generations where the "nexus" was this giant string flowing through space that could be affected by changes in gravity (e.g., the destruction of a star). Let's reverse the idea and make the string the gravity-bearing object. A super-massive molecular chain bookin' through space and wreaking havoc everywhere it goes. (Maybe a super-dense/massive fluid with high attraction and a parsec long.... Yeah... that's the ticket!) $\endgroup$ – JBH Dec 22 '17 at 1:21
  • $\begingroup$ @JBH And on the money too. Cosmic strings are hypermassive. Galactic masses per centimetre? (or is it per metre?). Anyway that's exceptionally massive. The main problem may be the way the cosmic string is arranged in space to cause gravitational catastrophe. $\endgroup$ – a4android Dec 22 '17 at 1:35

Enormous amounts of mass required

I'm not going to really address your first question of how an ubermassive black hole could exist. Within our current understanding of gravitational physics, black holes are formed by stuffing enough mass-energy together in one spot, and it doesn't sound like the humans of your story are rounding up all the stars. I'm going to assume for the sake of your story that a black hole is formed in some exotic manner that massively violates conservation of mass-energy, such as by somehow harvesting the zero point energy of the universe, or manipulating the cosmological constant (a kind of energy density of empty space), or something even whackier.

Black holes are not vacuum cleaners

The other problem is that black holes are surprisingly nondestructive. A black hole, once formed, doesn't really suck things in to their doom. Let's consider a relatively tame 1000-Sun black hole.

In the Newtonian regime (far away from the event horizon) black holes behave like any other mass, and objects will orbit around them as with any other mass. The Sun is heavy, but for example, rocks from outside the solar system tend to fall inwards, whip around the Sun, then head back out to deep space. Objects have a certain amount of potential energy relative to the central mass they're orbiting, and barring extraordinary events such as collisions, this energy is conserved. Unless an object is aimed directly at the central mass, i.e. it has nearly zero angular momentum, it won't get sucked in. If you were to magically plop this black hole in interstellar space, yes, things would be pulled, but by conservation of angular momentum and energy these things (stars, planets, comets, etc.) would end up either slingshot away from the black hole, in orbit of it, or relatively unperturbed further away. A black hole of 1000 solar masses would make a mess of stars' orbits around the galaxy, but each solar system would stay intact, and life would go on. If you're wondering about how many stars would be sucked in, I would estimate none. Space is very large and even 1000-Sun black holes are small, and it is really easy for an infalling star to miss the black hole and whip around back into deep space far away. Also, it would take objects about 10 million years to fall in from 20 light-years away, so this wouldn't exactly be a pressing issue.

The 1000-Sun mass isn't arbitrary, since it's just about the minimum mass to really start affecting the closest 1000 stars, and even then quite languidly. By a rough estimation, a black hole of 1000 Solar masses is sufficient to gravitationally dominate a roughly 45-ly radius, such that objects within that 45-ly radius would orbit the black hole, not the galaxy center (like the Moon orbits the Earth, rather than following its own independent orbit around the Sun). This is from a quick calculation of the Hill sphere, and the gravitational mass of the Milky Way. A radius of 45 light-years roughly encompasses the nearest 1000 stars, which should be a proxy for the number of planets, within a factor of 10 or so.

There's enough of that digression. If you want destruction, we'll exit the Newtonian regime. Let's make the event horizon really big, so all the stars are close to it. The Schwarzschild radius is $r_S=\frac{2GM}{c^2}$. To have an event horizon 4.22ly in radius, reaching the nearest habitable planet, we need a black hole of 4 times the mass of the Milky Way as a whole, at $2.688*10^{43}$ kg. This will instantly swallow Proxima Centauri, and the rest of the 1000 closest stars should fall in within about 230 years.

Alternatives to black holes

If I were you, I'd take an alternative path, since the physics is already being bent in knots. How about Modified Newtonian Dynamics?. MOND theories and other alternatives to general relativity were somewhat recently proposed as alternatives to dark matter and dark energy. You don't have to understand this math particularly well, especially since it's a quite abstruse field of active research.

My suggestion for the sake of your story is that you explain your humans' FTL-cheating tech as exploiting a localized deviation from general relativity that encompasses our 500-2000 star systems. Unfortunately, the FTL cheat only makes these extra terms in the equation bigger and bigger, so that after many uses this pocket starts collapsing in on itself. Where today the cosmological constant is positive, driving expansion, it gets altered locally to more and more negative values, causing contraction.

Another possibility is that where Newtonian gravity is purely $1/r^2=r^{-2}$, FTL drive use starts to add a $1/r=r^{-1}$ or constant $r^0$ term - if attractive, then planets and moons start drawing in closer; if repulsive, everything starts drifting away.

If you have any more questions, I've done more math I didn't bother writing up and would be glad to share.

If you'd like to play around with gravity, try this toy. As a warning, it's not to scale, but you can try spawning a ring of dust in a circular orbit around one star, and then adding a second star to see how most of it is kicked out of the system.

  • $\begingroup$ Welcome to WorldBuilding! Cool answer. If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! $\endgroup$ – Secespitus Dec 22 '17 at 11:06

Gravity effects all things equally, and the further you are from the center of pull the more gentle and evenly distributed is the pull. So, a supermassive black hole will move the whole star system - where you are asking to move planets individually.

You might be able to get away with a tiny black hole. Bear with me because this is weird - once matter goes past a critical density (about the mass of one hundred thousand super carriers squeezed into the volume of a raindrop) it has a sufficiently curved gravity to be called a black hole. As people were concerned about the possibility of tiny black holes being created by CERN, nothing rules out super dense, but not very massive, material from existing in the universe.

A meteor shower of this stuff would cause a localized effect, nudging a planet it is passing nearby by while leaving the star unaffected.

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    $\begingroup$ It can be tiny but it can't be too tiny "So, for instance, a 1-second-life black hole has a mass of 2.28×105 kg, equivalent to an energy of 2.05×1022 J that could be released by 5×106 megatons of TNT." en.wikipedia.org/wiki/Hawking_radiation $\endgroup$ – Slarty Dec 21 '17 at 20:57
  • $\begingroup$ "nothing rules out super dense, but not very massive, material from existing in the universe." — sorry, but that's just not true. @Slarty explains why black hole has its limits, but our understanding of quantum & particle physics also puts some limits on non-blackhole matter. $\endgroup$ – Mołot Dec 21 '17 at 22:53
  • $\begingroup$ 2,28E+05 kg is about 228 metric tons, right? $\endgroup$ – James McLellan Dec 22 '17 at 0:15
  • $\begingroup$ @JamesMcLellan yes correct and if memory serves from another question a 600 ton black hole would last about 20 sec. So as I said it can't be too tiny mass wise. Although obviously all of these black holes would be submicroscopic $\endgroup$ – Slarty Dec 22 '17 at 11:01

You might be able to achieve your big crunch type effect with just one handwave - and one you made at the beginning of your story. What is more common than hubris-filled scientists bringing disaster with their creation?

"Cheating" FTL implies to me a folding of space, like the Alcubierre drive does. People do not travel FTL but they pull space together so the distance is shorter. You must have some sort of mechanism in mind for how your folks have achieved this.

Now use that FTL-cheating mechanism to produce the Big Crunch you want. Space is folding up in a given area and carrying the contents of space (planets!) with it. You can have this occur as some sort of accident with your FTL-cheating tech.

A nice thing about this is the story arc.


You don't need an ubermassive, a supermassive is more than enough to destabilize an entire galaxy (Milky Way is actually heading to such a collision in the next few billion years, but it's akin to being on the highway and colliding with a merging car; not a head-on collision). And if it's moving at near-luminal velocities and on a direct collision course, what it hits wouldn't get much notice before it's too late as everything we see in the night sky is outdated by thousands upon thousands of years.

Heck, wouldn't even need to be supermassive, just near super-massive as 500 worlds is actually a really small segment of a galaxy.

So really, just a larger-than-average black hole moving near-luminal speeds and not originating in our universe on a direct collision course is way more than enough to do it. Just pull the entire star cluster in the black hole's wake out of protecive rim of the galaxy and into the more radiation-exposed above-disc regions.

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    $\begingroup$ Your idea about how outdated is what we see on the sky is really wrong. It's a common misconception. See this obligatory xkcd. $\endgroup$ – Mołot Dec 21 '17 at 22:57
  • $\begingroup$ You're right, not everything, just everything that's thousands of lightyears away is thousands of years outdated. $\endgroup$ – liljoshu Dec 22 '17 at 23:55

A "swarm" of small black holes may do the trick. Note that such swarm is highly unlikely to exist naturally as it would collapse, so it should be a creation of some advanced civilization. Such swarm would be devastating locally (when swaying across a planet or star), move at the speed you want, and have not special effect from far away, just the usual gravitational impact of their mass.

The exact size of these small black holes is to refine depending of the effects you want to achieve. The size of the swarm is also quite open. The stability over time of such swarm may need some handwaving, however. And well, regarding where it comes from...

... let's say that it is a minefield from of a Kardashev type III civilization !


There is this cool national geographic documentary available on youtube which explores the idea that our solar system is visited by a neutron star (from 1h24m onwards) having very high velocity. The devastation described comes from the radiation emitted from the neutron star as well as secondary effects from the gravitational field (e.g. meteors change their course and hit earth) Therefore I would guess that you don't need to literally crush those 500 colonized planets to make them uninhabitable.


Could an “ubermassive Black hole exist? To put it simply, there is nothing currently in scientific literature that says a really big black hole couldn’t exist. However, a black hole of that size is very unlikely to go unnoticed. Black holes moving through any dense parts of space (galaxies, gas clouds) become active (quasars) and very visible (usually in non-visible wavelengths but your humans are advanced enough to have telescopes that currently exist in our society. If you want to figure out calculations involving such a black hole, you can think about it as a really large planet.

One of your problems with making this plausible might be time dilation. Time slows down in gradient gravity fields, since objects under acceleration move relative to objects farther from the effects of gravity. Your planets being pulled (by any gravitationally large force) will be very delayed in letting anyone else know about it, depending on how close they are.

Dark Matter How about make the problem space itself? Instead of worrying about gravity and time, worry about some kind of asymmetric expansion of the local universe. Certainly, this would be unpredictable, visible only on a larger scale (people aren’t flying out of their houses but planets can move around), invisible to any current scientific tests, and, with the lack of actual scientific knowledge about the theory behind it, you can take a lot of creative liberties. Also, this kind of expansion wouldn’t really be new to scientific literature. The Big Bang Theory currently resides on a massive accelerated expansion some time in the early universe to work.

The one problem (is it really?) would be matter creation and annihilation near the expansion. Because of the non-uniform change in the fabric of space, the local universe will function under a time-dependent Hamiltonian, which allows for asymmetric virtual particle creation. Without going too much into detail, this effect could create large pockets of energy or matter, or annihilate energy or matter. This is mostly speculation at this point, so don’t take everything I’ve just said as fact.

It’s another galaxy If you don’t care about your humans seeing what’s messing with them, a “rogue” galaxy could rip apart the Milky Way very quickly. Andromeda, for instance, is scheduled to collide with us in the next couple billion years, so it isn’t unheard of. If you want to catch your humans by surprise, you’ll need a galaxy traveling fast enough that they won’t be able to see it until it’s too late. Maybe they just weren’t looking. Maybe the shockwave of gravity radiating from a relativistic black hole is pushing and pulling the Milky Way apart.

Hope this gives you some ideas.


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