Suppose in a very very long future, humans have become relative powerful military force in galactic politics due to a very powerful weapon: the ability of creating black holes wherever we want. Now we are safe from other aliens (except for very ancient species, which are peaceful anyway but they could level the entire galaxy if they wanted).

But the Interplanetary Security Council, follow a strict rule: for each weapon we create, we should develop and implement counter-measures on our most valorous planets (Earth first), and they are terrified that now we know it's possible to open a black hole wherever we want. They fear that hostile aliens could steal this tech, or that more advanced aliens might do to us since now we have powerful guns and a violent history, and not to say that we just started a galaxy-wide arms race to black-hole weapons.

Is there anything in the physics of we know now (in the history we do have FTL travel and communication, but other laws of physics are the same) that would prevent us from developing a defense against black hole weapons?

Assume that:

  • We have near limitless energy (thanks to fusion power, or whatever is plausible in a few centuries)

  • We do have teleportation tech, but is long known that teleporting anything alive results in horrible death due to quantum instability, and the "reaction" that produces the black hole can't be teleported either, the "quantum instability" is enough to trigger it.

  • We have the industrial capability to build entire mega-cities in weeks, and raw materials of any kind are not a problem because we have massive infrastructure to mine suns and planets (thanks to a few centuries of limitless energy)

  • With near infinite energy and raw materials, we can create all of elements we know of, including creating new elements (might not be stable but still), and we are capable of creating "dark matter" and "negative matter" (it's wonderful what near limitless energy allow you to do right?)

  • We have by now very smart A.I. and computers powerful enough to simulate an entire galaxy, and they can assist us on any research we want

  • The attack occurs outside our planet, but near or inside our solar system, and start very small (say a small 1m event horizon diameter, so it's needs a few time before eating an entire solar system)

  • We have long developed ways to heat our planet and keep an artificial atmosphere with near limitless energy, since we discovered that blowing up a sun is quite an efficient way to bring genocide upon other species, and since keeping Earth intact is our highest priority, any other body in our solar system can be consumed.

The defense can be either active (e.g. shooting something at the black-hole that might make it collapse) or passive (e.g. develop an anti-gravity shield around earth that would repeal the gravity pull from the black hole).

So what are the best bet for Earth to survive a black hole terrorist attack/alien attack? Or there is nothing that could stop the limitless hunger of a black hole?

Edit: It appears that I've overestimated the expansion capabilities of a black hole, but since the reaction that generates the black hole must be transported by a ship, and Earth already haves a massive layered planetary defense is unlikely that an enemy could trigger this reaction near Venus, Earth, Moon or Mars, but they would do their best to trigger the reaction the most closest possible and with enough mass to actually hit the Earth, it's essential that we have a plan if Earth gets into an event horizon from an enemy black hole or will do so in a few hours.

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    $\begingroup$ "an anti-gravity shield around earth" Really? Won't that also interfere with the Sun's gravitational pull too? The Earth would stop orbiting the Sun and spin off into space killing everyone and everything as it got cold and dark. $\endgroup$ Sep 27, 2015 at 18:28
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    $\begingroup$ Your paragraph that starts "is anything in the physics of we know now" doesn't really make any sense in the context of the "world" you're building. Physics as we know it doesn't allow for near limitless energy or near limitless resources. If you're going to use magic then you can make anything you like happen. Just create a "blackhole-away!" charm and cast it on the earth. $\endgroup$
    – Steve Bird
    Sep 27, 2015 at 18:50
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    $\begingroup$ NO! I target the sun with a white dwarf (if I can move a black hole I can move a white dwarf). Enjoy your nova. $\endgroup$
    – Joshua
    Sep 27, 2015 at 22:44
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    $\begingroup$ Probably the first rule of space warfare: if your opponent isn't interested in capturing an asset (like Earth), then if that asset can't accelerate, it will be destroyed. The fact of the matter is your opponent can calculate the trajectory of anything incapable of acceleration. They can then mount an overwhelming offensive strike against that location and they can do it over time (e.g. spend 1000 years to send 100,000 asteroids on a collision course with Earth, or relativistic weapons, or something else) $\endgroup$
    – Jim2B
    Sep 28, 2015 at 19:01
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    $\begingroup$ Probably the the most effective and simple answer will come from HOW these black holes are made in your story in the first place. Describe the creation process in detail on a page and then examine it closely. The creation process should give you clues on how to counter it. $\endgroup$ Sep 28, 2015 at 19:39

9 Answers 9


Physical Paradoxes

we do have FTL travel and communication, but other laws of physics are the same

They can't be the same, FTL breaks known physics. A lot.

The speed of light isn't really the speed of light, it's the speed of information exchange and the speed of causality.

FTL probably breaks causality, meaning you can time travel to the past. If so, the best defense against a black hole attack is to time travel back and prevent the attack.

Your FTL tech could be used to defeat a black hole, but we'd need to know more about how it works. FTL has some pretty funky consequences for black holes.

A black hole is a region of space where gravity is so strong not even light can escape. Imagine trying to swim against a river that's flowing faster than your top speed. But if you can exceed the speed of light you can escape a black hole.

The event horizon of a black hole is not just the point of no return, it defines an area of space which can never interact with our own. Inside the event horizon is, effectively, outside our universe. Once inside a black hole you are in your own universe.

Except with FTL you can come back. This means you can bring information out of a black hole. With FTL, we can interact with another universe.

We do have teleportation tech

Without putting some careful limits on it, teleportation means infinite energy. Teleport something high in the air, extract energy from it as it falls, repeat.

One way to avoid this is to make the energy cost of teleportation equal to or greater than its potential energy gain.

But let's forget about that for the rest of the answer.

Original Answer

You have bombs you can drop anywhere, there's no defense, and they're infinitely powerful. Not too different from reality. Just as in reality, you don't fight overwhelming military force with military force. You conduct asymmetric warfare.

  • Hide. You can't hit what you can't see.
  • Make the environmental cost too high (it eats planets).
  • Make the collateral damage too high (did we mention it eats planets).

But you obviously want a direct counter-measure. In order to do that we need to make some things clear about black holes.

Black holes don't have any more gravity than a normal object.

When a star collapses into a black hole, it doesn't suddenly suck everything in. It has exactly the same gravity as before. You wouldn't notice... gravitationally speaking. If you're not killed in the nova (which blows off mass reducing the gravity of the black hole) you'd freeze to death because your local star just went out.

This is the key to how we're going to get rid of this thing. A black hole can still be treated as a point mass. It's still subject to the law of gravity.

As an alternative weapon you could make neutron stars; they would do the trick just as well. Or you could create a normal star; just as much gravity, and it will burn you to a crisp from millions of miles away! It could be a wonderful terraforming device, or a hideous weapon.

Black holes evaporate.

The smaller the faster. Unfortunately your one meter black hole will take too long, probably longer than the age of the universe.

Black holes radiate tremendous energy.

...when stuff falls into them. This is called the accretion disc, and it's actually rather hard to fall into a black hole once you're orbiting it. To "fall" into a black hole you need to expend thrust to brake.

As it consumes stuff in our very, very cold outer solar system, our black hole will start to radiate energy from its growing accretion disc. This is how we can detect it, and we want to detect it while it's far away and small.

Black holes have only three properties.

Mass, charge and momentum. When matter falls into a black hole it retains these properties. This is very important for your question, especially momentum.

The Earth can be torn apart by its tidal forces.

The Earth doesn't need to cross the event horizon to be destroyed, it can be ripped apart by tidal forces. The side of the Earth facing the black hole is $13000 \; \text{km}$ closer to the black hole than the far side, meaning the gravitational pull is stronger. As the Earth gets closer to the black hole, this difference gets so strong it rips the Earth apart.

The point where a body is ripped apart by tidal forces is called the Roche Limit, and we don't want the Earth getting any closer to your black hole than that.

Now back to our problem.

The attack occurs outside our planet, but near or inside our solar system, and start very small (say a $1 \; \text{m}$ diameter, so it's needs a few time before eating an entire solar system).

Will this thing eat the entire solar system? Its gravity is related to its mass. So...

How much mass does a one meter diameter black hole have?

When you say "$1$ meter in diameter" I presume you mean the diameter of its event horizon. We need its mass. $R$ is the event horizon radius. $G$ is the gravitational constant. $M$ is the mass of the black hole, $c$ is the speed of light.

$R = \frac{2GM}{c^{2}}$

Solving for $M$...

$M = \frac{c^{2}\cdot R}{2G}$

Plug in the numbers, $R$ is $0.5\;\text{m}$, $G$ is $6.674\cdot 10^{-11} \; \text{N}\cdot \frac{\text{m}^{2}}{\text{kg}^{2}}$, $c$ is $3 \cdot 10^{8} \; \frac{\text{m}}{\text{s}}$.

$M = \frac{\left(3 \cdot 10^{8}\;\frac{\text{m}}{\text{s}}\right)^{2} \cdot 0.5 \; \text{m}}{ 1.3348 \cdot 10^{-10} \; \text{N}\cdot \frac{\text{m}^{2}}{\text{kg}^{2}}} = \frac{4.5 \cdot 10^{16}\;\frac{\text{m}^{3}}{\text{s}^{2}}}{ 1.3348 \cdot 10^{-10} \; \frac{\text{m}^3}{\text{kg} \cdot \text{s}^{2}}} = 3.37 \cdot 10^{26} \; \text{kg}$

That's a little more than half the mass of Saturn. Saturn is a big gravitational player, but it hasn't hoovered up the entire solar system. This thing is going to have to get pretty close to Earth to mess with it, and it's not going to be gaining much mass on the way.

What is the Roche Limit?

As mentioned earlier, a black hole can rip the Earth apart with tidal forces if it gets too close. How close? We need to calculate the Roche Limit.

$d = r \cdot (2 \frac{M}{m})^{\frac{1}{3}}$

Where $d$ is the Roche limit, $r$ is the radius of the Earth ($\approx 6300 \; \text{km}$), $M$ is the mass of the black hole ($3.37 \cdot 10^{26} \; \text{kg}$) and $m$ is the mass of the Earth ($6 \cdot 10^{24} \; \text{kg}$). I got $30400 \; \text{km}$.

Depending on its trajectory and speed, it will probably pass harmlessly through our Solar System, maybe mess up the orbits of a few planets that we can correct later. But let's assumed it's aimed at Earth. What can we do?

I'm going to ignore infinite energy and FTL and teleportation and anti-gravity and non-existent anti-particles, because once you have all that you might as well say "magic". Don't need them anyway, we have a perfectly fine way of handling rogue stellar objects using nothing more than Newton's Law of Gravitation.

Use a Gravity Tractor!

No, it's not a tractor beam.

Put a spacecraft in orbit around the black hole and, without touching it, use the spacecraft's gravity and thrust to change the black hole's trajectory to miss the Earth (and presumably go somewhere safe, like a parking orbit around a brown dwarf). Since we have infinite energy, it doesn't really matter how fast or how massive the black hole is, we can keep building bigger tractors.

Hmm... this black hole has the mass of Saturn. The Earth is about 100 times less massive than that. It would be easier to move the Earth (and anything thing smaller than Saturn) out of the way. Just be sure to remember to put it back.

This may sound crazy, but it's the most sensible way we currently have to defend against asteroids.

Knock it away by shooting it.

Since black holes have mass and momentum, you can treat them like a big, sticky billiard ball and smack it with another big billiard ball.

Let's say this $1 \; \text{m}$ black hole is coming toward Earth at high velocity and we detected it around the orbit of Neptune, about $30$ AU away. We'd probably notice distortions in the orbits of outer solar system objects. This gives us plenty of time to react. Even light takes four hours to reach the Earth from out there.

Momentum is mass x velocity. We have infinite energy to play with, and we don't want to make the black hole too much bigger, so we're going to fire a slug at near the speed of light. For best results, our anti-black-hole cannons are positioned above and below the plane of the solar system. That way the slug impacts at a 90 degree angle and bounces the black hole above or below the orbital plane where there's less for it to mess with.

How far do we have to knock the black hole? Will just one degree do? Let's say our slug impacts at $20 \; \text{AU}$ and knocks the black hole out of its course by $1$ degree. By the time it reaches the Earth it will be $\tan\left(1 \; \deg\right) \cdot 20 \; \text{AU} = 0.35 \; \text{AU}$ outside the plane of the solar system, or $52500000 \; \text{km}$. Safely outside the Roche Limit by three orders of magnitude.

What about shooting it with anti-matter?

Another answer said to shoot it with anti-matter, which is a really good idea, which is a really bad idea because it will just make the black hole bigger!

But for funsies, what happened if they did annihilate? How big an energy release would it be?

We know the mass of the black hole, $3.37 \cdot 10^{26} \; \text{kg}$, and we're going to shoot another object made of the same amount of anti-matter at it. Plug that into the old e=mc^hammer formula and we get... $3.03 \cdot 10^{43} \; \text{J}$. How much energy is that?

...going off inside the solar system.


Let's try the gravity tractor.


Blackhole Defense by Reclaiming the offending Blackhole

The OP makes it sound like arbitrarily sized black holes can be created at arbitrary locations whenever someone with sufficient authority says to do so. Presumably, these black holes can be removed when they are no longer necessary. If this removal capability exists then defense of the planet is pretty straight forward. Ring the Valorous Planet with gravity detectors linked with black hole reclaimers. Should a blackhole be detected, the reclaimers kick into action and sap away whatever energy/matter is being used to create the attacking blackhole.

The reclaimers could be based on the teleportation technology mentioned in the OP. Just teleport the blackhole (or parts of it) to Sagitarius-A and not worry about it again.

Make a backup of the planet then restore it when (not if) something happens. Granted, this isn't specifically a defense mechanism but it is a recovery mechanism. With unlimited storage, unlimited materials and effectively infinite industrial capacity, it shouldn't be hard to just build a new Earth and clone all the people on it.

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    $\begingroup$ I like the silent sarcasm. Upvote for thinking outside the box :D $\endgroup$ Sep 27, 2015 at 20:58
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    $\begingroup$ @YoustayIgo, it's the simplest solution. If you can make them anywhere you want, you should be able to remove them from anywhere you want. $\endgroup$
    – Green
    Sep 27, 2015 at 21:10
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    $\begingroup$ Backups. Right. I knew I was forgetting about something. $\endgroup$
    – user
    Sep 27, 2015 at 21:43
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    $\begingroup$ @Green: I was mentioning the planetary copy-paste and universal cloning thing :p $\endgroup$ Sep 28, 2015 at 5:28
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    $\begingroup$ Allow an opt-out of the "restore in event of sudden planet failure". It will take only one iteration to weed out all the people who believe cloning isn't possible. $\endgroup$
    – Erik
    Sep 28, 2015 at 12:30

Throw another blackhole near the incoming blackhole, and they would both happily spiral away to unknown destinations.

You have another option too:

Shoot the incoming blackhole with another blackhole made of antimatter. They would mutually annihilate each other. Just make sure you have your safety measures because a pair of matter-antimatter blackholes annihilating each other would create galaxy-shattering flash of energy.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ Sep 30, 2015 at 0:31

My answer: use the Force, i.e. the electromagnetic force.

Deflecting the trajectory of an incoming black hole is the right idea but it's far harder to do so with gravitation than with electromagnetism, which as a force is far more powerful. For protons it's 10^36 times stronger, which is enormous, even to a hyper advanced civilization.

Let's assume we have only mass, momentum and charge to work with in a black hole.

If the incoming black hole is charged, and especially if it has angular momentum, and thus a magnetic field, then it's rather easy. If the rules allow you to create your own counter-holes, make them with lots of charge and a high enough angular momentum to make a large magnetic field and shoot them in a trajectory near the incoming BH, and work out the classical electrodynamics to make them to attract the enemy BH away from its collision course.

Naked charges tend to end up shielded in Nature as other free charges rush in to counteract the charge, but the magnetic fields can't be (as there are no free magnetic monopoles as far as we know), so that would be the best bet for long-range interaction.

If the incoming BH is uncharged, then you'd have to first charge and spin it up by having some of your own charged BH's collide with it, and then send additional ones near by to deflect it.

  • $\begingroup$ IIRC, magnetic field interaction between small (relative to the distance) magnets drops off as 1/r^3, while electrostatic attraction/repulsion falls off as 1/r^2 (like gravity). In space, there isn't a lot of charged matter around to come and cancel out the charge on the black hole, so electrostatics are your best bet. You still need enough momentum (or impulse) to redirect the black hole, but it's easier to apply from a longer distance, so this is nice. $\endgroup$ Sep 29, 2015 at 4:06

There are two radically different cases here, based on the meaning of "wherever you want".

1) "Wherever" means a long range projection. In this case there's no defense, they create a black hole already on target, that's that. Your only defense is to have many planets and hidden bases capable of retaliation.

2) "Wherever" means you don't need local mass, you simply activate a machine and a black hole pops out. (Or perhaps eats the machine, the mechanics don't matter.)

This is not a meaningful threat to a competent defender with the same tech base. Defenses:

A) As others have said, throw another black hole at it. Unlike what others have said it doesn't matter if it's a near miss or a hit. In the case of a hit you now have a black hole with the combined mass and vector. Ensuring that vector misses anything vital should be child's play. In the case of a near miss the two black holes will do a gravity assist maneuver past each other, both will be deflected. Again, a miss.

B) If you can create a black hole you have the ability to create incredibly intense gravity. The same tech should be able to deflect a black hole's path.

c) The most hazardous of the three: You have the gravity tech, use it to move your planet out of the way. You certainly can do this, the hazard comes from how perfect your generators are. If they are anything less than 100.0000% even you're going to get earthquakes, volcanoes and tidal waves.

Edit: I thought of a story where black hole weapons were used. A key factor is that they did not have gravitic tech at all, the black holes were natural.

Life Probe.

The hostile forces aimed very carefully at their target and did not maneuver when they entered engagement range. Destroying them didn't destroy the black holes that powered their drives, turn the ships to vapor and the black holes still hit.

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    $\begingroup$ You don't need to throw a black hole at it. Anything with lots of mass will do. Probably easier to accelerate Saturn towards the black hole than accelerating a black hole. $\endgroup$
    – MichaelS
    Sep 28, 2015 at 4:48
  • $\begingroup$ @MichaelS It depends on how big it is. If it's small enough it will just punch through an asteroid with almost no slowing. $\endgroup$ Sep 28, 2015 at 4:59
  • $\begingroup$ I guess that's the case you'd rather miss. If you went straight at it you'd just slow it down ever so slightly, since the mass to the "left" or "right" would cancel out. But a Saturn-like planet passing two to three Saturn radii from the black hole should easily cause a miss if the black hole is any distance at all from the target planet. $\endgroup$
    – MichaelS
    Sep 28, 2015 at 5:39
  • $\begingroup$ The fix to the spoiler tag is to put the >! symbol in front of the blank line between the first and second. You need two spaces at the end of each line to make a newline. I submitted an edit recommendation you can check out. The fake tag is just because edits have to be 6 characters or more. $\endgroup$
    – MichaelS
    Sep 28, 2015 at 5:46
  • $\begingroup$ @MichaelS So it was my attempting to separately spoiler the book title and what I was saying about it that was messing it up? (And you could have removed the line asking for help to get the 6 characters) $\endgroup$ Sep 28, 2015 at 19:53

Perhaps you CAN create black holes, but if your enemy has more resources, perhaps he can create more of them at a time than you could conceivably counter.

So, I think the most asymmetric defense would be to move the Earth a few hundred thousand miles.


According to some theory, we can actually be on the horizon of any black hole without falling into it just like we do around the sun... Speed needed to do that would be big but as we have limitless energy it would not have problem..

We can make a shield around Earth to keep heat inside it, and increase speed of Earth so that we can be on horizon without being sucked into black hole. It is not currently possible as we don't currently have tech for that, but in future it might be possible.

Going through the wikipedia I found another term known as "ergosphere". It states that

Objects and radiation can escape normally from the ergosphere. Through the Penrose process, objects can emerge from the ergosphere with more energy than they entered. This energy is taken from the rotational energy of the black hole causing it to slow down

By which we may able to get out of black hole.


  • $\begingroup$ Can you provide a source for "be on the horizon without falling in" part? I agree that its possible to orbit a black hole (why not) but the event horizon is the point of no return. Not even light can escape from this point (radius of photon sphere is 1.5 times the radius of event horizon for non-rotating black holes). $\endgroup$
    – Arc676
    Sep 28, 2015 at 7:32
  • $\begingroup$ We might not able to escape it.. (Untill we find a way to disintegrate black hole) but earth will survive... Give me a min to find source $\endgroup$
    – Kesh
    Sep 28, 2015 at 7:33
  • $\begingroup$ You mentioned "some theory". Could you find some resources for your points? It would improve the quality of your answer. $\endgroup$ Sep 28, 2015 at 7:35
  • $\begingroup$ "space.stackexchange.com/questions/1911/is-there-any-stable-orbit-around-a-black-hole". The best proof it is possible is our own galaxy.. Where billions of stars of our galaxy(milkyway or any other galaxy) move around massive black hole which is in the middle of milkyway... $\endgroup$
    – Kesh
    Sep 28, 2015 at 7:39
  • $\begingroup$ I'm not at all sure that counts as proof at least of what you are implying, but I edited the link into the answer anyway so that the answer remains the authoritative source of what you feel is an answer to the question. (Comments are subject to deletion for a wide variety of reasons, but questions and answers remain.) $\endgroup$
    – user
    Sep 28, 2015 at 9:03

You have FTL travel and communication, so you also have time travel. Develop a "big reset button" like Janeway always managed to use in Voyager, to the detrement of real plotting.

Or if time is self consistent, make a chronology-active shield in the form of wormhole mouths on either side of the planet. Any incoming projectile would cause a paradox, unless it found a self-consistent solution that crashed with its "other" self. Forward used this device in the humorous Timemaster.

  • $\begingroup$ Timemaster required the objects have an elastic collision where they bounce off each other (forcing the trajectory of the young object into the wormhole). This wouldn't exactly work with black holes as they don't bounce off of each other (but merge and then you've got a 2M black hole that is half its future self... and... my head hurts). $\endgroup$
    – user487
    Sep 29, 2015 at 22:49
  • $\begingroup$ With the chronology protection principle watching your back, something will happen. It may need to affect events and prevent the launch from succeeding in the first place. The collision is simply the canonical example trotted out literally for humor. $\endgroup$
    – JDługosz
    Sep 30, 2015 at 0:32

If you can create unlimited negative-mass matter (a very different thing to antimatter, and often called "exotic matter", a substance which seems to be a prerequisite for warp drives and wormholes in the currently known laws of physics) then you can fire beams of it at the black hole to reduce its mass until it evaporates from Hawking radiation.

As everyone else has mentioned, a 1m black hole is absurdly large. Even a 1cm black hole would be pretty nasty.


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