# Black hole minefields

A civilization has developed black hole guns and Hawking Radiation stabilizers while warring with other civilizations in their stellar neighborhood. Currently, they can create black holes up to 1000 metric tons and launch them at $$0.9c$$, and create stationary ones up to 250000 tons.

There is a large battlefield that they wish to mine with black holes. They have thought of two options:

1. Placing tiny black holes with stabilizers that destroy themselves upon contact with an object, like an enemy spaceship. This will cause the micro-black hole to release all its energy in a millionth of a second, destroying the ship in a massive explosion.
2. Placing large black holes without anything. When an enemy spaceship nears the black hole, it will destroy the ship and consume it.

Are either of these feasible, and if not, are there any alternate solutions regarding minefields in space?

• Regarding minefields, there are many things to take into account : comparing costs with other believable techs of this level or below, space's size and 3rd dimension battle, how space expands, salvaging mines, friendly/enemy targeting... To improve the question, how are your ships (e.g. : their size and mass, approx) and how do they move : magic gateways, fasterthanlight or under light speed travel? Acceleration speed? – Tortliena May 18 at 17:19
• Small sizes of bh's won't make them undetectable, as small puff of plasma cloud if will not trigger them then give away their locatons, so it not necessarly that much better than fusion or antimater charge. Even if there are possible some semi-stelth approaches, do not rely on them for strategical stuff. Placing network of detectors with sparse torpedoes inbetween is a better way. – MolbOrg May 18 at 17:29
• Even if the black holes have "Hawking radiation stabilizers", that doesn't mask their gravity. If you seed a volume of space with enough black hole "mines" to be a threat to ships (i.e. a very, very large number because space is ridiculously huge), then 1) it's going to be detectable from quite a distance by the gravity and 2) the mines will start sucking each other together. – GrumpyYoungMan May 18 at 17:50
• And by itself, a 250,000 tonne black hole is not a significant gravitational threat. If 250,000 tonne black hole was a meter away, its gravitational acceleration would be 0.017 m/s^2. A spacecraft that doesn't hit a black hole that size dead-on can blithely ignore its gravitational effects, because they're effectively negligible for even early space-age space travel. – notovny May 18 at 19:38

First of all, a 1000 metric tonne black hole would evaporate in 84 seconds with a luminosity of 85000 megaton/s, while a 250000 metric tonne black hole would evaporate in 1.3 billion seconds with a luminosity of 1.3 megaton/s.

A black hole that evaporates in 1 millionth of a second has a mass of only 2.2 metric tonnes and a luminosity of $$10^{10}$$ megaton/s.

That aside, in space nothing can be still. It has to move, either in a closed orbit or in an open trajectory. You can't lay a bunch of objects and have them stay still there, like bottles in a pool.

What you can do is fire those black holes toward the targets and have them detonate when at range (wear protective glasses).

• I am wondering if for your party with bottles in the pool whether you open them before putting them in? Because if you didn't open them then after you swim up you would need to use your hands to open them before you drank. That seems like cheating. – Willk May 18 at 16:18
• @Willk how is this related to the answer or the question? – Trioxidane May 18 at 16:24
• @Trioxidane - I need to more thoroughly understand the bottles in the pool simile. – Willk May 18 at 16:34
• @Willk, if you put bottles to float in a pool, they tend to stay still, unless there is a strong wind or current – L.Dutch May 18 at 17:35
• I don't disagree with your comment but just wanted to mention that the "Hawking Radiation stabilizers" that the querent mentions are presumably intended to suspend or prevent evaporation of the black hole. – GrumpyYoungMan May 19 at 3:06

Space is very big.

Supose that you want to put a minefield around an entire star system at a distance of three billion miles from the star.

No, the numbers involved in the calculations would be far too big if mere miles were used as the units.

Make the minefield have a radius of 30 Astronomical Units (AU), to make the numbers smaller and more maneagable. An AU is 149,597,870.7 kilometers or 92,955,807 miles.

According to a surface area calculator, the surface area of the spherical shell around the star system would be 11,309.7336 square AU. So if there are 11,310 mines, each mine would have a surface area of almost 1 square AU to itself. If the average place on the surface of the sphere was was about 0.5 AU from the nearest mine, the mines would have to be powerful enough to damage or totally destroy a space battleship at at distance of at least 0.5 AU, which is 74,798,935.35 kilometers or 46,477,903.5 miles.

What is the biggest explosion which we are familiar with? The constant explosion of the Sun. The Sun emits the energy of countless hydrogen bombs ever seceond. And yet humans hae sent space probes to the planet Venus, only 0.72 AU from the Sun, and to the planet Mercury, only 0.39 AU from the Sun.

Space probes have passed much closer to the constantly exploding Sun than that. On April 17, 1976 the Helios 2 space probe reached "a distance of 0.29 AU (about 27 million miles or 43.432 million kilometers)" from the Sun. Helio s 2 survived that close pasage to the Sun by years:

Helios 2’s downlink transmitter, however, failed March 3, 1980, and no further usable data was received from the spacecraft. Ground controllers shut down the spacecraft Jan. 7, 1981, to preclude any possible radio interference with other spacecraft in the future.

https://solarsystem.nasa.gov/missions/helios-2/in-depth/[1]

Parker Solar Probe now holds the record for closest approach to the Sun by a human-made object. The spacecraft passed the current record of 26.55 million miles from the Sun's surface on Oct. 29, 2018, at about 1:04 p.m. EDT, as calculated by the Parker Solar Probe team.

The previous record for closest solar approach was set by the German-American Helios 2 spacecraft in April 1976. As the Parker Solar Probe mission progresses, the spacecraft will repeatedly break its own records, with a final close approach of 3.83 million miles from the Sun's surface expected in 2024.

https://www.nasa.gov/feature/goddard/2018/parker-solar-probe-breaks-record-becomes-closest-spacecraft-to-sun[2]

3.83 million miles is about 0.04120 Au. According to my rough calculations one AU would be about 24.27 times that distance. So one would need about 589 times 11,310 space mines, or 6,661,962 space mines each emitting as much energy in a second as the sun emits in a second, to deliver as much energy to the average spot on the surface of the sphere as the Parker Solar Probe is expected to experience and survive at its closest approach in 2024.

And the Parker Solar Probe will not merely experience that intensity of radiation for only one second, but will experience similar levels for a long time as it swings around the Sun.

So obviously your space mines will have to be built to deliver a much greater intensity of radiation at a distance of 0.04120 AU if they hope to disable a replica of the Parker Solar Probe. But presumably space battleships of the superadvanced cultures in the future would many times better protected against intense radiation than 21st century space probes.

So you will have to either make your mines explode with many times the energy that the Sun emits evry second, or else pack them much more densely in the shell around the star system, so that instead of mere millions, you will need "billions and billions" of space mines.

In Star Trek: Deep Space Nine:

An episode featured a mine field around a solar system. While this is much more plausible than mining a galaxy (in the way that swimming across Lake Erie is more plausible than swimming across the Pacific), they did it in a 2-D plane, so that anyone trying to avoid this minefield could simply fly over or under it. Even then, they were building it at a rate that would have taken them hundreds of years to complete. A later episode tackled the issue a bit more sensibly with a space minefield around a very small area (the opening of a wormhole) with a realistically long time spent laying it, using matter replicators to let it lay itself, but that gets into energy usage issues.

That plot element in the episode made the series seem more like BS9 than DS9.

https://tvtropes.org/pmwiki/pmwiki.php/SciFiWritersHave/NoSenseOfDistance[3]

In Blake's 7:

One scene showed someone laying mines around the Milky Way galaxy to keep out aliens. There are two ways to interpret this, neither of which is plausible: One: It's meant literally and the entire galaxy is surrounded by mines. By the time you gathered enough matter to build this minefield, there'd be no Milky Way galaxy left. Two: The mines aren't surrounding the entire galaxy, just on the likely invasion route. Of course this runs headlong into another trope as the alien invaders could just navigate around it.

https://tvtropes.org/pmwiki/pmwiki.php/SciFiWritersHave/NoSenseOfDistance[3]

What about building black hole mines with gravity strong enough to suck in any ship which tries to travel between? If the gravity of the black holes is so high, and the distance between them so small, that no ship can pass safely between them, the black holes will all move toward each other and merge into one giant black hole, which will leave a big gap in the minefield.

• +1 just for mentioning "Blake's 7." – JBH May 18 at 23:13

Unfeasible because of efficiency

Regardless how you place (black hole) mines in space, it is a huge waste of energy. Most mines are never "used". The ones that are used, are going without being used for long stretches of time. Also space it big, so you need a lot of them. This society can stabilise, control and fire black holes. With such technology it seems that mines would be a thing of the past. You don't need mines if you can target and destroy enemy ships with great accuracy. With the technology they have that should be very easy.

To still stick to your mine theme you can do the following. Use sentinels to monitor the space. When enemies are detected, a black hole is fired upon it. In a way the sentinels work as mines, as anything in the vicinity will get obliterated. But it'll take much less energy than just throwing black holes out there, stabilised or not. The astronomical energies for a single big black hole just can't be compared to even an army of huge complex and energy hungry sentinels. The army of sentinels just demands astronomically less power. It can also differentiate between friend or foe, allowing you to use the space yourself as well.

• Mines are meant as a passive area-of-denial weapon. You don't need to activate them (as in explode, or as in sucks ships in with a gravity straw) to be effective. Though... I agree that there are other, much less expensive ways of setting up minefields a civilization would have gained access long before and that'd become a staple of military strategies. Still, black hole minefields are sooo coool! – Tortliena May 18 at 17:06
• @Tortliena mines are also meant to be completely passive. Why would you want something as active as a black hole as a mine? Even though big ones have impressive lifetimes, they are also easier to spot (ironic I know) and are more likely to move or move other things. Sattelites seem so much easier to control than massive black holes. – Trioxidane May 18 at 18:07
• Yep, you lose the surprise advantage, which is useful during the first battle round. Maybe "space barbed wire" is a better term than minefield, in that regard. It's actually more passively active than passive until triggered, if you get what I mean. – Tortliena May 19 at 9:44

Minefields would be impractical at best, impossible at worst, and you'd see them from the proverbial mile away

• Black holes are the living epitome of gravity. That means you need to set up the field such that the field is in a stable orbit around itself — like a binary star on steroids. The distance between each mine must be enough that there is a point between each pair of mines (make sure you understand that... each pair of mines) that the gravity is basically zero (balanced forces between each BH pair). End result? Incoming ships gently dodge-and-weave through the field like a duck through water.

• And speaking of gravity, that minefield would create a gravitational field that would, IMO, be detectable from a very long way away. Minefields work because the enemy doesn't know where the mines are — but in this case, they might as well be glowing bright red.

• Black holes evaporate — which is the point of L.Dutch's answer. They don't last forever. The smaller the BH, the less time it will hang around. To add to this, minefields are temporary defenses (anyway) with long-term consequences. They're meant to stop people from using a particular approach — but what does that mean in space? But more to the point... how long is your war? The reason we still have problems with WWII mines today is that you can't estimate how long the mine will need to remain active. So it's designed to basically be active forever. But forever in space means subtle but still consequential gravitational forces are working on your mine field. That means even a century into the future your lovely minefield is now warped and winging off into who-knows-where (of course, that could be used as a subplot in your story).

• I've gotta second M.A. Golding's answer, too. The idea of mine fields has cropped up in SciFi now and again... but when you really think about it, you'd need to place billions... maybe even trillions of black holes to make any kind of defense that was worth a lick. If you had to travel 10 light years to get to the battle field, who cares if you need to travel an additional 0.1 light year to fly around a mine field? That's a mine field at least 8.951e+24 square kilometers in a circle that happens to block your path. A minefield of any value would have to encapsulate, for example, a planet (or a space station...), but that gets in the way of both incoming and outgoing ships. And then there's the problem of keeping everything in orbit (go re-read that first bullet).

• Finally... black holes don't, insofar as I think we know... explode. So what you're really doing is hoping the incoming ships are, well... stupid enough to fly close enough to your minefield to get sucked into one of the black holes. See that first bullet again.

But, the answer really should be from the perspective of your world... not ours

Yes! Both these solutions work great, one for short-term mining where you need to use a location in space and then move on, and the other for long-term mining where you need to defend, for example, a planetary resource. In keeping with the tag, both of the two mine types are consistent with the rules of your world.

Now, let's switch back to reality for just a moment, to make a point about telling stories

I'm a believer in the idea that you shouldn't get too hung up on reality when designing your world. In reality, the odds that there will ever be spaceship-centered combat is vanishingly small. Space is so big, the resources of a space ship so small, the ability to maneuver so good, and the speed generally so fast, etc... it all adds up to the reality that even if a space ship were actually in the right place at the right time, the odds of the battle taking place are darn close to zero.

Is the ship in orbit around your world? Come in from the poles. Is the ship out around one of the outer planets? Come in on the other side of the sun from that planet. You won't even be seen until it's far too late for the defending ships to return. I think I'm making my point. Whatever circumstance you come up with for spaceship-centered battle, I can come up with a perfectly reasonable counter-plan that completely nullifies your efforts. Build a minefield around your planet? Great! If I can build black holes I can probably move asteroids and use them as very efficient mine sweepers. All I need to do is upset one or two of the "mines" (by increasing their mass) to upset the gravitational balance and boom! All the black holes either absorb into a single mass or go winging off into space. And if that includes absorbing the planet they were protecting... so much the better!

My point is that it's very unlikely that space combat will take any form like we've seen to date. In fact, it would make a lot more sense to sling relativistic missiles at our enemy's planets than it would to engage in combat with big, slow, expensive space ships.

Which is why I'm recommending you not worry too much about "reality." I'm willing to suspend my disbelief and read a good story that has black hole minefields — because it won't be the minefields that hold my interest... it'll be the good story.

Remember that a black hole has the same overall gravitational field as anything else of the same mass -- the only difference is that it has a much smaller radius, so you can get really close to it and hence experience bigger gravitational effects at that small distance. Far away there's no difference between a 250000 ton black hole and a 250000 ton asteroid. The Schwarzschild radius of a 250000 ton black hole is smaller than an atom. If a spaceship happens to run into it it'll pass right through the ship, absorbing only a trivial amount of matter that happens to be in its path. There would be some disruption due to gravity on the rest of the ship, but if the ship was built to take significant acceleration it should handle this just fine. I'm not even sure if the resulting hole would be big enough for any air to leak out. So passive use of black holes for minefields is pretty much a non-starter.

If you have some "magic" way to keep the black holes from evaporating, then using them as Hawking radiation bombs seems much more promising, as they do produce very big explosions. But nuclear weapons or antimatter might be easier to produce and use in mines than black holes.