I still think one of the most terrifying tropes in sci fi is not alien invasion but destruction by black hole. It's impersonal, and once it starts it's inevitable, because of physics.

I really hope we never learn of the existence of black holes in the kiloton range. If you think intelligences can annihilate themselves with nukes... how about sucking your planet away from the inside out?

The cross-section for capturing on the Sun is much larger. The Sun is there, and it is old. There are many Suns, and they are old. I'm unaware of the observed sudden disappearance of a known star.

Ergo, the number of planet-destroying black holes running around is likely to be small.

The much bigger risk is having Earth's orbit disrupted slightly by a passing solar-mass scale black hole. We wouldn't get much warning that such a thing was coming, but it could alter our orbit enough to get us into thermal trouble.

So that'll depend on why the black holes arrive.

This article is about a binary system, which I would argue meets your criteria.

If the black hole were manufactured, and/or being used as a weapon, there's no reason it should hit the sun, and every reason it would hit a planet.

Now I think some of the other responses illustrated why this is not a problem if our current model of black holes is valid. The "what if" in one of those books I mentioned is "what if there was more exotic physics that allowed small black holes to be stable?"

Knocking a substantial number of small outer-solar-system objects onto hyperbolic orbits through the inner solar system would probably lead to a tremendous reorganization of humanity's space exploration priorities, and might achieve goals such as containment without escalating to ecocide.

Well, what about just a star or rogue planet that wanders into the solar system, either on a collision course with earth or close enough to disrupt orbits?

People dismiss that as astronomically unlikely, but it must have happened many times somewhere in the universe. The average person doesn't win the lottery once in their life, but someone does.

I believe we're beginning to appreciate that just like there are more small objects in the solar system than large ones, and there are more small stars than large ones, there are significantly more solitary planets than stars or even brown dwarfs, and they would be almost as hard to see coming as a black hole until they got close.

There are many disasters that at least microbes could recover from, but anything the size of Mars that wandered in would, I presume liquefy and sterilize the entire planet. It would be so unexotic and yet humans would be completely impotent to do anything.

It's much more statistically likely to happen closer to the center of the galaxy, where there is a much higher density of both star systems and black holes.

Charles Stross' book Incandescence is about (amongst other things) a race of sentient insects that hide inside a small asteroid orbiting a quasar that had had an encounter with a black hole sometime in the distant past, and had been ripped apart... but was now orbiting close enough to the star to be mostly safe. Maybe.

> People dismiss that as astronomically unlikely, but it must have happened many times somewhere in the universe

It's hypothesised that a Mars-sized planet once collided with Earth in the early history of the solar system (about 4.5 billion years ago), creating our moon today:

https://en.wikipedia.org/wiki/Giant-impact_hypothesis

Right, and a more recent paper suggests the reason for the seas on the near side of the moon is that the earth temporarily shone like a star, whereas the moon being smaller cooled faster overall. So the moon was melted by earthshine.

Not sure that quite meets this definition, but there's a newly named object Synestia that sounds pretty similar.

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

A very close sub-brown dwarf: https://en.wikipedia.org/wiki/WISE_0855%E2%88%920714

> So the moon was melted by earthshine.

Do you have any links with more info on this?

At what mass is Hawking Radiation a significant factor?

Toss something over that either hoovers up your planet, or bakes it if a near miss. Tricky to store, though.

Hawking Radiation is only a significant factor at far lower masses than the Earth. There's a nice calculator here [0], with an example 10^12 kg black hole producing ~350 megawatts.

Working backwards, 1 megaton per second (4x10^9 megawatts) gives us a black hole of 3x10^8 kg.

0: http://xaonon.dyndns.org/hawking/

Question: Suppose I wanted to replace my apartment with a black hole. I need about 1300 square feet of area, so how much mass do I need?

Answer: About 350 earth masses.

Question: After thinking about it, living on a black hole is clearly impractical, but what if I want to replace my heating system with one? What size black hole will produce a comfortable temperature of 70 F?

Answer: Roughly the mass of Ceres.

Question: I want to go see what an event horizon looks like up close, so what size black hole is needed for the gravity to be 1G at its "surface" for optimal sightseeing?

Answer: About a trillion solar masses. It might be a little tricky finding or making one that large, as the largest known in galaxy centers are on the order of 10^10, not 10^12.

> to be 1G at its "surface"

This doesn't make any sense to me. By definition at the event horizon gravity is enough to keep light from escaping. Light can easily escape from 1G.

Well, as we all know, on earth's surface, the gravity is 1G, and rockets do escape it. So if the gravity at the event horizon was about 1G, then why couldn't you accelerate temporarily, from inside to outside, until at least your fuel ran out?

Without thinking too deeply, I suspect the issue is that you could never be stopped at that location in the first place, unlike Earth's surface.

It can escape from a local 1G gravity if the black hole were as small as the Earth. As soon as you move 6400 km away, that 1G gravity turns into 0.25G.

But the event horizon of such a black hole would be far larger than that of the Earth. Move 6400 km away, and you'll still have a gravity of ~1G.

Rinse and repeat, and you'll find that even light won't be able to escape.

The handy calculator says 1G at the event horizon implies a radius of half a light-year.

Which means it'll be at 1/4G after you travel a further half light year.

That's a hell of a lot of energy. (If I'm doing the math right, the potential energy difference is on the order of ~20% of the rest mass at the event horizon)

Yeah either the calculator is wrong or I don’t know what I’m talking about. Gravity at the surface of the black hole or the event horizon may well be 1G but all world lines point towards the singularity. I think what is missing is a sufficiently large black hole would have a really low average density at the event horizon thus you wouldn’t get speghettified instantly due to the huge discrepancy of gravity on different parts of your body.

Yeah, black hole "radius" (not volume) grows linearly with it's mass. So very heavy black holes aren't that "dense".

Light doesn't really accelerate, so if you put it in any gravity well that's wide enough, it will eventually fall in.

Question: Suppose I want to use a black hole to replace a 3 minute egg timer, what kind should I look for?

Answer: The appropriate mass is roughly the size of a WWII destroyer, or somewhat more than the largest dump truck in the world. There might be a drawback in that the energy release will be about 50,000 megatons per second. That might affect how fast your egg cooks.

Even better way to think of it - the energy release will total that of the mass of a WWII destroyer over the course of three minutes :)

Since 50,000 megatons/sec is the starting value, and it'll get bigger as those three minutes count down.

I wonder how far away you need to be from that thing to survive its last few seconds. Is a few million miles and maybe a good-sized moon to hide behind sufficient?

I love that calculator.

I asked "so if I want to use a black hole to replace a 60 watt light bulb, how much mass do I need?" And the answer is, roughly that of Phobos. It would also last over 10^22 years, so it might even be cost effective!

And unlike your standard light bulb, there's no need to worry about it getting dimmer with time - this one'll keep getting brighter until it disappears entirely!

Manufacturer's notice: the event horizon temperature goes up as well. By the time it has just five thousand years left, it'll be radiating super-energetic x-rays at (an average) 1.57 giga-electronvolts each. https://arxiv.org/pdf/0908.1803v1.pdf#15

Yeah, but that's after about ten trillion times the current age of the universe. For the next few billion years, it's going to remain 60 watts.

How would you detect something like that? A primordial black hole the mass of a small asteroid with a tiny absolute magnitude?

If such a thing existed, there would likely be more than 1. How long would it take such an object to consume a star like our sun if it grazed or hit the star, vs a harmless hyperbolic flyby?

Here's a Hawking radiation calculator that you can use to get an idea of some of the numbers involved: https://www.vttoth.com/CMS/physics-notes/311-hawking-radiati...

Using this calculator, we find that a black hole with the mass of 433 Eros, 6.687E15 kg, would be quite hot, some 18 million kelvin, but emitting just 8 watts of power, and be 0.009 nanometres in radius. Pretty hard to find.

Let's try something smaller. 162173 Ryugu, the rock that Hayabusa2 landed on, is only 4.5E11 kg. This gives you a much brighter hole: 1758 megawatts at 2.72657E11 kelvin.

Interestingly, this may make it harder to detect! Its blackbody emission peak is at 9,211 KeV, and the Chandra X-Ray observatory only goes up to 10 KeV. (Focusing very-high-energy photons is not easy) You'd have to detect it from the fraction of total power it emits lower down in the spectrum.

Maybe some gamma-ray bursts are primordial black holes having their final death cry, with their emission spectrum moving into higher and higher frequencies?

Although that pretty much means that Hawking radiation very much is a significant factor on any humanly achievable mass.

You are looking at something that weighs less than mount Everest and thinking that's an unrealistic amount of mass to control.

In actual fact, moving mount Everest To The Moon is trivial in comparison with the task of creating a black hole. It's a feat only accessible to Kardashev type 2, at the very least 1.5, civilisations. We are talking about civilisations that can move small planets at will.

The most credible approach is to build giant gamma-ray lasers, and focus so much energy into a spot to small, that the weight of light itself will collapse the spot into a tiny black hole made of pure energy, Kugelblitz.

Once you would have a black hole formed that way, you could continue force-feeding it with lasers or particle beams to weigh whatever amount you desire. It would be a very energy-inefficient process.

At some point the black hole reaches a mass where it's Eddington limit allows it to feed by 'normal' means faster than it evaporates by hawking radiation. At that point it could server to convert matter to energy.

Sounds like fusion: there’s a big gap between making it happen for a moment and actually using it.

... Ignoring the HUGE gap between smashing together a few atoms so a small fraction of their bits evaporate and smashing them so hard they pretty much disappear.

And puts some hard, theoretical limits on using black holes as total conversion reactors. Reducing the power output to a terawatt requires 2x10^10kg... Probably not practical. (It would also have a radius of 0.03 femtometers. A proton? ~0.84)

Is there a Planck constant type value for minimal singularities?

The mass of the smallest-possible black hole is called the Planck Mass (~21 micrograms). Such a black hole would have a Schwarzschild radius of 1 planck length, and would evaporate in 1 planck time. The planck mass is also considered to be the largest-possible mass of an elementary particle.

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

A note for this and other comments below. Hawking radiation is more violent (energetic) for smaller black holes. Galactic black holes emit radiation at a much smaller temperature (roughly inversely as mass) while tiny black holes are very hot and evaporate quickly in a runaway “explosion”. However, note that this is only the temperature. The surface area of the BH event horizon (which emits the radiation) scales as mass^2 — that should determine the number of photons/particles.

There's an old sci-fi book called Ring of Charon you might not like then :)

https://www.goodreads.com/book/show/513160.The_Ring_of_Charo...

The low-mass black hole is thought to be 3.3 times the mass of our sun. Previously, the smallest black hole discovered was 3.8 times the mass of our sun. I guess this opens up a new breed of stars that are somewhere between massive neutron star and low-mass black hole.

A completely non-radiating mass — that’s smaller than any black hole we’ve seen — in a binary orbit with a radiating star.

Doesn’t this fit the general criteria of a Dyson Sphere or Swarm?

Not to be all tin foil hat, just neat to think about.

Dyson spheres/swarms do radiate heat (infrared).

Why? Couldn’t it absorb almost all energy that “touches” it? Making it impossible for us to detect the tiny amount of heat generated? Genuine question.

Energy can't disappear or be consumed - if energy doesn't get released out then it gets absorbed i.e. accumulates; if it accumulates then the Dyson sphere heats up a lot, if it heats up then it glows with blackbody radiation.

Can we detect tiny amounts of blackbody radiation? Not that such a sphere would release tiny amounts, but I'm curious about this as well.

We can detect the CMB which, using the Stefan-Boltzmann law [s-b] and a CMB temperature of 2.725 K, is about 3.12e-6 Watts/square meter. But, the CMB is pervasive. The blackbody sphere you're asking about is very far away, and the amount of power you'd receive scales like the inverse square of the distance from the sphere to us here. So forget it, the CMB's blackbody energy flux on/near Earth is ENORMOUS compared to the star's.

[s-b] https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law

No; not without heating up (since the inside of the sphere can't radiate it back to the star at the center). As the sphere heats up it radiates outwards in increasingly energetic spectra. Since entropy (in this case in the form of unusable heat) always goes up, the sphere couldn't stay dark forever.

What about radiating it in a specific direction (one that just happens to be away from us)?

Heat radiates in all directions by definition.

It is theoretically possible to direct it to some extent (at the cost of raising entropy elsewhere) but it would require massive effort for no good reason.

You could never direct 100% of it, since any matter not at absolute zero has to radiate longer wavelengths.

Once you do direct it, someone will still see you if they are in the path of the beam. It appears to be an exercise in futility.

> You could never direct 100% of it

No, but you could direct enough of it that anything left is too dim to see unless you've got really good eyes ("eyes" in this case being whatever sensors we're using to detect black-body radiation).

> someone will still see you if they are in the path of the beam.

Space is big. What's the chance that we'd actually cross paths with that beam?

On the other hand, space is big. That beam would probably hit something eventually, and I suspect in that case we might pick up a reflection.

> It appears to be an exercise in futility.

Assuming the point is actually to hide. Maybe instead it's an attempt to recapture some of that energy otherwise lost as heat?

Beam it at a black hole

Hopefully it's a special kind of black hole like the one mentioned in the article. Wait...

That was one of the alternatives that I was thinking as well :)

> Why?

Physics. Entropy.

If it absorbed energy that touched it, it would heat up. You could then use the difference between it and its cooler surroundings to run an engine. Waste heat from the engine could then be absorbed to keep the engine running. You'd have created a perpetual motion machine, which is considered impossible.

But how hot does it really need to get? A solar panel doesn't heat up to sun temperatures, and a heat pump operating between the sun and the vacuum of space could be really efficient. The heat could be rejected elsewhere, no?

The important thing about space is that's there's no elsewhere.

In a closed system of a star, the sphere, and the outer space eventually energy will flow from the star to the outer space region.

There's no perfect insulation. (See also Maxwell's demon.)

Of course detection is a different matter altogether. Hiding better and better is possible. But if you really want to hide a Dyson swarm is not the best way :)

Anyone who figures out how to cheat the 2nd Law of Thermodynamics probably has no interest talking to riffraff like us. Just be glad they haven't harvested us for something and move on with your life and civilization. And whatever you do, don't make [eye] contact.

Reading the article, it seems that non-interacting is referring to the fact that the black hole is hard to detect as there is no other star or gas source to cause it to generate X-Rays as matter falls into it.

I find the name interesting since, mini black holes are one hypothesis concerning Dark Matter, and also Weakly Interacting Massive Particles. Google also doesn't seem like Non-interacting black holes are common nomenclature. How do these names get created?

In the case of WIMPs, the authors thought the name was funny, and published several conference posters before trying to publish in ApJ. The editor of ApJ normally rejects cute names, but let this one through because it was already in the literature. He regretted that decision when the MACHO paper arrived...

Someone writes a paper using the terminology, others refer to it, and it sticks.

Considering "weakly interacting" is already a very stuck name. By most accounts these are typical black holes, which do not interact weakly, let alone not at all. I don't think this name will stick.

"quiescent" is already in wide use referring to black holes, and "quiescent" appears in the 2nd paragraph of this paper. Some stellar binary systems are called "interacting".

And according to the full title of the paper, it is not the black hole that is noninteracting, it is the binary system.

While the title of the paper says a low-mass black hole, the abstract does say it could also be a high mass neutron star:

> Constraints on the giant’s mass and radius imply that the unseen companion is 3.3+2.8−0.7 solar masses, indicating that it is a noninteracting low-mass black hole or an unexpectedly massive neutron star.

Presumably, further observations will help determine what it is.

This is happening in binary systems. The black hole is not emitting any detectable X-rays from objects falling into it. There are none falling into it then. It is non interacting.

But we know it is non interacting because it Doppler shifts a nearby object in the binary systems these scientists are observing.

Being a binary system, unless the objects are at beyond perfect equilibrium with respect to their accelerations, the objects will eventually collide (in a true binary system). In other words, the Doppler shifting object must be actually falling, however slowly, into the black hole.

My question is this: Can we assume that the object is heating up and emiting additional x rays