What if Planet 9 is a Primordial Black Hole? (2019) | Hacker News Reader

What if Planet 9 is a Primordial Black Hole? (2019)

arxiv.org

159 pointsrbanffy153 comments

there's a 1:1 scale picture of the black hole fig 1: https://arxiv.org/pdf/1909.11090.pdf

It is one of my most-favorite figures in a physics paper. I'm still bummed that PRL made them remove it in the final version.

What, really? Why? It's one of my go-to examples of clear, impactful data presentation.

More precisely, a black hole five times as massive as the Earth. And it fits on a page. I think I have to print that out and pin it to a wall somewhere...

At what range would the gravity differential kill you? Would you be able to see it before it kills you? If it was close to the sun we should be seeing some kind of halo around it due to gravitational lensing right?

edit: ok maybe we'd need it to be between us and the sun for that to work, the lensing would be that of other stars I guess. I suppose the important question is, since it's relatively close to us, would it deform space hard enough for the gravitational lensing to be noticeable at such close proximity.

edit2: more train of thought, just because it's fun, I suppose since it's so small and the gravity relatively weak (i.e. not as strong as some huge stars or galaxies) any halo or distortion would not extend more than a couple meter, which wouldn't be noticeable from our distance, right?

As usual the answer is - it depends, but we could estimate forces in question. Assuming my math is correct, at 67 kilometers getting 1 meter closer will results in gravitational pull difference of 137kg. At 40 kilometers, this number increases to ~623kg. I am assuming bad things will happen somewhere between those two ranges.

F = G * (m1 * m2) / r^2 . G = 6.674 / 10^11 (m^3 / (kg * s^2)) | m1 = 5 * 5.972 * 10^24 kg | m2 = 100 kg

Only r in the equation above is not a constant, so we can compute F(r) - F(r+1) to get a difference in force that being 1 meter closer to the black hole would make at a distance r. Basically, this is the difference between how hard your toes would be pulled down vs your heard if you curl into a ball.

  Distance Force          Force + 1      Diff in kg
  1        1.99286E+17    4.98214E+16    1.49464E+16
  1000     1.99286E+11    1.98888E+11    39797421.92
  10000    1992856400     1992457888     39851.15023
  20000    498214100      498164282.3    4981.767364
  30000    221428488.9    221413727.7    1476.11612
  40000    124553525      124547297.6    622.744272
  50000    79714256       79711067.53    318.8474585
  60000    55357122.22    55355277.03    184.5191277
  70000    40670538.78    40669376.79    116.1990494

Very cool, thanks! It's closer than I thought, yet just too far to see it unless it's causing things to happen around it. I suppose if it would crash (slowly :p) into the earth and you were at 67 kilometers you'd see some crazy things.

Worth remember for comparison that the Kármán line is at 100 kilometers. ISS orbits at ~400 kilometers. So, if such a hole were to appear on the surface of the planet, even the crew of the ISS would have a very bad day.

Well the ISS orbits at R_earth + 400 km. So it wouldn't be nearly as bad as that number makes it sound.

Before you got very far into this it would be really hard to stay curled up in a ball.

Does anyone know how fast one would fall in from a 100km initial distance?

This answer: https://physics.stackexchange.com/a/332510/236187 seems to suggest that it takes mere milliseconds once one is within three times the Schwarzschild radius of the black hole. And that dozens of radii outward, one already moves at a significant fraction of the speed of light.

I believe that the Roche limit would be roughly the same as it is on earth. However, the gravity constant would be the same. So for something as massive as a person, you should not experience any destructive forces until you reached extremely close to the object. Probably measured in a handful of miles or less.

Einstein: "Time and space are not conditions in which we live, but modes by which we think. Physical concepts are free creations of the human mind, and are not, however it may seem, determined by the external world."

Paul Brunton: "The most valuable metaphysical fruit of the quantum theory is its finding that the processes of the universe which occur in space and time, emanate from what is fundamentally not in space and time."

Nisargadatta Maharaj: "In reality, time and space exist in you. You do not exist in them."

And furthermore, "God does not play dice with the universe."

So basically, Kant called it

Kind of cute. Immediately reminded me of this (for you Beatles fans out there).

http://www.westfieldcomics.com/blog/wp-content/uploads/2018/...

I used to read that to my daughter and make all the voices from the movie.

It's a great kids' book.

Go lasso that horse and ride it to the stars!

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

Halo Drive: Fire a laser around the Halo and get more energy reflected back than you put in.

https://www.youtube.com/watch?v=rFqL9CkNxXw&t=2s

> FIG. 1. Exact scale (1:1) illustration of a 5M⊕ PBH. Note that a 10M⊕ PBH is roughly the size of a ten pin bowling ball.

And it's oddly terrifying.

One of these things intersecting with Earth would be an interesting way for humanity to end.

The real question is, with the warping of space-time, how long would it take for us to perceive the end? Would it seem like the black hole started moving incredibly slowly, or would our planet start getting torn apart before then?

This is the premise of a book by Thomas Wren:

https://www.goodreads.com/book/show/2335064.The_Doomsday_Eff...

From (distant) memory it was a fun read with some science but not overly realistic wrt to how humanity would react.

A very large black hole would have a hard time sneaking up on me, but one the size of a bowling ball on the other hand...

Nibbler would be proud.

Wouldn't there be life-ending amounts of Hawking radiation coming from it? In other words, not a black circle :-)

From the PDF:

> On its own, a PBH of mass 5M⊕ has a Hawking temperature of 0.004 K, making it colder than the CMB, and since it’s radius is rBH ∼ 5 cm, the power radiated by the PBH alone is minuscule.

Also wolfram alpha says it would take 7e52 years to evaporate from hawking radiation, so it's quite stable: https://www.wolframalpha.com/input/?i=%285120*pi*G%5E2*%285+...

My understanding is that a black hole large enough to be colder than the CMB will grow (gain more mass energy by absorbing CMB photons, even in "empty" space, than it loses by Hawking radiation), continuing to cool.

I assume but am not sure that the cooling of an already-sufficiently-cold black hole in this manner is slower than the cooling of the CMB itself due to expansion, so that at some point as the universe winds down it will begin to shrink again and eventually evaporate, but definitely over the lifetime of any star we see a black hole this side won't radiate significantly, or evaporate.

I did the figures a while back on what it would take to build a Hawking-radiation-basedv mass-to-energy power plant (in orbit, obviously), and I was astounded at how tiny the black holes have to be in order to get appreciable power output from them.

I can resize pdfs...

How far from this black hole would the event horizon be?

In the current theory of GR (which predicts the existence of black holes), black holes themselves are point singularities - so the only meaning for "size" is the event horizon.

Note that physicists don't believe this 0-size point is actually correct - the expectation is that a theory of quantum gravity would predict some fixed size for the black hole, one that also prevents the infinities inherent in the GR description of black holes.

Not quite. Point singularities can't spin, and since a black hole with no spin as about as possible as real mass with a temperature of absolute zero, every black hole we're ever likely to observe is actually a ringularity or other two-dimensional configuration.

Thanks for this comment, it was enlightening. I'd never much thought about this because physics is really not my forte, but after a few minutes of thinking about it, it's really intuitive why a point singularity can't spin. I can't explain it in any properly rigorous or technical way, but it makes no sense that someone could (by convention, of course) designate a "front" and "rear" to a mass with zero length in every direction.

What does it mean “to spin” in this case?

It means the black hole has angular momentum, which is detectable due to the resultant frame dragging (effect of the spinning on the spacetime around it) and presumably also due to the flattening of the sphere of the event horizon.

As I understand it, the fact that black holes are rotating precludes them from actually being a point singularity -- "rotation" having no meaning for a non-dimensional point. So they need to have some kind of disk or ring structure (still infinitely thin, however). (Note: I'm not a physicist, could be completely wrong).

That is the event horizon: when discussing a black hole's "size", you're discussing the event horizon.

Worth noting: despite the fact that this black hole would have an event horizon 4.5 cm in diameter, that doesn't mean you could get anywhere near it or put it in a box or anything.

I want to touch it.

and when you close to it it's almost impossible to resist.

"when you stare into the abyss..."

I'd rather hit it with a baseball bat, personally...

rip my ink cartridge trying to print that out

It would be a funny hack to try to trick a printer with malformed content to over-print that section of paper a multitude of times.

This is absolutely something you can do with ASCII dot-matrix printers, and it's not even malformed! It was common to end a line with a CR rather than a CRLF in order to overstrike some characters three times for bold, or overstrike them with _ for underlining; somewhat less common was to overstrike several different characters to get a solid block (such as 0, #, and *). (Accented characters were likewise achieved by overstriking `, ', and ~, but usually using backspace rather than CR.) With a normal amount of ink on the ribbon, you would punch through the paper after less than ten overstrikes.

I understand that line printers (those that printed a line at a time rather than a character at a time) also supported overstrike sequences, but I never punched through the paper on one. Likewise, presumably the same approach would work with a daisy-wheel or typeball printer, but I can't testify.

So small, can I make a ball peen hammer out of it and be the... lesser Thor?

If you can lift it, er... before it absorbs you, then you must already have supernatural powers, so you can join the Norse gods.

And absorbed by black hole would pretty memorable at my memorial. Its win-win.

Probably the only way to die that would also result in you outliving everyone you ever knew.

Unless they followed you

FTR, if you wondered what they meant by "annihilation signals from the dark matter microhalo", apparently one of the theories for what makes up dark matter is WIMPs which are special particles that interact weakly with normal particles but can interact with each other and result in creation of other, better observable particles.

The theory is that in the early universe, WIMPs were created spontaneously due to massive temperatures of the primordial plasma, but later as the universe expanded and cooled, this creation didn't work. However, the WIMPs would still be able to react with each other, reducing their numbers, until they'd be so rare that annihilations are such a rare event that they don't significantly reduce their numbers any more. Then space expanded even further.

https://arxiv.org/pdf/1403.4495.pdf

Apparently due to the strong perturbances of the dark matter that this black hole creates, it creates potentially detectable annihilations, and they discuss which scenarios can be excluded or detected with data generated by the FERMI-LAT gamma ray telescope.

Not a physicist though, so please correct me if I'm wrong.

Note for the curious: WIMP simply means "weakly interactive massive particle". And "massive" here means "has mass," not that they're gigantic.

Although they're theorized to have a mass closer to the Higgs boson than a proton, which is a lot of mass as far as subatomic particles go.

As opposed to MACHOs ("massive compact halo object") where the "massive" actually does mean "planetary to stellar mass".

is that synonymous with dark matter as described in galaxy structure models?

IANAP, but they're (among others, like MACHOs, primordial BHs, etc.) potential candidates for dark matter, yes.

Huh, so the detection of a MACHO candidate would be a WIMP?

That is some delicious astrophysics smorgasborg

I think MACHO and WIMP are alternative theories. MACHO came first IIRC

GP was not well-phrased. I think they meant "the first detection of a MACHO candidates could be via a WIMP detection" (a black hole planet 9 would likely be in the last uneliminated sliver of MACHO candidate space and we'd see it via WIMP annihilation)

IIRC, the other way around. The editor who greenlighted the WIMP paper for print (and only because it was already an established name) regretted this decision once he saw the MACHO paper. Apparently he doesn't like cute names.

"annihilation signals" sounds very troubling

matter / antimatter annihilation is the perfectly normal state of affairs, we only exist as a fluke because it did not happen perfectly.

Should have (2019) in the title.

Previous discussion: https://news.ycombinator.com/item?id=21078068

For those who, like me, didn’t know what planet 9 was:

> Planet Nine is a hypothetical planet in the outer region of the Solar System. Its gravitational effects could explain the unlikely clustering of orbits for a group of extreme trans-Neptunian objects (ETNOs), bodies beyond Neptune that orbit the Sun (Wikipedia)

I sort of love this train of thought because it's simultaneously not impossible and hilariously hands-up shruggy about the problem of why we can't find planet 9.

"What if the reason the only evidence of planet 9 we have is gravity is that gravity is the most observable aspect of planet 9" would be expert-level trolling if it weren't actual science. :)

"Note that a 10M⊕ PBH is roughly the size of a ten pin bowling ball"

How to tell something is written by an American :)

The paper has two authors, one American and one from the UK. If I had to guess I'd guess that was from the UK author.

Why? Because in the US ten pin bowling is the only kind of bowling most people are aware of, except perhaps in New England where candlepin bowling is popular or central Texas where nine pin is played. I'd thus expect an American author to simply say "bowling ball".

The UK is mostly ten pin, but in the non-English speaking parts of Europe nine pin is big. I'd expect a UK author to be aware of that and so specify the type of bowling ball.

There is a whole nuther type of bowling in the UK, played on grass, no pins, you aim to get your balls closest to a jack.

One of the deadliest sports too IIRC, as in, highest chance of dying while playing.

Less to do with the dangerous action and more with the 80+ average player age, but still.

In the future our descendents will play galactic boules, testing who can place the celestial body in the closest orbit to the black hole

Is that Bocci Ball?

A variant:

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

Sounds right but for the wrong reason possibly.

Lawn bowls is very common. Thus a bowling ball may be taken as some to mean one of these[0]. Which is very different in size to a ten pin bowling ball.

[0]https://i.ebayimg.com/images/g/tRIAAOSwJddfCxXt/s-l1600.jpg

As a Brit, my first thought for 'bowling ball' would be lawn bowls. But then I'm Gen-X, maybe for British younglings it's different¿

Those are called woods not balls. And are not in fact spherical.

However, it's semantics. I agree with the logic.

Would not a brit compare with a football?

asoccer or ruggers?

That would make physicists happy. Once you locate it, you could observe and study it with satellites.

Yep dropping matter in it would allow us to study physics at much higher energy than we can generate here on earth.

plus: a way of generating energy (I think that one depends on spin?) and a way of accelerating spacecraft.

Matter tends to glow in agony when falling into a black hole while the tidal forces shred its atoms apart. Tidal forces should be pretty rough at this size.

The hard part of using a black hole for propulsion is that you’ll need to drag a planetary mass along. It’s an interesting engineering problem.

There's also the Penrose process which is theoretically useful on spinning black holes. I also watched this in the past, which has some fun ideas (that are probably a little fanciful?) https://m.youtube.com/watch?v=ulCdoCfw-bY tl;dw: put a mirror around it and shine light in, get back lots of energy and/or blow up your whole solar system.

I'm not sure if any of that would apply to a ~planet mass black hole spinning with ~planet sized angular momentum, or how solid the physics even is, but it sounds cool anyway.

> get back lots of energy and/or blow up your whole solar system.

Fun concept - a bomb you can't throw without it dragging your world along.

wrt propulsion I meant a gravity assist, passing by the BH much closer than you would a planet.

A [2019] on the title is justified I think.

If these are common would not our exoplanets hunt have found a bunch of these? Especially when looking at star wobble.

We have visual detection of very few of the exoplanets we have identified. Detection by wobble gives mass and orbital period. Detection by transit gives cross sectional area. Anything detected by transit is not a black hole, but anything we have only a detection by wobble we don't know the size, it could be a black hole.

Maybe we have, but without a visual detection, we don't realize what we're seeing in the wobble.

“This scenario could be confirmed through annihilation signals from the dark matter microhalo around the PBH.”

Wouldn’t that require locating a mirror behind the PBH and bouncing signals back off of it?

No, just like you can slingshot around planets, blackholes are massive enough they can do that to light and other matter. It's the primary reason we can "see" them.

No you might see gamma rays coming from it.

If there were a black hole in the solar system, is there something interesting we could do with it apart from studying?

You could drop matter in there and get 40% of the mass back as energy. Not that it's useful energy for us at the moment, but a more advanced civilization could probably learn how to ensure most comes out as polar jets and harvest that somehow.

https://web.stanford.edu/~wilkinsd/docs/posters/BlackHolesPo...

Easier to build a Dyson sphere around it than the sun.

A Dyson sphere around a body with 4.5cm radius sounds quite manageable. I wonder how small a Dyson sphere could be with current materials. I imagine it's mostly limited by gravitational forces at that point?

or some sort of roman war bird.

> roman war bird

Roman? Did you maybe mean Romulan?

I think you can also extract energy from it by slingshoting an object off the black hole to draw energy from the black hole's spin.

Would it be useful to have a large power plant at the edge of the solar system? There's not much out there to power.

Microwave transmission could send it somewhere else... wirelessly: https://en.wikipedia.org/wiki/Wireless_power_transfer#Microw...

You could use it to execute prisoners perhaps. Death by blackhole

The CO2 cost of getting the prisoners from the prison outside earth's gravity well and on their way to planet 9 seems prohibitive.

One more reason for rehabilitation as prison policy.

you'd have to space bus the prisoners, a few hundred thousand at a time. during the journey they might mutiny, take over the ship and set a course for a nearby livable planet.

One of aphorisms of Kozma Prutkov[1] was this: "Throwing pebbles into the water, look at the ripples they form on the surface. Otherwise this activity will be an empty amusement."

I believe, that throwing pebbles into black hole, and watching the ripples they form would be very amusing.

[1] https://en.wikipedia.org/wiki/Kozma_Prutkov

It would be a cheap source of additional speed for spacecraft in the outer solar system, by means of powered gravity assists. See https://en.wikipedia.org/wiki/Oberth_effect

> additional speed for spacecraft

Extreme rip-your-craft-to-bits tidal effects [0] would make life exciting. Also, see 'Neutron Star' by Larry Niven for a description of such a manoeuvre, albeit a less dangerous one than than sling-shoting a black Hole because of the larger size of the neutron star.

[0] https://en.wikipedia.org/wiki/Tidal_force

If you were really precise, you could tune your assist based on distance. You could get a pretty sick change in angular momentum relative to the sun since you wouldn't have to worry about hitting atmosphere or terrain. If you're far away you feel nothing, if you're too close you join the singularity, and everything in between is gravy.

Imagine getting a 1970's style planetary alignment, going with a Voyager style slingshot and your final assist is a couple thousand miles away from a black hole.

If it can cause some good amount of time dilation, then send people into future by having them orbit around it.

It might be better to wait

Maybe more easily study the effects of relativity by sending a spacecraft to it?

Sending a spacecraft out hundreds of AU is doable with current technology, but it would take many decades.

Depending on the size of it, harvest energy from its Hawking radiation.

As stated in the paper, the amount of Hawking radiation coming off it is absolutely miniscule. The black-body temperature of the emissions is much much lower than the temperature if you just look into space.

That doesn’t look like a lot of energy, but we could enclose it in a dishwasher sized box, which is kind of a funny concept.

As for the size, the paper has a 1:1 scale drawing of it.

It would have to be a pretty sturdy box. Even if such a box could be built, it would be tricky to keep the black hole in the exact center.

I would like to upvote this 1000 times to underline "pretty sturdy box." It can be a particularly amusing exercise to do the math for things like this, calculate the gravitational forces involved, and compare the results to e.g. the strength of steel.

The net gravitational force is zero: https://en.wikipedia.org/wiki/Shell_theorem

I don't see how Shell theory applies here. The box wouldn't be inside the blackhole, it'd be outside it.

If the box were a perfect sphere then yes it would have zero net gravitational pull on the black hole, but the black hole would still have an extremely strong pull on it. And that's still assuming it's perfectly centered which would be almost impossible to maintain.

> but the black hole would still have an extremely strong pull on it

No, it would not. Newton's third law: if an object has zero net pull on the black hole, the black hole also has to have a zero net pull on the object. The shell theorem is more general than you are assuming: it doesn’t matter if the heavier object is inside or outside, the surrounding object doesn't need to be spherical (it can be any shape as long as it fully envelopes the black hole) and the black hole doesn't need to be centered. I know it is a counter-intuitive result, but it can be easily proven with calculus.

See for instance this sentence about Dyson shells: "Such a shell would have no net gravitational interaction with its englobed star (see shell theorem)" https://en.wikipedia.org/wiki/Dyson_sphere#Dyson_shell

Edit: Related, what is bigger? The gravitational force the Moon exerts on Earth or the gravitational force the Earth exerts on the Moon? Veritasium has a nice video explaining the answer: https://www.youtube.com/watch?v=8bTdMmNZm2M

The net pull on the _center of mass_ of a shell would be zero. The pull on any local region would still exist though. All that the shell theorem says is that the sum of all those local directional forces is zero.

Reading the exact passage you quoted, "the compressive strength of the material forming the sphere would have to be immense to prevent implosion due to the star's gravity."

You're wrong despite your confidence. Cage will have zero net gravitational effect on the black hole (assuming the cage is perfectly symmetrical), but that just means it won't by itself exert gravitational force on the black hole (and vice versa) causing it to move in any direction.

Cage's material will still get attracted to the black hole and close to the event horizon structural forces exerted by gravity will be hard / impossible to counteract with current materials: https://en.wikipedia.org/wiki/Strength_of_materials

I also exert zero net pull when I tear open a package of snacks. Yet the package is destroyed in the process.

You'd probably want to use active control systems to move the box around to follow the black hole, with rockets when necessary. However, by lanna's reasoning below from the shell theorem, if the box were a spherical shell, it wouldn't be an unstable equilibrium like a ringworld, so your active control system could be pretty sluggish.

Such a box can definitely be built if you make it much larger than a dishwasher.

A box made of any known material that close in would get ripped to shreds by tidal forces.

I was under the impression that from 50 cm (middle of the sides) to about 70 cm (the corners of the dishwasher) it wouldn't be that bad, but I'll need to do the math.

Finally solves the problem of what to do with cable news pundits.

It would be amazing if something like this was a necessary factor for the emergence of life in a solar system

interesting ... in what way?

It would mean the great filter could be behind us.

I'm not sure how a black hole would help create life, an inbound Planet 9 would certainly be a hinderince to life's existence though.

Maybe it doesn't help create life, but prevents its destruction, like Jupiter protects Earth from asteroids, or has another shielding effect.

Iirc it’s a bit of a myth that Jupiter protects the Earth. It does some sometimes capture objects and send them away from us, but it also sometimes hurls objects straight towards us

Wouldn't the capture of a PBH by Sol be more like forming a binary system with a large distance between the two bodies? Intuitively it feels like capturing something with that much mass would result in our sun "wobbling" with respect to its expected path through the Milky Way.

The Sun also wobbles because of the planets. Primordial black holes are (depending on theory of creation and path in later life) smaller than stellar black holes, maybe far smaller, in the order of lunar mass or even particle magnitude mass. So the wobble from a PBH will be overshadowed by that of Jupiter and the other planets. The sun doing a wobble of more than 30AU (what you would expect for a 1 solar mass PBH somewhere past neptune) with respect to the galaxy would have been detected ages ago, even the parallax measurements of yore are precise enough for that. From that fact, the magnitude of disturbance in other planets' and TNOs' orbits and the absence of gravitational lensing observations, our PBH should be in the order of planetary masses.

> particle magnitude mass

IIRC, that small would evaporate almost instantly[0]. I think you’re thinking of particle radius, which would be mountain mass[1].

[0] this formula says less than a plank time and is therefore wrong: https://www.wolframalpha.com/input/?i=5120π%28G%29%5E2%28mas...

[1] https://www.wolframalpha.com/input/?i=0.1nm*c%5E2%2F2G

Yes, if it would have particle mass at the time of the big bang it would have evaporated long ago. But all small black holes do evaporate (the smaller, the faster), so if PBHs were e.g. car-mass at the big bang, they could just be the right size for particle-masses right now (and gone in a few years).

1.7e11 kg is a pretty big car: https://www.wolframalpha.com/input/?i=%28%28%28h-bar%29%28c%...

And again, the predicted lifespan of a proton-mass black hole is significantly less than a plank time, which in turn is therefore a sign that — one way another — the maths doesn’t actually apply to that situation in the first place.

The minimum mass for a black hole is the Planck mass, approximately 21 micrograms. Such a black hole is a Planck length in radius, has a Hawking temperature of the Planck temperature, and will decay in Planck time, releasing Planck energy of 1.9 GJ (about the same as a tank of gasoline).

I think it depends on the mass of the object so captured? AIUI the mass of planet 9/PBH is estimated at 5-10 earth masses, which at that distance would not wobble sol (330,000 earth masses) very much.

Not all black holes have to be big; a 10 earth mass black hole would be quite tiny.

The paper actually depicts a 5 earth mass black hole at 1:1 scale, if you are curious.

Black holes in general are much smaller than people think. The event horizon of a 10 solar mass black hole is only something like 12 km across. That’s basically the head of a pin at space scale.

Supermassive black holes are on a whole different level however.

The most mind-boggling bit is the inverse relation between mass and density. Quoting[1]:

> A black hole of 387 million solar masses would have the average density of water and would be comparable to a giant water balloon extending from the sun almost to Jupiter.

[1]: https://arxiv.org/abs/1312.0340

A sphere of water extending to the orbit of Jupiter is a LOT of water.

Average density. What would the density be within the orbit of Mercury?

By nature of black holes: we don't know. For us it is pretty much a mass removed from this universe. For all we know, it has the average density everywhere. But the currently prevalent theory posits the density of the volume of the black hole is actually zero, with all the mass in a single point (singularity).

Interesting! Thank you for explaining.

If the event horizon of a 5-earth BH hole is several inches across as the paper states then doubling the mass/volume of said sphere would less than double the radius methinks ... maybe the figure is 12cm?

It's not any different (at large distance) than any other object of the same mass.

A previous thread https://news.ycombinator.com/item?id=23132488

I still like to think if it existed it could be our ticket out of the solar system.

(or a braking system for reentering if we ever made it that far)

A similar concept is explored in the sci-fi novel Hole, by Brandon Q. Morris.

I'd give it a 3/5 - fun read, not a classic.

https://www.goodreads.com/book/show/43566986-the-hole

He claims hard sci-fi but his stuff is actually pretty soft.

At first I read the title as Plan 9

That would explain why it is not more popular. If you install your software in a black hole, you cannot get any computed data from it out again.

If that thing were to appear on Earth, wonder how long before it becomes 6M⊕.

... and Expanse fans everywhere cheered!

Reply on news.ycombinator.com