Thanks for reminding me I needed to blog about why I think Hawking radiation drives aren’t really as good as they look: https://kitsunesoftware.wordpress.com/2022/05/14/no-a-black-...
(I agree that Isaac Arthur’s channel is good).
The paper you linked, https://arxiv.org/abs/0908.1803, is also a fun read.
We could throw garbage or nuclear waste into it
From the article: "if the black hole’s temperature is high, the radiation is composed of all elementary particles, photons, electrons, quarks, and so on. It’s really unhealthy. And a small black hole converts energy into a lot of those particles very quickly. This means a small black hole is black basically a bomb."
The nuclear waste thrown into it may be much cleaner than the stuff it will throw back out.
True, but that's tunable, the bigger it is the less comes out.
Also a great (short) one by Kurtzgesagt
Extremely Understated summary answer (to the first question), from the article:
> So some engineering challenges that remain to be solved.
"indeed, we need to fit half the universe in a grain of sand, more funding is required to overcome this challenge".
How many story points would you allocate for this one?
Anything over 8 is probably an epic.
Could you please split it into multiple tickets that we no bigger than 3 points
Splitting it into 3-point tickets an 11 point ticket all by itself.
I sure love doing things that aren't actual work
Twitter takeover funding has entered the chat.
“If you wish to make an apple pie from scratch, you must first invent the universe”
I admit I know next to nothing about this stuff, but something doesn't add up. If everything has a Schwarzchild radius determined by its mass, then should we conclude that particles like electrons and protons also have a (very small) Schwarzchild radius? If the smaller it is, the sooner it explodes, then shouldn't atomic particles have all finished exploding a long time ago? When they explode, what do they eject, if not more subatomic particles like themselves? Alternatively, is the explanation that atomic particles are extended bodies whose sizes exceed their Schwarzchild radii instead being of point masses? If so, then what kind of stuff fills the interior of an electron? I don't have any answers but I have a feeling we're on shaky ground when we start trying to extrapolate general relativity concepts to atomic scales.
> If everything has a Schwarzchild radius determined by its mass, then should we conclude that particles like electrons and protons also have a (very small) Schwarzchild radius?
> If the smaller it is, the sooner it explodes, then shouldn't atomic particles have all finished exploding a long time ago?
See my other comment here: https://news.ycombinator.com/item?id=31378092
> I have a feeling we're on shaky ground when we start trying to extrapolate general relativity concepts to atomic scales.
Correct. We know nothing about how to marry General Relativity with atomic-scale physics (quantum mechanics). That's why everyone and their dog are looking for a theory of quantum gravity.
Very interesting link - I suppose this could potentially make the problem slightly moot for electrons. Still, I don't think this works for other elementary particles, as black holes can't have color charge or weak hypercharge as far as I know (so they can't behave like quarks, gluons, W or Z bosons etc.)
> We know nothing about how to marry General Relativity with atomic-scale physics (quantum mechanics). That's why everyone and their dog are looking for a theory of quantum gravity.
True, though I think this is not even a problem in matching GR and QM, it is a problem in GR itself. The math of GR has infinities when looking at the center of a black hole, so we know there must be some other math that prevents the curvature from reaching infinity. We can of course easily invent infinitely many solutions to this problem, but there is no way to choose between them on an empirical basis, even in principle (since we can't ever experiment with the inside of a black hole).
A theory of quantum gravity would solve a different problem: GR is nonlinear, while QM is linear (if we ignore the Born rule) - so they can't describe the same system. Relatedly, if applying GR to a system described by a wave function, we are not able to compute how space time will curve given that a single particle(with its mass) is usually present at many points in space-time.
It is hoped that solving the second problem will also solve the first, but I'm not sure this is guaranteed.
> Still, I don't think this works for other elementary particles, as black holes can't have color charge or weak hypercharge as far as I know (so they can't behave like quarks, gluons, W or Z bosons etc.)
I think it is expected they can. The simple reason there are no explicit BH solutions with color charge is that, in contrast to electrodynamics, there's no classic field theory for the strong interaction that we could put into our Einstein-Hilbert action.
> I think this is not even a problem in matching GR and QM, it is a problem in GR itself.
Yes and no.
All kinds of theories have singularities and infinities. Classic electrodynamics is full of them and quantum field theory is, too. Nevertheless we still say the theories are fine and treat the singularities as pretty much nonphysical. ("Point particles don't really exist / a better theory will get rid of them", "We don't see the bare particles anyway, so let's remove the infinities using renormalization", et cetera.) Yes, spacetime singularities seem somewhat more severe, but I think we have good reasons to believe (e.g. the uncertainty relations) that a theory of quantum gravity would solve this conundrum. I mean, every single singularity we worry about in GR comes with infinite curvature and/or infinite energy densities, hence necessarily requires quantum mechanics to study.
On an unrelated note: Why is no one complaining that quantum field theory, from a mathematical point of view, is completely ill-defined? It surprises me time and again that people ascribe severe issues to GR ("It has singularities", "It's not quantum") and yet completely forget that the issues in quantum mechanics (both philophical and mathematical) are much more severe. GR, at the very least, is a mathematically absolutely rigorous theory, with well-defined objects and axioms and such. QFT, in turn, to this day is a toolbox of weird "shut-up-and-calculate" heuristics.
> We can of course easily invent infinitely many solutions to this problem, but there is no way to choose between them on an empirical basis, even in principle (since we can't ever experiment with the inside of a black hole).
There is one way: Come up with candidate theories of quantum gravity and with experiments to test quantum-gravitational effects outside a black hole (there are a few ideas) and select the right theory based on the experimental results and then have the theory predict what happens inside a black hole. Boom. If you say this approach is not valid as it'll remain a theoretical prediction and we still won't be able to peek inside a black hole, you're somewhat right. But right now we're having a discussion about spacetime singularities, which are a purely theoretical problem, too. No one has ever seen them.
> GR is nonlinear, while QM is linear (if we ignore the Born rule) - so they can't describe the same system.
We already know they are incompatible but linearity has nothing to do with it. The equations of motion of interacting quantum fields are non-linear, too. In fact, electrodynamics is, too, in some sense (backreaction & self-force), and we still managed to quantize it.
> Relatedly, if applying GR to a system described by a wave function, we are not able to compute how space time will curve given that a single particle(with its mass) is usually present at many points in space-time.
I wouldn't say this is just a related problem. This is the problem of quantum gravity.
> It is hoped that solving the second problem will also solve the first, but I'm not sure this is guaranteed.
Again, I think the reason people are hopeful are the uncertainty relations. A theory of quantum gravity necessarily has to incorporate them somehow.
I'm curious if anyone has seriously explored the GR math of space with "bubbles" that may turn pretty big in case of black holes. The spacetime wraps around those bubbles, so the only way to detect such bubbles is observing the GR effects. My impression so far has been that GR theorists assume that the spacetime is continuous - it may be distorted here and there, but overall it's a smooth "pile of space" homeomorphic to a sphere.
That is a very good question which I've been thinking about for years: How is it that at cosmological scales it is reasonable (apparently) to assume that the matter density of the universe is homogenous (i.e. the same everywhere) in space, yet at a local level, we have Schwarzschild/Kerr spacetimes around every spherically symmetric body (whether black hole, star, planet or atom)? How does a homogeneous universe emerge from this "bubbly" spacetime, as you call it, at larger scales?
Unfortunately, Einstein's field equations are not linear, so in contrast to other (linear) field theories, this case is not as simple as superposing several black hole solutions to a global solution and then averaging or zooming out in an appropriate way, since the sum of two solutions won't give another solution.
I'm wondering whether anyone has ever looked into the scaling behavior of the Einstein field equations but the answer from most people in the community that I've talked to has been no.
/? Quantum gravity fluid
"Gravity as a fluid dynamic phenomenon in a superfluid quantum space. Fluid quantum gravity and relativity." (2017)
> The hypothesis starts from considering the physical vacuum as a superfluid quantum medium, that we call superfluid quantum space (SQS), close to the previous concepts of quantum vacuum, quantum foam, superfluid vacuum etc. We usually believe that quantum vacuum is populated by an enormous amount of particle-antiparticle pairs whose life is extremely short, in a continuous foaming of formation and annihilation. Here we move further and we hypothesize that these particles are superfluid symmetric vortices of those quanta constituting the cosmic superfluid (probably dark energy). Because of superfluidity, these vortices can have an indeterminately long life. Vorticity is interpreted as spin (a particle's internal motion). Due to non-zero, positive viscosity of the SQS, and to Bernoulli pressure, these vortices attract the surrounding quanta, pressure decreases and the consequent incoming flow of quanta lets arise a gravitational potential. This is called superfluid quantum gravity. In this model we don't resort to gravitons. Once comparing superfluid quantum gravity with general relativity, it is evident how a hydrodynamic gravity could fully account for the relativistic effects attributed to spacetime distortion, where the space curvature is substituted by flows of quanta. Also special relativity can be merged in the hydrodynamics of a SQS and we obtain a general simplification of Einstein's relativity under the single effect of superfluid quantum gravity.
IIRC, when I searched gscholar for "wave-particle-[fluid]" duality" a few weeks ago there were even more recent papers.
Does Quantum Chaos describe fluids or superfluids? https://en.wikipedia.org/wiki/Quantum_chaos
Do CAS tools must stop reducing symbolic expressions describe infinity such that?:
Conway's surreal numbers of infinity aren't quite it, I'm afraid. Countability or continuum? Did Hilbert spaces (described here in SymPy with degree n) quite exist back then? Degrees of curl; divergence and convergence https://docs.sympy.org/latest/modules/physics/quantum/hilber...
assert n*x*oo == oo
Sorry, but I'm afraid you're not making a lot of sense.
How is "GR on Bernoulli", GM cannot describe nxoo more precisely than oo, and Conway's surreal infinities aren't good axioms either (for GR or for QM with (chaotic) fluids which perhaps need either infinities plural or superfluid QG (instead of QFT fwics); not making sense?
No, I'm afraid not. You might want to provide some background on what you're trying to say and, also, on your notation.
You have some misconceptions.
1) For all that we have been able to measure it, the electron is a point particle. It does not have a radius. The concept of radius does not apply. Every time we try to measure it, we just end up setting a smaller upper bound for the radius than last time. This is true of all of the leptons ("lightweight particles"). The same sorts of probes of electrons suggest that there is no "stuff" in them. That's all you get, this point with some numbers associated with it (charge, mass, angular momentum, lepton number, etc).
2) Black holes -- and I am going to constrain myself to a "no-hair" situation for those of you in the know -- have only three variables that describe them: mass, charge, and angular momentum. Anything else describes its position and how it is moving at the time. They're really quite dull. (Exploration of where the information that fell into the black hole went is ... contentious, abandoned, frustrating, etc). Radius is a function of mass (and angular momentum, you can distort the event horizon if it had enough spin).
3) They don't "explode." The theorized-but-not-yet-observed Hawking radiation is about chucking out the occasional particle and "borrowing" it from the black hole. This is done under conservation of the above mass, charge, and angular momentum. The smaller they get, the more chance they throw something out, so it is really a runaway process that only looks like an explosion at the end.
4) Due to this conservation, if you somehow made a single electron into a black hole, that black hole could only ever spit out one thing in its lifetime: an electron.
5) The proton is quite different. It is not the opposite of an electron. It is known as what is called a baryon ("heavyweight particle") and it has a size. It is also composed of smaller things, unlike the electron, three quarks and some gluons (which serve to hold the whole thing together).
6) Atomic scales are fine. We can understand things about relativity at the atomic scale. For example, we use the surprisingly extended half-lives of certain incoming particles to verify time dilation. Or just look up how relativity affects the orbital radii of very heavy atoms, in particular gold. Subatomic scales are more interesting.
Thank you for your comments. I might have at least one other misconception in need of clearing up. My impression from reading about it somewhere was that the Hawking radiation is predicted to happen as a consequence of vacuum fluctuations. When an electron-positron pair spontaneously forms close to the event horizon, and one particle falls in but the other doesn't, they can't annihilate so the one that's left outside appears to emanate from the black hole. Is that not the consensus, or if it is, why should the amount of radiation depend on anything but the surface area of the event horizon?
Your description of hawking radiation isn't quite accurate. It's a popular misconception. the actual process is not as easy to understand. see below:
That said, what the OP said about "borrowing" electrons I am not sure about.
It's more of an accounting deal. There's mass where there didn't used to be, and it was pretty near here so ... we gotta make the books balance.
You are pretty close. It's the curvature of the surface area. Smaller black holes have a less ... homogenous orbital space near the event horizon. More tidal forces, etc, so a particle-antiparticle production would be more likely to be torn apart.
> If everything has a Schwarzchild radius determined by its mass
It doesn't, not in the sense you mean. You can calculate a Schwarzschild radius for any mass, but that radius only means something physically for an actual black hole. You can use the calculated radius to estimate how hard it would be to turn some ordinary object into a black hole; that's what the article does by comparing the Schwarzschild radius for various masses or energies to the actual radius within which we can compress them by processes we can currently control (and of course the latter radius is very, very much larger than the Schwarzschild radius for those masses or energies, which means we have no feasible way of turning any of those objects into black holes). But that in no way means that those ordinary objects have some actual, physical Schwarzschild radius that acts like the horizon of a black hole. They don't.
(Not a physicist myself)
Blackholes are just a solution to Einstein equations for an object in which all its mass is concentrated in its Schwarzchild radius. Protons and electrons are bigger than that so they are not Blackholes and they will not "explode".
> When they explode, what do they eject
If it was possible to concentrate a proton to make a blackhole, when it evaporates, I'd say it "eject" itself (a proton)
That said, Einstein's equations do not really apply at quantum scales. So what happens with such blackhole is unknown. We never observed micro blackholes, and the Hawking radiation is just a theory which may or may not be true.
> If it was possible to concentrate a proton to make a blackhole, when it evaporates, I'd say it "eject" itself (a proton)
> Quantum gravity (via virtual black holes and Hawking radiation) may also provide a venue of proton decay at magnitudes or lifetimes well beyond the GUT scale decay range above, as well as extra dimensions in supersymmetry.
Perhaps it's possible that a proton get transformed into a black hole and then the black hole decays into a positron and a pion (or a positron and a few photons). Nobody is sure about this, and nobody has seen this or other decays of protons. More speculative details in https://en.wikipedia.org/wiki/Proton_decay#Theoretical_motiv...
I'm not sure what the issue you see here is: a very small Schwarzchild radius would be smaller then the size of the particle, and as a result the particle cannot collapse itself into a black hole.
The problem is that electrons and quarks and other leptons are considered 0-size (point like) particles, but they do have mass - so, according to GR, they should "collapse" into black holes.
Of course, experiments so far are also consistent with leptons having very small but non-0 size. Since their Schwarzschild radius is much smaller than a Planck length, we will probably never be able to design an experiment that would show a disagreement here.
It's also notable that GR predicting a mathematical singularity at the center of a black hole shows that it can't be right at such extreme scales - there must be some unknown limit that prevents the density of a back hole from reaching infinity, and that would probably solve this issue as well.
If they're incompressible (i.e. fundamental) particles though, then there's no inconsistency: any single electron can't compress itself into a black hole, because it's experienced gravitational attraction can't increase - it doesn't can't pull on itself because it has no internal structure.
Two electrons on the other hand can, because above some point when you push them close together the force between them rises above electrostatic repulsive and they'll pull their 0-size closer and closer until a singularity forms.
Of note, black holes on this scale aren't going to be stable though: they'll evaporate pretty much as fast as they form from Hawking radiation.
EDIT: Of note - at this sort of scale it's not entirely clear to me that whether an electron is a black hole is a meaningful question either. Black holes can have spin and charge, so an electron and an black hole masquerading as an electron would be superficially indistinguishable - it would weigh the same as an electron, and so electrostatic force would dominate all its interactions. This has been speculated: https://en.wikipedia.org/wiki/Black_hole_electron though not observed at the moment. But the inconsistency isn't because it would not be sufficiently "electron-like".
Is a black hole electron consistent with hawking radiation theory? or is that the naked singularity part; since there is no event horizon, they dont radiate. it seems strange to even call it a black hole at that point.
You're forgetting quantum mechanical effects. Effectively, a electron is constantly quantum-tunneling out of its own event horizon. (Or, equivalently, a electron, considered as a black hole, always immediately decays into Hawking radiation consisting of exactly one electron (with the same position, momentum, electric charge, etc, as the supposed black hole, since black holes aren't exempt from the various conservaton laws).)
Every mass has a corresponding Schwarzchild radius because mass is just a variable in the equation. Almost everything in the universe has an actual radius far far larger than it's Schwartzchild radius, hence most objects in the universe are not black holes.
According to my calculations, schwarzschild radius of electron is 1.4e-59 m, and electron radius is 2.82e-15 m, so electron is huge and electron density (if such thing exists) is not enough to form black hole.
your "electron radius" is the "classical electron radius" which is a ficticious radius one uses i lf disires. in modern theory electrons have no radius.
> edit: typo
Nitpick, but you missed one ;)
To add some understanding: the name does not mean "the child of Schwarz".
It's composed of two German words: "schwarz" which means "black" and "Schild" which means "shield". So "Blackshield". No children involved here.
Seems like an ultraviolet catastrophe situation
im not even sure how to begin there... probably the only wait contain a black hole would be... warping space-time negatively? like kind of warp bubble?
> Slight problem with this is that you can’t touch black holes, so there’s nothing to hold them with. A black hole isn’t really anything, it’s just strongly curved space. They can be electrically charged but since they radiate they’ll shed their electric charge quickly, and then they are neutral again and electric fields won’t hold them. So some engineering challenges that remain to be solved.
It's just a gravitationally attractive body: it moves towards the net gravitational force acting on it. So you could control it by trucking sufficiently large masses closer or farther away within a suitably large containment area, provided it wasn't too massive.
Easiest thing to do though would be to build it in orbit.
Good way to clean up space junk
No better than any random orbiting lump with the same mass, and quite possibly worse. If you want to clean space debris put up something like a big disc of aerogel.
Unlike the trash bucket in Linux. 1 time out of 20 you go back and restore that thing you trashed. And damn glad you are to have that power.
Black holes, otoh, are forever.
How on Earth did you twist this discussion about blackholes to bring up Linux?
The whole idea of irrevocable trash disposal vs the less irrevocable kind.
they are not forever.
> They can be electrically charged but since they radiate they’ll shed their electric charge quickly
This view is not right, e.g. electrons radiate and keep their charge. Black holes lose their electric charge when opposite charged matter falls in.
Electrons can’t lose their charge through radiation because they can’t have fractional charge. They radiate photons to lose momentum. (They can become uncharged by combining, eg e+P = N).
A black hole can presumably radiate charged Hawkings radiation? I.e. if an electron-positron pair is created near the event horizon of a negatively-charged black hole then it would disproportionately capture positrons and repel electrons, thus radiating away its charge. (Could be wrong here, I’ve not looked into charged black holes before). I would assume here that charge radiates away at a different rate than mass, and by her statements it sounds like ch argue evaporates away quicker.
She’s not making a general claim that “everything that radiates loses charge”, that would be silly.
I recall reading that any magnetic fields present when the black hole formed would be "frozen in place". If so, couldn't that be used?
That said, I found that very surprising and expected the magnetic fields to disappear, so maybe I misunderstood something.
I’d imagine you could keep feeding it an electric charge to counter any charge it loses.
You can put a small one in orbit around a body of mass. Or let it orbit inside and wait for the spectacle.
I skimmed over Crane and Westmoreland's paper and an article that discusses specific designs, and I'm still unsure how one would keep the vehicle and black hole moving together while also generating thrust. It seems like there would be a conservation of energy problem, regardless of the design. I'm not a physicist, so I'm probably missing something.
Most things one would use for thrust in space are inherently repulsive. The part I'm having trouble with is that it seems like even though a black hole would put out a lot of energy, it wouldn't be inherently repulsive, and so any vehicle would have to exert a station-keeping (or orbit-keeping) force equivalent to whatever the over-all black hole engine was emitting, making the engine itself useless. This seems especially true for the design in the second article, where the Dyson shell weighs 600 times as much as the singularity. But again, I'm probably missing something obvious since it's outside my areas of expertise.
Maybe we could make Black Hole Computers?
♩ ♪ ♫ ♬ Black hole sun, won't you come And wash away the rain? Black hole sun, won't you come? Won't you come? Won't you come? ♩ ♪ ♫ ♬
One of my favorite science fiction stories, "The Krone Experiment", has this question as a central plot element. https://www.goodreads.com/book/show/16032842-the-krone-exper...
One question I had, prompted by a fever dream in which a black hole spawned in my house...
If a black hole were to come into sustained existence, assume the smallest one. How long could we stand near it before being unable to escape? And how far is that distance?
The equation for escape velocity is: v = √(2GM/R)
R = distance (radius, really) M = Mass of the body G = Universal gravitation constant
We can modify this equation to find for the distance at which you can escape:
r = 2GM/v^2
The answer is largely: it depends on how fast you can go, at the speed of light you can escape from further away, since the pull will increase the closer your are to the "event horizon".
I'm in a car right now (as a passenger ofc) doing this from my phone so not in a situation where I can put together a model, but you should be able to plug in some numbers and estimate a result, just make sure you convert to SI units so you don't accidentally end up 3 orders of magnitude off.
A black hole with the mass of the earth would have a radius of about 2cm, so things less massive than a planet start to get very small, very fast, and you end up fighting quantum effects which become less intuitive.
There is no "smallest one". The small ones evaporate to smaller and then nothing very quickly. "Evaporate" as in, emit huge amounts of radiation -- like a bomb.
> The small ones evaporate to smaller and then nothing very quickly.
This is not at all known, as we have no idea what a theory of quantum gravity would look like (which would necessarily enter the game here). We might end up with a black hole remnant, or Hawking radiation might behave differently for microscopic black holes etc.
It's theorized that there is a grapefruit-sized black hole orbiting our sun in the outer reaches of our solar system with a mass of 5 to 10 earths. A black hole spontaneously appearing in your home small enough to not rip apart the entire planet immediately would probably be too small to notice without some sort of detection equipment.
> It's theorized that there is a grapefruit-sized black hole orbiting our sun in the outer reaches of our solar system with a mass of 5 to 10 earths.
Source: https://arxiv.org/abs/2004.14192 (There was also a pretty good discussion about it here on HN.)
> would probably be too small to notice without some sort of detection equipment.
What makes you think that?
Theoretically, black holes can have a mass of the tiniest fraction of a gram which would be unimaginabley small. It's my own speculation that you wouldn't be able to detect that with a naked eye.
Thanks for the link!
> you wouldn't be able to detect that with a naked eye.
What if you touched it? No idea what the spacetime would look like near a gram-sized black hole with lots of heavier matter surrounding it but I suppose there would still be pretty severe tidal forces.
Anything coming anywhere close to the event horizon would be removed, and some fraction of that mass would be converted into energy an irradiate the surrounding. So the best you could hope for is that it's moving fast, leaves a hole in you and leaves fast before you die.
Keep in mind that event horizon isn't a shell, just a point at which your future (which is in the singularity) is certain.
> Keep in mind that event horizon isn't a shell, just a point at which your future (which is in the singularity) is certain.
Yeah, exactly my thought. Then again, we're silently assuming here that spacetime would pretty much look like one of the vacuum black hole solutions plus some additional matter (our body) near it. That doesn't seem too likely, given that our body is much heavier and can't just be treated as a test particle. OTOH it doesn't seem too likely, either, that the actual spacetime would look completely different: There will surely still be a black hole and an event horizon.
Ah yes, the tachyon detection grid. ;)
Follow-up question: are black holes generally a question of density and not mass? If I could take a laptop and squeeze it hard enough to overcome various nuclear forces, would I get a black hole with an event horizon the size of a laptop's gravitational field?
What would it take to get an event horizon on a human scale (a feet or two across?)
Sort of. It depends on both density and mass. The more mass there is, the less density is required to make a black hole.
A solar mass black hole is stupid dense. But a supermassive black hole is less dense than the earth, and can be less dense than water. That's still an insane amount of mass, but it's not really all that dense.
A human scale black hole would be even denser than a solar mass black hole. It would require over 200 earth masses, though that's still a tiny fraction of a solar mass.
In this comment, are you defining density as mass per volume contained by event horizon? Or do we know how the mass is distributed inside the black hole? Does it even make sense to discuss distribution of mass in a black hole? Would clues about that leak out through dynamics like rotation?
You only need total mass and total volume. No other details leak out.
A non spinning black hole is an absolutely perfect sphere, with no "hair". A spinning black hole is flattened, or maybe even a torus, but is still mathematically perfect.
Unless quantum mechanics intervenes in ways nobody has yet figured out.
Total volume of... what? Contained within event horizon? Surely not singularity? (Which I would assume to be a... well, singularity.)
Yep, event horizon.
The singularity occurs at a nominal point at the center (or a ring for a rotating black hole). It has no volume, but all of the mass ends up there, causing divide by zero errors.
> are black holes generally a question of density and not mass?
Correct. Any mass M taking up a spherical volume of radius less than 2GM/c² (the Schwarzschild radius) will necessarily be a black hole. Black holes are thus the objects in the universe with the highest mass density and, coincidentally, the highest entropy density.
> What would it take to get an event horizon on a human scale (a feet or two across?)
A mass M = Lc²/2G, where L = 1ft for a black hole 2ft across.
The event horizon is the least of your worries: the real problem is tidal forces.
The calculation in that document is representative: for a solar-mass hole (event horizon radius 2.9km) the tidal forces on a human are 51000x Earth gravity at 100km away!
You may enjoy The Aleph. https://en.m.wikipedia.org/wiki/The_Aleph_(short_story)
Well, if a black hole somehow appeared on Earth, with a low-ish velocity relative to the ground, it would immediately fall inside Earth so "standing" near it would be pretty difficult.
That's called the event horizon, which is proportional to its mass.
When I was a kid I was really afraid that particle colliders would create a black hole that would sink to the center of the earth and eventually eat us all, so I find this article quite soothing.
Cosmic rays with higher energies than that from human particle accelerators hit the Earth on a regular basis. If Black holes could have been formed by them, we wouldn't be around to ask, so actually not much to worry about particle accelerators generating Black holes.
You can't infer from our failure to not exist that cosmic rays hitting Earth never create black holes. All you can infer is that any such black holes aren't dangerous. That still is sufficient to support your overall thesis that since cosmic ray black holes are not a problem we don't have to worry about black holes from colliders so your overall point stands.
Black holes formed by cosmic rays hitting Earth would not be dangerous because they would be very very small. Most likely they would very quickly decay via Hawking radiation, but even if they did not decay for some reason they would be so small that very little would actually fall into them.
Small black holes created in the early universe, big enough to not noticeably decay in the billions of years since and so much bigger than those cosmic rays hitting Earth might create, are actually taken seriously as one of the candidates for dark matter. Even those, which would be much larger than anything cosmic rays or colliders might make, would be sufficiently hard for things to actually fall into that they could pass right through you without you noticing.
> So, if you hold the mass fixed and compress an object into a smaller and smaller radius, then the gravitational pull gets stronger.
I find this bit interesting because I'm pretty sure I've read the exact opposite before. My previous understanding was that the gravitational pull is only determined mass - but a black hole can put an almost arbitrary amount of matter into the same space, therefore the gravitational pull is factually much stronger than for any "ordinary" object of the same radius.
However she is saying the compression itself is already increasing the pull.
So as an example, suppose our sun got replaced by a black hole of identical mass (but much smaller radius). Would this cause orbits of the planets to shrink (increased gravitational pull) or stay the same (identical gravitational pull)?
She's cheating a bit, to make a valid point. The gravitational force exerted by an object is due, as you say, to the object's mass-energy. So, a solar mass black hole centered on the center of mass of the sun would have the same gravitational effect on earth as the sun does. But the force a test particle experiences due to the gravity of either object depends on the the square of distance of the test particle from the center of mass of the the object. The physical expanse of the object doesn't really matter, if you're outside of the object. But, with the sun, the closest you can get to the center of mass is roughly 700,000 km. Any closer than that and you're inside the sun. Once you're inside the mass radius of an object, the force you experience is due only to the proportion of the object's mass that is closer than you to the object's center of mass. So the gravitational force you experience (if you could survive being inside the sun) declines as you get closer to the center of mass, until it's zero at the center (the pressure you experience is a different matter - it steadily increases as you journey down the mass). The black hole's radius, though is only about 3km, and so you can approach to within that distance of its center of mass. At that distance from a solar mass, the gravitational force is enormous - sufficient to overcome the momentum of a photon, "drag" it back into the black hole. So, the gravitational force you can experience from an object does depend on it's size, even though the total force at astronomical and mere macro scale distances does not.
Gravitationally, nothing would change if the sun was replaced by a black hole of the same mass, at least for objects above the surface of the current sun. But being 1 kilometer above the event horizon of that black hole would be very different to being 1 km above the surface of the sun.
As the radius of a object shrinks (but with mass held constant), the _surface_ gravity increases. Remember that the pull of gravity decreases with the square of the distance away from the object. With a smaller object, you can get a lot 'closer' to all that mass, so gravity is stronger at its surface.
> If you remember Newton’s gravitational law, then, sure, a higher mass means a higher gravitational pull. But a smaller radius also means a higher gravitational pull. So, if you hold the mass fixed and compress an object into a smaller and smaller radius, then the gravitational pull gets stronger. Eventually, it becomes so strong that not even light can escape. You’ve made a black hole.
I think this is discussing the gravity on itself — or the peak gravitational pull, for nearby objects.
Compressing the Earth wouldn’t make far away objects experience it differently, but compressing Earth would increase the peak pull nearby — to the point of creating a black hole. Much more gravitational pull than anywhere on Earth experiences now. But that radius would be far, far inside of where the surface currently is.
Density increases nearby gravity by focusing mass.
Mass and energy are same thing. They effect gravitational pull the same. If you compressed the Earth the energy it takes to compress the Earth would increase the mass-energy of the Earth, and this would change the orbits of planets.
If you make an object smaller, you can get closer to it's center of mass, increasing your experience of it's gravitational force.
your experience of its gravitional force is dependant on distance.
the description of force experienced being spoken about is from the frame of a variable distance observer, not the gravitating body.
Black holes created from energy are called Kugelblitz black holes . The article correctly points out the difficulty of doing so with traditional lasers.
But if we can ever figure out a way to reflect (and thus lase) gamma rays (or some other much higher energy radiation) this then becomes possible.
Of course we don't even have a plausible theory on how we might construct "grasers" .
Are free electron lasers fundamentally incapable of operating up into gamma? Maybe we just need to wiggle harder.
This is the kind of question that makes me believe in the Great Filter¹.
This is the kind if questions which justifies the need for humanity to colonize multiple planets if it doesn't want to go extinct.
Came here to post this but couldn't remember the name "Great filter", thank you. :)
I remember back in the day when cartoons had black holes you pull out of a pocket and throw down to use either as a portal to a different part of the cartoon or as a way to send your antagonists somewhere else.
I think that's where we should start in looking at potential use cases for personal, instant black holes.
I'm not young any more so I would volunteer to help in development and testing of any portals as long as I get good Cajun coffee and a smoked brisket sandwich in the lunchbox with a blood orange and a slice of Mom's apple pie.
While that is a hole, and is black, it's a totally different and unrelated kind of black hole from what TFA is talking about (and is more commonly called a "portable hole", after the version in Dungeons and Dragons).
I understand that there may be other things to consider here. But, if given the opportunity to dink around with the physics and end up with an instant pocket portal that can do the cool cartoony stuff I am willing to give it a whirl.
I can see where it could be used in hot climates to help avoid energy waste. Buildings could be built with no doors and using UV reflecting glass for windows and people would enter and exit with their own personal pocket portals. Had enough for one day and feel like a cold brew? Flip that pocket portal like a disc golf disc and when it sticks to the wall just jump thru it to the street. It closes up behind you allowing for the absolute lowest conditioned air loss situation. No more door and threshold gaskets to replace, ever. Just keep your portal tuned to the right energy level and make sure that you never let it work like your money, burning a hole in your pocket.
> if given the opportunity
You don't have that opprotunity. Black holes and portable holes have as much to do with each other as computer chips and potato chips.
The actual title of the post asks the hypothetical questions: "Can we make a black hole? And if we could, what could we do with it?"
From where I'm at, that gives me all the opportunity that I need. I described the type of black hole that I would like to create, gave evidence from the historical record for some of its attributes, and described how it would or should be used.
I'm not sure why you are unable to acknowledge this. OP asked people to think about it. I thought about it and replied. Perhaps you didn't see the same cartoons or are not seeing the utility of the personal instant black hole.
Now you're thinking with portals.
Not from the video… What blew my mind was to learn that a black hole get bigger when it absorbs light. Light from a campfire you had as a kid could be feeding a black hole right now.
> Light from a campfire you had as a kid could be feeding a black hole right now.
I don't think we have a close enough black hole for this to be true for anyone reading this right now.
Maybe, there's a theory that the 9th planet is a black hole, and that's why it's very hard to detect outside of the gravitational effects. Amusingly there's a diagram in the paper that's a 1:1 diagram (life size) in the paper.
Would be crazy if true, we could verify all kinds of things by tinkering with a live black hole.
Well, yeah, it is weird at first. But once you think about it, it is not unintuitive: if the light was just gone, that would be wrong, too, right? Also, energy == mass, so of course, it is equivalent to throw mass into a black hole or energy. The longer I think about it, the more it is mass that is the weird thing. What is mass? Something like frozen energy...
The Earth gets bigger as well, and by the same mass per photon, when it absorbs light.
It loses this mass through radiation, but then, so does a black hole. That's fancy Proper Noun Hawking Radiation, but radiation it remains.
Not trying to be a downer, I find the fact that a photon doesn't /have/ mass but still /is/ mass endlessly fascinating.
> Roger Penrose already pointed out in the early 1970s that it’s possible to extract energy from a big, spinning black hole by throwing an object just past it. This slows down the black hole by a tiny bit, but speeds up the object you’ve thrown
Black hole railguns/artillery?
Or, in the name of safety, mobile satellites in low earth orbit armed with hard tungsten rods, accelerated by temporarily generated black holes to relativistic velocities for prompt global strikes on time sensitive targets. Could make for a good movie.
surely the law of conservation applies, in that it would be more efficient to take the energy used to generate the black hole and apply it directly to launching the projectile instead?
It would, although the utility of using a black hole has a universal mass-energy converter would be substantial. Take any matter you want, toss it in to be crushed and then extract it back out as kinetic energy you can use to make electrical power.
Well, its essentially a black hole railgun. Exept a railgun uses the magnetic force instead of gravity. and black holes are 'theoretically' really efficient at converting mass to energy.
How would one "rods from god" on a timely manner..
You cannot harvest the energy given to you by a blackhole... unless the impacts of tungsten objects yield harvestable energy.
You don't have to chuck in rods, that's just the intuitive explanation for why you oughta be able to take energy back out. Realistically (for a certain value of realism) you'd use the magnetic field generated by charged particles accelerated in such a fashion, or something like that
small black holes are there for nanoseconds, im not really sure you could find a good method to "shoot" them
the whole "you can shoot somin near a black hole and speed it wayyyyy up" reminded me of the three body problem. some advanced species just tossing crap at black holes and blowing up stars
Surprised no one has mentioned the Kugelblitz yet, so I’ll drop this fun hit of exotic theoretical engineering here.
I dunno what you could do with it but the black hole laser bit blew my mind https://arxiv.org/abs/1409.6550
The horrible but obviously accurate answer is it would be weaponized.
This is what I worry about with fusion, it's not going to be used for free power for the world, it's going to be used to power war-machines.
The only value of a black hole you can build would be as a doomsday weapon: do what we want or we end the world. Except...that's been the case since the Cold War with regular nuclear weapons.
As for fusion: you need to do more research. We've had fusion bombs since 1952. Practical fusion power for electrical generation is what we don't have since the constraints are very different.
It's handier interstellar if you can find a way to accelerate it at any real velocity I guess. At that point I can't imagine it's easier than just chucking a projectile at some absurd fraction of c
you are suggesting singularity munitions, as others have before. the process is not known but the desired product is- arbitrarily create an unstable singularity that converts surroundings out to a radius into energetic content leading to explosive "jetting" and gravity wave propagation until spacetime re-normalizes.
It’d be cool if we made a black hole type bullet that would suck someone or something in completely on impact and dissolve. Not sure if feasible though.
yes, but no :)
so as the blackhole gets smaller the more energy it radiates, eventually basically blowing up. so simply put a small BH in a magnetic trap next to someone.
but if you shoot it it'll go too fast to stay put.
though it might be possible to release a small one next to someone slowly.
small means ~ 1 million kg, which evaporates in 84 seconds, though it will emit so much energy that... well it will turn a city into plasma almost instantly
basically the problem is that either general relativity and Hawking are correct, which mean that there is simply no way to have a small (compared to human mass, so like a big bomb, eg a few metric tons) black hole that doesn't violently want to turn back into a non-blackhole, or if it's possible then our theories are incorrect and all bets are off :)
Seems just like a grenade. You time the throw so your target gets destroyed. Incredibly efficient, since unlike nuclear or fusion nukes 100% of the matter gets turned into energy.
Vastly increasing the target's density would do for that; no need to go past that into full singularity. cf "Neutronium Alchemist" in Peter F Hamilton stories.
It'll never be as simple or satisfying as the old school hammer.
Some parts of heyjackass.com immediately sprang to mind.
Outlaw Star had it!
> So, if you hold the mass fixed and compress an object into a smaller and smaller radius, then the gravitational pull gets stronger. Eventually, it becomes so strong that not even light can escape. You’ve made a black hole.
In Newtonian doctrine, a spherical object, like earth, attracts -as if- all its mass is concentrated at its center. So, if her reasoning is correct, the earth must already be a black hole, because all its mass is supposed to be concentrated at its center.
No, because that approximation only works when you're outside the object. Once you're inside the object, any shell of mass outside your distance from the center cancels itself out (produces zero net force).
So the escape velocity from earth at its surface is well below the speed of light. And below the surface, gravity is even less. Only a black hole packs enough mass into a small enough place to get the escape velocity above the speed of light.
If GP wants to read about this, the name is actually "shell theorem":
That's only 100% true for a radius _outside_ of the object.
attracts -as if- all its mass is concentrated at its center
At a radius _inside_ the object, only the mass closer to the origin counts so the "effective" mass of the object drops smoothly to zero.
Actually it's something like 2 radius, unlike the earth there's no stable orbits under 1.5 or 2 radii (I forget which). Outside of 2 radii there's no differences.
Yes, you can compress any object small enough and you end up with a black hole.
And yes as far as anything else in the solar system is concerned, if the earth was compressed into a blackhole no orbits would change. Well relativistic effects mean there's no stable orbits below 2 radii (maybe it's 1.5), so various sats would get sucked in.
However just because that's true, doesn't imply the earth is a blackhole, just that the orbits in the solar system wouldn't change. Similarly if the sun collapsed into a blackhole the earth's orbit wouldn't change, but it would get much colder.
That's an approximation that works at some (very small) distance from the surface. It does really not work at the center of the object.
If we had "reverse centrifugal force" you could create a black hole by "spinning" the thing really fast.
Just spin it the other way around :^)
Maybe a donut shaped black hole could be made by spinning. Or a vortex.
Maybe flip it in 4d
I think this discussion is too focused towards stable blackholes. How useful can unstable (even nanosecs) blacholes be?
If a ‘Type III’ civilization is meaningful it could be one that uses a quasar the way we use a star.
They should harness it's energy instead.
Would be neat to suck up all the garbage
Sure, but the weight would still be there, and whatever containment you had would be more stressed, till it fell into the local gravity well, which near earth would be bad.
"Io, corner pocket."
there's a potential video game there. planet pool. dropping things into the sun would be a "scratch"?
Send trash in it.
First two sentences of the article:
> Wouldn’t it be cool to have a little black hole in your office? You know, maybe as a trash bin.
> what could we do with it
Putin merely has access to nuclear weapons. I suppose the “I win or the earth gets it” is the same whether we’re talking nukes or a black hole
We could put a black hole over Russia, so that any ballistic missiles they launch get sucked up in it.