And itself is based on two things: that they're absolutely black as in they don't reflect anything (that's what they believed when then the name came about), and that things can fall into them.
>A hole is not something, it is a cavity in something else
Well, and black holes can behave like cavities in the universe. They bend the surrounding area with their gravity so much that they end up forming a kind of cavity.
Compound names mostly come from associations, not careful logic.
Names in general are arbitrary symbols, not definitions.
Black holes, as we think of them as something, are really shadows. It'd be more accurate and less confusing to call them singularity shadows.
E.g. there are several theories about how our universe itself is the inside of a black hole, and so on.
But a well IS a hole in the ground and a pretty deep subject.
A water well is a special type of hole known as an excavation, meaning, of course, that it was excavated. Wells are not made by accumulating so many things that they just sink into the ground, and nothing is taken out of a black hole to create it.
An object with a relatively much smaller mass can take a hyperbolic orbit arbitrarily close to a star or planet without "falling in". Practical examples include <https://en.wikipedia.org/wiki/List_of_hyperbolic_comets> and many small near-earth objects. Theoretical details in Newtonian gravity at <https://en.wikipedia.org/wiki/Hyperbolic_trajectory> and there is a literature exploring post-Newtonian corrections for such "orbits" (in e.g. General (or Numerical) Relativity, or formalisms like gravitoelectromagnetism or effective one body, for cases where one or both bodies are "compact" (like white dwarfs or neutron stars), the mass-ratio of the bodies is close to 1, or one or both bodies are moving at large fractions of c, or there is some combination of these features).
An object can take a hyperbolic orbit arbitrarily close to a black hole without falling in.
Black holes were first formalized in the context of General Relativity; commonly one would use a different term to describe something phenomenologically similar but set in a different theory ("fuzzballs", gravastars, and so on). Fevers and spots appear in diseases with very different causal agents (any number of quite different bacteria, viruses, and other things may cause grossly similar symptoms in the victim). Likewise, apparent trapping surfaces can appear in many ways in different theories of gravitation or in different configurations of variables within a single theory of gravitation.
The important word in the previous sentence is "trapping": anything crossing that surface from the outside to the inside cannot cross back to the outside for arbitrarily long times. Differences in configurations on the inside of a trapping surface do not materially affect the outside at all. The "apparent" qualifier captures the possibility that the trapping surface does not go to the eternal future because of (for example) instabilities from quantum effects (Hawking-style), so we can take at all as meaning "for a very very very very verrrrrrrrry long time".
The observables of a black hole in General Relativity (in its form as a physical theory that adequately represents many physical features of our universe) are all outside the event horizon. Anything inside the horizon stays permanently inside. From the outside one cannot test the internal configuration. While the internal configuration is described in several black hole solutions to the Einstein Field Equations, nobody expects that just because the external configuration (outside the black hole) is a good physical model, that therefore the internal configuration must be a good physical model too. Roy Kerr makes this point almost every time he lectures about his solution for black holes with nonzero angular momentum (example: 48m04s mark <https://youtu.be/nypav68tq8Q?t=2884>, where he points out that the Kerr solution is a vacuum solution, and that adding matter inside the horizons is likely to dramatically change the black hole internal configuration. Note however that adding matter to the outside part of the Kerr solution is highly likely to be undramatic, and that is one reason why the Kerr solution is astrophysically useful).
Stars and planets differ from black holes in that there is no apparent trapping surface. You can shoot an electron neutrino right through the Earth or the Sun. You can't shoot an electron neutrino through a black hole: if it goes in, it stays trapped inside.
Your term "singularity shadows" presupposes that as we develop better solutions (numerical or analytical) of the Einstein Field Equations for (apparent) astrophysical black holes, the singularities that appear in e.g. the Schwarzschild or Kerr solutions will remain. That may not be true. I don't think the "shadow" part adds any accuracy.
It is not true that "gravity always implies matter". In General Relativity there exist several exact solutions to the Einstein Field Equations where there is significant spacetime curvature but no mass. Some of these usefully approximate features of the universe we observe, even though as far as we can tell there is no part of our universe that is completely free of matter (in the most general sense, including electromagnetic radiation), although large and growing regions are so sparse that the matter in them does not collapse gravitationally into clumps. This trend is just as much an effect of spacetime curvature as is the gravitational collapse of dust clouds into stars.
Finally, you can use whatever nomenclature makes you happy. It's just a fanciful term that covers a wide range of theoretical descriptions and astrophysical phenomena. Astrophysicists and theorists use "black hole" knowing that they may be talking about quantitatively and qualitatively different objects with fairly similar symptoms being presented. But they also know how to find, read, and understand a precise mathematical description that removes the ambiguities of English (and other languages) and any inaccuracies (in some settings a black hole may be a very weak greybody radiator; and in some settings "hole" may be less poetic and more descriptive, e.g. in Wheeler's bag-of-gold solution). Substituting some other pithy name for "black hole" doesn't help these physicists, and is unlikely to help anyone who doesn't know how to deal with the formal, unambiguous descriptions of them.
The same is true of a black hole. 90% of the matter orbiting a black hole will never fall into it.
Yes, as I wrote in the second paragraph:
>> An object can take a hyperbolic orbit arbitrarily close to a black hole without falling in.
It occurs to me that in your various comments here you are thinking of the point mass (or divergence of the Kretschmann scalar or whatever) as the black hole. Conventionally, and for good practical reasons, practically everyone working with astrophysical and theoretical black holes define the horizon as the black hole.
It's frequently tempting to think of the point-mass in Schwarzschild as the generator of the event horizon. After all, it's usually described as being a surface at r = 2GM/c^2, with "M" doing the heavy lifting, if you'll pardon the expression. However, Schwarzschild is an eternal black hole, rather than one that forms by gravitational collapse. For the case where there is some matter and no black hole -> some matter + a black hole, it is the early configuration of the "some matter" that generates the event horizon.
If one, following Lemaître-Tolman <https://en.wikipedia.org/wiki/Lema%C3%AEtre%E2%80%93Tolman_m...>, takes a spherical shell of radiation with a total momentum-energy comparable to a galaxy all moving radially inwards, and starts the spherical shell at billions of light-years from the shell's centre, then anything already at the centre (even a small interplanetary civilization) is already inside the event horizon before the civilization's home planet formed. Barring faster-than-light travel, nothing within (or produced by) the "victim" solar system will be able to cross outside a surface near the trailling edge of the inrushing radiation: at early times all possible low-speed trajectories from the victim solar system ultimately recurve back to a (set of) point(s) within it and at late times all possible high-speed trajectories recurve inwards.
Near the latest time in the collapse, everyone outside the victim solar system can conclude the victims are inside a black hole, even though for hours to days (and much longer, if we make the total mass of the shell extremely large) the victims inside will still be going about their business wholly unaware (because "c") of their fate. Horizon = yes. Singularity = no ("not yet", perhaps).
Finally,
> 90% of the matter orbiting a black hole will never fall into it.
is probably wrong, especially if one takes "never" literally. The dynamical spacetime in the near horizon region is on its own probably enough for orbital decay of anything in close orbit (a few tens of R_{crit} ~ R_{Schwarz.}) and there are plenty of forcing functions on bodies in elliptical orbits at greater remove, particularly in galaxy cenrtres and globular clusters.
What am I thinking now?
Technically, the meaning of the name is "defender of men".
- "Hole" — things fall in and don't come out (more or less).
Plus they look like holes on those 2d representations of space-time.
Seems reasonable to me.
I wasn't willing to argue this before because my issue was not with the adjective. But actually black holes are not really black, they emit radiation.
> - "Hole" — things fall in and don't come out (more or less).
But things fall into any massive object like planets or stars, so "hole" is ambiguous. Things also fall out of holes, such as holes in a ceiling or screw holes.
> Plus they look like holes on those 2d representations of space-time. Seems reasonable to me.
It is a fair point to say that the hole of a black hole is a 4 dimensional hole, except that space-time is itself a 4 dimensional hole, leading to more ambiguity.
You keep repeating that spacetime were a hole. What makes you think that?
Definitely not true in math - if you take a circle (S^1 = { (x, y) : x^2 + y^2 == 1 }) it has a hole as defined by homotopy in the middle and there is no 'space' there. If you fill it with space you get a disk (D^2 = { (x, y) : x^2 + y^2 <= 1 }) there is no longer a hole as defined by homotopy/is contractible.
The one that defines the noun, "hole," as a hollow place in a solid body or surface.
> if you take a circle (S^1 = { (x, y) : x^2 + y^2 == 1 })
Black holes are real, but your circle can only "exist" in mathematics.
You are aware that General Relativity is about the most mathematical (and mathematically rigorous and mathematically advanced) theory we have in physics? Several predictions were made purely based upon mathematical arguments, without much physical input. So your argument that math were a "different realm" with no connection to reality is full of, uhh, holes.
In fact, I would argue mathematics is mainly a language that provides us with the precision that everyday language lacks, it's a tool that allows us to make precise arguments while ruling out logical fallacies.
In the present case it allows us to say very precisely what we mean by a hole. This definition[0] has been employed in countless predictions in physics and its usefulness has been confirmed by experiment. Meanwhile, your definition is vague at best.
[0]: Look up "simple-connectedness" or, more generally, "homotopy groups".
This straw man was not my argument. My argument is that circles only exist mathematically.
But it doesn't matter. The definition of a hole doesn't rest on the definition of a circle.
No I don't, and this is still a straw man.
1) If a hole is made of space, it doesn't follow that space is a hole.
2) Space is not spacetime.
Yes, it absolutely does, so long as there is no category error, and there is none here. The Universe is just an inconceivably large, curved hole with a tiny little bit of stuff in it.
> 2) Space is not spacetime.
Einstein and I disagree with you. Prior to Einstein's work, space and time were independent dimensions. But over 100 years ago Einstein showed that relativity of motion mathematically combines space and time into one, spacetime. For all intents and purposes, anyone talking about space today under any circumstances is also talking about spacetime whether time is specifically mentioned or not it is always there along with space.
There are lots of different definitions of "hole", and I don't think black hole is misleading to the layfolk. They're typically dark, like some holes. And like some holes, stuff falls in and generally doesn't survive the journey. To the physicist, they know that the words "black hole" just reference a physical object with lots of mathy description.
You can say black holes are holes in spacetime with a big rock at the bottom.
The more I've thought about this, the more it seems wrong. Black holes are not a hole in spacetime like a puncture hole in fabric. There is no missing spacetime like there would be missing fabric. Black holes are more accurately a round 4D valley in spacetime rather than a hole in spacetime.
Who said puncture holes? Ground holes also exist you know.
this reasoning also applies to "infinity". it literally means NOT-finite. it refers to the lack of a thing: namely a biggest number.
then again, freaking language and maths sure make us able to think about these 'lack of [blank]' as if they were actual things (to the point that abstractly they are as real as it gets)
"Infinity" applies to the reasoning of what a hole is, but not to what a black hole is. A black hole is not not something. It is something, yet a hole is not something. Unlike a hole, a black hole is defined by its inherent characteristics, mass, spin and electric charge. A hole can only be defined by characteristics of what it is not, the empty volume of missing substrate.
my larger point was about how language 'creates' such real abstractions; sometimes with terrible names (your original complaint).