@Howwhye 1. As of current knowledge, electron has no estimated radius, only the upper limit measured. 2. Black hole can be considered a fundamental particle in a sense, the smallest possible black hole is called "planckon" or "Planck particle". 3. Look into math on black holes. The insides of black hole have imaginary time, so there is no singularity at present.

Isn't that the average density within the event horizon, rather than the density of the "singularity"? AIUI the space just inside the event horizon is very similar to the space just outside it and most of the mass is further in.

@Howwhye it is usually considered that Hawking radiation cannot be emitted without the horizon. On the other hand, there is no way in which event horizon can be formed in finite time. So, it is likely that the very idea of a black hole is hypothetical: physics.stackexchange.com/questions/21319/…

@Howwhye no, Hawking radiation has not been experimentally detected and likely will not be, unless we create a black hole in an accelerator. Any stellar-mass black hole emits much less radiation than the cosmic microwave background, so the black holes would look really black to us compared to the surrounding area (assuming they have no accretion disc or other induced radiation sources).

@Howwhye light is distorted near all massive bodies, including Earth and the Sun (distortion around the Sun has been observed). The discs around black holes are accretion disks, composed of hot gas spiraling around black holes.

@DikranMarsupial at any given time t there is no inside of the horizon. Look at this black hole space diagram: researchgate.net/profile/Jean-Pierre-Luminet/publication/…

@Anixx please can you give a full reference to the paper. The "another universe" induces some skepticism. It is also at variance with a lot of other work I have read from e.g. Susskind.

@DikranMarsupial it is absolutely standard Black Hole Penrose diagram. en.wikipedia.org/wiki/Penrose_diagram

@Anixx I suspect by "at any time t" you mean time from the perspective of a viewer outside the event horizon. That doesn't mean that the person falling into the event horizon (preferably of a large black hole) wouldn't just see relatively normal space on the other side. I'd still like a reference to the paper by Luminet

@Howwhye accretion disk has complex dynamics, but basically it is matter in the orbit of black hole that got warmed up by huge pressure and friction. en.wikipedia.org/wiki/Accretion_disk

@DikranMarsupial I just looked up for a Penrose diagram picture. You can see the "other universe" area in the Wikipedia article as well. A person falling into the black hole cannot reach the event horizon before the black hole complete evaporation. The Penrose diagram represents an idealized case of eternal, non-evaporating black hole (Schwartzschild space). In this space any moment of time is represented by a line, crossing the origin of the coordinates. Those lines that go inside the horizon cone represent imaginary time.

@Anixx you used the diagram to imply that there is no inside to a black hole, which I don't think is true, so I want to see if that is what Luminet was claiming with that diagram or whether it is your claim. You have not provided me with the reference to support that claim, just a diagram that may or may not support it (it doesn't as far as I can see). "A person falling into the black hole cannot reach the event horizon before the black hole complete evaporation." they can in hypotheticals, so that is just evasion.

@DikranMarsupial there is no inside of black hole at any given real finite time t. It can be seen from the diagram (it has a line representing an arbitrary time moment t).

@Anixx "given real finite time t" that is an obvious equivocation and doesn't mean that there is not physical inside to a black hole, just that arguments about time from our perspective run into difficulties. You still haven't provided a reference to the paper where the diagram came from. That rings alarm bells in any scientific discussion.

@DikranMarsupial here is the paper researchgate.net/publication/… and here is the article about Kruskal coordinates with a similar diagram: en.wikipedia.org/wiki/Kruskal%E2%80%93Szekeres_coordinates

@Anixx I am reading the paper now, it clearly doesn't support your contention that there is no inside to a black hole. Fig 4 is a space time diagram that depicts a black hole having space within the event horizon. It also talks of traversing this space "inside the event horizon, the radial coordinate r becomes timelike. Hence every particle that crosses the event horizon is unavoidably catched by the central singularity. "

LOL, the text below the Penrose diagram in Luminet's paper discusses traversing the event horizon and the internal structure of the black hole "Nevertheless the study of the internal structure of black holes is a fascinating subject ..." I'm calling it as a bluff called at this point and have no need to continue. I don't know why so many online forums indulge in this kind of thing.

@DikranMarsupial the paper depicts ideal Schwartzschield space, that is eternal, static, non-evaporating black hole. If the black hole evaporates, it will evaporate before the observer reaches the event horizon. The time at the event horizon corresponds to t=∞ (this can be seen on the diagram). No object can reach that point in finite time.

@Anixx that is from the perspective of an observer outside the event horizon. It is not what would be experienced by the observer passing through the event horizon. In Luminet's paper he describes this as the difference between "proper time" (experienced by the observer falling into the EH) and "apparent time" (experience by observer outside the EH). This is all standard stuff. The gravitational field means that the two observers experience time differently, but that doesn't mean that one doesn't fall into the black hole, it is just the other never sees it happen.

@DikranMarsupial no observer can pass the horizon in finite time. Before he reaches the horizon the black hole will evaporate. The text you are reading describes static Swartzschild space (without evaporation). If you see an infinity appearing in a theory, this means the theory is not applicable to the chosen scale.

O.K. so you don't understand the difference between proper time and apparent time. I suggest you read Luminet's paper again (or any of the books written by Susskind or other black hole researchers). "in finite time FROM THE PERSPECTIVE OF AN OBSERVER OUTSIDE THE EVENT HORIZON". Note Luminet's paper confirms what I wrote earlier "Crossing of the event horizon (black holes have not a hard surface) is not accompanied by any particular event." Clearly Luminet thiks the Even Horizon can be crossed, and so does more or less every other BH researcher I have read.

@DikranMarsupial Should I repeat? This paper describes static Schwatzschield space with eternal black hole. Real black hole in theory should evaporate, and will not exist all the time till the object reaches the horizon. This paper does not account for the finite time of black hole existence.

"FROM THE PERSPECTIVE OF AN OBSERVER OUTSIDE THE EVENT HORIZON" Your error has been pointed out, and you have not responded to it. Note that you are still not making a distinction between proper and apparent time. I'm not interested in attrition loop arguments, you can continue repeating yourself, but it won't make it true.

@DikranMarsupial by the perspective of falling observer the black hole also will evaporate before he reaches the horizon. Because the black hole from his perspective accelerates evaporation. It is impossible that from the point of view of one observer he crossed the horizon, but from the point of view of distant observer the black hole evaporated before that and they could cross hands afterwards. The model discussed in the text does not account for finite time of BH existence.

@DikranMarsupial you are talking about static Schwartzschield space, I am talking about real world. It is believed that in real world black holes should evaporate. This text does not take into account thermodynamic and quantum effects.

"Because the black hole from his perspective accelerates evaporation. " citation required. I note you are STILL failing to distinguish between proper time and apparent time.

@DikranMarsupial you can see the time by the clock of infinitely distant static observer on the given diagram as diagnal lines crossing the origin, it is represented by the hyperbolic angle (split-complex argument), area between the diagonal line and hyperbola. And you can see the proper time of the falling observer as the y axis. But again, this is diagram of a static space. Not counting evaporation.

""Because the black hole from his perspective accelerates evaporation. " citation required. " citation still required.

BTW, you earlier introduced the wikipedia page here en.wikipedia.org/wiki/Kruskal%E2%80%93Szekeres_coordinates . If you look at the caption for the diagram, it includes " The quadrants are the black hole interior (II)" yes, you introduced a web page that directly contradicts your claim that black holes do not have an inside. Sorry, life is too short to waste on demonstrations that Brandolini's law is true. I've wasted too much (proper ;o) time already.

@DikranMarsupial @DikranMarsupial black hole accelerates evaporation from the point of view of any observer, but by the proper time of the falling observer it will evaporate faster. The smaller a black hole, the faster it evaporates en.wikipedia.org/wiki/Hawking_radiation#Black_hole_evaporation

@DikranMarsupial the luminocity (radiated power) is inversely proportional to the square of mass.

@DikranMarsupial Since we are on a philosophc forum, I should add that anything inside the event horizon is not physical, because it cannot be established with scientific method.

@DikranMarsupial For the purposes of science, the BH behaves as if it was homogenous viscous liquid. Or if all its mass was concentrated on its shell.

Sorry, scientism is the antithesis of science. " BH behaves as if it was homogenous viscous liquid" how can it be a liquid if it has no inside, you are contradicting yourself now.

@DikranMarsupial because it depends on interpretation. If we take Kruskal coordinates, there is no inside, but at the same time for the outside observer it behaves as a viscous liquid.

@DikranMarsupial Due to the Newton Shell heorem, BH behaves equally regardless whether it is a point mass, an uniform ball or empty shell, as long as the mass is the same.