Why are black holes so special? [closed]

Question

When reading about black holes, I have also read about things relating to black holes like holographic principles, parallel universes, black hole connecting multiple universes, etc. But black holes are just very dense stars which bend space-time like ordinary stars. (It just bends more because it's much denser). But still, why do we see black holes as a portal or holographic stuff when in the end it's just another star?

Answer 1

why do we see black holes as [...]

Well that's the first issue. We cannot see black holes. Light cannot escape the event horizon, so talking about anything that goes on inside is hypothesis (more or less compatible with available experimental data depending on which theory) or speculation.

black holes are just very dense stars which bend space-time like ordinary stars.

Black holes are not strictly speaking stars, they are regions of spacetime where gravity is so strong that not even light can escape. Black holes of stellar mass are expected to form when very massive stars collapse at the end of their life cycle, making the connection between gravitational collapse and black holes. But still, there is no nuclear fusion going on, so you can't classify them as stars. They might be a possible ending of the evolution of some stars.

But the spacetime is not just "more curved" than usual. At $r=0$, the curvature is infinite. This is what is known as a singularity. For that point, GR gives $\infty$. When a theory gives $\infty$, it usually means it has reached the end of its applicability (see e.g. the ultraviolet catastrophe which, among others, spurred the invention of quantum mechanics). Hence the search of extensions or alternatives to GR.

Answer 2

But black holes are just very dense...

They actually do not need to be dense if they are very massive. There is quantity called Schwarzschild radius, which tells you for the given mass what is the radius of created black hole: $$r_s=\frac{2G}{c^2}M,$$ with standard meaning of the symbols. You can rewrite this using average density instead of mass and you will get: $$r_s=\frac{2G}{c^2}\frac{4\pi r_s^3}{3}\rho \Rightarrow r_s\approx\sqrt{\frac{c^2}{8G\rho}},$$ which means, that for example if you fill ball of radius around $100AU$ with air of density around $1\text{kg m}^{-3}$ you will get a black hole. (A lot of approximations are going on though, but the main idea of creating black hole by filling huge area with low density material should be valid)

But still, why do we see black holes as a portal or holographic stuff when in the end it's just another star?

The thing is that black hole is not really a star or any object in the usual sense, it is just some area of universe which is curved by gravitation so much, it cannot communicate with outside word. That is all. And it is very important and general property.

Note: the actual definition of black hole is probably more complex than just inability of communication - I am not sure about that. But for the purposes of the answer, I think what I wrote suffice.

I have also read about things relating to black holes like holographic principles

I am by far not an expert on holographic principle, but from what I understand, the whole principle tries to solve problem of information lost in black hole. There are some theorems in general relativity which tells you, that there is very few information that are left once the event horizon forms. Some information remain like mass of the black hole, or its rotation or charge, but the information wheter this mass is result of sending a book to the black hole or a rock is supposed to be lost. In quantum physics however information is conserved and cannot disappear and it is thought that in quantum gravity this conservation should still remain valid. Holographic principle tries to solve this paradox. So it is rooted very deeply with black holes as this is where the information paradox arises.

parallel universes

I do not know the context in which this was written, but remember that black hole is not an object but area of spacetime which cannot communicate with outer word. This area might be area between two (or more) parts of the overall universes. The two parts cannot communicate between each other though as the signal would need to go through the area from which there is no escape. Therefore each part is independent and we can call one part the universe and other part the parallel universe. You can google maximally extended schwarschild solution, or Kruskal-Szekeres coordinates to get more precise informations. I do not know, wheter you will find anything useful at layman level though.

black hole connecting multiple universes

Again the same theme as in parallel universes, but this time the black hole is not black hole in both parts of the universes, but black hole in one and white hole in another. Just like black hole is part of the universe that cannot communicate with outside world - or in another words part of the universe that cannot send signal outside, the white hole is part of the universe to which no signal can be sent from outside (and all the signal sent from inside must travel outside). This simply means, that any signal sent inside from "our" universe cannot return back (because it is black hole), but the signal inside can go outside in another universe (because here it is white hole) and thus you have a portal. Again you can google maximally extended reissner-nordstrom solution for more precise information.

Answer 3

I would like to answer with a quote by Subrahmanyan Chandrasekhar, a Nobel laureate notable for his contributions to the study of black holes:

Macroscopic objects, as we see them all around us, are governed by a variety of forces, derived from a variety of approximations to a variety of physical theories. In contrast, the only elements in the construction of black holes are our basic concepts of space and time. They are, thus, almost by definition, the most perfect macroscopic objects there are in the universe.

Answer 4

Unlike in electromagnetism, the Einstein's gravitational equations are non-linear. This non-linearity becomes apparent when the field is strong and causes effects of the orbit precession, time dialtion, frame dragging, gravitational lensing and others. The field becomes especially strong near neutron stars, but reaches the extreme in black holes. For this reason black holes are perfect objects to study gravitation, understand its unique effects and validate the Einstein's theory of General Relativity.