# black hole

1. ### I An observer passing the event horizon of a black hole

Hi all, I've just read this entire thread and watched the videos about black holes posted by @PeroK, which I liked very much (thanks @PeroK! :smile:). I am not particularly well aquainted with GR and my questions are concerning the often mentioned statement that an observer that passes the...
2. ### I Does Dark matter get trapped in black holes and increase their mass?

Dark matter passes through everything, but is only influenced by gravity, so in the case of a neutron star, since dark matter doesn't interfere with ordinary matter, it can just pass through it, but neutrinos might be stopped by it's density, as neutrinos can just pass through stars almost as if...
3. ### Why doesn't a real Infinite Redshift Limit occur at R+ for Kerr BHs?

As I have studied before, I found that Infinite Red Shift occurs where gtt = 0 but this exercise says that on Kerr's Black Hole it doesn't really work like that. Right now I'm blocked because I didn't find anything on the internet about it so I don't know how to show this phenomenon. Any help...
4. ### I A Planet or a Black Hole?

According to MIT Technology Review, the Solar System's conjectured 'ninth planet' might be a Black Hole. https://www.technologyreview.com/s/614441/is-planet-9-actually-a-primordial-black-hole/
5. ### A Derivation of radial momentum equation in Kerr geometry

I am trying to derive the radial momentum equation in the equatorial Kerr geometry obtained from the equation $$(P+\rho)u^\nu u^r_{;\nu}+(g^{r\nu}+u^ru^\nu)P_{,r}=0 \qquad$$. Expressing the first term in the equation as $$(P+\rho)u^\nu u^r_{;\nu}=(P+\rho)u^r u^r_{;r}$$ I obtained the...
6. ### 4-acceleration in a circular orbit around a black hole

In a circular orbit, the 4-velocity is given by (I have already normalized it) $$u^{\mu} = \left(1-\frac{3M}{r}\right)^{-\frac{1}{2}} (1,0,0,\Omega)$$ Now, taking the covariant derivative, the only non vanishing term will be  a^{1} = \Gamma^{1}_{00}u^{0}u^{0} + \Gamma^{1}_{33}u^{3}u^{3}...
7. ### B What would happen if a black hole made entirely from anti-matter merges with another black hole made from normal matter?

What would happen if a black hole made entirely from anti-matter merges with another black hole made from normal matter? Since most of the product left over from the annihilation is in the form of light (gamma rays) would the black hole lose mass?
8. ### A Understanding gravitational waves (GR)

I am reading the following paper on the basic physics of a binary black hole merger: https://arxiv.org/ftp/arxiv/papers/1608/1608.01940.pdf Imagine two black holes orbiting each other until a point they merge. As you can see in Figure 1, the wave period is decreasing and thus the frequency...
9. ### B Black holes and the first law of thermodynamics

The first law of thermodynamics states that matter can only be transferred from one state to another, and cannot be truly destroyed. What happens to matter consumed by a black hole? What happens to it and where does it go? Does the first law still hold true?
10. ### B How can string theory be proved

The fundamental building blocks of the universe is thought of super strings, if proved can solve the mysteries of the universe but if proved than how? And how can it solve the mysteries of dark energy &dark matter and black holes?
11. ### I Is time bidirectional in a black hole?

I just watched a youtube video saying that somewhere in a blackhole, time is bidirectional (can go to the future and can go to the passed), but space is one-directional. Any introduction material on this subject? Thanks in advance!

13. ### A What does the image of the black hole say about general relativity?

We all heard about the "photo of black hole". My question is: did this image show any hints of physics beyond general relativity? I read once (not sure where) that the apparent size of black hole shadow is very sensitive to corrections to general relativity. Add some minor correction and the...
14. ### I M87 Black Hole Photos and the Spin Orientations

M87 Black Hole Photos and the Orientations the Angular Momentum of the Black Hole and that of the Accretion Disk. I’m trying to understand the orientations the angular momentum (let’s call it spin) of the black hole and that of the accretion disk. Veritasium has good video on the topic , but...
15. ### I Black hole image: What are those "lobes"? [M87 10April2019]

Lobes Did anyone hear an explanation of the appearance of the lobes visible in the published image? I only caught part of the Q&A. (Visible most prominently at at eleven o'clock and four o'clock positions, and to lesser degree at three and nine) Do these type of observations generate "lense...
16. ### A Penrose diagram of black hole with a changing event horizon

Dear all, I have a question on Penrose diagrams. Consider a collapsing star that forms a black hole with a Schwarzschild radius normalized to 1. What happens in the Penrose diagram when additional matter falls in? I suspect the diagram then has to look like this : When the outer shell (second...
17. ### I Black hole orbital mechanics questions

What are some of the challenges associated with calculating orbital parameters of objects around a black hole (I.e. Orbital velocity, period, semi major axis, kinetic energy). At what point can classical physics no longer provide accurate results?
18. ### A Quantum amplitude for a particle falling into a black hole

Here we consider a black hole formed by gravitational collapse classically. We also consider a scalar massless Klein-Gordon field propagating on this background. To quantize the field we expand it in appropriate modes. The three sets of modes required are: The incoming modes, appropriate for...
19. ### B Can event horizons cross for passing black holes at high speed

Scenario: You have two black holes approaching, one from the left (A), one from the right (B), each at speed S. They are offset vertically. S is sufficiently high that they will deflect passed each other without merging. Question: Suppose the speed S is high enough so that the event...
20. ### I Uniqueness theorems for black holes

I am under the impression, there is no unique solutions to Einstein's field equations for a cosmological constant, or for higher dimensional spacetimes. Has anybody got a counter example for a solution including the cosmo constant to show there are multiple solutions, for example, i know of the...
21. ### I Massive particle equivalent in sonic black holes

I am working on a research project where I intend to describe what the Penrose process would like in a sonic black hole. I have found what a rotating (Kerr) black hole looks like in the sonic analog: https://arxiv.org/pdf/1802.08306.pdf I have also found that the analog of massless particles...
22. ### I Harnessing energy from black holes?

Whether it be through Hawking radiation, miniature black holes, or even white holes, is it possible that one day energy could be harnessed from black holes and used on earth?
23. ### I Supermassive black hole eating a stellar black hole

I'm trying to visualize what a fast spinning supermassive black hole slowly eating a stellar black hole should "look" like; how would the mass flow between the two? Could enough mass be removed from the stellar black hole, that it loses it's event horizon before entering the supermassive black...
24. ### A Unification of dark energy and dark matter

Dr James Farnes of Oxford's e-Research Centre recently proposed a new model which unifies dark energy and dark matter into a single phenomenon - a fluid which possesses "negative mass". https://news.sky.com/story/mystery-of-dark-matter-may-have-been-solved-by-oxford-scientists-11572089 The paper...
25. ### B Black Holes and Time Dilation

When we approach a black hole, the effect of gravity is such that relative to us, time far from the black hole would approach infinity as the distance to the Event Horizon approaches zero. But what happens when we cross the Horizon? How do we measure time outside? Will infinite time have have...
26. ### B Dark matter and time dilation

Hello everyone, By considering the effects of the gravitational time dilation the speed of the inner stars must be higher for the local observer than for the external one. So why the gravitational time dilation can not potentially explain the galaxy rotation curve? I already read that the...
27. ### B Dark matter is "normal" matter in black holes?

I read that 1. dark matter has to be concentrated in galaxies; 2. McGaugh & Co discovered a precise relationship between visible-ordinary matter and the calculated sum of ordinary + dark matter from thorough observation of actual acceleration of more than 150 galaxies. 3. First experiments to...
28. ### B How to annihilate black holes?

Hi Everyone, I'm interested in how to annihilate black holes (they are like blenders separating all the things apart and agglomerate like "tumors/cancers" in the universe). However, I'm new to this field and need some clarifications on some of the basic concepts like "annihilation" and...
29. ### A How Long To 'FreeFall' Into TON 618 From ISCO

So I'm trying to help someone get an idea of the 'scale' of some of the most massive black holes, but, I'm at best a laymen when it comes to this stuff. Because ISCO for a non-rotating black hole is just 3 times it's radius, and I can calculate apparent arc radius from ISCO, I could tell them...
30. ### I Do black holes accumulate neutrinos?

Do black holes accumulate neutrinos ? Do the neutrinos that fall in, decay in some way , maybe interact with the condensed matter, or just stay "parked" in perpetual loops ?