Black hole Definition and 999 Threads

I've been working through Leonard Susskind's "The Theoretical Minimum" lecture series (which are a fantastic introduction to the topics covered by the way) and a couple of his comments confused me when he was covering the Kruskal-Szekeres metric/coordinates in General Relativity. The end of the...

We know that light's speed gets slowed down when traveling through a medium, and the more dense the medium the slower light can travel (of course c remains constant, but it takes longer to travel due to the continuous scatterings, absorbtions and re-emissions). Inside a black hole, just below...

Hey so i have a naive question that i always had since i was young, but i never really could find an answer to it. Lets say you have a black hole isolated in a vacuum, in a closed system. Dark matter is supposed to be a sort of weakly interacting massive particle, which exerts a gravitational...

Homework Statement Trajectories around a black hole can be described by ## \frac{d^2u}{d\theta^2} + u = \alpha \epsilon u^2 ##, where ##u = \frac{1}{r}## and ##\theta## is azimuthal angle. (a) By using ##v = \frac{du}{d\theta}##, reduce system to 2D and find fixed points and their stability...

Question! So Alice falls into a black hole, instead of the volume increasing for the black hole, it actually increases proportional to it's area. Thus one can draw the conclusion that 3 dimensional information can be fully explained by the information encoded on the surface area at the boundary...

What is the size of the singularity? 1. Is it 0 cm? 2. Is it below Planck length? 3. Is "the size of singularity" the wrong question, such as asking "what is the length of 500 celcius"? What does that mean? From event horizon all the way, just before, the centre, is vacuum? And suddenly there...

Hello, I had a thought about the formation of white holes off of black holes and i wanted to ask if it is possible, if a massive black hole is surrounded by a lot of matter(a lot of giant stars etc.) and it consumes so much matter that even a aquasar is not sufficient enough in disposing all of...

SR implies that mass can't reach c, but mass does reach a black hole's event horizon. How is this reconciled with SR?

Dear PF Forum, In less than 1 second after big bang, baryons were created. And there's asymmetry in it. Can anyone help me? 1. Is it physically possible for a galaxy made entirely from anti matter? 2. If it's true, is it statistically possible for a galaxy made entirely from anti matter? If...

ok, hear me out on a lymph, because I'm going to either be talking crap or something that sounds crazy either way... my question is: why does the phenomena of a black hole look so perfectly circular/spherical? now here's my reasoning spheres are the shape that takes the least energy to be...

Homework Statement A sub-atomic particle is near the event horizon of a black hole. Due to the nearby gravitational field, the Ricci Curvature Tensor is changing rapidly. The particle then performs quantum tunneling. Homework Equations Which version of spacetime does the tunneling particle...

I still don't understand this fully. Can someone please explain? I saw a video where an analogy was made, the distant observer is like an accelerating observer in flat spacetime and guy falling in is like an inertial observer, is it just like the coordinates of the distant observer from the...

Can someone please explain what a black hole is? I hear everyone speaking about it but have no idea what it is. Is it something where even speed of light becomes 0? How?

Hi, Einstein once showed that if we assume elementary particles to be singularities in spacetime (e.g. black hole electrons), then it is unnecessary to postulate geodesic motion, which in standard GR has to be introduced somewhat inelegantly by the geodesic equation. I don't have access to...

I've read that a stellar-mass black hole has a lifespan on the order of 10^67 years. Does this mean that a clock which is at rest with respect to (and sufficiently far away from) a stellar-mass black hole will tick off 10^67 years before the black hole evaporates? Also, will shell observers...

Hello all, please forgive me if I seem somewhat unaccustomed to this site, I have just registered. I suffer from an illness called catastrophic thinking syndrome, simply put this means that I turn everything I read into a disaster until I become ill due to anxiety / depression . I recently...

Sorry for a bit of a sci fi question but are anti matter black holes likely, presumably they would need to come from whole antimatter stars in antimatter galaxies? otherwise they would already have destroyed themselves?

this seems like a dumb question but i just can't think of a solution. black holes can suck photons, that's why they are black. but there are charged black holes, and the EM forces is mediated by photons, so how can the mediating photon escape the gravity? also arent the gravitons (i know...

Hello to all ! I wanted to ask a question in this forum. I am french, and I have discussed this topic in a french physics forum, but with no clear conclusion. I hope I'll have another insights in this forum, which seems very well frequented. I know the underlying subject has been discussed in...

I was thinking today about black holes. I was imagining how they formed a singularity, not mathematically, but physically and I got stuck at the Planck density. It's not a singularity yet and even with the entire weight of the rest of the object on top of it, you shouldn't be able to pack more...

How the black hole acquire greater mass than any other objects in the universe...?

I need to find the vectors for time and radius that describe a space-like 4-acceleration of an observer falling radially into a spherically-symmetric black hole. Previous to this question, the values of the real time derivatives for time and radius were derived to be: dt/dτ = (1-2m/r)-1 and...

Wouldn't this explain why galaxies with no black holes are so small?

I'm currently teaching a gen ed course called Relativity for Poets. This is the first semester I've taught it, and it's been a ton of fun so far. If anyone is curious, http://www.lightandmatter.com/area3phys120.html is the class's web page with links to the syllabus and lecture notes. The...

Which is the most powerful black hole or a worm hole and why, also what is the difference between them?

The force in the centre of the earth, assuming it was a perfect sphere and the density was the same everywhere, would be zero as the pull from all directions would cancel. Why isn't it like this for a black hole? Surely the forces from each direction should cancel leaving zero resultant force at...

what does happen if the black hole died Or disappeared??

what does happen if i threw my watch into a black hole??

Would the universe end if you entered a black hole? What I mean by this is that due to time dilation would time elapse so fast for the universe outside the black hole relative to you inside it that all the stars would burn out and all that wouild be left would be other black holes?

I am confused about black hole horizons and such common statements as "light cannot escape from inside the horizon". The way I currently understand it is as follows : 1. Horizons are always relative to an observer, and what is called "the black hole horizon" is just a shorthand for "the black...

As multiple stars would collapse into a black hole, are the electrons shot outwards? Or are they converted into mass with infinite density (what a black hole is right?) Thanks.

Hello. If time is infinitely dilated at the edge of the EH, how do we observe accretion?

How close do you have to be from a black hole to experience noticeable time dilation. I always believed you will not experience noticeable dilation until you are at or in the event horizon is this correct? Also does rotation/spin of mass such as a planet decrease it's inherent gravitational...

Rovelli & Vidotto's Planck Stars describes a possible quantum black hole - white hole transition through a quantum bounce somewhat analogous to the LQC bounce. In another thread, @marcus pointed out to me that this was not necessarilly considered the most likely scenario for a QG black hole. Is...

What would happen if there was a supernova explosion near a black hole ? Would it just sit there and absorb all the energy incident on it ? Or would it simply vaporize into elementary particles ? And if it does vaporize, could the remnants give us a clue as to the quantum state of matter inside...

Black holes grow by absorbing matter, which includes galaxies and black holes. Would the growth of black holes overtake the expansion of space time and collapse the entire universe?

So I was reading threads and i came across a question on what happens when a black hole collapses. As I was reading the responses, I saw some people commenting in which they asked if the mass the black hole absorbed was released when the black hole collapsed. If it did would the black hole...

What do you put inside einstein field equation for black holes? Why is it that such black hole solution is not feasible? Isnt the schwarzschild metric a solution for black holes? How is it not feasible?

I know that black hole have strong absorb force even light can't escape,so if we get near i think we can't also escape,but i want to know what will happen when we reach inside the black hole? and what happen if two black holes meets,what will happen if it come near to earth?

I'm curious about what others think. As I believe that you fall indefinitely in a black hole, and since you don't feel the gravity when falling, you fall until your incinerated by faster moving electromagnetic radiation falling on you. But you can only see what is above you, assuming your eyes...

Is there a physical boundary that is the event horizon? Or is there not? The reason I'm asking is because texts say that the event horizon that appears in Schwarzschild's metric is a result of the coordinate choice, and it disappears by choosing some other coordinates.

(I don't know if this is the best place to ask this, if not please feel free to move it elsewhere or delete it.) A number of Quantum Gravity papers explore the evolution of Black Holes and their potential transition to White Holes, and some discuss the possibility of astronomical observation of...

I thought that this might be a concern for the standard Cosmological model? An extremely large Black Hole discovered to have existed just 900M years after the Big Bang...

If the Singularity Has Infinite Mass, How Does Merging With Another Black Hole Create "Suoermassive" Black Holes? Infinite Plus Infinite Is Infinite. No Increase.

I am fifteen and I attend High Tech High International and I have to teach a hour long lesson on Black Hole Buoyancy. And I was wondering how to do that. Can I have some suggestions or help?

Hi there, I was reading one of my textbooks and I had a thought. For a black hole, there is minimum orbiting radius of ##R_{min}=3R_s## where ##R_s## is the Schwarzschild Radius. This minimum orbit is created by the fact that in order to obtain an orbit of that radius around a black hole, you...

is a dark matter black hole possible, and what would it be like ?

How exactly did Hawking compute that black hole entropy is 1/4 that of a Planck area and concluded about the holographic principle where information of a volume is located on the area of black hole? And if there was no holographic principle, how big should entropy of the black hole be with...

Hello :) I had a question , i recently red a lot about black holes and i had a question about the Schwarzschild Radius . How does that come that π is not in the equation ? The black hole is a sphere so instinctly one could think that Pi should be in the equations.. ? P.S: I'm french and 16 so...

Hello. I'm thinking about Hawking radation and there's one thing I find a bit strange. The radiation is usually described as originating at he even horizon. The explanation makes sense and all, but still how can this locally non-special place become the seat of a local phenomenon(emitting...