Horizon Definition and 438 Threads

Hi everyone, and happy new year if you happen to be reading this tomorrow. Rather than partying, I am writing up 100+ pages of astrophysics lecture notes, which I think will take infinite time as I keep getting stuck on every other line. My current problem is with the equation for the...

What's the difference between apparent horizon and event horizon? I checked Wikipedia but I still don't understand. Could anyone give a short explanation? Thanks!

Hi everyone, I have read a few different ways of looking at this problem, and it's one of those things where I am happywith the answer, just not how to get there using proper mathematics. My lecturer described this with some complex integrals involving E (but I'm not sure what that is!) but I...

I am under the impression that the event horizon radius of a non-rotating black hole is equal to its Schwarzschild radius. Is this correct? If yes, then I have a mixed bag of questions: Is the event horizon radius always calculated using the Schwarzschild metric, no matter what model we are...

The event horizon of a Kerr black hole is often depicted as being spherical, but this seems to be a reference to the horizon as defined in Boyer-Lindquist coordinates, where horizons appear at a constant value of r. However, Thorne describes the "black hole's horizon bulg[ing] out at its...

If I flew over to a the nearest black hole with the Hubble scope on a trailer (cough), how would the performance of the scope differ from current, particularly with regards to observing extremely distant objects. In particular, when time dilation becomes extreme as my orbit of the BH nears...

The event horizon Wouldn't it be possible for light to orbit a black hole inside the event horizon? In other words, the event horizon, while being a point of no return, is not necessarily a point of ultimate doom.

I am trying to understand things around singularities and related to this I have a question. What kind of singularity is Rindler horizon? Wikipedia (Rindler coordinates) says that: "The Rindler coordinate chart has a coordinate singularity at x = 0," But if Rindler coordinates are not...

Black hole drive in the film "Event Horizon" Cheesy movie, right? A lot of fun though. For those who don't know, there is a starship in the film called the Event Horizon, which utilizes an artificial black hole drive/engine in order to allow it to fold space, although it's probably more...

Hi , There's something I don't get regarding the orbits of photons in Schwarzschild geometry. As well known, by solving geodesics equation for null rays, you get that photons can be in a (unstable) circular orbit at r=3 M. However, if you look at the causal diagram for Schwarzschild...

I recently saw a video where it was stated that black holes may have an inner 2nd event horizon where beyond it is trapped light and energy! I have searched the web for an explanation pertinent to this hypothesis but found nothing. Can anyone shed some light (if possible)?

Cosmo calculators and tabulators a primarily about the PAST expansion history and they give learners hands-on understanding by being able to vary the model parameters and see change. That's good. I sometimes notice a difference here at PF between how posters with mainly verbal understanding...

Hi Guys I've just made a sheet illustrating the horizon problem and how it is solved by inflation. I thought it might be handy for anyone interested in it, or having trouble with it. I'd also appreciate it if some of you could check it over and see if there's anything I have gotten wrong...

Varying coordinate systems in GR has given me a new perspective that may help to resolve a problem that has been nagging at me ever since I began working with GR. In every problem I've ever dealt with, a complex mathematical result describes an impossible scenario, something that cannot occur...

Is there an event horizon beyond the visible universe where the laws of physics kind of get cut off, a limitation to the reach of gravity. I ask this because if space is expanding there must be a point that if light left Earth it could never come back because the return distance is expanding...

hi, I am not a physicist so sorry if this is a stupid question, its just curiosity. how thick is the light like event horizon of a schwarzschild black hole, for instance, what the closest distance scale that an infalling photon and an escaping photon be, and whatI is "inbetween"? I've heard...

How can inflation explain the particle horizon's problem?

In the book "The Black Hole War" by Leonard Susskind, he states that a person can live past the event horizon (to a certain point, of course) in a massive black hole because "the horizon of the larger black hole would be so large that it would almost appear flat. Near the horizon, the...

I'm curious - once an object passes the event horizon the image of that object remains on the event horizon only to become more redshifted rather than dissipating. two questions: 1) why does the image remain if the light stops traveling? if the light cannot travel to the observer, than there...

We currently have a thread about logarithmic corrections to the basic black hole entropy formula. I was thinking about attempts to relate the magnitude of dark energy to the area of the cosmological horizon, and about the various analogies made between the cosmological horizon and the horizons...

Hey guys this is my first post so I hope this doesn't come out stupid since I only know slightly more than a layman. Recently I have been trying to wrap my mind around what happens inside an event horizon. Specifally I have been confused with the matter which resided inside the schwarzchild...

Hi, I've heard a fair bit about the event horizon of a black hole. What exactly is the event horizon? Thanks,

Good morning. I am wondering what is the nature of virtual particle production beyond the event horizon of a black hole. When a particle-antiparticle pair is created from the vacuum, it takes time for them to attract electromagnetically and annihilate; but since the event horizon separates...

In class, our instructor talked about a pecularity of black holes. When virtual particles come into existence for an instant at the event horizon, sometimes one is trapped by the black hole, while the other is able to escape. My question is, what determines the rate at which particles are...

I'm not sure why the other thread was locked, unless you're banning questions? I'll ask it in as clear a way as possible. When an observer approaches an event horizon to one plank length away one of two things must happen: 1). Any observers previously falling towards the black hole...

Homework Statement Calculate the angular size of the comoving horizon at the z=1100 last scattering surface, as projected on to the current (CMB) sky. Assume flat FRW cosmology and no cosmological constant. First calculate angular diameter distance to last scattering, then the particle...

Is it true that massive observers travel with c when passing an event horizion? I know that light cones get tilted at the event horizon. But every observers travels at light speed there? thanks in advance

When classically deriving the Schwarzschild radius of a black hole, the kinetic energy of an outgoing particle (moving at the impossible-to-achieve maximum of the speed of light) is equated with the gravitational potential of the black hole at that point. \frac{1}{2}mc^2 = \frac{GMm}{r}...

What is the exact calculation of Particle horizon in Lemaître model? Does it exist? Is it finite or infinite? Can anyone calculate that integral? Thanks

It is said nothing can escape the Event Horizon, not even light. How about an entangled pair that is inside the event horizon and outside it. Would they still be entangled such that they still form correlations?

It is often said that passing the event horizon for a large black hole is basically a non event as the size of the tidal acceleration at the event horizon depends on the mass of the black hole. But let's consider something else; what happens to the stars in front of a free falling observer...

Hello, Wald defines, on page 203 the future Cauchy Horizon of a set S\subset M as: H^+(S)=\overline{D^+(S)}-I^-[D^+(S)] Where the overline means the closure of the set. D+ is the future domain of dependence (i.e. all points in the manifold which can be connected to S by a past inextendible...

As I currently understand it from the point of view of an observer falling into a black hole it takes a finite time to cross the event horizon and reach the singularity. From the point of view of a far away observer the person falling into the black hole never actually crosses the event horizon...

Hi, I am trying to show that timelike geodesics reach the Rindler horizon (X=0) in a finite proper time. The spacetime line element is ds^{2} = -\frac{g^{2}}{c^{2}}X^{2}dT^{2}+dX^{2}+dY^{2}+dZ^{2} Ive found something helpful here...

A clock falling towards the event horizon of a black hole would appear slowed down to the point of being frozen in time (or almost). But I'd like to understand properly what happens to the length contraction experienced by an observer falling together with that clock. Would he experience...

Hello everyone. I'm working on a video-game that takes place on a distant Earth like planet. A key part of the gameplay revolves around a special day and night cycle. I'd like this planet to be tidally locked to it's red dwarf star making it's habitable zone limited to the terminator. Of...

Homework Statement This is really a horizon broading, even music students and histroy students can take this course. So I think this HW problem is very simple, but it is too simple that I am afraid I will do it wrongly. Homework Equations My Dr. said we don't need to know the C++...

I am wondering which black hole horizons might be observed experimentally. a comment in another thread...(PAllen) Kip Thorne in BLACK HOLES AND TIME WARPS has a nice spacetime diagram for what he calls the absolute (current) and apparent horizons of a black hole. The apparent or...

At last someone has made a point of broadcasting that colour is in your head and not wavelength. The recent BBC Horizon programme on TV, whilst being a bit fanciful and touchy feely, made it quite clear that the colour we see / appreciate is affected by many factors and totally depends on the...

I recently read the new Scientific American and in one of the articles it says that the observable universe can see a distance of 42 billion light-years away but my question is that the big bang is predicted to have occurred at approximately 14 billion years ago, so how can the observable...

How can we still receive photons from last scattering, i.e. the CMB? Does our receiving the photons (not other evidence from CMB) require a constraint on the curvature of the universe or the speed of expansion? I can see how a curvature that described a closed universe would have CMB around...

In the popular physics books that I enjoy reading, black holes are described as containing a singularity of zero volume that contains 100% of the mass. I can't envision this, since 100% of the spacetime inside the event horizon would then be empty space except for virtual particles. Is this...

Here's a problem from an astronomy book "the top of the mountain 1000 m in height can just be visible seen from a ship approaching the land where the mountain is situated. . If the observer's eye is 30 m above sea level. Then how far the ship is from the mountain? " The main problem is i...

I have a question sort of related the size of the observable universe. I know that we are limited by the speed of light in how far we can see, but what I am wondering is... In the early universe when the first light could finally break free and travel in straight lines did it travel in...

I have a question about the horizon size of the empty Milne Universe. The empty Milne Universe expands linearly with time. This implies that the horizon size for the Milne Universe is given by: d_H = Integral [ t=t_early to t_0 ] ( dt / t ) d_H = log t_0 - log t_early Thus the...

Are there any known metrics in which black holes do not have the Schwarzschild radius? Specifically, I'm interested in whether it's possible for a black hole to have an event horizon which is not of the form: constant * mass.

If the postulated Higgs field has a non-zero vacuum expectation, shouldn't the typical black hole be ingesting Higgs particles at an alarming rate?

What is Expanding Horizon of a black hole? Recently, I want to read something general on Expanding Horizon...

The Usselo horizon is a well known dark sandy or peat layer extending from west to central Europe. It's from Allerod age and it's described well here by some leading Dutch researchers. Unfortunately the nestor of Dutch Palaeobotany and Palynology reseach -specialist on the Usselo horizon -...

Ok, although I had hoped to avoid having to ask a stupid question, it seems as though I'm just too dense to figure this one out on my own. Generally I can, with the help of Google, and the combined wisdom of the internet, deduce an answer. But either Google, the internet, or my brain has failed...