Horizon Definition and 438 Threads

Hi, one of the problems that inflation right after the Big Bang solves is the horizon problem. While this post is not really related to inflation, I was wondering why a closed universe is not a more favorable candidate for the solution to that problem, rather than inflation. Perhaps I...

From a far away observer, thing falling into a BH takes infinite time to cross the horizon. At the same time, the horizon radius is proportional to BH's mass. But if we never really see any energy fall into a BH, how did it acquire a horizon in the first place as seem from outside? i.e. What...

I understand why you would be ripped apart if you enter a black hole, but I don't understand the fact that if the black hole is large enough, You would not be ripped apart if you passed the event horizon. And, some black holes you would be ripped apart outside of the event horizon? Can...

What would you see if you were on an aircraft near it that could hypothetically survive? Would you see light frozen in time, but technically gone? By that I mean would you just see an afterimage of it even though technically it's gone? Or Would you see light just vanish into the black...

I've got two questions here 1a What is the definition of an event horizon of black hole? In Carroll, he defines an event horizon as a boundary of causal past of future null infinitiy. What is the physical interpretation of that? 1b What is the difference between event horizon and Killing...

So, I remember once watching a lecture series on relativity. In one of the lectures which discussed black holes, the lecturer spoke of what it would be like to actually fall into the black hole. Of course, he did the whole talk about the outside observer seeing the person falling in slowing to a...

Hi, If a particle falls radially into a Reissner-Nordstrom BH with Q<M (i.e the case where you have an inner and and out horizon), then where in the conformal diagram does the test particle cross the inner horizon r_{-}? See for example the righthand Penrose diagram of...

In every animated depiction of a Black Hole, we are lead to believe that the Event Horizon forms along the Equitorial Plane of the Black Hole. Is this true, or can the Event Horizon form anywhere around the Black Hole?

As I understand it, time (as seen by a distant observer) near event horizon of BH slows to "zero". It makes me wonder how long (as seen by a distant observer) it takes for matter to fall to the event horizon. I would guess this would be calculated via some appropriate integral (I am not...

Hi, I think I have I little misunderstood about these three concepts: Ecliptic Celestial equator Horizon Can someone explain me the difference between them, specially between the second and the third ones? Thank you carlos

Can a freely floating observer do an experiment that tells him when he is crossing the event horizon?

Does an infinite plane Have a horizon, forgive me if I'm asking it wrong.

Hi, I'm trying to figure out how inflation (just deSitter) solves the horizon problem, but I am stuck. I understand the solution in terms of conformal coordinates, allowing for a negative conformal time let's the lightcones of CMB intersect. Fine. But how do I see "physically" what is going on...

Let's just say I was on a ship in the middle of the ocean. I am at a certain Longitude and Latitude with a Right Ascension (RA0) and Declination (DEC0) looking straight up into the heavens. If I look East how many degrees (RA and DEC) difference from my initial location to the horizon? Also...

Consider a double slit experiment at the event horizon of a black hole, with 1 slit on each side of the horizon, one observer inside and 1 outside, inside observer should observe interference by equivalence principle, whereas outside one should not, since the photons can't enter the second slit...

We have talked about black hole, Unruh and cosmological horizons in these forums...I am wondering if a simple example [thought experiment] might usher in an introduction to such rather abstract horizons: the sphercial hollow shell of matter...from the outside, a test particle is attracted...

hello, what does exactly mean geometrically that time and space switch roles at the event horizon of a nonrotating black hole?. I understand that the - for time becomes a + and the + for space becomes -, but how to interpret it geometrically? also I want to know if after the event horizon...

does anyone actually know what happens when something reaches the event horizon?

It takes an infinite amount of time for any particle to cross the event horizon of a black hole, from our point of view as an outside observer. Which means that since the big bang, not a single particle has ever crossed the event horizon of a black hole. They just come closer and closer to it...

Hello, I know that time dilates while approaching the event horizon of a black hole, but explanations failed to make me understand HOW MUCH of each phenomena causes this as an object approaches the EH. On the one hand there is photon delay due to gravity acceleration approaching the speed of...

Well, with some degree of uncertainty that this thread is going to start any kind of conversation that could go anywhere – I am not really posing any questions, I’m not sure if it is proscribed or even intended that all threads should start with a question – I wanted to mention something I saw...

Don't know where I picked it up, but something indicated to me that inside R_eh = 2GM/c^2 lies a black hole whose R_bh = GM/c^2. And that at R_bh lies the energy singularity. And that at R_eh there is not an energy singularity, but only an end to communication with the world outside. I...

Lets say for a place that is at a latitude of 55 degrees, how high above the horizon the sun would be on the vernal equinox, summer solstice and winter solstice. Any hints on how to figure this out would be helpful I Know Vernal Equinox-When the sun crosses the Celestial Equator moving...

The Schwarzschild metric, described in Schwarzschild coordinates, has a Killing vector \partial_t. This vector is timelike outside the horizon, but spacelike inside it. Therefore I would think that a Schwarzschild spacetime should not be considered stationary (which also means it can't be...

Here is an example, where time dilation is coordinate-independent. Alice freely falls into the rotating BH. When she crosses the second horizon, she stops and hovers (enjoying the spectacular view of the naked ring singularity). As ring actually repels in some places, she would be safe and...

Hi everybody, The framework of infitesimally thin shells is the well known Israel junction formalism. Let us suppose motion of a thin in Schwarzschild spacetime. I mean both side of the shell is desribed by Schwarzschild metric. Let us suppose that the shell consists non-massless...

Here I had been thinking the Unruh horizons and specifically black hole horizons were the only (thermal) radiators. Now I notice Leonard Susskind says in THE BLACK HOLE WAR that the cosmological horizon also radiates...but "inward" towards us. If a thermometer on a string were extended close...

Perhaps you know the question what the captain of a spaceship trapped inside the black hole event horizon shall do in order to maximize the time left to being sucked into the singularity. I know the following idea (and I used to believe it over years :-) The geodesic equation of...

Hey folks I'm totally knew to this forums but i assure you I'm a science junky (especially Physics) I figured i post my first question here as it seemed pretty general so I'll give it a go I've heard many statements about the universe being finite, in fact, it seems to be regarded as truth...

I'm having no end of trouble with this seemingly simple problem: Homework Statement What's the minimum mass of a black hole for which you could survive a fall through the event horizon without being ripped to shreds? Why would you be ripped to shreds for smaller black holes? Homework...

Homework Statement Show that a black hole's event horizon will never shrink due to a Penrose process. Homework Equations M_{ir}^2 =\frac{1}{2}(M^2 + \sqrt{M^4-J^2}) The Attempt at a Solution It is easy to show that the irreducible mass will never decrease, and I know of a result that...

Consider an electron traveling through space at 99% of the speed of light. It passes within the event horizon of a black hole. We know that it can not escape. This implies something to me and I am wondering if it is correct: Since it does not come out, that means that the acceleration by the...

Sorry for what may be very basic and unscientific questions, I'm a brand new poster! Regarding black holes: It is my understanding (please correct me if I am wrong) that if an astronaut were to approach and enter the event horizon of the black hole, those watching from the outside would...

as "tidal gravitational forces" ripple outward, does that not change the curvature of space, making the region around the black hole very dynamic? so wouldn't the event horizon not be a sphere, but rather a wavy dynamic structure? because the "measured gravity" at a specific radius out at any...

Cosmologists seem to refer interchangeably both to a big-bang origin just beyond the limit of present observation, and to an observational horizon where the rate of expansion of the universe exceeds the speed of light. The notion that looking out in space entails looking back in time gives...

How can i find out aircraft's bank angle knowing it's local frame in horizon frame? Specifically, the horizon frame is y for up, z for north, x for east, the plane frame is x for right wing-wards, z for nose-wards, y for up. I know unit vectors of plane frame in horizon frame, and want to...

Homework Statement Find the acceleration of the crate when it is pushed by the same force at an angle of 30 degree below the horizon. Given mass = 2.0kg Applied Force = 10 N, and uk = 0.047 Homework Equations Fx/y = ma = sum of all force vectors Fk = ukNThe Attempt at a Solution So I did...

In a spacetime with a black hole, where is the horizon in the CFT? Also, if a black hole spacetime corresponds to a thermal CFT, doesn't that mean the CFT is in a box where there is still a universe outside to provide a temperature?

Fortunately, not me. But, it's going to be for someone I know this year. It's hard to watch even though it is mostly due to her bad decisions. I have given her advice over the last four years (when she asked) but, she always finds a reason why she can't do what she needs to do. Today, I sent...

Has anyone ever considered the outer event horizon of a point particle (classical electron perhaps...)? Does it make sense to consider it Kerr and charged because of spin? Is it comparable with a Planck length? I know we would need a quantum gravity to deal with it, I'm just curious to see what...

Kevin Brown http://www.mathpages.com/rr/s6-04/6-04.htm gives the acceleration of particle in the Schwarzschild metric as measured in terms of the proper time of the particle as: \frac{d^2 r}{d\tau^2} = -\frac{m}{r^2} Does this not cause a problem for those that assert Schwarzschild...

I have recently finished "Gravity from the Ground Up: An Introductory Guide to Gravity and General Relativity". Great book, but I am confused about the behavior of time at the event horizon of a black hole. I spent some time looking at existing threads on this site, but was unable to find...

Hi! There's got to be something wrong with the following method to take photographs of the area inside a supermassive black hole, but I can't figure it out. 1. Lower your spacecraft to just outside the event horizon. 2. Extend a uniform steel bar with cameras embedded in it...

Why do we need an inflation to solve the horizon problem? http://en.wikipedia.org/wiki/Horizon_problem If O(t) is omnuim ((c) Frederik) - wavefunction of the whole Universe then initial conditions like O(t=0) = const solve the problem?

All falls out of this universe. Over the horizon. No longer 'part of this' Yet what has fallen out. Pulls more in. Thus it is part of this. What am i missing here? Is em a nice outfit clothing the brute power of gravity. The destroyer and the maker. Physics without philosophy is sterile...

Reading "Exploring Black Holes" I find that a particle following a geodesic towards a black hole always reaches the speed of light c when crossing the "event horizon" regardless of the reduced circumference that the particle begins at (1mm or 10 million light years). This would also...

Homework Statement This is what the question says exactly: Assume the universe today is flat with both matter and a cosmological constant but no radiation. Compute the horizon of the Universe as a function of \OmegaM and sketch it. (You will need a computer or calculator to do this).Homework...

I saw this question posted in a Yahoo forum. I would be interested in the answers from the people here: "We can all agree that the more massive a black hole is the smaller the circumference of the black hole's event horizon will be. Therefore, can a black hole ever have enough mass to where...

If this is in the wrong section, my apologies, but I'm wondering if it's possible, with known equations, to calculate the final velocity of matter accelerating into the event horizon of a black hole, and what those equations would be. I don't think this is an easy question, so any help is...

As I understand it, an Event Horizon is a boundary surrounding a black hole where light is held 'still'. If the photons never get beyond this boundary, how can they be observed?