Horizon Definition and 446 Threads

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?

if i ran towards the cosmic horizon (really fast) will i see it receding?

In flat space-time, an accelerated observer sees a horizon behind him. But any inertial observer sees the events behind that horizon. Around a black hole, a distant observer sees a horizon, a freely falling observer sees none. In all these cases, one observer can say "there is nothing behind...

Black holes, Time technically stops after the event horizon correct? So, Let's say, We had a string.. That cannot be ripped, broken, streched, or anything that would harm or alter it's position. Let's say, It was 20,000 Light years long and on each end of it was a black hole pulling. Now, We'd...

I would be grateful if someone could point me to a description of how a sphere of freely infalling matter ---- say equivalent to that of a collapsing massive star --- generates an finite-sized event horizon, as observed far from the incipient black hole. I can only imagine that the event...

Some thought experiments and questions. I would much appreciate any comments on whether my analyses below are correct. 1. I am falling feet first into a black hole. An external observer will never see me fall past the event horizon. However, in my frame of reference I will fall past it in...

Given present theory, how would a laser photon stream behave at a tangent point to an event horizon (EH)? Is it possible for a photon stream to orbit a black hole? Could the beam be split at the EH with one branch spiraling into the black hole while one branch follows some geodesic (perhaps not...

Special relativity shows that any accelerated observer sees an event horizon. In fact, if an observer is accelerated by a, the horizon is at distance l=c^2/a in the direction opposite to a.

Special relativity shows that any accelerated observer sees an event horizon. In fact, if an observer is accelerated by a, the horizon is at distance l=c^2/a in the direction opposite to a. If an observer is on a carousel or merry-go-round, he is accelerated inwards. Does he then see a horizon...

Here's what for me was a fascinating (and straight forward) description of How Jacob Beckenstein concluded information is displayed on the horizon of the black hole. The formulas are so simple and the results so profound I wanted to post them here...I'd never seen this before... This is...

Kip Thorne says the following in his book BLACK HOLES AND TIME WARPS,1994, page 417, It may be "simple", but that explanation sounds flimsy at best...can anyone help me understand it? How did effect precede cause in classical general relativity or does this have some other origin? It...

Hi, I'm totally lost on how inflation solves the horizon and flatness problem. Flatness Problem Explanation I have d/dt(1/Ha)<0 and therefore |\Omega-1| is driven towards zero rather than away from it. My Confusion Doesn't inflation increase the volume of the universe...

'Poking' the Event Horizon Forgive what is probably a ridiculous scenario, I was having a discussion earlier :smile: We have Black Hole A and Black Hole B of completely (or near) equal gravitational field strength. http://img148.imageshack.us/my.php?image=blackhole1.gif A...

According to an outside observer, an object slows down as it approaches the event horizon but never reaches it. Why does it slow down? Thanks.

blechman's statement: Gravitons are emitted FROM THE SURFACE of the event horizon (remember: gravitons are massless, and therefore move at the speed of light). THAT's what we see. There is NO information (gravitational or otherwise) that can escape from INSIDE the black hole. This is not a...

This question may have been asked before becuase it seems like one of the first questions that would be asked after learning of black holes. My question is, how do gravitons escape the event horizon? They must somehow escape, otherwise the black hole could not influence anything with its...

In another thread it was said: (my bold) In Rindler coordinates a "Rindler horizon" forms at a fixed distance below the Rindler observer (accelerating uniformly "vertically upwards" and Born-rigidly in flat spacetime, in the absence of gravity). This horizon has many of the properties of a...

Is there a theory about the relationship, a ratio perhaps, for a Black Hole's event horizon dia. and its actual internal mass dia.? It would seem that there must be a physical mass in there somewhere, of some particular size for a given mass. This ratio may vary proportional to the mass of the...

Consider a particle that has fallen inside the event horizon of a black hole. You can show that it must have a minimum radial velocity that scales as \frac{1}{\sqrt{r}} for small r. Where, by radial velocity I mean \frac{dr}{d \tau} and tau is the proper time. Doesn't this mean that as...

I posted a similar question under cosmology but the question was unable to be answered. I thought I would try a reframe the question. When approaching a black holes event horizon, the exit cone for light become smaller until it is eliminated at the event horizon itself. But how can gravity...

Hello! In New Scientist this week (actually next week!), there was a question concerning the curvature at the horizon. The formula is as follows for the distance to the horizon: (2*6373*h)^1/2 km; where h is the height of the individual from the ground. Using the exaple it states a towe...

A question that's been bugging me: Why doesn't the gravitational influence of a black hole end abruptly at the event horizon? Supposedly nothing can escape a black hole, so how does gravity itself escape? Light cannot escape a black hole, and gravity travels at the speed of light...

Suppose this happens: 10 billion people are taken to a large black hole. The people are thrown into the black hole as we fly around it in the large spaceship. I have read that when an object goes past the event horizon, we as observers away from the black hole will see the people freeze...

My understanding is that, according to the Equivalence Principle, accelerated motion is indistinguishable from that of being in a gravitational field. So then, at the Event Horizon of a black hole, where gravity is too great for light to escape, wouldn't that be equivalent to accelerated motion...

I am struggling with an understanding on what the longest proper time an observer can spend before he will be destroyed into the singularity. How should I approach this problem?

My question is: Photons that orbit at the event horizon, require relativistic time to circumvent the circumference of the black hole relative to an observer external to the black hole, keeping Mr. Einsteins second postulate in mind, that being the velocity of light is constant through vacuum (...

I am not clear on how |g_{rr}| \neq 1 in the Schwarzschild solution manifests itself. If I approach the event horizon of a black hole while holding a meter stick "vertically", that is, with d\theta =0 and d\phi =0 between its endpoints, will it appear to someone far away (r \gg r_{s}) that its...

The BBC are http://news.bbc.co.uk/2/low/science/nature/7440217.stm" by Erickcek, Kamionkowski, and Carroll of Stanford in which it is stated that: "There is an anomaly in the CMB: measurements from the Wilkinson Microwave Anisotropy Probe (WMAP) ... indicate that the...

I wonder about something. How big would the event horizon of the universe be when all matter was infinitely packed together?

This is one question in a past test without solution: Calculate the size of our horizon in an omega = 1 Universe. Give your answer in light years or kpc. Thanks.

(the story so far … I maintain that, inside an event horizon, there is a useful distinction between "space" and "space-like" dimensions, and that in any realistic coordinate system, time is space-like. JesseM maintains that, in any realistic coordinate system, time must be time-like. JesseM...

I'm currently in the process of understanding the properties of the Cauchy (or inner event) horizon in a rotating Kerr black hole. Initially, I thought the horizon was a result of centrifugal forces from the rotating ring singularity and extreme frame dragging that forced matter outwards...

Researchers at St. Andrews University, Scotland, claim to have found a way to simulate an event horizon of a black hole - not through a new cosmic observation technique, and not by a high powered supercomputer… but in the laboratory. Using lasers, a length of optical fiber and depending on some...

Homework Statement The following statements may or may not accurately describe what you would find just outside the event horizon of an extremely massive black hole like Gargantua (fake black hole), at a distance corresponding to an orbital circumference of 1.0001 times the horizon...

http://upload.wikimedia.org/wikipedia/commons/1/1d/Event-horizon-particle.svg Would there still be a t axis? Would the yellow cone get squished (become one-dimensional) and stretched over the x axis? Or would it stretch to infinity along the t axis and spread over the entire (x,t) plane?

Okay, let's say I'm orbiting a black hole right near the event horizon. If I take a long stick and poke the black hole what happens? If time stops for the end of my stick as it reaches the event horizon how can I keep pushing on my end? For that matter if I jump in feet first doesn't my head see...

Hi, I'm working on a lengthy problem, and one part asks to find the height of the Sun above the horizon as a function of time. I came up to this solution: \tan (\theta+h) = \frac{\cos (\omega t)}{\tan \phi} where \theta is Sun's height below (or above) the celestial equator (i.e. -23.4...

arXiv:0711.4810 Title: Dark Energy in Light of the Cosmic Horizon Authors: Fulvio Melia Comments: Submitted to MNRAS Subjects: Astrophysics (astro-ph) Based on dramatic observations of the CMB with WMAP and of Type Ia supernovae with the Hubble Space Telescope and ground-based...

Hello, I's a spead though that black holes swallow everything in it's vicinity. But, something bothers me : when we close on the event horizon of a black hole from the outside, the gravitational field tends to be infinite, and at the surface of the horizon it is infinte. So to say, that...

For a while I've been wondering why the moon looks so big on the horizon. I looked around the web and found 2 answers: 1) At the horizon the brain thinks that it's very far way compared to when it's above your head in the sky, but since they're really the same size in both places the brain...

To Physicists/Administrators/Moderators: The following two facts raises a doubt 1) "No influence can travel from inside the Event horizon of a black hole to the outside of it AT or below the speed of LIGHT (V<=C)." 2) "Gravitational influence travels AT (or below?) the speed of light...

As the tile says, anyone's got an idea what the g-forces might be at the boundary of a black hole's event horizon? I got a formula to calculate the event horizon radius but not the the gravitational forces. Equivalence to Earth G's would be nice or in m/s2 = meter per second squared! Is it...

Is it true that if a luminous object is captured inside the cosmic horizon , then it stays in the horizon forever? And is it also true that if the accelerating expansion of the universe continues forever , then galaxies outside our local supercluster will move beyond the cosmic horizon and will...

this is a case where it could be very helpful if someone (Pervect? Wallace? hellfire?) who has the numbers handy could tell us how far away the CEH is at present according to the usual LCDM model I don't know the exact figure. I think it is somewhere around 16 Gly. that is, slightly further...

Homework Statement In my GR final, there was a question that asked to show that the horizon r=2GM is a null surface, i.e. d\tau^2=0. How does that work, since just plugging r=2GM and dr=0 in the Schwarzschild metric yields d\tau^2=-(2GM)^2d\Omega^2 which is obviously not zero. :confused: