Horizon Definition and 446 Threads

I am reading the textbook by Ryden and I just ran into a statement that puzzles me (on page 201 if you have it at hand): "For instance, we found that, in the absence of inflation, the horizon size at the time of last scattering was ##d_{\rm hor}(t_{\rm ls}) \approx 0.4\,\rm Mpc##. Given a...

The event horizon, or schwarzschild radius for a black hole with the mass of the Earth is 3 km. But according to http://hubblesite.org/reference_desk/faq/all.php.cat=exotic, objects would have to be as close as about 6.2 miles (10 km) to the black hole's center before they began spiraling in...

I am currently reading "Our Mathematical Universe" by Max Tegmark. From the book I learned a couple of things I find startling: 1. The event horizon, beyond which galaxies run away from us faster than light and hence cannot be seen by us, is about 14 billion light years away. 2. Our...

say you fell beyond the event horizon of a black hole can another black hole passing close enough pull you out?

I'm looking for proof that Wolfgang Rindler coined the term Event Horizon. I believe that it was Rindler because that's what I've heard from reliable sources, e.g. http://www.mathpages.com/home/kmath339/kmath339.htm Thank you in advance.

Theoretical question... If an extremely long object approached the event horizon, let’s say a torpedo shaped craft 1 mile long. The craft would get stretched and the observer’s time would become dilated as the craft/object approached the event horizon correct? Would it ever be possible for an...

http://arxiv.org/abs/1407.0989 White hole anyone?..^^ "...intense gravity creates a horizon, but it is not an event horizon. It is locally like an horizon, but not globally. So, matter is trapped for a while, but not forever; it is called sometimes a “trapping” horizon."-CR

In "The Hidden Reality", Brian Greene mentions almost off-hand that inside the event horizon of a black hole, space and time are reversed. But no details are given. What, precisely, does this mean? Does it mean that in one's equations, if one is using a (-,-,-,+) signature, then everything...

So according to Stephen Hawking, non-penetrable event horizons don't really exist. So by the same argument the cosmic event horizon can't exist either right? Only an "apparent" horizon which may hold information from outside the visible Universe for a short while until it enters the visible...

Although the subject line might seem to put this question inside general relativity, the reason I put it in quantum physics is because I would like to know what happens when one treats a singularity as a particle. Obviously from outside the event horizon, one cannot do this, but inside the event...

Hawking said that the horizon area of a black hole never decreases and illustrated that in his Hawking Are Theorem: dA/dt ≥ 0 Does anyone know why is it like that. Why doesn't the area decrease?

This is amazing. Dieudonne, Erdos, Atiyah - it's mathematical heroes galore. (heart)

Hi friends, I was wondering about the following - in GR texts we always see these penrose diagrams and some line representing the horizon and all these timelike , spacelike curves and all that ... but the picture that I have of GR is just that of a smooth 4 manifold endowed with a metric . Can...

Lets say we have a couple objects, one very close to a black hole, one a little farther out and then one a good distance away. Would the one very close to the black hole see the other ones time moving much quicker than their own time? How about just as they are going to hit the event horizon...

I have a doubt/question/idea what ever it may be some thing like this Theoretically is it possible if we place one of twin electrons(Quantum entanglement)into event horizon of Black hole and observe the second one on earth, so that what is happening in Black holes? i.e how electron's inside...

I have read that one of the problems with the big bang theory, which inflation solved/explained, is that although the universe seems to be isotonic, the different regions are so far apart from each other that they would have been unable to "talk" to each other, and so how is it that they "know"...

I have always assumed once we pass through a black hole's event horizon, we can throw away the return ticket. But I have been thinking - is it really so final? Consider two identical black holes close to each other with their even horizons overlapping, or nearly so. Is the spacetime on a...

Here's the question: One of the brightest stars in our night sky is a red supergiant with a mass about 10× the Sun’s mass and a radius about 1000× the Sun’s radius. At the end of its life it will explode as a supernova and then collapse and become a black hole. How large will the black...

Why does time stop in the event horizon of a black hole? Or at least slow down, from what i know.

Lets say I fall into a super-massive black hole's event horizon. Facing outwards towards the event horizon (with the singularity directly at my back), turn on my flashlight. Will those photons emitted by my flashlight actually be able get any further away from the singularity than I was...

Hi, I read a while back that astronomers were able to view an old supernova explosion using a technique that involved looking for light reflected off some hydrogen gas. After thinking about this for a while, I decided to come here with my question. Is it possible to use this technique to view...

When I look at a minkowski diagram for a black hole I can see that time goes to infinity for the outside observer while the infalling object approaches the black hole. That means that for an outside observer, it takes an infinite amount of time until the infalling object reaches the event...

What are the odds that there is a different event horizon for dark matter?

Hello, I am trying to draw an analogy, which just came with a flash in my mind. Please clarify me, if it is wrong. Kindly note that it is an analogy only. An event horizon -- Where the light emitting is not strong enough to go inside the black hole also it cannot go outside the zone and...

If you click on Jorries calculator http://www.einsteins-theory-of-relativity-4engineers.com/LightCone7/LightCone.html you see in the default table that immediately comes up (without your doing anything) that the present-day CEH is 16.5 billion ly. What does that mean to you? How do you picture...

Homework Statement I got a problem wrong on a quiz and I'm pretty positive on all the other questions as being correct but maybe this one.. How does the gravitational force at the event horizon (Schwarzschild radius) behave as the black hole mass increases? Homework Equations...

I just registered and am not entirely sure this is the right section of the forum to ask, but regardless... My understanding of gravity is that it is a wavelike distortion in spacetime which travels not instantaneously but at the speed of light, and may also be describable in terms of a stream...

From a distant frame of reference a falling object never reaches the event horizon due to time dilation. If I drop a meter stick into a black hole lengthwise I should see both ends of the stick getting asymptotically closer and closer but never reaching the horizon, thus the stick should appear...

Hi every body! I'm a new guy here, and want to applologize for my Not perfect English. From the name of the topic you can understand what 'm trying to do here. ok here is an exercise: We have tennis ball m=57gr. and diameter of the ball D=6.7sm. from the start point coordinates Xo=0 & Yo=0.3m...

Hi, I'm new and uneducated. Would like to know how to reconcile space-time being considered asymptotically flat near the event horizon and beyond, with the observable fact that stars do indeed orbit the black hole. I understand those orbits are not near the horizon, and I believe all the...

Basic question I'm sure, but here goes; If the following is correct; An object may be considered to be 'at rest' when there are no inertial forces acting upon it (ie; it is not accelerating). A satellite is at rest because it is in freefall. A person standing still on the Earth's...

hi some black holes are inner and outer horizon! what's this means? inner and outer? what happen between them?

From previous threads I have understood that crossing the event horizon of a supermassive black hole is nothing very unusual for the falling observer locally. Usually in these considerations the falling observer has been thought as a "point" without much dimension. How about if Earth (and...

Wikipedia says, "Likewise, any object approaching the horizon from the observer's side appears to slow down and never quite pass through the horizon, with its image becoming more and more redshifted as time elapses." So does this mean from the outside observer's perspective, that nothing ever...

I've heard two hypothesis: 1. The air causes the image of the moon to literally appear larger 2. The increased size of the moon comes purely from illusion of perspective Which one is it?

Why is it said that we won't notice anything special crossing the event horizon of BH? I agree that there is no singularity and the curvature there is not infinite etc. but inside the event horizon everything moves only in one direction, towards the singularity r=0. Therefore it's possible to...

Where is the boundary that light will never reach us, and will light seem to stand still at this boundary if so will there be a cosmic ring of light from some observer?

Homework Statement The maximum proper distance a photon can travel in the interval (0,t) is given by the horizon size h(t) = R(t) ∫0t dt' / R(t') Show that, for a matter dominated universe h(z) = H0-1(1+z)-1(Ω-1)-1/2cos-1(1-2(Ω-1)/(Ω(1+z))) for Ω>1 = 2H0-1(1+z)-3/2...

We start by defining two coordinates ##u=t+r^*## and ##v=t-r^*##. Then we define another two coordinates ##u'=e^{u/4GM}## and ##v'=-e^{-v/4GM}##. But from what I have understood this is true for ##r>2GM##. How do we define ##u'## and ##v'## for ##r<2GM##? I think it's ##u'=e^{u/4GM}## and...

I have been watching Susskind's lectures on Cosmology which are great. There is something that I can't wrap my head around. I know that if we look far away enough into the past (about 100,000 years after the big bang I think he said) , the radiation that is being emitted comes from plasma and...

Im sure there must be something I'm missing here. Can someone please explain to me? You're outside a black hole event horizon and you have two identical objects. If one object were released from only 1 meter above the event horizon, and another were released from a million km away...

http://news.cnet.com/8301-11386_3-57579003-76/stephen-hawking-predicts-end-of-earth-scenario/

You may be weary of the repeated questions about event horizons. The concepts are slippery. Imagine Bob and Alice. Bob free falls into the black hole. For simplicity, assume he falls along a radial geodesic with no tangential component. Alice remains outside to observe. Alice observes...

I've been told that time slows down so much inside a black hole that an event horizon never actually comes into existence and that we don't know what happens inside a black hole. Is it true that the event horizon never comes into existence - or at least, if time slows down like general...

This is a question someone asked me today and it's bugging me allot. If the acceleration caused by gravity is greater than the speed of light at a black hole event horizon then does this mean that the matter is falling at faster than light speed?

PLEASE LOOK ATTACHMENT! Star mass affects event horizons diameter.But we know that Black hole's foundation is singularity so we can understand that singularity affects event horizon diameter. Example:Imagine there are two stars, first star mass is 5 star mass second one is 7 star mass...

The universe is expanding as described by Hubble's law, which means that at a certain distance from an observer, expansion exceeds the speed of light, so all waves become infinitely red-shifted. In other words, if an goes beyond this point, no information about it can ever come back to the...

1. Homework Statement Prove the following- \kappa^2=-1/2(\bigtriangledown_{\mu}V_{\nu})(\bigtriangledown^{\mu}V^{\nu}) Given, the following, \chi^{\lambda}\bigtriangledown_{\lambda}\chi^{\nu}=-\kappa\chi^{\nu} \bigtriangledown_{(\mu}\chi_{\nu)}=0...

This is a puzzle based on a purported "refutation of GR" on http://finbot.wordpress.com/2008/03/05/no-black-holes/, which is also the subject of a subthread in this Hacker News discussion. (Btw, the Hacker News discussion contains my answer to the puzzle if you scroll down enough, so you're on...

From a GR perspective, how does the event horizon of a black hole know how to behave? Consider a simple scenario of a shell of material outside the event horizon of a black hole, in free fall. Once the material is consumed by the black hole, the event horizon will be greater, but my...