Horizon Definition and 438 Threads

One way that people introduce the Hawking temperature of an event horizon, is by taking the near-horizon limit of the BH metric and then do a Wick rotation of the time coordinate. Then, the regularity of the metric requires that the Euclidean time to be periodic. But how can this give us the...

I read that time dilation near a black hole's event horizon causes the infalling matter to "freeze" just above the event horizon and never cross it (in a distant observer's frame of reference). Doesn't the same phenomenon prevent the event horizon and singularity from being formed in the first...

Having discussed recently https://www.physicsforums.com/threads/rod-falling-radially-towards-the-center-of-a-mass.871169/ I'm now puzzled by the question, what happens to the rod during his radial fall through the event horizon and what would the hypothetical observer at ##r=2M## measure...

Dear PF Forum, What happens inside Event Horizon? 1. Will clock stop inside EH? An object crosses the EH of a black hole around 1 billions solar mass which its Schwarzschild Radius is 3 billions km. It takes light to cross that distance (in 'normal' space) 30 thousands seconds. The time for an...

I have a question about the cosmic horizon. If theoretically I go out 20 billion light years to the cosmic horizon as there a point in space where there is a huge build up of light where space is moving away at exactly the speed of light? Is this this the 2d representation of reality they talk...

In this thread let's forget math, we don't have math to describe these energies anyway. Observation tells us that at extremely high energy densities there is some expansive force that vastly overwhelms everything else. Expansion has become the only viable explanation to what we observe. So I...

i recently learned that according to maths, the horizon for a human is about 5klms, and if this human was 100meters tall, horizon would be 40 klm away. in that case, if we are standing at the beach, how can we see a boat (or flat land) that is more than 40 klms away?

Basically, if the size of the event horizon(EH) of a black hole is R (radius) can the rate of change of this R or dR/dt be faster than the speed of light(c)? When it comes to expansion of an EH of a black hole: Does the event horizon immediately expand? For example, if one black hole consumes...

I am trying to find the declination at the horizon in the east at a particular latitude. The equation is: delta = asin(sin(lat)*sin(Alt) - cos(lat)*cos(Alt)*cos(Azi)) where Azi is the azimuth and is equal to 90 degrees in the eastern most direction. Alt is the Altitude and is equal to 0...

Here http://www.damtp.cam.ac.uk/user/db275/Cosmology/Lectures.pdf, I find on page 31 in (2.1.5) I assume that it is childish calculus that connects both sides of this equation. But still, can someone help me why the integral can be rewritten like that? thanks!

One of the benefits of inflation often mentioned in beginner treatments is that it solves the horizon problem by taking a volume which was in thermal equilibrium and expanding it so much that it is now larger than the observable universe thus explaining how the CMB temperature is so uniform...

Scenario 1: Alice drops into the event horizon at free fall and notices nothing as she crosses it. Bob looks on from the outside and sees her flattened against the event horizon. Scenario 2: Alice drops into the event horizon from the outside and notices nothing. But as she nears it, she...

its commonly stated that no particle accelerator will ever be built that can probe the Planck scale. what about an can an event horizon telescope observe quantum gravity effects near a black hole, such as Sagittarius A* do the various candidates of QG such as string theory LQG asf gravity...

Based on my understanding, Hubble sphere is the region that contains GALAXIES receding subluminally while GALAXIES outside are receding superluminally (obviously on the border, GALAXIES recede at the speed of light). Particle horizon is the limit of what we have seen so far since the (big...

This is probably something I should know already, but... (sigh). :blushing: OK, so we can't just look at a metric (e.g., black hole in Schwarzschild coords) and deduce something physically weird at ##r = 2M##, because that's just an artifact of the coordinate system -- the curvature tensor...

I was just reading a review http://physics.aps.org/articles/v9/62 of a recent paper authored by Physics luminaries (http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.231301). The paper itself is behind a paywall - and I suspect the math is more than I have the energy for. The gist...

Lets look at the known story of Alice and Bob outside a Black hole. Alice are falling inn to the hole, and Bob are outside. What would Alice observe from the event horizon of a Black hole. Looking outward. We know Bob who is outside, se Allices clock standing still, but Allices clock are not...

I am puzzled. Please help me out. Thanks. If we cross the horizon of Schwarzschild black hole horizon, we know that r becomes to timelike. Why the fututre is direction of decreasing r rather than increasing r?

I know that Killing horizon is the hypersurface on which timelike Killing vector field becomes null. Beyond that surface Killing vector field becomes spacelike. But Stationary Limit Surface has also such a property. I wonder, if they are the same thing, if so, why is there different names for...

I'm reading Cosmology by Harrison and in Chapter 21 he discussed the horizon riddle, I understood the problem that he posed but his solution was confusing. "Consider two widely separated observers, A (for Albert) and B (for Bertha). We suppose they can see each other. Each has a horizon such...

I live about 1/4 mile from the Interstate Highway. I am far enough away the traffic can not be heard until the sun is very low on the horizon. Every even for about 1 hour when the sun is very low it sounds like the interstate traffic is in my back yard. It is so loud it is just like standing...

Assume we drill a hole through the Earth - through the center towards the other side. Then we use a telescope, point it through the hole and observe stars on the other side of the earth. The telescope experiences a constant acceleration from Earth's gravitation. Accelerated movements have an...

The question is simple, and I have checked the "similar discussions" and googled, but I still come out with various replies. I also know that experimental evidence that could back up the selected choice would be tricky, not to say impossible. But at least according to theory: the entangled...

Assume a spherical black hole that is eating matter from its surroundings. Then its Schwarzschild radius will increase with a speed proportional to the mass flux that enters the black hole. The question is: is this speed limited by the speed of light in vacuum c? If the event horizon is a...

In the opening paragraph of section 6.3 Carroll defines a killing horizon to be a null hypersurface Σ where some killing vector field χμ becomes null. Later (on page 247 if you have the book) when distinguishing between static and stationary space times, he says that in a stationary, but not...

The Schwarzschild radial coordinate ##r## is defined in such a way that the proper circumference of a sphere at radial coordinate ##r## is ##2\pi r##. This simplifies some maths but creates some rather odd side-effects, so to get a more physical picture I like to use isotropic coordinates...

In various explanations of the event horizon which do not invoke the existence of a firewall (thereby upholding the dictum that an observer would not notice any difference upon passing the event horizon until she looked out the window), one uses the concept of a theoretical observer passing the...

The question is to resolve a logical conflict. GR says as we fall into a black hole, an outside observer will see that event come to a stand still as if the falling object is hovering at the horizon. This stand still extends to infinite time. Unfortunately, I've read and hear the term...

Refer to this image: http://imgur.com/xT20HOv The white nodes are 'stations' which broadcast signals at light speed. The blue circles around them represent the particle horizon relative to these nodes. The pink nodes are intermediary stations in between these stations that pass on the signal...

Hello, a long-awaited fan film has been released on Friday. I'd say it's pretty good.

The question is as follows: suppose I throw a metal bar 1m long inside the event horizon of a supermassive black hole of 1 million solar masses. At both ends of the metal bar there is a light source. (I chose a supermassive black hole to rule out any spaghettification process: with some quick...

http://cdn4.sci-news.com/images/2016/02/image_3628-Gravitational-Waves.jpg In the above image, i can see that the event horizon of each black hole has decreased in size. Does that mean that the event horizon can shrink in the presence of another black hole?

Hi. From an outside observer's view, any object approaching the event horizon of a black hole appears to slow down and never quite pass through the horizon. So information about those objects can always be retrieved (if you correct for the redshift). So what actually is the information paradox...

From another discussion, I realize my understanding of Event Horizons is quite lacking, but I am unable to find a definitive, reliable answer to fillbthe gaps. My issue is essentially that of trying to resolve the apparent conflict between an observer, Alice who remains motionless wrt a BH at...

It seems that every description providing ways to visualise or form analogies or presernt a simple model for infalling to Black Hole causal event hortizons rely on two observers, one aporoaching and potentially 'crossing' the horizon, whilst the other 'observes' from a "safe" distance. It makes...

If there is a large mass near a black hole, is the event horizon teardrop shaped? Today I was thinking if it was possible to violate the idea that anything that passes the event horizon is gone forever. I forgot to label my picture, Figure 1 is the top and Figure 2 is the bottom. I imagine a...

http://thenextweb.com/us/2016/01/22/trumpscript-is-a-programming-language-that-thinks-and-acts-like-donald/?utm_medium=email&utm_source=flipboard Some Features No floating point numbers, only integers. America never does anything halfway. All numbers must be strictly greater than 1 million...

It's been a while since I took any QM so I'm fairly rusty...not even sure that I'm asking this in the right way. How does one set up the equations to determine the characteristics of quantum tunneling if you have a particle with a particular energy inside an event horizon? For example, suppose...

If an object is an infinite distance from a black hole and falls directly to the black hole without being affected by any other force, what is its velocity at the event horizon?

Homework Statement Soon astronomers will be imaging the “shadow” of light from the event horizons of black holes. Since black holes are very small, this achievement seems impossible. Nevertheless, it is possible for three reasons: 1. Supermassive black holes have large event horizons. 2. Radio...

I have a question regarding the exact formulation of the mechanism of Inflation. In thehttp://www.damtp.cam.ac.uk/user/db275/Cosmology/Lectures.pdf he uses ##\frac{d}{dt} \frac{1}{aH} < 0## as an definition of inflation. I see that it yields ## \ddot a > 0##, but my confusion lies in the...

Hello. In oral exams my professor likes to ask if Alice and Bob can communicate, if Alice ist just above the event horizon of a schwarzschild black hole and Bob ist just below. He wants to hear: Communication is possible, because the event horizon is observer dependent. Only an observer...

Dear all, As far as I understand, for a distant observer, time stands still at the event horizon of a black hole, right? In particular, nothing will ever pass the EH. Instead, everything that approaches the BH will get stuck at the EH and stay there forever from the perspective of the distant...

Homework Statement Numerically integrate and report the particle horizon distance today for the currently fa- vored model \Omega_M=1-\Omega_{DE}=0.25,\omega=-1. Assume the scaled Hubble constant to be h = 0.72, and report the particle horizon in billions of lyr (Gyr). Homework EquationsThe...

I am by no means an expert in physics being largely self taught (day job is a medical student) but have an avid interest in trying to understand the fundamental nature of our universe. In doing so I'm currently reading the book "Trespassing on Einstein's Lawn" by Amanda Gefter and came across...

For an observer far away, nothing ever seems to actually cross the event horizon of a black hole, but to "freeze" right at the event horizon. Does this mean that if we could observe a black hole, we would be able to still see everything that has ever entered the black hole? Would every object...

Can horizon complementarity apply to a horizon between de Sitter and Minkowski space?

When I observe the sunset from my balcony, the sun appears far away near the horizon. But when I watch sunset at a beach the sun appears closer to me at the horizon. Why so?

As I understand it, the statement of the horizon problem assumes that the uniformity of the CBR measured at opposite directions in the sky needs a mechanism to create this uniformity. I also understand that many cosmologists do not share this assumption. The purpose of this thread is to seek...

I am clearly talking about black holes. The event horizon is the limit where even a photon won't escape it. I tried to calculate it in the easy way using enegry calculation m * MG/R = mc^2 / 2 but I do not know if I am using the right equation or even if I can divide by the m because it...