Horizon Definition and 438 Threads

Media Advisory: Press Conference on First Result from the Event Horizon Telescope April 10, 15:00 CEST (13:00 UTC. In 8 days and 13 hours) Livestream links are on that website. The Event Horizon Telescope is a collection of radio telescopes all over the world which recorded data from the...

A few years ago I became intrigued by articles reporting the discovery of stars very close to the purported Big Bang; 400 million years seems an awful short time for a star to evolve. Then more recently the discovery of 2nd generation - hydrogen, carbon stars - in the same proximity, supposedly...

Dear all, I have a question on Penrose diagrams. Consider a collapsing star that forms a black hole with a Schwarzschild radius normalized to 1. What happens in the Penrose diagram when additional matter falls in? I suspect the diagram then has to look like this : When the outer shell (second...

Let's say a rocket carrying a positively charged balloon starts to accelerate with a constant proper acceleration at time t. After a long time another rocket carrying a positively charged balloon is launched. The crew of this rocket drives the rocket to a position right below the...

At the event horizon of a black hole, the curvature of space is infinite. Matter falling in therefore becomes accelerated to the speed of light. General relativity says infinite energy need to accelerate mass to the speed of light. Comments please

I am reading an article, Inflation and CMBR by Charles H. Lineweaver. He explains the inflation period as the shrinking of the event horizon in the comoving coordinate system. Which it makes sense since the inflation was a period of ##\Lambda##. And In this period of time event horizon shrinks...

Can anyone please explain how inflation removes the horizon. problem of standard cosmology . As much I know inflation is a period of accelerated expansion which can be approximated by ~exp(βt) which can be achieved if the energy density remains constt. throughout (e.g ≠ε(t)),which is the case...

The furthest distance that we can see is defined by the Radius of the Particle Horizon which its nearly 46 Gly. However, the cosmic event horizon is nearly 16 Gly. Is this means the galaxies that further than the 16 Gly are just will stay the same in the sky? Since their light can never reach...

So when an object is falling towards a black hole, it's clock relative to us, outside observers, is slowing down. Until said object reaches the Schwarzschild radius and it stops completely. So how can that object ever cross the event horizon, if it is frozen in time?

@Grinkle recently asked a question about detecting crossing the event horizon, which got me thinking. I think that, at least in principle, I can deduce when I cross an event horizon with a "closed box" experiment, basically by measuring tidal forces. I'm planning to see if the maths works, but...

Folks usually point out that tidal forces make questions like the one I am asking below hypothetical at best, I understand that. I am taking as an axiom that a free-falling observer observes nothing unusual when crossing an event horizon. More strongly, the free-falling observer cannot detect...

i got a bit lost in the responses to my last question so I am guessing this one is really going to be beyond me. Assumptions I have used for my questions are: · Speed of light = 299792.458 km/s · Hubble constant = 71 km/s/Mpc (I know about the tension of H0 being 68 and 73 but...

Homework Statement For a power spectrum density fluctuations ##P(k) \propto k^n##, I need to find the scaling (with respect to ##a##) of the horizon wavenumber ##\frac{2\pi}{\chi_H}## in a matter dominated universe in terms of ##n##. ##\chi_H(a)## is the evolving particle horizon, in a flat...

Hello people, I have been thinking about a concept that I was taught whilst learning GR, If I understand correctly it is that Lorentz symmetry becomes local when we consider GR. This makes sense to me as then the metric is generally speaking not Minkowski, only for a...

I would like to know which of the following interpretations of what happens when a local observer with a non-zero mass (i.e. not a photon) crosses the event horizon of a black hole: 1. Not only does the falling observer not *notice* anything strange (because his/her clocks run proportionally...

Leonard Susskind said "everything that ever fell in, to make the black hole, [..] [is] all contained in [...] progressively thinner and thinner shells that approach the horizon asymptotically, never quite getting there" and from the perspective of someone outside the black hole "a shell, called...

In p.244 of Carroll's "Spacetime and Geometry," the Killing horizon ##\Sigma## of a Killing vector ##\chi## is defined by a null hypersurface on which ##\chi## is null. Then it says this ##\chi## is in fact normal to ## \Sigma## since a null surface cannot have two linearly independent null...

Does the event horizon of a black hole really represents the surface of the "star" (or mass) itself? What I mean to say is: That the event horizon is the (let's say it this way) sphere surface where the scape velocity is => than the speed of light. So it is not necessary for event horizon to be...

Hello physics forum. I am not very well versed in physics, so this question could be a misfire, but I just wanted to clear this up. I watched one of Susskinds holographic principle lectures. So I get that Bob would see Alice turn into a hot mush of energy as she approaches an event horizon...

Is is possible for Alice and Bob to find themselves on opposite sides of an event horizon and go about their experiments, without the fear that one of them might be mangled to death by tidal / differential gravitation effects?

My question is regarding how spacetime looks like beyond the event horizon of a black hole, in particular how distances behave. In the Minkowski diagram of a black hole, all paths leads to the singularity. But what is the magnitude of the distances involved here? Let's say a neutron star is...

Homework Statement If I am standing on the ocean shore, and my eye level is 6 feet above sea level, how far above the horizon would an island appear that is 15 miles away and whose highest point is 200 feet above sea level (assuming the earth’s diameter is 3,960 miles?) In other words, how many...

I was just wondering as it seems pretty counter intuitive that there is a really defined horizon where light can't escape from a black hole. It would make more sense to me if light gradually curved into one. Or does it do this? Please enlighten me ;)!

Hi, Assuming an average Neutrino mass of 2.0 Electron volts how much mass will a black hole with an event horizon the size of the one at the center of our galaxy accrete in a year? I've seen estimates of how many Neutrinos pass through our bodies each second and it seems like the mas would...

Suppose we have monthly totals of observed data for last 35 years. That data is of inflow of a river in a reservoir and monthly demands from the reservoir. We are interested to check the effect of construction of a dam in the upstream. The effect is, whether the downstream reservoir will have...

If you are observing a particle enter a black hole, you watch its proper time go to zero at the event horizon as it is 'frozen' there from your frame of reference. What happens in your reference frame as the black hole evaporates? While you can't illuminate where the particle is from your frame...

I have some questions regarding the temperature of empty space in a de Sitter universe or to say it better - the Hawking radiation emmited from the cosmological horizon: 1) Do particles that make up the radiation get produced by the empty space inside the patch (the Bunch Davies vacuum) or by...

How much is the length of horizon that human's eye can see at sea level? How much is the length of horizon that human's eye can see at higher level? any formula? I am not sure i should use wide or length. i mean the rightest point of horizon to the leftest point of it for human' eye angles.

Is there more to our universe than what we can observe? If so, does that mean that photons from the CMB are traveling towards us from beyond our cosmological horizon?

In a paper from 2003, it has been shown that the particle horizon is about 14300 Mpc, or 46.6 billion light years (and it has been recalculated in 2016 with more accurate parameters values at around 14200 Mpc = 46.3 billion ly). From what I understand, the calculation proposed in 2003 takes the...

Under the holographic principle, the physical description of the 3d world at a particular location in spacetime is encoded on the two dimensional cosmological horizon that encapsulates it. Does that imply that the "measurements" taking place on a 2d shell that is the horizon is creating new...

I have come across the following multi-explanations of how Hawking radiation/evaporation of a black hole happens: Particle/anti-particle story: particle/antiparticle pair creation from vacuum near the event get torn apart - one going into black hole, the other away; in some of these...

What occupies the space between a black hole's event horizon and the singularity? Stuff hurtling to the singularity? Vacuum?

I didnt understand a concept in black holes,So I ll try to make a vısualization to the process to explain my ideas properly. Let's suppose we have a star with mass ##6M_ο##.We know that this star will turn to black hole,So Let's come to the end of the life of the star.It will explode as...

If I passed within one meter of the event horizon of a supermassive black hole (where the tidal forces are trivial) and stuck out a two meter rod such that one meter of the rod was inside the event horizon, what would I see happen to the rod?

The Kerr metric for a black hole of mass ##M## and angular momentum ##J = aM## is $$ds^{2} = - \frac{\Delta(r)}{\rho^{2}}(dt-a\sin^{2}\theta d\phi)^{2} + \frac{\rho^{2}}{\Delta(r)}dr^{2} + \rho^{2} d\theta^{2} + \frac{1}{\rho^{2}}\sin^{2}\theta (adt - (r^{2}+a^{2}) d\phi)^{2},$$ where...

Suppose I am orbiting a black hole (BH) at some distance d outside its event horizon (EH), and with orbital velocity v. I do not like where I am, so I try to increase d by using an amount E of energy to increase my orbital velocity to v', where v' is whatever is necessary to escape the BH and...

We can write down the metric of the Schwarzschild black hole in Schwarzschild coordinates. Which aspect of the metric in Schwarzschild coordinates indicates that the coordinates are only valid outside the event horizon?

There is Hawking radiation associated with black hole event horizons. And there is Unruh radiation associated with horizons produced by acceleration. I've also heard some suggest that there is radiation associated with the cosmological event horizon due to space itself accelerating in its...

I have made graph of event horizon of Kerr black hole by giving simple command of polar plot. The problem is that the point where the event horizon and static limit meets should be along y-axis but instead its on x-axis. I have tried everything but not getting it right. What mistake I am...

Hello there, Suppose ## \Delta = r^2 + 2GMr + a^2## and ## \rho^2 = r^2 + a^2 \cos ^2 \theta ##. The Kerr metric is $$ ds^2 = - (1 - \frac{2GMr}{\rho^2})dt^2 - \frac{4GMar\sin^2 \theta}{\rho^2} d t d \phi + \frac{\rho^2}{\Delta} dr^2 + \rho^2 d \theta^2 + \frac{\sin^2 \theta}{\rho^2} \left[...

Assume a spherical region of breathable air just under the density needed to form a low density supermassive black hole. Two people float 20 feet apart from each other exchanging small talk. As gravity does its work, the spherical region holding the person closer to the center reaches the...

Ok..I know that at some point, from the Hubble Law , galaxies will seem to moving away from us speed of light,But actaully they can't because the space-time itself expands so it will be like a black hole horizon,which within that radius its , c/H , we can observe things etc.But out of that...

Hello! I'm having a hard time finding realistic black hole simulations, but I saw one recently (black hole size comparison on youtube) that showed the following 3 black holes (attached). I noticed that the larger the black hole, the smaller the "distortion zone" was relative to the radius of...

So in GR, for a classical black hole, if A is approaching the event horizon, to an observer far away, let say B, B would never observe A crossing the event horizon as B would observe A's time slow down in the limit to 0 and A's length contract in the limit to 0. In fact, according to B, A never...

Do laws of physics apply below the event horizon? It appears as if black holes had such gravity as to have an escapr velocity higher than c, which means that things are pulled inwards at higher speeds than the speed of light. Or am I overlooking something?

This question is regarding classical black holes. So inside the event horizon, spacetime behaves strangely. Space is now one dimensional and only in the forward direction(that is, into the singularity) and time of events is preceived as forward and backwards since light from a source comes...

I know this is (probably) not going to work.. But I can't figure out why not :-) So here is the theoretical situation... Lets say we have 2 black holes that, somehow, we can perfectly control (velocity, position, rotation etc. etc.). Now, one of my probes accidentally falls into one of the...

If there were 2 observers, 1 at rest with CMB, and the other falling into a black hole, how much time passes for the observer at rest with CMB while the other falls past the event horizon? I ask because I keep reading that time dilation (due to gravity) will cause the object falling into appear...

I have a doubt about black holes. (I'm sorrying for my poor English.): 1 - Does an observer outside a black hole see forever (i.e. his lifetime) a object stationary when it reaches the event horizon? 2 - Or photons emitted by the object have its wavelength so red-shifted that the observer...