Event horizon Definition and 306 Threads

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...

The event horizon of a black hole appears to be plastered with 'afterimages' of everything that ever fell into it. (Because gravitational time dilation makes every such object appear to stop at the event horizon.) Now, suppose an event horizon is 'full' as defined by the Pauli exclusion...

According to this video, , if a black hole is large enough you could actually travel for some time within the event horizon without dying because the event horizon is so far from the actual singularity. So, assuming that's true, what would you see while you were inside the black hole? Here's...

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...

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...

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...

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...

OK, so it's time to start a new thread. I heard many times that there exists only one black hole solution for a given mass and angular momentum, but I know already that this is not true. We all know that if we throw something into an existing black hole, its event horizon starts to ripple. So...

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...

Hey guys, I'm currently doing a project on black holes and need some input on what you believe to be the biggest misconceptions about black holes, thanks! You can read more about the guidelines of the project at: http://www.aqa.org.uk/subjects/projects/aqa-certificate/EPQ-7993

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...

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...

OK, in solar mass terms, 29+36=62 with 3 left over. Or make that 3 left out. This is a little-advertised mechanism for pulling mass from a black hole. And, since the output is gravity waves, it doesn't seem to be directly related to Hawking Radiation. Somehow intense gravity can scoop out...

This question is in context of the recent gravitational wave detection by aLigo. Apparently aLigo has detected the entire process, including the before merger, during merger, and aftermath of the completed merger. My understanding is that two black holes should not be seen to be merging in...

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...

Is this the proper formula for calculating the Schwarzschild radius of a black hole? rs = 2GM / c2 If it is not, or if anyone has one that might work better, could you refer it to me?

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...

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...

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?

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...

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...

There are many videos and articles about this topic (what it looks like if you fall into a black hole). I remember hearing that, inside the event horizon of a black hole, time has essentially stopped for an outside observer. However, if you fell into one that would mean that any amount of time...

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...

What's the best way to explain why tidal forces for an observer free-falling through an event horizon are finite? My first thought was to say that "gravity isn't a force, it's a curved space-time". On further thought, however, it seems to me that consideration of the Rindler horizon shows...

Dear PF Forum, Can we avoid spaghettification for some times, once we're inside event horizon? I choose a rather massive black hole, so the tidal force won't be so big at EH. Mass: 1 trillion solar mass Schwarzschild radius: 2.950 trillion KM What is the gravitational force at EH? I calculate...

Questions about black holes: Various articles mention that it takes infinite amount of time to observe something pass through the event horizon. Does this imply that the redshift observed from afar would carry on forever, that the infalling object would just become dimmer and dimmer, but never...

Homework Statement I am preparing a report on black holes and I recently learned about a phenomenon I was previously unaware of: the photon sphere of a black hole. While reading an article on said occurrence (I have now confirmed this on multiple sources) the photon sphere which is the minimum...

In classical mechanics, to raise from some height h_{0} to infinity over a gravitational body, takes a certain amount of energy, the energy associated with escape velocity, let's just call it ε. ε = \lim_{t\rightarrow +\infty} \int_{h_{0}}^t ƒ(h)dh Likewise, it's time-reversible, so dropping...

I've been working through Leonard Susskind's "The Theoretical Minimum" lecture series (which are a fantastic introduction to the topics covered by the way) and a couple of his comments confused me when he was covering the Kruskal-Szekeres metric/coordinates in General Relativity. The end of the...

Forgive my ignorance here, I'm not a physicist, but the exact nature of gravity beyond the Event Horizon has me a bit stumped. Say I cross the Horizon of a Supermassive Black Hole, meaning the pull on me isn't really that much stronger than Earth's pull initially - why can't I then escape...

So the event horizon is the distance from the singularity where escape velocity exceeds the speed of light, right? Well let's say I am standing next to the event horizon, RIGHT next to it. What if I stick my arm in past the event horizon, then pull it out. I can't do that, right? Why not...

SR implies that mass can't reach c, but mass does reach a black hole's event horizon. How is this reconciled with SR?

According to Wikipedia, the gravitational time dilation formula is given by t_0 = t_f \sqrt{1 - \frac{2GM}{rc^2}} = t_f \sqrt{1 - \frac{r_0}{r}} where t0 is the proper time between events A and B for a slow-ticking observer within the gravitational field, tf is the coordinate time between...

Homework Statement A sub-atomic particle is near the event horizon of a black hole. Due to the nearby gravitational field, the Ricci Curvature Tensor is changing rapidly. The particle then performs quantum tunneling. Homework Equations Which version of spacetime does the tunneling particle...

Hello, I remember hearing somewhere that it is possible to calculate the event horizon, I was just wondering if this is possible and what equations are used to calculate it.

No one seems to be bothered by this except me: Black holes have a finite lifetime measured in Schwartzchild time due to Hawking radiation. Similarly, the universe probably has a finite lifetime measured in Schwartzchild time. In that case, nothing ever falls through the event horizon of a black...

Is there a physical boundary that is the event horizon? Or is there not? The reason I'm asking is because texts say that the event horizon that appears in Schwarzschild's metric is a result of the coordinate choice, and it disappears by choosing some other coordinates.

Hi! It is stated in V. Mukhanov's book "Physical foundations of Cosmology" the following (page 44, after equation 2.25): "In contrast, for the dust dominated universe, where ηmax=2π, the event horizon exists only during the contraction phase when η>π." could someone please explain why is this...

Hi guys, I have a GR question. It is usually said that black holes have event horizons in which time freezes/stops relative to an outside observer. This happens in the Schwarzschild coordinate system. But are there any coordinate systems in which the coordinate time of the black hole and its...

Stephen hawking came to know of a study that stated that surface area of an event horizon increases. So he said that since area has increased Entropy also had increased. But why does surface area of an Event Horizon increase?