Event horizon Definition and 308 Threads

I want to show that it is impossible to construct a timelike curve between two points on the event horizon of a black hole. This should be an obvious fact, for example, by considering any particular model of a black hole, one can go to coordinates that extend over the horizon and by observing...

Posting this as I have so far not been able to find a straightforward answer to the following question. The formula for the outer event horizon of a kerr black hole is given by the following equation: $$r_+ = \frac{GM}{c^2}\left(1+\sqrt{1-\frac{J^2c^2}{M^4G^2}}\right)$$ Where ##J## is the...

When objects/galaxies (particles and antiparticles) move across the cosmological event horizon, do they leave behind an “image” on the horizon, such that when we look at the image, we can tell what kinds of objects went through to the other side (ie, we would know information such as the mass...

There is good enough experimental evidence for the existence of a Photon Sphere. Is there clear unequivocal experimental evidence for the existence of an Event Horizon ?

Let's say Bob falls into a blackhole, and Alice is sufficiently far away that she is not falling in. She sees Bob's clock stop and his image fade away and all that. However, from Alice's perspective Bob never actually crosses the horizon. From her perspective, Bob is sitting frozen and invisible...

If, from an outsider's perspective it takes an infinite amount of time to cross the event horizon, but a finite amount of time for the black hole to evaporate, how does that look like from the perspective of the falling observer. For that matter, how does it look like from the perspective of the...

So I just had a question. In a Netflix documentary, they showed these people trying to simulate the conditions of a black hole by having a vortex in a pool of water. they said that light cannot pass through the hole and the physics around is essentially adjacent to the event horizon and what...

I realize that something I thought I understood about a beacon approaching a black hole I am unsure of. My hypothetical - I am at a safe distance from as simple a black hole as it makes sense to discuss. I launch a beacon at it, and I calculate that in one hour of my own proper time the...

I have heard that an person falling toward a black hole would 1) start to "freeze" from the viewpoint of an outside observer as the infalling person's (call him Bob) time dilation slowed Bob's time to a crawl, and 2) from Bob's viewpoint it would seem that he simply falls through the horizon...

From what I understand about time dilation and the relativity of simultaneity; if we imagine two people near a black hole and one of them begins to approach the black hole on a trajectory that crosses the event horizon. The stationary observer will never see the moving observer enter the black...

Looking at Kruskal diagram, it appears that light from all previous events of something crossing event horizon at that same point, reaches the falling observer when it crosses the event horizon. Is my interpretation correct?

As I understand things if you're hanging out in your space suit some distance away from a black holes event horizon and your buddy decides to dive on in you will never see him cross the event horizon. You'll see him approach the event horizon but never cross it. It would seem the time needed to...

as I understand time stops at the event horizon of a black hole for the far away observer; so can we actually see 2 black holes orbiting eachother? I also understood that the singularity is a moment in future. so what happens when 2 black holes orbit eachother with the singularites? from the...

The presence of a big mass (BH or neutron star) in the vicinity of a BH must have an effect on the shape of the event horizon, an indentation comes logically to mind. When the system is rotating, it would lead to tidal effects on the horizon that would disclose internal material of the BH. This...

Thought experiment: What will happen if you place a mirror behind the event horizon and turn on the light in its direction?

The title is a direct quote of this video by Dr. Becky Smethurst, an astrophysicist specializing in black hole research. This is a mistake, right? Supermassive black holes, for example, don't have tiny radii, compared to stellar mass BHs. Then there's the equation she presents seconds later...

Here's my reasoning. The event horizon is the point where the escape velocity becomes greater than the speed of light. This results in the event horizon spacetime boundary having infinite time dilation. So, that must mean that inside the boundary of the event horizon, time dilation must...

As far as I know, entropy could be reversed by the Poincaré recurrence theorem if it had a finite horizon given by some amount of vacuum energy causing an accelerating expansion. However, I found this lecture by Leonard Susskind () where he tells a way through which the vacuum could decay into...

Please hear me out; I've referenced some good papers below, and I think you'll find the movie clip intriguing. Is there anything solid to this movie? The faster than light travel. The dimensional gateway. The folding of spacetime, passing through the hole, and then the return of spacetime to...

For me, Event Horizon was very scary but had enough sci-fi and sci-real to it that it captivated me.

The Bekenstein Bound places a upper limit on the amount of entropy that a given volume of space may contain. This limit was described by Jacob Bekenstein who tied it quite closely to the Black Hole Event Horizon. Put simply, black holes hold the maximum entropy allowed for their volume. If you...

Has anyone done this calculation? It seems to me that if light and time disappear at the event horizon, it's a gravitation limit as well.

I've heard quite frequently that events inside the event horizon of a black hole are causally disconnected from the rest of the universe. I take it to mean that while outside events can interact with the events inside of the horizon, the reverse is not true i.e. inside events cannot interact...

What does and observer inside of a collapsing shell observe? Lets say we have a shell of matter collapsing to a black hole. What would observers near the center see? How would the rest of the universe appear when, The shell is approaching the Schwarzschild radius? After the shell passes the...

Using LightCone8 Cosmological Calculator and PLANCK(2018+BAO) data as input, we can get the following result: In the figure, Dhor is the event horizon and Dpar is the radius of the observable universe. Currently (t = 13.79 Gyr) Dhor has a value of 16.58 Gly. Does the event horizon have any...

I was reading a paper written by George Smoot [1], which assumes the holographic principle as true and conjectures that our universe would be encoded on the "surface" of an apparent horizon as the weighted average of all possible histories. In that way, there would be one world (or universe)...

Hi Everyone, I'm hoping someone can share an equation that would give the distance of the cosmic event horizon for a given time after the big bang. Thanks for any help! Jay

I have read about the spaghettification of objects due to tidal forces as they get close to the singularity. Gravity at your feet is stronger than at your head, so you get stretched and pulled apart. In this case, the singularity is a point in space. But I also read about the time coordinate...

Streaming now:

I understood that the event horizon is a null surface and not a place in space, what is the relationship between it and the Schwarzschild radius? Also, what does the Schwarzschild radius physically represent for example for an object such as a star?

Are the atoms of objects ripped off when they cross the event horizon? Does a metal rod that partially crosses the event horizon maintain its lattice structure of atoms? If I put a sound wave generator on the end of the bar that has crossed the event horizon how far can the waves be detected?

Homework Statement:: See below. Relevant Equations:: See below. I am trying to calculate the event horizon and ergosphere of the Kerr metric. However, I could not seem to find a proper derivation or formula to calculate the event horizon and ergosphere. Could someone point me to the...

Recently I have seen a number of General Relativity visualisations that show spacetime flowing towards any mass, similar to water flowing into a sink hole. ScienceClic's video is an example. That model is also used in the "waterfall model" to explain the event horizon of a black hole, as the...

Let ##\mathscr{H}## be a constant-##v## cross-section of the event horizon (area ##A##). The expansion is the fractional rate of change of the surface element, ##\theta = \frac{1}{\delta S} \frac{d(\delta S)}{dv}##. The problem asks to prove the formula ##\frac{dA}{dv} = \frac{8\pi}{\kappa}...

Hello everyone, I have a hard time to conceptualize the case of a moving black hole. We know from SR that time slows down for moving objects; but time dilation at the event horizon is already equal (tends) to zero. It seems that it can create some sort of conflict for the black hole movement...

[This thread can be considered the A-level footnote to https://www.physicsforums.com/threads/is-there-an-inside-to-a-black-hole.1007588/] For a static [admits a hypersurface orthogonal timelike Killing field ##k##], spherically symmetric spacetime, a time coordinate ##t## can be chosen as the...

As closer the observer will be to the event horizon, the more the time dilatation will be. As we know, if the observer O1 has a clock, another observer O2 very far from the black hole will se the O1 clock "slowing" down as O1 approach the event horizon. The limit is that the O1 clock "stops" at...

As you fall through the event horizon, for a time one part of your body is inside the event horizon and the other part is outside. At that point the two parts of your body are casually disconnected. So shouldn't it be severing the chemical bonds holding your body together at the point of the...

Physics is not my area of expertise. That being said, philosophy of science is, but I'm not here to discuss philosophy. I recently found myself trying to imagine how light behaves once it crosses the event horizon of a black hole. Presumably, between the event horizon and the singularity...

The event horizon of a black hole is defined with respect to observers far away, and we know that light from within the horizon can't reach a distant observer. But what if an observer is within the "main" event horizon? Presumably, there will be another horizon nearer to the center, such that...

I saw a fascinating video from PBS space time about dissolving an event horizon. See here for reference: The video addresses rotating kerr black holes and charged black holes, but doesn't talk about the combination of rotation and charge. So what happens when you spin up the black hole as...

Curious if the time dilation at the edge of an event horizon would have the apparent effect of prolonging the life of the star to an outside observer - so for example a blue hyper giant at the edge of an event horizon with an expected main sequence time of, say, 500 million years, would remain...

Can electromagnetic radiation escape from the event horizon of a Black Hole if the wavelength is long enough? What if a Black Hole contains electric charge, hypothetically supposing we dumped a large number of protons into it? Electric charge is mediated by the electromagnetic force. So the...

Main Question or Discussion Point Wouldn't the definition of the event horizon of a black hole be the radius at which the acceleration of gravity exceeds the speed of light, instead of the radius at which the escape velocity exceeds the speed of light?It's very clear to me that a...

Let's say there is a black hole billions of miles away from earth, a hefty one such that a careless traveler could end up inside the horizon before noticing he'd been swallowed by the BH. Based on Earth observations the BH event horizon radius is r. We hop in a ship and go to a safe escape...

Hi guys. Imagine that in the exact instant when a massive particle A crosses the event horizon of a black hole, a Photon does the same,so that they have a race toward the singularity. Who will win the race? Will they have still different velocities?

Hi there! I have a question for anyone; If we could have built a shielded spaceship that can withstand all the radiation etc from a supermassive black hole. And we managed to park at the event horizon. And we want to collect all the visual data that's there in laters, how could we get all...

Acceleration due to gravity at the Event Horizon of a Black Hole.

In my browsing around various science forums a have come across the comment that the gravity field becomes infinite at the event horizon. I have always thought that this is a misunderstanding, and that it only becomes infinite at the central singularity. Then I found this same statement in...

Hi all, this is a new scenario I got thinking about after having received great feedback and corrections from other PF'ers in this thread. Thanks again for the great help! This new scenario is similar to the previous one, but with a twist including a mirror. And as I said in that thread, I am...