# What is Horizon: Definition and 446 Discussions

The horizon is the apparent line that separates earth from sky, the line that divides all viewing directions based on whether it intersects the Earth's surface or not.
The true horizon is actually a theoretical line, which can only be observed when it lies on the sea surface. At many locations, this line is obscured by land, trees, buildings, mountains, etc., and the resulting intersection of earth and sky is called the visible horizon. When looking at a sea from a shore, the part of the sea closest to the horizon is called the offing.The true horizon surrounds the observer and it is typically assumed to be a circle, drawn on the surface of a perfectly spherical model of the Earth. Its center is below the observer and below sea level. Its distance from the observer varies from day to day due to atmospheric refraction, which is greatly affected by weather conditions. Also, the higher the observer's eyes are from sea level, the farther away the horizon is from the observer. For instance, in standard atmospheric conditions, for an observer with eye level above sea level by 1.70 metres (5 ft 7 in), the horizon is at a distance of about 5 kilometres (3.1 mi).
When observed from very high standpoints, such as a space station, the horizon is much farther away and it encompasses a much larger area of Earth's surface. In this case, the horizon would no longer be a perfect circle, not even a plane curve such as an ellipse, especially when the observer is above the equator, as the Earth's surface can be better modeled as an ellipsoid than as a sphere.

View More On Wikipedia.org
1. ### B Horizon and Curvature Observations

Thinking last night about what I should expect see of the Toronto skyline, which is 40 miles across the lake from me. Starting with an ideal scenario (nape-of-shore to nape-of-shore), a chord calculator tells me that for r=3900 mi. d=40 mi. the chord height will be 270 ft. I was surprised at...
2. ### B Crossing the Bekenstein Bound at Black Hole Event Horizon

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...
3. ### B Does Gravitation Start at Black Hole Event Horizon?

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.
4. ### I Black Holes: Event Horizon Causally Disconnected?

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...
5. ### I Question on the horizon problem

Hi Physics Forums, I had a question on the horizon problem (explanation for apparent thermal equilibrium between distal points on the CMB, anisotropies notwithstanding). Just as background, I just have an amateur interest in cosmology with no formal training (biophysist by training and...
6. ### A Can a black hole horizon act as a source of Chaos?

I was going through this paper where on page 5 they argue that in the given Poincare section: I am a bit confused by this statement. How does the given saddle point correspond to the black hole horizon and is it necessary that it acts as a source of chaos? Any explanation would be truly...
7. ### I Black hole horizon for different observers

The paper is The Volume Inside a Black Hole (0801.1734) Looking at the abstract, I have a question already. It is stated: Because the light rays are orthogonal to the spatial 2-dimensional surface at one instant of time, the surface of the black hole is the same for all observers (i.e. the...
8. ### I Cosmic Event Horizon – Does it have any real effects?

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...
9. ### Two persons headed towards the black hole horizon

I have tried inserting in the above formula 2m for r, but I get a huge answer. The correct answer is ##1.28*10^9 N/kg## for person A and ##1.28*10^-3 N/kg## for person B. I also suspect that the formula for the Lorentz factor (##1/\sqrt{1-2GM/r*c^2}##) has some relevancy here, but I cannot...
10. ### I Equation - cosmic event horizon

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
11. ### I What Happens Inside BH Event Horizon?

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...
12. ### B Event Horizon Telescope, Milky Way supermassive black hole stream

Streaming now:
13. ### I Differences b/w Schwarzschild Radius & Event Horizon

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?
14. ### I European Southern Observatory (Event Horizon Project team) – picture of Sgr A*

This alert regarding an announcement from the European Southern Observatory – announcement 12th May The results are from the Even Horizon Telescope project which was responsible for producing the first-ever image of a black hole in 2019. The video of that announcement is in the link. Worth...
15. ### B Black Hole Event Horizon: Atom Effects & Wave Detection

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?
16. ### I Black Hole: Knock on the horizon

First of all, I want to note that geometry is being discussed, which in fact is the General Theory of Relativity. And in any geometry, there are infinitely thin, weightless, etc. lines, rulers, and so on. In the future I will remind you about this. The system of units is meters. There is a...
17. ### I How is the sound horizon at recombination and BAO measured in the CMB?

So I'm trying to understand the sound horizon measured at recombination and the sound horizon measured with BAO. Here is what I've gathered (PLEASE tell me if I'm wrong and if you could please explain, I've been trying to read but can't find a clear explanation): ~r(z*) is measured using CMB...
18. ### Finding Event Horizon & Ergosphere: Derivations & Formulas

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...
19. ### I Schwarzschild Radius vs Event Horizon: Black Hole?

Real quick, are the terms "Schwarzschild Radius" and "Event Horizon" for a black hole interchangeable or is there some subtle difference between the two? Just looking for a ballpark answer here
20. ### I Quasi-Static Change of Event Horizon Area

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}...
21. ### I Time Dilation at Moving Black Hole Event Horizon

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...
22. ### I Definition of Rindler Horizon: A Global View

[Moderator's note: Thread spun off from previous one due to closure of the previous thread.] I have been thinking about this off and on, and though late to the thread, want to propose another way of looking at this that can be presented both at B level or A level. I post here at B level, and...
23. ### A Rindler Horizon Subtleties: Advanced Discussion

[Mentors' note: thread spun off from https://www.physicsforums.com/threads/equivalence-principle-and-rindler-horizons.1007879/] [Disclaimer] Definitely not B Level, but possibly of interest: https://journals.aps.org/prd/abstract/10.1103/PhysRevD.100.084029 discusses the Rindler horizons of an...
24. ### Math Treatment of Approaching Event Horizon

[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...
25. ### A Orbiting spaceship just above a black hole horizon

The experiment I am thinking about is a spaceship that approaches the horizon of a supermassive black hole by firing its engines in the opposite direction of its motion. I have the following questions: 1. When the ship is in a stable orbit, just above the horizon, how would an observer far away...
26. ### B Can you fall below the horizon on an expanding balloon?

This scenario came up (as a dream) in a sci-fi novel (Robert J. Sawyer's The Terminal Man). Two people of height h, initially within sight of each other, at d distance apart , on a huge balloon of radius r. As the balloon expands, they might or might not lose sight of each other over the...
27. ### B Exploring Time and Space Dilatation Near Black Hole Event Horizon

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...
28. ### I Slow-roll Inflation and Horizon Problem

I am a bit confused regarding the concept of the horizon problem. I have studied that the background radiation data implies that the radiations were not in causal contact at the beginning of the universe as from the big bang model. I want to know that how inflation is solving that problem? To...
29. ### Trigonometry - Horizon Related Word Problem

Hello! I'm trying to solve this problem. Here's the diagram I tried to make. I have difficulty understanding this math problem.. I've tried to solve the problem using the symmetry of the triangles but I didn't get the right answer, and I can't seem to understand the "concept" of the horizon...
30. ### I Crossing Event Horizon: Physics of Disintegration?

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...
31. ### Just got into Horizon Zero Dawn (PC)

Many, many wonderful ideas in that game. What sticks out most immediately is how a society which needs to deal with an infestation of large, occasionally hostile autonomous machines whose only common weakness is an inability to fly will "naturally" develop "structural engineering" that resembles...
32. ### B Event horizon for observer within main horizon

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...
33. ### I Dissolving Event Horizon w/Charged & Rotating BH

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...
34. ### B Star at black hole event horizon

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...
35. ### B Particle Horizon & Coformal Time Near c - Observer Effects

How would an observer's particle horizon and coformal time be affected by her traveling at very near c (relative to the CMB), both in and opposite the direction of travel (ahead and behind)? Also, how would Hubble expansion be impacted in her frame of reference? (I apologize in advance for...
36. ### B Can it escape the event horizon if the wavelength is long enough?

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...
37. ### B Black hole horizon and Hawking evaporation

Hello, I take the example of two observers : - A distant observer - A falling observer For the distant observer, the formation of the horizon is not part of his future cone of light, we agree. For the falling observer, the consensus says it is crossing the horizon. First question: the...
38. ### I Black Hole Event Horizon Hypothesis

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...
39. ### I Event Horizon Radius: Is it Constant?

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...
40. ### B Particle with negative mass inside a black hole horizon

[Moderator's note: Spin off from previous thread due to topic change.] Just as an aside and not related to the OP, would a real particle with negative mass inside the event horizon follow the runaway motion? Would it be ejected?
41. ### I Who Will Win the Race at the Event Horizon?

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?
42. ### I To collect all info at the event horizon

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...
43. ### I Gravity at the Event Horizon of a Black Hole

Acceleration due to gravity at the Event Horizon of a Black Hole.
44. ### I Event Horizon Gravity: Infinite or Not?

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...
45. ### I An observer passing the event horizon of a black hole (Part 2)

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...
46. ### I An observer passing the event horizon of a black hole

Hi all, I've just read this entire thread and watched the videos about black holes posted by @PeroK, which I liked very much (thanks @PeroK! :smile:). I am not particularly well aquainted with GR and my questions are concerning the often mentioned statement that an observer that passes the...
47. ### B Kinetic Energy of an Object Falling Into an Event Horizon

Hi, When objects fall in a gravitational field, they convert gravitational potential energy into kinetic energy. Because energy is always conserved: amount of kinetic energy gained = amount of gravitational potential energy lost. Now the gravitational energy lost should be equal to the amount...
48. ### B Can we see matter crossing the Event Horizon with a powerful telescope?

There is some confusion on my part what the actual reality is at the Event Horizon, since there appears to be different answers in using Kruskal-Szerrkes coordinates or Schwartzschild coordinates. Reality does not have two answers. There is only one right one. Asked multiple times on this...
49. ### I Solving the Horizon Problem: 1 Source or 105?

It says that since there is homogeneity in the Universe's temperature, all these points must have come from one source (or a source close to each other?) at a certain time. Then it also calculates the number of these sources and it's ~105. But isn't that very dense mass right before the Big...
50. ### B Physics near the event horizon

Understanding that I might be pushing the limits into heuristic territory, I'm wondering how much agreement exists on whether the theory holds up in the proximity of an event horizon. This came up during a recent discussion about matter falling into the black hole and the Schwarzschild solution...