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.

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  1. jbriggs444

    I Inside the Schwarzschild horizon

    Mostly correct. You can observe stuff that is closer to the singularity than you are. It is just that by the time you observe it, you'll be closer to the singularity than the object you observed was when you observed it. The above description is not quite right either. It is just my own...
  2. mfb

    Stargazing Event Horizon Telescope Results Released Yesterday (April 10, 2019)

    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...
  3. G

    B 83 quasars right up against the Event Horizon

    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...
  4. S

    A Penrose diagram of black hole with a changing event horizon

    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...
  5. J

    B Coulomb force over Rindler horizon

    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...
  6. S

    B Exploring the Physics of a Black Hole's Event Horizon

    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
  7. Arman777

    B How exactly inflation solves the Horizon Problem

    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...
  8. A

    I Inflation resolves the horizon problem

    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...
  9. Arman777

    I Event Horizon and Particle Horizon

    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...
  10. M

    I If time doesn't pass at the event horizon, how is it crossed?

    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?
  11. Ibix

    I Detecting Crossing Event Horizon w/ "Closed Box" Experiment

    @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...
  12. Grinkle

    B Conundrum thinking about crossing an Event Horizon

    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...
  13. A

    B Relationship between the age of the Universe and the Hubble horizon

    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...
  14. T

    How is the horizon length related to the power law spectrum

    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...
  15. Milsomonk

    I Lorentz Symmetry at Black Hole Event Horizon

    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...
  16. N

    B Gravitational Event Horizon: Two Interpretations

    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...
  17. M

    I Black hole event horizon confusion

    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...
  18. J

    A Why is Killing vector field normal to Killing horizon?

    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...
  19. P

    Exploring the Surface of a Black Hole's Event Horizon

    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...
  20. F

    I Alice Observes Bob at Event Horizon: DarkLink's Question

    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...
  21. S

    B People-Safe Event Horizon: Can Alice & Bob Experiment?

    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?
  22. D

    I Minkowski metric beyond the event horizon

    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...
  23. F

    Height of an island beyond the horizon

    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...
  24. M

    B How is light influenced really close to the event horizon?

    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 ;)!
  25. C

    A Neutrino flux at event horizon

    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...
  26. S

    A Reasonable length of forecast horizon in a time series

    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...
  27. C

    I Particle at event horizon as black hole evaporates

    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...
  28. durant35

    I Hawking radiation from de Sitter horizon

    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...
  29. A

    I The length of the visible horizon

    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.
  30. T

    I Are photons from the CMB being emitted from beyond our cosmological horizon?

    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?
  31. DoobleD

    I The particle horizon value is ignoring inflation?

    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...
  32. FallenApple

    I Measurements at the Cosmological Horizon?

    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...
  33. A

    I Black hole - event horizon and Hawking radiation

    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...
  34. A

    I Black hole - between horizon and singularity

    What occupies the space between a black hole's event horizon and the singularity? Stuff hurtling to the singularity? Vacuum?
  35. Arman777

    I Event Horizon and the Mass of a Black Hole

    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...
  36. C

    I Rod Near Event Horizon: What Would Happen?

    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?
  37. A

    I Area of the event horizon of a rotating black hole

    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...
  38. T

    I Can a cosmologist cross an event horizon?

    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...
  39. A

    A Schwarzschild coordinates beyond the event horizon

    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?
  40. F

    A The CC and cosmological event horizon radiation

    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...
  41. S

    Polar plot for the event horizon of a Kerr BH

    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...
  42. J

    A Compute Induced Metric on Kerr Event Horizon

    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[...
  43. H

    A The formation of the event horizon.

    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...
  44. Arman777

    I Exploring the Black Hole Horizon of Our Universe

    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...
  45. Chris G

    A Q: Black hole event horizon vs distortion effects

    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...
  46. FallenApple

    I Is Causality Relative in Black Holes?

    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...
  47. T

    I Do laws of physics apply below the event horizon?

    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?
  48. FallenApple

    I Strange Spacetime behavior inside Event Horizon

    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...
  49. D

    I Is it possible to distort a black hole's event horizon

    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...
  50. BenAS

    B Confused about how long it takes to cross an event horizon

    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...
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