Hole Definition and 1000 Threads

A hole is an opening in or through a particular medium, usually a solid body. Holes occur through natural and artificial processes, and may be useful for various purposes, or may represent a problem needing to be addressed in many fields of engineering. Depending on the material and the placement, a hole may be an indentation in a surface (such as a hole in the ground), or may pass completely through that surface (such as a hole created by a hole puncher in a piece of paper). In engineering, a hole may be blind or through if it is partial or complete depth.

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  1. T

    I Are Two Supermassive Black Holes on a Collision Course?

    Well, not right away. It's about 10 000yrs in the future. From: THE ASTROPHYSICAL JOURNAL LETTERS The Unanticipated Phenomenology of the Blazar PKS 2131–021: A Unique Supermassive Black Hole Binary Candidate Popular version...
  2. G

    B Black Hole Entropy: Basis of Logarithm Explored

    In textbooks, Bekenstein-Hawking entropy of a black hole is given as the area of the horizon divided by 4 times the Planck length squared. But the corresponding basis of the logarithm and exponantial is not written out explicitly. Rather, one oftenly can see drawings where such elementary area...
  3. G

    I Spin (or not) of a Kerr Black Hole

    The Kerr solution describes the gravitational field of a rotating black hole. Oftenly, the hole is said to be „spinning“, what appears as misleading to me. My questions: 1.) Is it correct to say that angular momentum in this way is treated like orbital angular momentum, not like spin? 2.) Can...
  4. Carlos Torres

    B Does the inertia of matter change in a black hole?

    Hi everyone, I was in physics class and the professor asked if the inertia of matter changes in a black hole and I would like to know if anyone has the answer to this question.
  5. M

    I Measuring Light Reflection in a Black Hole

    To keep things "simple", the black hole is 1E30 kilograms. The statite (stationary satellite, blue) hovers above the hole at a fixed location (twice the Schwarzschild radius from the singularity) by tremendous acceleration. The statite drops a probe (green) that begins to fall toward the hole at...
  6. Physics Slayer

    B Black hole at the beginning of time

    If all the matter was condensed to a single point at the beginning of the universe, then why didn't it all collapse into a black hole? I have heard speculation that the laws of physics change with time, is this the reason why there was no black hole at the beginning or is the reason more...
  7. R

    I Black Hole Waterfall Analogy & Light Speed

    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...
  8. L

    I Do Stars Fuse Elements Heavier Than Iron Before Imploding?

    I know black holes are stars that fuse together elements until they reach iron which doesn't radiate energy to counterbalance the gravity, but do any stars fuse elements heavier than iron that would once again give off energy prior to it imploding? By the way, why doesn't iron creation create...
  9. J

    Understanding the Chemical Bonding in Tire Plug Repair

    This is one method to fix a tire with hole. I'd like to understand what happens mechanically to the tire belts and plies when plug method was used to fix the hole. This is part of a tire. Now imagine you push a tool into the hole (like in the video), would the belt and ply simply have...
  10. PeterDonis

    I Kinematic Decomposition for "Rod and Hole" Relativity Paradox

    In a recent thread, I said that if there was interest, I would post in a separate thread the calculations for the kinematic decomposition of the congruence of worldlines describing the rod in the "rod and hole" relativity paradox discussed in that thread. Since there was interest, I am posting...
  11. S

    B So, a black hole and an antimatter star bump into each other....

    This is a bit hypothetical obviously as I doubt the conditions for this scenario would ever occur in the real universe. Imagine a black hole, about 10 solar masses. It is, amazingly, sitting in an area of space that is a perfect vacuum. Just by chance, a rogue antimatter star of exactly the...
  12. S

    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...
  13. B

    Find hole size for allowable leak in pressure vessel

    Hello, all. I am completely new here so please be gentle. I have a small, cylindrical, sealed ("no leak") pressure vessel with an approximate volume of 27 cm3. It has a constant supply of air pressure at 8"WC. I'm trying to figure out how big of a hole it can have if there is an allowable...
  14. Astronuc

    B Observational bias? Lack of massive black hole observations....

    https://phys.org/news/2021-11-lack-massive-black-holes-telescope.html I wonder how this affects the 'known', or rather, 'observed', mass in the galaxy and universe.
  15. A

    I Relativistic Doppler Effect near a Black Hole

    Hey everyone, if I were to view a shining person rotating near a black hole at near the speed of light there would be 2 kinds of redshifts: gravitational redshift and relativistic doppler effect redshift. Right? But, say at some point, the person is traveling towards me, then the doppler effect...
  16. G

    I Falling into a Black Hole: Blueshift Questions Explored

    I am under the impression that an outside observer would see things redshifted as the person they are observing approaches the event horizon. So, it seems reasonable that someone from inside the black hole would see incoming light blueshifted. Is this inaccurate? Why or why not? If it is...
  17. G

    I Black hole inside of a black hole.... can it be done?

    Let's say you have an absolutely giant black hole, so big that items inside of it leisurely approach the singularity, reaching it in about a million years (or whatever time it takes for a black hole to form from matter accumulation). Could matter slowly accumulating somehow form its own black...
  18. S

    B Is there an inside to a Black Hole?

    Disclaimer: I'm not a scientist, please excuse my ignorance if the following seems stupid. Since time slows down as an object gets nearer to the EH, would an outside observer, living and observing for an almost infinite amount of time ever see an object/particle cross the event horizon...
  19. Vandenburg

    I Can Gravity Escape a Black Hole?

    If: Gravity propagates via "particles" of gravity, and The particles of gravity are massless and so move at the speed of light then how can the gravity particles escape from a black hole? Aren't they as trapped as the photons? It seems as if black holes should present no gravitational field...
  20. V

    B Air escaping out of can with hole

    In the first figure, air goes forward and the can goes backward. My question is why does the air exert the pressure gradient force on the can. Can I have a hint for why the can doesn't move when air is being sucked in in the second photo.
  21. A

    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...
  22. haushofer

    I Black hole formation watched from a distance

    Dear all, For a new book I'm writing I'm investigating some common misconceptions in physics. And of course, that means confronting myself with my own confusion. One thing I've never got clear in my head, and which I find hard to answer using google and my textbooks on GR, is the following: how...
  23. .Scott

    I Does Time Drive Black Hole Travelers to a Central Singularity?

    Since my understanding of these geometries is wrong, I'll do this in numbered steps - the easier to correct my logic. I think the big problem I have is with the time dimension. There seems to be a presumption that the time vector will drive a falling object into a central singularity. But how...
  24. Charles_Xu

    I The definition of volume inside black hole?

    What is the definition of volume inside a black hole? we know the grr element of Schwarzschild metric is negative inside event horizon, so how to define a volume inside event horizon? if there is no definition of volume, is there the definition of density?
  25. R

    B GR and closeness to a black hole singularity

    Assume a Schwarzschild black hole. Near the event horizon other than Hawking radiation the behavior of matter and energy are fairly accuratly described by general relativity. How close can one get to the center (the singularity based on GR) before one must switch to a quantum gravity theory...
  26. Roberto Pavani

    I Gravity at Schwarzschild Radius of a Black Hole

    From "standard" formula we have that the gravity acceleration a = GM/r^2 and that the Schwarzschild radius rs = 2 GM / c^2 Is it possible to compute the gravity acceleration at Schwarzschild radius putting r = rs? In this case we will have a = c^4 / (4GM) This mean that a very very...
  27. D

    I Maximum hole diameter to prevent water leakage

    I have a empty bottle and immersed it into the water, where the pressure inside the bottle is equal to air pressure. If I want to make a hole on the bottle, how big is the hole to prevent the water leak into the bottle at different depth? what kind of parameter is involve in this phenomena? can...
  28. tmalcolm

    Force pressure variance after hole is plugged?

    There is a tall cylinder filled with water. And there is a 3 in diameter hole near the bottom and water is gushing out. (assume the cylinder is continually being re-filled from the top) You work to plug the hole with a 10 inch long cylinder that is exactly the perfect diameter fit to plug the...
  29. nomadreid

    I Entangled particles in black hole decay?

    First, I was not sure whether this should go into the Relativity or the Quantum Physics rubric, but since the central question is about entanglement, I opted for the Quantum. I do not have the necessary sophistication to follow string theory arguments, and even most explanations in...
  30. Roberto Pavani

    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...
  31. D

    Supermassive black hole in the Large Magellanic Cloud?

    Ok, I know, it's science fiction, you can make anything work if you really want it to. I'm planning out a sci-fi story which I wanted to try and keep as grounded as possible in believable scientific concepts. For context, the basic premise is: humanity detects a wormhole on the outer edge of the...
  32. S

    Question about water container with hole at the bottom

    (a) $$V=\int_{0}^{x} A~dx$$ $$=\int_{0}^{x} f(u) dx , \text{u is dummy variable}$$ Is this the answer? Or there is something else I can do to continue the working? (b) $$\frac{dV}{dt}=\frac{d}{dt} \int_{0}^{x} f(u) dx=f(x).\frac{dx}{dt}$$ Is this correct? (c) $$\text{time}=\frac{\text{rate...
  33. J

    I Black Hole Diameter: Is It Finite or Infinite?

    Hi All I'm sure this question has been covered previously , but when searching I do not find a definitive answers. I recently watch some talks given by Kip Thorne that had me thinking about black holes and their densities. So my deduction is as follows . Using General relativity, and...
  34. E

    I To derive an equation of black hole thermodynamics

    This is the problem statement: We can start by writing ## (\star d \star d \xi)_a = - \nabla^b (d\xi)_{ab} = - \nabla^b \nabla_a \xi_b + \nabla^b \nabla_b \xi_a = 2\nabla^b \nabla_b \xi_a ##. Then with ##\nabla_a \nabla_b \xi_c = R_{cbad} \xi^d = -R_{bcad} \xi^d## we can contract over...
  35. Ilya B

    I Can a Black Hole be Destroyed by Electron Beam?

    Electrostatic repulsion of two electrons is about 4.17*10^42 stronger than their gravitational attraction, and is mediated by massless carriers. Black holes preserve charge, and charging a BH with even a moderate electric (negative) charge will result in BH repulsing electrons instead of...
  36. Kyran

    B Is a black hole a massive ball of photons?

    Black holes suck things in and the current explanation is that they bend spacetime. I have my own hypothesis though. If electrons shoot out photons when they switch positions in the atom that would mean that at the very least electronshave photons in them. Atoms always try to have the right...
  37. H

    B Spinning Black Hole Drags Space-Time: What Causes Friction?

    If a Black Hole is spinning (perhaps they all do) I have heard it distorts the 'fabric' of Space-time in the vicinity. What is the 'friction' component which allows the distortion?
  38. S

    Mesh hole size calculation for RFID signal blocking

    For a project I am currently facing a problem that I find difficult to solve. I am developing a shielding portal that uses a mesh to blocks UHF RFID signals. I would like to use aluminium mesh due to it being lightweight and its attenuation properties. The frequency of UHF RFID signals in the...
  39. E

    I Understanding White Holes to Region III in GR

    Trying to follow Townsend's notes; section 2.3 is discussing ways of dealing with the co-ordinate singularity at ##r=2M##, i.e. either by transforming to ingoing EF which cover I & II, outgoing EF which cover I & III, or KS which cover the entire manifold. I got a bit preoccupied with the...
  40. E

    I Finding the angular momentum of a Kerr black hole

    [Mentor Note -- Specialized question moved to the general technical forums] Homework Statement:: To show that ##J = Ma## for the charged Kerr metric [Wald Ch. 11 Pr. 6] Relevant Equations:: \begin{align*} \mathrm{d}s^2 = &- \left( \frac{\Delta - a^2 \sin^2{\theta}}{\Sigma}\right) \mathrm{d}t^2...
  41. M

    I Observe Hawking Rad. in Black Hole?

    Theoretically could an observer in a black hole perceive hawking radiation escaping the black hole as a black hole within the black hole? Also if so maybe that black hole could produce a radiation similar to or related to hawking radiation (Making a strange entangled system for conservation of...
  42. C

    Particle falling radially into a black hole

    I've been stuck starting anywhere with this. I need to finish this class for graduation and i'd like a safety net of a passing grade with this.
  43. D.S.Beyer

    I Observing Christmas Lights in a Black Hole: What You See

    If a string of blinking Christmas lights extends from the center of a black hole out to a large radius r. What do I see, if I am perpendicular to the line of lights, at radius r? Experiment specifics 3 solar mass, non-charged, non spinning black hole. Observer is 1,000 Au from the center of...
  44. icalle3

    Condition to form a primordial black hole that I don't understand

    Hi guys, I'm new here. I am doing my final degree project and it's hard for me to understand what this paragraph means in one of the papers that I'm reading, it's about primordial black hole formation. [Talking about a spherically density perturbation] The rarefaction wave starts at the surface...
  45. ScruffyNerf

    B Black Hole Observation: Outside Observer & Spherical Symmetry

    I know that for the infalling observer the horizon is a fake singularity that can be removed via the Eddington-Finkelstein co-ordinates but wouldn't the classic Swartsheild co-ordinates still apply for the outside observer? So, while for the infaller it takes a finite time, the outside...
  46. complete_noob

    B What Happens When Light Approaches a Black Hole?

    Hello I am not a physics student and i don't know anything about science, but i was curious if someone could tell me about what happens when light approaches a black hole i have heard that nothing goes faster than light but i have also heard that black holes can suck in light, combined with the...
  47. J

    I How Long Does it Take to Fall into a Black Hole?

    Gratings The following is an example of a question that I have been battling to get my mind around for some time: How long, in gravity-free time (Tᴓ-time), would it take for a stationary, one kilogram, point-mass to fall directly towards a non-spinning, accretion-disk-free, singularity in a...
  48. Psnarf

    I Follow an object into a black hole

    In Stephen Hawking's Brief History of Time, an astronaut is stretched head to toe by the gravitational gradient of a black hole. Let us replace the astronaut with a large object and follow it inside as it travels along a radial line toward the center. I assume the gradient increases with, say...
  49. P

    Trivial question about units (Hans Bethe Small Hole Diffraction)

    http://www.physics.miami.edu/~curtright/Diffraction/Bethe1944.pdf Hans Bethe's paper Jackson doesn't take any issue with Hans Bethe's paper as far as I can tell when he references it. That leads me to believe the two sets of dipoles are equivalent and there happens to be an "old school"...
  50. Haorong Wu

    I Calculating Distance Light Travels Near a Black Hole

    Suppose that we tangentially send a light from an orbit of radius ##h## to another orbit of radius ##l## near a black hole. I would like to calculate the distance that the light travels. I start from the Schwarzschild metric, $$ ds^2=-(1-\frac m r) dt^2+\frac 1 {1- \frac m r} dr^2 +r^2 d \theta...
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