What is Black hole: Definition and 1000 Discussions
A black hole is a region of spacetime where gravity is so strong that nothing—no particles or even electromagnetic radiation such as light—can escape from it. The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole. The boundary of no escape is called the event horizon. Although it has an enormous effect on the fate and circumstances of an object crossing it, according to general relativity it has no locally detectable features. In many ways, a black hole acts like an ideal black body, as it reflects no light. Moreover, quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation, with the same spectrum as a black body of a temperature inversely proportional to its mass. This temperature is on the order of billionths of a kelvin for black holes of stellar mass, making it essentially impossible to observe directly.
Objects whose gravitational fields are too strong for light to escape were first considered in the 18th century by John Michell and Pierre-Simon Laplace. The first modern solution of general relativity that would characterize a black hole was found by Karl Schwarzschild in 1916, and its interpretation as a region of space from which nothing can escape was first published by David Finkelstein in 1958. Black holes were long considered a mathematical curiosity; it was not until the 1960s that theoretical work showed they were a generic prediction of general relativity. The discovery of neutron stars by Jocelyn Bell Burnell in 1967 sparked interest in gravitationally collapsed compact objects as a possible astrophysical reality. The first black hole known as such was Cygnus X-1, identified by several researchers independently in 1971.Black holes of stellar mass form when very massive stars collapse at the end of their life cycle. After a black hole has formed, it can continue to grow by absorbing mass from its surroundings. By absorbing other stars and merging with other black holes, supermassive black holes of millions of solar masses (M☉) may form. There is consensus that supermassive black holes exist in the centers of most galaxies.
The presence of a black hole can be inferred through its interaction with other matter and with electromagnetic radiation such as visible light. Matter that falls onto a black hole can form an external accretion disk heated by friction, forming quasars, some of the brightest objects in the universe. Stars passing too close to a supermassive black hole can be shred into streamers that shine very brightly before being "swallowed." If there are other stars orbiting a black hole, their orbits can be used to determine the black hole's mass and location. Such observations can be used to exclude possible alternatives such as neutron stars. In this way, astronomers have identified numerous stellar black hole candidates in binary systems, and established that the radio source known as Sagittarius A*, at the core of the Milky Way galaxy, contains a supermassive black hole of about 4.3 million solar masses.
On 11 February 2016, the LIGO Scientific Collaboration and the Virgo collaboration announced the first direct detection of gravitational waves, which also represented the first observation of a black hole merger. As of December 2018, eleven gravitational wave events have been observed that originated from ten merging black holes (along with one binary neutron star merger). On 10 April 2019, the first direct image of a black hole and its vicinity was published, following observations made by the Event Horizon Telescope (EHT) in 2017 of the supermassive black hole in Messier 87's galactic centre. In March 2021, the EHT Collaboration presented, for the first time, a polarized-based image of the black hole which may help better reveal the forces giving rise to quasars.
As of 2021, the nearest known body thought to be a black hole is around 1500 light-years away (see List of nearest black holes). Though only a couple dozen black holes have been found so far in the Milky Way, there are thought to be hundreds of millions, most of which are solitary and do not cause emission of radiation, so would only be detectable by gravitational lensing.
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...
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...
Could dark matter consist of black holes formed shortly after the big bang? They would form the perfect development seed. If they all have Sun-like masses then they are not detectable from here (they are just 3 kilometers wide!). They have virtually no collisions with stars and could form a...
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?
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...
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...
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...
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...
It's one of those staples of sci-fi. Unless you're going for a hard sci-fi that uses rotation or linear acceleration to achieve simulated gravity, your starship will likely use "gravity plating". As we all know, gravity plating is just a plot device, usually for shows with more limited budgets...
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...
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...
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...
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...
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...
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?
[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...
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...
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...
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...
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...
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...
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...
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...
Hi,
I'm a Portuguese translator working on a documentary about black holes and there is a specific bit of dialogue between Stephen Hawking and his colleagues that I'm having a hard time translating. Basically, Hawking says: "So, it could be the F plus minus
term takes this away." Is this the...
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...
For a planet to be able to support life, it needs to have a source of energy. In our case this energy comes from the sun.
But in this paper, the author argues that a rogue planet (a planet that has been ejected from its stellar system and no longer orbits any star and is wandering in...
I just learned from the American Journal of Physics that the two books
Space Time Physics by Taylor and Wheeler
and
Exploring Black Holes by Tayor, Wheeler, and Bertschinger
are for free now! What a nice Christmas gift!
http://www.eftaylor.com/spacetimephysics/...
I have a problem understanding what is going on in the region called the ergosphere of a "fast" Kerr black hole.
- Relativity teaches us that no frame of reference can have relative displacements greater than the speed of light, ok.
- The ergosphere of a fast Kerr black hole can contain light...
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...
If singularities don't exist in QG then what prevents particles from just collapsing falling further until they collapse into a singularity? Is there a repulsive force in QG ? Is time infinitely stretched near a singularities? What else could be happening?
When calculating the proper time along a timelike radial geodesic, with the initial condition that object the starts at rest at some Schwarzschild coordinate ##r_0>r_S##, i.e.
\frac{\mathrm{d}r}{\mathrm{d}\tau}\Bigg|_{r=r_0}=0\;,
after using the equations of motion one finds...
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...
Paradoxical scenario. Suppose Jack and Jill are sitting safely a kilometer above the event horizon (EH) of a large black hole. Now suppose:
Jack decides to head toward the center of the black hole, traveling at an easy pace (say 10 km per hour).
Jill sees Jack (with her ultrasensitive infrared...
The following paper appeared earlier this year on arxiv, entitled "Islands in Schwarzschild Black Holes":
https://arxiv.org/pdf/2004.05863.pdf
First, a bit of background: this paper appears to be part of a larger research effort aimed at resolving the black hole information paradox by showing...
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...
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...
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...
My fascination with black holes runs very deep. I was wondering if anyone could shed some light on two questions I have:
Do black holes orbit around another object, or are they "free-roaming"? If they do, what determines which object they rotate around? The place where they form, or some other...
When something approaches black hole time dilation slows the event down from our frame of reference such that nothing seems to cross the event horizon. How is it then we can observe two black holes colliding? From our frame of reference wouldn’t it seem the event never happens?
Hi All,
This is my first post, so please bare with me and if I am going all wrong about, please let me know.
The definition of a black hole according NASA; 'A black hole is a place in space where gravity pulls so much that even light can not get out'. Now I am not challenging this at all...
Mentors' note: This thread has been split off from https://www.physicsforums.com/threads/can-you-even-fall-into-a-black-hole.992212/ because it's a different interesting question that deserves it's own thread
That applies to a static black hole. But if something falls in it isn't static...
It takes infinite amount of time to cross the event horizon from an outsider's perspective. But black holes eventually decay from Hawking radiation. So if you wait long enough a black hole won't exist anymore, as it would have decayed into nothing.
The in-falling observer witnesses infinite...
Consider an observer starting a purely radial free fall from rest at infinity in outgoing Vaidya spacetime - this being a simple model for a radiating black hole. Does anyone have an explicit expression for the coordinate in-fall time (assuming purely radial motion) from infinity to event...
I notice that in a Schwarzschild black hole, at r=r_{s}/2, the c dt and dr terms are exactly the opposite of what they are in external, normal flat space (Minkowski metric). That is, one gets them by multiplying both terms by negative one. I'm having trouble grasping what this means. An...
Recently, I was tasked to find the surface area of the Schwarzschild Black Hole. I have managed to do so using spherical and prolate spheroidal coordinates. However, my lecturer insists on only using Weyl canonical coordinates to directly calculate the surface area.
The apparent problem arises...
Hello--
I have a question about space observations. It was only 4 or 5 years ago that I was taught that nothing could escape a black hole. More recently I have seen information and images of black holes. What has changed that now we have black hole obseravtions. Most recently, a black hole...