Black hole

Simulated image of a black hole.

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The abyss stares back

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“”They are wild waves of the sea, foaming up their shame; wandering stars, for whom blackest darkness has been reserved forever.

—Jude 1:13

A black hole is an astronomical body so dense that its escape velocity is greater than the speed of light. The most common explanatory mechanism for producing new black holes is the core collapse of a massive star, which produces black holes of at least ca. three times the mass of the Sun, although other mechanisms, such as the merger of two neutron stars, are possible. Black holes can merge and produce supermassive black holes, which have been found in the dense central regions of galaxies. Science long regarded black holes as theoretical objects — scientific consensus held that black holes existed because of compelling indirect evidence from multiple lines of research: accretion disk spectroscopy, stellar orbits, and the detection of gravitational waves from a black hole merger. However, an example one was finally imaged in April 2019 (see picture further down), and remaining doubts finally dissipated. (Older sources may still refer to black holes as purely hypothetical objects.)

While astrophysical evidence demonstrates the existence of black holes, or at least of something that acts exactly like, theoretically, black holes are paradoxical objects, and give rise to a lot of theoretical discussions that are leave some people freely confused about the astrophysical reality of their existence. Even physicists may commit the sin of unhinged speculation containing unprovable or infeasibly difficult-to-prove claims. General relativity (GR) essentially predicts that it can't predict what goes on at the very centre of a black hole. According to GR, an infinitely dense singularity would form. But if quantum physics is valid, such an object couldn't exist; it would have to have a physical size, even if extremely small. This incompatibility would need to be solved with a theory of quantum gravity. Stephen Hawking predicted that black holes would emit very faint radiation (Hawking radiation) due to quantum effects. The black hole information paradox is that no mathematically rigorous way has been found to correlate the information that falls into a black hole and the information embedded in Hawking radiation. Hawking's proposed solution led him to proclaim that "black holes don't exist", which is not the case; it is true only in a very technical sense which only physicists can appreciate. Nobody has solved the paradox. Nobel prizes expected for anyone who does.

However, objects with such extreme properties have also attracted varying breeds of pseudoscientists. It is as if these people want to trigger bullshit detectors to spare the experts from trying to dissect their (s)crap.

Existence and Formation

The first true image of a black hole, captured by the Event Horizon Telescope.

The first image of Saggitarius A*, the supermassive blackhole at the center of the Milky Way, captured by the EHT in 2017 and released to the public in 2022.

There are black holes that are as large as thousands to billions of solar masses at the centre of our and most other galaxies. These may have been created by the compression caused by the enormous central mass of stars mutually attracting each other until the gravity exceeded the radiation pressure. These supermassive black holes are so gravitationally powerful that their effect on the real motion of stars in close orbit around them can be (easily — with the right equipment) detected.

Black holes have been conjectured to form through the collapse of large stellar objects when the gravity of the object becomes sufficiently greater than the radiation pressure or by the huge compressive forces inside (exploding) supernovas.

It has been hypothesised that black holes might have been created at or about the time of the Big Bang. These black holes could be tiny in comparison with stellar black holes and many would probably have decayed to ordinary matter by now.

According to general relativity, black holes have only three distinguishing characteristics: mass, angular momentum, and electric charge. Two black holes with the same mass, angular momentum, and charge are truly identical objects. For this reason, some physicists speculate that black holes may be a kind of elementary particle, though a macroscopic one.

Observable properties

What gravitational lensing looks like.

Black holes have, theoretically, many outré properties of which possibly the most notable is relativistic time and space distortion. They have almost certainly been detected, by observing the radiation emitted from streams of matter from binary stars flowing into a black hole at velocities approaching light speed and by the lensing effect on light from more distant bodies as it passes close by on its way to Earth. Another method of finding black holes is examining their effect on the orbits of nearby objects, particularly stars.

Tidal forces, or rather their strength as anything with mass can produce them, are also another notable property of black holes. However, the more massive the hole the weaker they are, meaning that according to calculations, a hypothetical astronaut falling into one would be spaghettified (ie, reduced to a long, thin, strand of flesh and bones) long before it reached the event horizon of a stellar-mass black hole, but (s)he could pass through the event horizon of a supermassive black hole such as Sagittarius A* with no effects.

In 2015, the gravitational waves caused by two distant black holes colliding were observed for the first time using two Laser Interferometer Gravitational-wave Observatory (LIGO) detectors, thus validating the existence of gravity waves, a major prediction of general relativity (and, since those gravitational waves coincided with the predicted ones for a collision of two black holes, a very strong proof of the existence of these celestial bodies). The observation was announced in 2016.

In 2019, the first image of a black hole (or rather what is known as its shadow), in the center of the giant galaxy Messier 87, was captured by the Event Horizon Telescope (see the second image of this article)

In July 2021, Stephen Hawking's Black Hole Area Theorem was experimentally confirmed by observations of gravitational waves coming from merging black holes. The theorem states that the surface area of black holes is a non-decreasing function in time under specific energy conditions. Such conditions arise, for example, when two black holes merge, as has been observed. The total surface area of the resultant black hole was greater than the sum of the areas of the two black holes that formed it, which is exactly what was predicted by the theorem.

In May 2022, the first image of Sagittarius A^* (the supermassive black hole thought to exist at the heart of our galaxy, the Milky Way) was released by the Event Horizon Telescope.

Non-observable properties

Black holes are characterised by an event horizon. This is an imaginary spheroid around the object, of radius proportional to the mass of the object, at which communication from inside to outside becomes impossible (but see note below). Many relativistic effects become apparent at this distance.

The distance of the event horizon from the center of the black hole is called the Schwarzschild Radius (r_s), and is directly proportional to the black hole's mass. r_s = 2.95 kilometers per solar mass.

Hypothetical properties

It is hypothetically possible that black holes have no "physical dimensions" — that is, the object inside the event horizon is a true quantum singularity: an infinitely dense, infinitely small point of mass. To date, though, we know of no way to test this.

One of the latest hypotheses ("Brane theory") regarding the basic structure of the universe implies that black holes might contain "junction points" known as wormholes between universes at which matter/energy is passed from one universe to another. This is pure conjecture and will probably remain so. Also note, if this idea got your hopes up, that any matter going through such a junction point would be shredded by the immense tidal forces of the black hole before it could go through and the wormhole's throat would be short-lived, so you'd end up crushed to subatomic size.

Another supposition is that the dark matter of which the universe is largely composed, could consist of a very large number of microscopic black holes. This, however, has been ruled out or at the very least them being a significant part of it.

What a black hole ISN'T

A black hole is not:

• A hole (as in the equivalent of a two-dimensional (flat) hole in a wall). Not only would one in space look like a sphere or at most as a distorted spheroid if it was rotating fast enough, but also, barring wormholes as commented above, it does not carry anywhere, the afterlife included. If a black hole sucks you in, your mass is added to the black hole's one, so if there's an afterlife, then you'll go there in the usual fashion... assuming souls aren't affected by gravity. This also means that it is wrong to see them as a sort of funnel in a 2-D space (accretion disks around them have nothing to do with that).
• Totally black. If you've seen the movie "Interstellar" (if not, see image here), or the above picture of an actual black hole, you can see how we can detect one thanks to all that matter falling into one and the gravitational lensing it induces on whatever is behind it, as well as the effect that its gravitational field has on more distant objects. Even where none of these three techniques is applicable, one could still be spotted thanks to the feeble radiation emitted by matter being absorbed by it (remember that even interstellar space is not 100% empty of matter). And then there's Hawking radiation, even if it's so weak (and the more massive the hole is, the fainter it is and thus the harder to spot) that most of the time it is not noticeable at all, and, as the other Wiki states, it's indeed a very good approximation to say that a black hole is "black".
• A cosmic vacuum cleaner. Barring tidal forces and that even light cannot escape from one, a black hole has the same gravitational attraction of an object of similar mass (a star, etc), not gaining some kind of magical suction powers just because, and the only difference is that the hole being so small and not emitting radiation of any kind by itself except very feeble Hawking one would allow a hypothetical spacecraft to orbit it way closer than to a star of similar mass (assuming a stellar-mass black hole), tidal forces aside, close enough for to leave said orbit being very difficult or downright impossible due to the hole's gravitational field. A star or even an entire galaxy of them can orbit one or vice-versa with no ill effects (best example: our very galaxy, even if being strictly accurate everything, our local supermassive black hole, included, actually orbit the galactic barycenter, same in a system of a star plus a black hole), except on the former case if it's too close the hole will draw down matter from the star (just ask Cygnus X-1) or worse.
• Magic. Black holes, for all their weird properties, are still objects of this Universe that are subjected to the same laws of physics all things in the latter must obey, thus they can be explained by said laws, even if the ones we currently have break down when considering what happens on their very centers.

Hawking radiation, and black hole evaporation

See the main article on this topic: Hawking radiation

Physicist Stephen Hawking theorized that black holes slowly give up mass through a process dubbed Hawking radiation, a complicated theory involving quantum fields that he oversimplified into an explanation with virtual particles for laypeople. It is thought that through this process, even the largest of black holes would eventually disappear (where 'eventually' refers to mind-twistingly long timescales, even by cosmological standards).

The time that it would take a black hole to completely evaporate via Hawking radiation depends entirely on its mass:

$t_{\operatorname {ev} }={\frac {5120\pi G^{2}M_{0}^{3}}{\hbar c^{4}}}\;$

Thus, a black hole with one Solar mass (2 x 10³⁰ kg) would last for 2 x 10⁶⁷ years, while a black hole with a mass of 1 gram would last for 8 x 10^-26 seconds.

It's important to note that even if the maths behind it work Hawking radiation is a theoretical concept so far, as its detection is far beyond our current technological capabilities.

It might seem that the Hawking Radiation has been disproved by the Hawking area theorem which was recently proven correct. However, this is not true. Since the surface area of a black hole is proportional to its entropy and the entropy always grows, if the entropy carried out by the resultant radiation is higher than that which would result from a change in the increase in surface area, there is no contradiction. The second law of thermodynamics stays satisfied. When two black holes merge, entropy carried out by the radiation is lower than that of the area change, causing the black hole to grow rather than shrink.

Micro black holes

Hypothetically, any process — at any scale — that squeezes matter into a space smaller than its own Schwarzchild Radius will create a black hole. This could happen if subatomic particles were smashed into each other with sufficient energy, thereby creating a microscopic black hole.

Some opponents of the Large Hadron Collider (LHC) predicted that it would create such a micro black hole, which would swallow the Earth and doom us all. They have nothing to worry about. For one thing, some of the cosmic rays bombarding the Earth's atmosphere impact with more energy than the collisions in the LHC, and none of those have swallowed the Earth yet. For another, even if a microscopic black hole did pop into existence, it would evaporate via Hawking radiation so rapidly that it wouldn't have time to swallow anything. (A micro black hole weighing 1000 proton masses would evaporate in ~10^-88 seconds.)

At 10³² K each particle of matter becomes its own black hole, and the usual understanding of space and time is at an beginning end.

History

Surprisingly:

“”A paper written by the Rev John Michell in 1783 [published in 1784] was discovered in the 1970s. This is the first known discussion of the concept of a black hole. John Michell (1724-1793) was born three years before the death of Isaac Newton. He became a well-known British geologist and astronomer and was later regarded as the 'Father of Seismology' in his study of Earthquakes. He is also credited with the idea of Binary Stars, the demonstration of an inverse square law in magnetism, and was the inventor of the torsion balance before instigating the experiment, later completed by Cavendish, to weigh the Earth.

However, there are distinctions between Michell's "black hole" and modern black holes. In Michell's model, the star is still shining, but the particles of light are pulled back to the surface of the star. This is like a ball thrown on the surface of the earth with velocity less than the escape velocity. In general relativity, nothing can get past the event horizon. Moreover, in GR, all the mass is in a single point. In Michell's theory, it isn't.

Black hole-powered stars

Some theoretical research has been carried to study what effects would have small primordial black holes, from these with the mass of an asteroid to those with masses comparable to Earth, that lay in the cores of stars. For example, a Sun-like star would shine normally until after hydrogen exhaustion in the core due to standard fusion reactions, when accretion into the black hole would quench out nuclear reactions and become the main energy source of the star, which would shine for several billion years as a subgiant-like star first and a red giant-like one later, longer than a standard fusion-powered star, before becoming a black hole.

External links

• How Black Holes Work
• Hubble.org on the subject