Black Hole

History of the black hole

The concept of a body so massive that even light could not escape was put forward by the geologist John Michell in a letter written to Henry Cavendish in 1783.

“If the semi-diameter of a sphere of the same density as the Sun were to exceed that of the Sun in the proportion of 500 to 1, a body falling from an infinite height towards it would have acquired at its surface greater velocity than that of light, and consequently supposing light to be attracted by the same force in proportion to its vis inertiae, with other bodies, all light emitted from such a body would be made to return towards it by its own proper gravity.”

This assumes that light is influenced by gravity in the same way as massive objects.

The idea of black holes was largely ignored in the nineteenth century, since light was then thought to be a massless wave and therefore not influenced by gravity.

Definition

Black holes are the evolutionary endpoints of stars at least 10 to 15 times as massive as the Sun. If a star that massive or larger undergoes a supernova explosion, it may leave behind a fairly massive burned out stellar remnant. With no outward forces to oppose gravitational forces, the remnant will collapse in on itself. The star eventually collapses to the point of zero volume and infinite density, creating what is known as a “singularity “. As the density increases, the path of light rays emitted from the star are bent and eventually wrapped irrevocably around the star. Any emitted photons are trapped into an orbit by the intense gravitational field; they will never leave it. Because no light escapes after the star reaches this infinite density, it is called a black hole.

A black hole is a region of space in which the gravitational field is so powerful that nothing can escape after having fallen past the event horizon. The name comes from the fact that even electromagnetic radiation (e.g. light) is unable to escape, rendering the interior invisible. However, black holes can be detected if they interact with matter outside the event horizon, for example by drawing in gas from an orbiting star. The gas spirals inward, heating up to very high temperatures and emitting large amounts of radiation in the process.

Size of Black Holes

There are at least two different ways to describe how big something is. We can say how much mass it has, or we can say how much space it takes up. If we think about the masses of black holes, there is no limit in principle to how much or how little mass a black hole can have. Any amount of mass at all can in principle be made to form a black hole if we compress it to a high enough density. We suppose that most of the black holes that actually exist out there were produced in the deaths of massive stars, and so we expect those black holes to weigh about as much as a massive star.

Black holes can be divided into several size categories:

  • Super massive black holes that contain millions to billions of times the mass of the sun are believed to exist in the center of most galaxies, including our own Milky Way. They are thought to be responsible for active galactic nuclei.
  • Intermediate-mass black holes, whose size is measured in thousands of solar masses, may exist. Intermediate-mass black holes have been proposed as a possible power source for ultra-luminous X ray sources.
  • Stellar-mass black holes have masses ranging from about 1.5-3.0 solar masses (the Tolman-Oppenheimer-Volkoff limit) to 15 solar masses. These black holes are created by the collapse of individual stars. Stars above about 20 solar masses may collapse to form black holes; the cores of lighter stars form neutron stars or white dwarf stars. In all cases some of the star’s material is lost (blown away during the red giant stage for stars that turn into white dwarfs, or lost in a supernova explosion for stars that turn into neutron stars or black holes).
  • Micro black holes, which have masses at which the effects of quantum mechanics are expected to become very important. This is usually assumed to be near the Planck mass. Alternatively, the term micro black hole or mini black hole may refer to any black hole with mass much less than that of a star. Black holes of this type have been proposed to have formed during the Big Bang (primordial black holes), but no such holes have been detected as of 2007.

Types of Black Holes

Astrophysicists currently classify black holes according to their angular momentum (non-zero angular momentum means the black hole is rotating) and electric charge:

Non Rotating Rotating
Uncharged Schwarzschild Kerr
Charged Reissner-Nordstorm Kerr-Newman

Astrophysicists expect that almost all black holes will rotate, because the stars from which they are formed rotate. In fact most black holes are expected to spin very rapidly, because they retain most of the angular momentum of the stars from which they were formed but concentrated into a much smaller radius.

Major features of non-rotating, uncharged black holes

Event horizon  

This is the boundary of the region from which not even light can escape, but at the same time, light does not get sucked into the black hole. Stephen Hawking, in his book, “A Brief History of Time”, describes the event horizon as “the point at which light is just barely unable to escape.” The event horizon is the defining feature of a black hole – it is black because no light or other radiation can escape from inside it. So the event horizon hides whatever happens inside it and we can only calculate what happens by using the best theory available, which at present is general relativity. The gravitational field outside the event horizon is identical to the field produced by any other spherically symmetric object of the same mass. The popular conception of black holes as “sucking” things in is false. Objects can maintain an orbit around black holes indefinitely provided they stay outside the photon sphere.

Singularity

According to general relativity, a black hole’s mass is entirely compressed into a region with zero volume, which means its density and gravitational pull are infinite, and so is the curvature of space-time which it causes. These infinite values cause most physical equations, including those of general relativity, to stop working at the center of a black hole. So physicists call the zero-volume, infinitely dense region at the center of a black hole a “singularity”.

A photon sphere

A non-rotating black hole’s photon sphere is a spherical boundary of zero thickness such that photons moving along tangents to the sphere will be trapped in a circular orbit. For non-rotating black holes, the photon sphere has a radius 1.5 times larger than the radius of the event horizon.

Accretion disk

Space is not a pure vacuum even interstellar space contains a few atoms of Hydrogen per cubic centimeter. The powerful gravity field of a black hole pulls this towards and then into the black hole. The gas nearest the event horizon forms a disk and, at this short range, the black hole’s gravity is strong enough to compress the gas to a relatively high density. The pressure, friction and other mechanisms within the disk generate enormous energy. Infect they convert matter to energy more efficiently than the nuclear fusion processes that power stars. As a result, the disk glows very brightly, although disks around black holes radiate mainly X-rays rather than visible light. However Accretion disks are not proof of the presence of black holes, because other massive, ultra-dense objects such as neutron stars and white dwarfs cause accretion disks to form and to behave in the same ways as those around black holes.

Major features of rotating black holes

Rotating black holes share many of the features of non-rotating black holes – inability of light or anything else to escape from within their event horizons, accretion disks, etc. But general relativity predicts that rapid rotation of a large mass produces further distortions of space-time in addition to those which a non-rotating large mass produces, and these additional effects make rotating black holes strikingly different from non-rotating ones.

Two important surfaces around a rotating black hole. The inner sphere is the static limit (the event horizon). It is the inner boundary of a region called the ergosphere. The oval-shaped surface, touching the event horizon at the poles, is the outer boundary of the ergosphere. Within the ergosphere a particle is forced to rotate and may gain energy at the cost of the rotational energy of the black hole.

What happens if something falls into a black hole?

An object in very strong gravitational field feels a tidal force stretching it in the direction of the object generating the gravitational field. This is because of the inverse square law. It causes nearer parts of the stretched object to feel a stronger attraction than farther parts. Near black holes, the tidal force is expected to be strong enough to deform any object falling into it, even atoms or composite nucleons, this is called spaghettification.

An object in a gravitational field experiences a slowing down of time, called gravitational time dilation, relative to observers outside the field. The observer will see that physical processes in the object, including clocks, appear to run slowly. As a test object approaches the event horizon, its gravitational time dilation (as measured by an observer far from the hole) would approach infinity.

From the viewpoint of a distant observer, an object falling into a black hole appears to slow down, approaching but never quite reaching the event horizon: and it appears to become redder and dimmer, because of the extreme gravitational red shift caused by the gravity of the black hole. Eventually, the falling object becomes so dim that it can no longer be seen, at a point just before it reaches the event horizon. All of this is a consequence of time dilation: the object’s movement is one of the processes that appear to run slower and slower, and the time dilation effect is more significant than the acceleration due to gravity; the frequency of light from the object appears to decrease, making it look redder, because the light appears to complete fewer cycles per “tick” of the observer’s clock; lower-frequency light has less energy and therefore appears dimmer. As an in falling object approaches the singularity, tidal forces acting on it approach infinity. All components of the object, including atoms and subatomic particles, are torn away from each other before striking the singularity.

How can we know Black Holes exists ?

Since black holes are small (only a few to a few tens of kilometers in size), and light that would allow us to see them cannot escape, a black hole floating alone in space would be hard, if not impossible, to see. We can’t see a black hole directly since light can’t get past the horizon. That means that we have to rely on indirect evidence that black holes exist.

Accretion disks and gas jets

Extremely large accretion disks and gas jets may be good evidence for the presence of super massive black holes, because as far as we know any mass large enough to power these phenomena must be a black hole.

Strong radiation emissions

Neutron stars and other very dense stars have surfaces, and matter colliding with the surface at a high percentage of the speed of light will produce intense flares of radiation at irregular intervals. Black holes have no material surface, so the absence of irregular flares round a massive, ultra-dense object suggests that there is a good chance of finding a black hole there.

Intense but one-time gamma ray bursts (GRBs) may signal the birth of “new” black holes, because astrophysicists think that GRBs are caused either by the gravitational collapse of giant stars or by collisions between neutron stars and both types of event involve sufficient mass and pressure to produce black holes. But it appears that a collision between a neutron star and a black hole can also cause a GRB, so a GRB is not proof that a “new” black hole has been formed. All known GRBs come from outside our own galaxy, and most come from billions of light years away so the black holes associated with them are actually billions of years old.

Gravitational lensing

A gravitational lens is formed when the light from a very distant, bright source (such as a quasar) is “bent” around a massive object (such as a black hole) between the source object and the observer. The process is known as gravitational lensing, and is one of the predictions of Albert Einstein’s general theory of relativity. According to this theory, mass “warps” space-time to create gravitational fields and therefore bend light as a result.

Objects orbiting possible black holes

Many stars come in binary systems — pairs of stars in orbit around each other. If one of the stars in such a binary system becomes a black hole, we might be able to detect it. In particular, in some binary systems containing a compact object such as a black hole, matter is sucked off of the other object and forms an “accretion disk” of stuff swirling into the black hole. The matter in the accretion disk gets very hot as it falls closer and closer to the black hole, and it emits copious amounts of radiation, mostly in the X-ray part of the spectrum. Many such “X-ray binary systems” are known, and some of them are thought to be likely black-hole candidates.

Micro black holes

There is a theoretical possibility that a micro black hole might be created inside a particle accelerator. These black holes are not the same as gravitational black holes, but they are vital testing grounds for quantum theories of gravity.

 

Black Holes evaporation?

In 1970’s, Stephen Hawking came up with theoretical arguments showing that black holes are not really entirely black: due to quantum-mechanical effects, they emit radiation. The energy that produces the radiation comes from the mass of the black hole. Consequently, the black hole gradually shrinks. It turns out that the rate of radiation increases as the mass decreases, so the black hole continues to radiate more and more intensely and to shrink more and more rapidly until it presumably vanishes entirely.

 

White Holes

The equations of general relativity have an interesting mathematical property: they are symmetric in time. That means that you can take any solution to the equations and imagine that time flows backwards rather than forwards, and you’ll get another valid solution to the equations. If you apply this rule to the solution that describes black holes, you get an object known as a white hole. Since a black hole is a region of space from which nothing can escape, the time-reversed version of a black hole is a region of space into which nothing can fall. In fact, just as a black hole can only suck things in, a white hole can only spit things out.

White holes are a perfectly valid mathematical solution to the equations of general relativity, but that doesn’t mean that they actually exist in nature. In fact, they almost certainly do not exist, since there’s no way to produce one. Producing a white hole is just as impossible as destroying a black hole.

 

Wormhole

We have been talking all along about black holes that are not rotating and have no electric charge. If we consider black holes that rotate and/or have charge, things get more complicated. In particular, it is possible to fall into such a black hole and not hit the singularity. In effect, the interior of a charged or rotating black hole can “join up” with a corresponding white hole in such a way that you can fall into the black hole and pop out of the white hole. This combination of black and white holes is called a wormhole.

Unfortunately, worm holes are more science fiction than they are science fact. A wormhole is a theoretical opening in space-time that one could use to travel to far away places very quickly. The wormhole itself is two copies of the black hole geometry connected by a throat – the throat, or passageway, is called an Einstein-Rosen bridge. It has never been proved that worm holes exist and there is no experimental evidence for them.

 

Black holes, Solar System and Earth

Black holes are sometimes listed among the most serious potential threats to Earth and humanity, on the grounds that a naturally-produced black hole could pass through our Solar System.

Stellar-mass black holes travel through the Milky Way just like stars. Consequently, they may collide with the Solar System or another planetary system in the galaxy, although the probability of this happening is very small. Significant gravitational interactions between the Sun and any other star in the Milky Way (including a black hole) are expected to occur approximately once every 1019 years. For comparison, the Sun has an age of only 5 × 109 years, and is expected to become a red giant about 5 × 109 years from now, incinerating the surface of the Earth. Hence it is extremely unlikely that a black hole will pass through the Solar System before the Sun exterminates life on Earth.

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