Black Holes

Physics 1040 2012James Chambers

What is a Black hole?

• The theory of general relativity predicts that a sufficiently compact mass will deform space-time to form a black hole.

• A black hole is an object that has collapsed under its own weight to a point, creating an object that is fantastically small, yet enormously dense. It sucks in everything it can absorb, and once formed, nothing, not even light, can escape its gravitational pull.

There are two main types of black holes.

• 1. Stellar Black Holes are born from the death of stars, which are roughly a few times the mass of our sun. These stars end their lives when the hydrogen fuel that makes up the star's interior burns off, causing the star to collapse.

• 2. And supermassive black holes, which range in mass from a few hundred thousand times the mass of our sun to a few billion times that mass, and exist at the center of galaxies.

• The black hole that lives at the center of the Milky Way is 4 million times the mass of our sun.

There are two schools of thought on how supermassive black holes form.

• It's possible that they are seeded from the death of the earliest stars of the universe, which were massively large.

• Another, recent theory involves discs of gas that swirl and funnel like a tornado. The early universe was filled with gas and radiation. In some spots, gravity caused gas to fall into halos of dark matter and form into gas discs. Instabilities in these discs caused the swirling effect, in which the gas begins to funnel from the outside in.

Evidence of Black Holes have been theorized about around for hundreds of years.

• Newton showed there was evidence of black holes. In his theory, an energy argument tells us that there is an escape velocity from the surface of any spherical object of mass. If this velocity is greater than the speed of light then light from this object cannot escape to infinity. Thus the condition for such an "unseeable" object is “a black hole”

The difference between Einstein and Newton’s theory’s on gravity and black holes

• in Newton's theory a particle could overcome a black holes gravity with strong enough engines to provide the energy needed for escape.

• This is not so in general relativity, Einstein's theory of gravitation. In that theory, escaping the black hole is equivalent to moving faster than light, an impossibility in relativity.

• 1. Quiet region• 2. Ergosphere• 3. Event Horizon• 4. Singularity

•1. The quiet zone is the area just inside the black holes influence but still far enough away that its pull still negligible.

2. The Ergosphere

• a region of space-time in which it is Impossible to stand still.

• “Frame-dragging” General relativity predicts that any rotating mass will tend to slightly "drag" along the space-time immediately surrounding it. Any object near the rotating mass will tend to start moving in the direction of rotation.

For a rotating black hole, this effect become so strong near the event horizon that an object would have to move faster than the speed of light in the opposite direction to just stand still.

3. The Point of No Return “The Event Horizon”

• Where reality starts to come apart

• Around a black hole there is a mathematically defined surface called an event horizon that marks the point of no return.

• It is called "black" because it absorbs all the light that hits the horizon, reflecting nothing, just like a perfect black body in thermodynamics.

• Quantum mechanics predicts that event horizons emit radiation like a black body with a finite temperature.

• The event horizon is referred to as such because if an event occurs within the boundary, information from that event cannot reach an outside observer, making it impossible to determine if such an event occurred.

• To a distant observer, clocks near a black hole appear to tick more slowly than those further away from the black hole. Due to this effect, known as gravitational time dilation, an object falling into a black hole appears to slow down as it approaches the event horizon, taking an infinite time to reach it.

• At the same time, all processes on this object slow down causing emitted light to appear redder and dimmer, an effect known as gravitational redshift. Eventually, at a point just before it reaches the event horizon, the falling object becomes so dim that it can no longer be seen.

• Although someone entering a black hole would not notice any of these effects as they crossed the event horizon. According to his own clock, he crossed the event horizon after a finite time.

How matter is pulled into a Black hole• Matter will orbit a black hole slowly in

a decaying orbit.

If a craft were to enter the event horizon it would be subject to Spaghettification

• It would be disassembled.• The process of spaghettification is as follows. • First, the object that is falling into the black hole splits in

two. Then the two pieces each split themselves, rendering a total of four pieces. Then the four pieces split to form eight. This process of bifurcation continues up to and past the point in which the split-up pieces of the original object are at the order of magnitude of the constituents of atoms. At the end of the spaghettification process, the object is a string of elementary particles.

4. The Singularity

• At the center of a black hole lies the singularity, where matter is crushed to infinite density, the pull of gravity is infinitely strong, and space-time has infinite curvature.

• Space-time curvature becomes so strong that the general relativistic laws break down and laws of quantum gravity take over.

• Here it's no longer meaningful to speak of space and time, much less space-time. Space and time cease to exist as we know them.

• Enter the strange world of quantum gravity.

Birth of a Black Hole • Gravitational Collapse When a gigantic star reaches the final stage of its life and is about to go supernova as it spent all of it’s nuclear fuel. So it stops burning and heating up and cannot create the nuclear energy required to feed the star and let it to continue the balance to support its own gravitational draw against the intense pressures brewing inside.

• The radius of the star shrinks to a critical size, called the Schwarzschild radius and it starts to devour anything and everything that comes a bit too close, including light. Gravity does its job and the core of the star caves in and implodes. The outer shells of the star explode into the space. They may even fall into the already dense black hole making it even heavier and denser.

• Basically what happens is that nuclear fusion reactions take place in the core of the star which causes an acute outward pressure but that pressure is optimally balanced by the intense inward pull of gravity by the star’s mass. But when a star is in its death throes, the fusion reactions combining hydrogen into helium (like in the Sun) stop and a new kind of nuclear reaction take place that convert helium into carbon. This is followed by carbon turning into oxygen and oxygen to silicon and then to iron.

• That’s the point where nuclear fusion stops and the outer layers of all the elements produced (hydrogen, helium, carbon & silicon) keep burning around the central core of iron.

• The mammoth iron core builds up and finally explodes which is called a supernova explosion.

• A star with a mass greater than 2 times that of our sun, after going supernova its shrinks and compresses into a new black hole.

A Supernova

Supermassive black holes • Are present in the centers of most galaxies, including our very own the

Milky Way.

• If you put billions of suns together, you will know the mass of one supermassive black hole. And that’s why the name as they are super-colossal in size.

• Astronomers aren’t very sure as to how black holes are formed of the supermassive variety. Maybe a couple of smaller black holes coagulate together, or humongous gas clouds cave in to form them.

• The British Scientist Stephen Hawking proposed that trillions of nonstellar black holes or mini or primordial black holes were created along with the universe in accordance with the ‘big bang’ theory

How a Black Hole dies!• This discovery came about when Stephen

Hawking discovered that black holes should radiate energy due to quantum mechanical processes. This radiation is called Hawking radiation. As a black hole radiates energy, it shrinks and the more it shrinks, the more it radiates and so finally it will completely evaporate.

• However, the timescale for this is extremely long: a black hole of the mass of the Sun will take more than a billion times a billion times etc. of the age of the universe to evaporate completely!

A lot of the information used was from the class text book Understanding Our Universe by Palen, Kay, Smith, Bluemthal, Nov. 2011