Assignments:

Unfinished Assignment Study Questions for Lesson 49

Lesson Objectives:

- What is a black hole?
- The event horizon and singularity
- Do black holes exist?
- Gamma ray bursts and gravitational waves

A black hole is what happens when gravity in an area gets so strong that not even light can escape. That is because as we have learned before, the stronger the gravitational force in an area, for example, around a planet, the higher the escape velocity needed to escape its gravity. In a black hole, there is so much mass packed into such a small area that the escape velocity has become greater than the speed of light. Since nothing can move faster than the speed of light, nothing can escape from within a black hole.

According to Einstein, space and time are bound together as four-dimensional spacetime, and a black hole is a curvature of spacetime that is so great that it essentially forms a bottomless pit. As you get closer to a black hole and the pull of its gravity gets stronger, you will eventually reach what is known as the event horizon. That is the boundary where the escape velocity has reached the speed of light. Beyond the event horizon, nothing can escape.

From the outside, the event horizon is actually shaped like a sphere. The size of this sphere is called the Schwarzschild radius of the black hole and depends on the black hole's mass. Basically, any collapsing stellar core becomes a black hole at the moment it shrinks to a size smaller than its Schwarzschild radius. At that point, the core disappears from view since no light can escape from it and the outward appearance tells us nothing about what falls in. For a black hole with the mass of our Sun, the Schwarzschild radius would be about three kilometers.

It is impossible to observe what happens inside of a black hole, but scientists believe that all the matter in a black hole should be crushed by gravity into an infinitely tiny and dense point in the center. This point is called a singularity. According to Einstein, spacetime should grow infinitely curved as it enters the pointlike singularity.

A black hole could be orbited just like any other object with the same mass, but if a person were to fall toward the black hole, a couple of things would happen. First, the light would become increasingly redshifted as he got closer to the event horizon until it eventually becomes so redshifted that we cannot see him. Second, time would seem to slow down until it comes to a complete stop for the person and he would seem to take forever to fall in.

From the perspective of the person falling in, time would appear to pass normally. However, the strength of the tidal force near the black hole would be so strong that no human could survive the stretching and pulling forces.

As we have learned, white dwarfs cannot exceed 1.4 times the mass of the Sun because that is when gravity would overcome electron degeneracy pressure. Calculations show that for neutron stars, around 3 solar masses is the limit before gravity overcomes *neutron* degeneracy pressure and the core collapses.

In a high-mass star, life ends when its iron core collapses and explodes as a supernova, leaving behind a neutron star. It is believed that if the star is massive enough, some of its outer layers will remain and fall back onto the neutron core, causing its mass to rise above the 3 solar mass limit. When the neutron star's mass exceeds this mass limit, gravity overcomes neutron degeneracy pressure, the core collapses in on itself, and what results is a black hole.

Another way for neutron stars to exceed the neutron-star mass limit is when two neutron stars in a close binary system merge.

Observed evidence of black holes is found in studies of X-ray binaries in close binary systems. Just as neutron stars in binary systems can form accretion disks that emit strong X-ray radiation, black holes should also form accretion disks that emit X-rays. Astronomers have found X-ray binaries where the object emitting the X-rays has a mass clearly exceeding the 3 solar mass neutron star limit, indicating it would have to be a black hole.

Gamma ray bursts are the most powerful and luminous bursts of energy known in the universe and most of them are believed to come from extremely powerful supernova explosions that form black holes.

Other gamma ray bursts may occur when two neutron stars or a neutron star and a black hole collide in a binary star system.

When two black holes merge, scientists believe they release massive waves of gravitational radiation that can be detected from billions of light-years away. What are gravitational waves?

They are ripples in spacetime caused when a massive object is accelerated. Since black holes are so massive, they would pour out gravitational waves as they orbit each other faster and faster, losing orbital energy. Right before they merge, the gravitational waves would be strongest. The Laser Interferometer Gravitational-Wave Observatory (LIGO) first detected such gravitational waves in 2015. These waves represent the strongest evidence yet for the existence of black holes.