How Things Work: Black Holes

Up there with the aliens from War of the Worlds, black holes are probably some of the more terrifying aspects of the universe.

Black holes form following the deaths of massive stars.

Throughout their lifetimes, stars are subject to their own gravitational fields in space. The gravitational fields create forces on the star.

One would expect this force to cause the star to collapse inward. However, there is an equally strong force in opposition to this gravitational force.

Elements of a star

This latter force is caused by the fusion reactions taking place within the star. For the majority of the star’s lifetime, hydrogen fuses inside its core to create helium.

When the star’s supply of hydrogen runs out, it becomes a red giant. The star initially expands and then shrinks to a helium core, at which point the helium atoms begin to fuse with other helium atoms to form carbon and oxygen.

In massive stars (those at least nine times as big as the sun) this process of expansion and contraction keeps going; the red giant phase typically lasts between a few hundred thousand to one million years.

Following oxygen, the star produces neon, silicon, sulfur, and then iron. The star begins fusing the element that it has just produced after each expansion and contraction.

At the end of its life, the star will fuse multiple elements at once in onion-layer shells. The outermost shell will fuse hydrogen, the original element.

Unlike the fusion of hydrogen, helium, and the other previous elements, the fusion of iron does not release energy. This is because each iron nucleus has a high binding energy, which is the amount of energy necessary to separate a nucleus into distinct parts.

Since energy is not released, the iron core continues to grow, eventually becoming so massive that it collapses inward. Following this collapse, a shockwave causes a supernova, which is an extremely bright explosion of matter.

This explosion causes the majority of the star’s matter to blow off into space, leaving a core that is both massive and compressed.
Decreasing to a radius of 10 to 20 kilometers, most massive stars collapse to form neutron stars, which are small but extremely massive (1.3 to two solar masses). Very large stars, however, collapse to form black holes.

Hence, a black hole is an enormously massive region condensed to a small radius. The effect of this dense strucure is an extremely strong gravitational pull that even light cannot escape.

The great escape

Most students in elementary physics are taught that what comes up must come down. In other words, if one throws a ball into the air, one expects it to fall back down towards the ground.

Well, if one were to launch that same ball at a speed of 11.2 kilometers per second (roughly 25,000 mph), it would not return. This value is what is known as the Earth’s escape velocity, the speed necessary for an object to overcome the Earth’s gravitational force.

Escape velocities are dependent on the distance to an object and the object’s mass. If the Earth were more massive, it would have a higher escape velocity. Similarly, if the Earth were condensed to a smaller radius, its escape velocity would also increase.

Combining large masses with small radii, black holes have escape velocities larger than even the speed of light. The speed of light is basically the de facto speed limit for the known universe; since nothing can surpass it, nothing can escape a black hole.

Black holes have a spherical surface called an event horizon, which is basically the point of no return. One way to think of the event horizon is as the region where the escape velocity is exactly equal to the speed of light.

Thus, it is impossible to exit a black hole after passing through its event horizon. Once inside a black hole, an object approaches the “singularity,” or center. The travel time from the horizon to the singularity takes fewer than 10 seconds.

At first, the object is in free fall. Free fall is the same effect that makes astronauts orbiting the Earth feel weightless. Astronauts in free fall are unable to feel the Earth’s gravitational force because the force acts evenly throughout their bodies and on their surroundings.

If an astronaut were floating in a black hole, however, such weightlessness would not last for long. As the astronaut approached the singularity, the intense gravitational force would act much more strongly on whichever half of his or her body was closer.

Eventually, this difference in forces across the body would rip the astronaut into pieces.

Physicists typically use space-time coordinates, or a coordinate system that tracks an object’s position in both space and time. Once inside the event horizon of a black hole, though, these two quantities exchange roles. That is, through the event horizon, space becomes timelike and time becomes space-like.

Normally, the progression of time is inevitable. A person continually approaches the future, no matter what.

Inside of a black hole, however, the shrinking of distance, rather than the passing of time, is the inevitable quantity. The distance between a trapped object and the singularity continues to shrink, and this process is as definite as seconds ticking off a clock. Similarly, the event horizon becomes the definite “past,” something the object inside may never return to.

The moral of the story is that when the future appears to be looming in the distance, be glad it’s not a black hole. You might have to pull an all nighter, but at least you won’t get ripped in half.