1.2 A History of Black Holes

summary

Aristotle believed that we are at the center of the universe, and space and time are absolute.

Copernicus said that we are not at the center of the universe, and that elsewhere must be similar to here.
Galileo proposed that there is no such state as absolute rest and moving observers see the same physics as stationary observers. He also believed that forces act instantaneously across space.
Newton proposed that gravity acts on all objects equivalently.
Maxwell showed that electricity and magnetism are the same force, which propagates at the speed of light, not instantaneously.
Einstein realized that in order for the speed of light to be the same for everyone, neither space nor time could be absolute, and must exist as a single construct called spacetime.
A decade later, Einstein proposed that spacetime itself was warped by matter and energy, creating gravity.

In ordinary life, we normally don’t notice relativistic effects.

However, the GPS in your cell phone would accumulate errors at a rate of 20 feet per second if it were not accounted for.
The effects become increasingly more dramatic on a cosmic scale.
While we can’t see inside black holes, we can see the warping of light produced by their gravitational lensing.

We can visualize spacetime on a graph by taking one of the spatial dimensions to represent time.

Using this set up, we can think of light outward over time as creating a ‘light cone’
As a light cone moves closer to a black hole, it tilts, until at the edge of a black hole one of its edges is vertical.
General relativity cannot predict what happens at a singularity in spacetime, and in this sense, predicts its own downfall.

Black holes are formed through the gravitational collapse of a star.

Nuclear fusion converts lighter elements into heavier ones.
If it grows dense enough, pressure will no longer be able to counteract gravity.
Gravitational potential energy will then be released in a supernova as the star collapses, creating a neutron star, or, if dense enough, a black hole.

It is impossible to observe black holes directly, but we can observe their effects on surrounding regions.

They alter the orbits of stars, and, if the stars get close enough, can tear them apart.
The merger of two black holes can create a gravitational wave, the first of which was detected in 2015.
Using these observations, we can estimate how many black holes are in the universe.
In our galaxy alone, there are over 100,000,000 black holes. The universe has 10¹⁷.
A new black hole is born in a supernova every second. However, the nearest observed black hole is 1600 light years away.