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 10
^{17}. - A new black hole is born in a supernova every second. However, the nearest observed black hole is 1600 light years away.