Black Holes and General Relativity

Learning Objectives

Define a black hole and an event horizon.

Explain how black holes are formed from the collapse of massive stars.

Describe the principle of equivalence from Einstein's General Relativity.

Explain gravitational lensing as a key piece of evidence for general relativity.

The Edge of Spacetime

Black Holes

A black hole is a region of spacetime where gravity is so strong that nothing—not even light—can escape.

Event Horizon: The 'point of no return'. It is the boundary surrounding a black hole beyond which the escape velocity exceeds the speed of light. Anything that crosses the event horizon cannot get out.

Singularity: At the center of a black hole is a point of infinite density where the known laws of physics break down.

Formation: Stellar-mass black holes are formed when a very massive star (> 20-25 times the Sun's mass) exhausts its nuclear fuel. The core collapses under its own immense gravity, crushing it down to a singularity. The outer layers of the star are blown away in a supernova explosion.

Einstein's General Relativity

Published in 1915, General Relativity is our modern theory of gravitation.

Core Idea: Gravity is not a force, but a consequence of the curvature of spacetime. Massive objects warp or curve the fabric of spacetime around them, and other objects simply follow these curves. As John Wheeler famously put it, 'Spacetime tells matter how to move; matter tells spacetime how to curve.'

Principle of Equivalence: This is the core idea that led Einstein to his theory. It states that there is no experiment an observer can perform to distinguish between being in a uniform gravitational field and being in a uniformly accelerating reference frame. Gravity and acceleration are equivalent.

Evidence for General Relativity

Gravitational Lensing: The path of light is bent as it passes by a massive object. This is because the light is following the curved spacetime around the object. This can cause the images of distant galaxies to appear distorted, magnified, or as multiple images (an 'Einstein Cross'). This has been observed and confirmed many times.

Gravitational Waves: Ripples in the fabric of spacetime caused by cataclysmic events, like the merger of two black holes. They were first directly detected in 2015 by LIGO.

Key Terms

Black Hole: A region of spacetime where gravity is so strong that nothing, not even light, can escape from it.
Event Horizon: The boundary defining the region of space around a black hole from which nothing (not even light) can escape.
Spacetime: The four-dimensional continuum in which all events in the universe take place, combining the three dimensions of space and the one dimension of time.
General Relativity: The geometric theory of gravitation developed by Albert Einstein, which describes gravity as a curvature of spacetime caused by mass and energy.
Gravitational Lensing: A distribution of matter (such as a cluster of galaxies) between a distant light source and an observer, that is capable of bending the light from the source as the light travels towards the observer.