# 2: Schwarzschild Geometry


• 2.1: Non-Euclidean Geometry
A space in which the rules of Euclidean space don't apply is called non-Euclidean. The reason for bringing this up is because our modern understanding of gravity is that particles subject to gravity exhibit curved motion not because there is a force acting on them but because spacetime is non-Euclidean.
• 2.2: The Schwarzschild Metric
The Schwarzschild metric is the metric for a (non-spinning) spherically symmetric mass in an otherwise empty spacetime.
• 2.3: The Schwarzschild t-coordinate
In general, the proper time is not the same as the Schwarzschild spacetime t-coordinate separation.
• 2.4: The Schwarzschild r-coordinate
To see what the r-coordinate means in the Schwarzschild metric, consider two events with different r-coordinates but the same t-coordinate, θ-coordinate, and ϕ-coordinate. This is essentially like measuring the distance between the edges of two concentric circles.
• 2.5: Spacetime Diagrams
In Special Relativity, we constructed light cones using lines at 45 degree angles.
• 2.6: Global vs. Local Coordinates
A global coordinate system embraces a large portion of spacetime that is curved in general. Global coordinate separations don't necessarily correspond to measurable quantities.  A local inertial reference frame is a patch of curved spacetime that is small enough to be considered flat. The rules of Special Relativity can be used on an LIRF.
• 2.7: Black Hole Formation
Under normal circumstances, the gravitational attraction of the gas molecules in a star is held at bay by an outward thermal pressure. When a star runs out of fuel, however, the equilibrium is destroyed and the gas will collapse. The resulting object is called a stellar remnant, and it can take one of three forms: White Dwarf, Neutron star, or Black Hole.
• 2.8: The Global Positioning System
The Global Positioning System uses the positions of satellites along with very accurate clocks to determine the location of a GPS receiver.
• 2.9: Video Resources

Thumbnail: The supermassive black hole at the core of supergiant elliptical galaxy Messier 87, with a mass about 7 billion times that of the Sun, as depicted in the first false-colour image in radio waves released by the Event Horizon Telescope (10 April 2019). Visible are the crescent-shaped emission ring and central shadow,[19] which are gravitationally magnified views of the black hole's photon ring and the photon capture zone of its event horizon. The crescent shape arises from the black hole's rotation and relativistic beaming; the shadow is about 2.6 times the diameter of the event horizon. (CC BY 4.0; Event Horizon Telescope)

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