Overview
This lecture explains the major effects of general relativity, focusing on what happens when someone falls into a black hole, with emphasis on different points of view.
Main Effects of General Relativity
- Mass in space bends spacetime, creating gravity.
- Gravity slows down time near massive objects.
- Gravity causes light's wavelength to stretch (redshift) as it moves away from massive objects.
Falling Into a Black Hole: The Scenario
- A person in a box is dropped towards a black hole, with observers in a distant spaceship.
- The gravitational force increases dramatically as the box approaches the black hole.
- The front of the box (closer to the black hole) feels more gravity than the back, causing stretching.
Tidal Effects: Spaghettification
- The box and person stretch into long, thin shapes as gravity differences increase—called spaghettification.
- Spaghettification occurs from all perspectives, not due to relativity.
- The human body cannot survive this stretching and is torn apart before reaching the black hole.
Spaceship Point of View
- Observers see the box get spaghettified.
- The box's color shifts from blue through the spectrum to infrared, then becomes invisible to the human eye (extreme redshift).
- Time in the box appears to slow down near the event horizon and stops at the singularity.
- The person seems to freeze in time and disappear into the black hole.
Box (Falling Person) Point of View
- Experiences normal time for themselves, even as they are spaghettified.
- Looking back at the spaceship, external time appears to speed up dramatically.
- The box's color appears unchanged to the falling person.
- After crossing the event horizon, can still see out, but cannot escape.
Inside the Event Horizon
- Falling person can see out of the event horizon, but no information can escape from within.
- Approaching the singularity, time stops, and the person is crushed by infinite gravity.
Key Terms & Definitions
- General Relativity — Einstein’s theory describing how mass bends spacetime and affects gravity and time.
- Spaghettification — Extreme tidal stretching near a black hole that elongates objects.
- Redshift — The increase in wavelength (shift to red) of light as it moves away from a massive object.
- Event Horizon — The boundary around a black hole beyond which nothing can escape.
- Singularity — The point at the center of a black hole where gravity is infinite and time stops.
Action Items / Next Steps
- Review the effects of general relativity and tidal forces near black holes.
- Prepare for test questions comparing different observers' perspectives (spaceship vs. falling person).