Overview

Relativity is Einstein’s masterpiece. It changed our understanding of space, time, and gravity.

  • Special Relativity (1905): Speed of light is constant; time and space are relative.
  • General Relativity (1915): Gravity is the curvature of spacetime.

Core Idea

The core idea is invariant light speed. No matter how fast you move, light always passes you at $c$ (300,000 km/s). This forces space and time to warp to keep the math working.

Formal Definition

  • Time Dilation: Moving clocks run slow.
  • Length Contraction: Moving objects shrink.
  • Mass-Energy Equivalence: $E=mc^2$. Mass and energy are the same thing.
  • Gravity: Mass tells spacetime how to curve; spacetime tells mass how to move.

Intuition

  • Special: If you fly past me at 99% light speed, I see you moving in slow motion and squashed flat. You see me moving in slow motion and squashed flat. Both are right.
  • General: Imagine a bowling ball on a trampoline. It curves the fabric. A marble rolls toward it not because of a “force,” but because it’s following the curve.

Examples

  • GPS: Satellites move fast (Special Relativity says time slows) and are far from Earth (General Relativity says time speeds up). Engineers must correct for both, or GPS would fail in minutes.
  • Black Holes: Gravity so strong that spacetime curves infinitely; not even light can escape.
  • Twin Paradox: One twin flies to space, the other stays. The space twin returns younger.

Common Misconceptions

  • Misconception: “Everything is relative.”
    • Correction: No. The speed of light is absolute. The laws of physics are the same for everyone.
  • Misconception: It proves Newton wrong.
    • Correction: It refines Newton. At slow speeds (everyday life), Newton is still accurate.
  • Spacetime: The 4D fabric of the universe.
  • Cosmology: The Big Bang is a prediction of General Relativity.
  • Gravitational Waves: Ripples in spacetime caused by colliding black holes (detected in 2015).

Applications

  • Nuclear Power: $E=mc^2$ explains the massive energy in atoms.
  • Astronomy: Understanding the universe.

Criticism and Limitations

  • Singularities: The theory breaks down at the center of a black hole (infinite density). It needs Quantum Gravity to fix this.

Further Reading

  • Relativity: The Special and the General Theory by Albert Einstein
  • Spacetime and Geometry by Sean Carroll