(6) GRAVITY "General relativity" (Brian Green's clip - Elegant Universe)

The Discovery of Gravity

Isaac Newton and the Concept of Gravity

  • Year: 1665

  • A young man, Isaac Newton, witnesses an apple fall from a tree, leading to a groundbreaking theory.

  • Proposal: The force pulling the apple to the ground and the force keeping the moon in orbit are the same.

  • Unified the heavens and the Earth under a single theory called gravity.

Implications of Newton's Theory

  • Unification: The same laws governing planets also govern tides and falling objects.

  • Gravity was the first scientific force understood; its laws were remarkably accurate even after over 300 years.

  • Newton's equations enabled precise predictions, such as calculating rocket trajectories necessary for moon landings.

The Problem with Newton's Theory

Limitations of Newton's Understanding

  • Newton accurately described the strength of gravity but could not explain its mechanism.

  • For nearly 250 years, scientists did not question this limitation.

Albert Einstein's Revolutionary Insights

  • Early 1900s: Albert Einstein, a clerk in the Swiss Patent Office, begins contemplating light's behavior and its implications.

  • At age 26, Einstein discovers that the speed of light is a universal limit; nothing can surpass this speed.

  • Conflict arises: Einstein’s theory contradicts Newton's instantaneous gravity.

Experiments to Understand Gravity

The Cosmic Catastrophe Experiment

  • Hypothetical scenario: The sudden disappearance of the Sun.

  • According to Newton: planets would instantly fly out of their orbits.

  • Einstein's challenge: Light takes 8 minutes to reach Earth, meaning gravity cannot operate faster than this information transfer.

Einstein's Reinterpretation

  • To reconcile Newtonian physics with light's finite speed, Einstein posits a new understanding of gravity.

  • Space-Time Concept: Einstein theorizes that space and time are interconnected in a single fabric.

  • The presence of mass warps this fabric, creating what we experience as gravity.

The New Picture of Gravity

Rethinking the Cosmic Catastrophe

  • In Einstein's model, if the Sun vanishes, it creates ripples in the fabric of space-time, similar to ripples in water.

  • These ripples travel at the speed of light, explaining why we wouldn't instantly feel the effects of the Sun's disappearance.

  • Gravity is redefined as the curvature and warping of the spatial fabric caused by mass.

General Relativity

  • Einstein's new theory is called general relativity.

  • This discovery revolutionized the understanding of gravity and earned Einstein household recognition shortly thereafter.