The Cavendish Experiment and Other Landmark Experiments in Gravity

Other Foundational Gravity Experiments

Table of Contents
  • Introduction
  • The Cavendish Experiment (1798)

  • Galileo's Inclined Plane (1604)

  • Newton’s Pendulum and Orbital Analogies (1687)
  • Eötvös Torsion Balance (1889)
  • Eddington's Solar Eclipse (1919)
  • Pound–Rebka Experiment (1959)
  • Gravity Probe B (2004–2011)
  • LIGO and Gravitational Wave Detection (2015–Present)
  • Summary
Introduction

Experiments are the pillars of physics—especially in gravitational science, where theory and observation are tightly interwoven. From Galileo’s inclined planes to Einstein’s predictions confirmed by radio telescopes and interferometers, our understanding of gravity has matured through carefully designed experiments. Among them, the Cavendish experiment stands out as the first direct measurement of gravitational attraction between two masses, enabling the first calculation of Earth's mass and validating Newton’s Law of Universal Gravitation.

The Cavendish Experiment (1798)

Performed by: Henry Cavendish Objective: To “weigh the Earth”—i.e., to determine the gravitational constant $G$

Experimental Design:

Cavendish used a torsion balance consisting of:

  • A horizontal rod suspended from a thin wire.
  • Two small lead spheres attached to the ends of the rod.
  • Two much larger lead spheres placed nearby.

As the large spheres attracted the small ones via gravity, the torsion wire twisted, and the degree of rotation was used to calculate the very small gravitational force between the masses.

Results:

Cavendish did not explicitly calculate $G$, but his data allowed later scientists to derive:

$$ G \approx 6.674 \times 10^{-11} \, \text{N·m}^2/\text{kg}^2 $$

This was the first laboratory-scale confirmation of Newton’s theory and laid the groundwork for gravitational constant measurements.

Other Foundational Gravity Experiments

1. Galileo's Inclined Plane Experiments (1604)

Goal: To study the motion of falling bodies. Key Insight: Objects fall with uniform acceleration, independent of mass (neglecting air resistance).

  • Used inclined planes to slow down motion for precise timing.
  • Refuted Aristotle’s claim that heavier objects fall faster.
  • Introduced the concept of acceleration due to gravity, foundational to kinematics.
2. Newton’s Pendulum and Orbital Analogies (1687)

Key Tool: Pendulums and geometric reasoning Key Insight: The same force that pulls an apple downward keeps the Moon in orbit.

  • Used pendulum experiments to measure local gravity.
  • Demonstrated inverse-square law by comparing free fall and lunar motion.
  • Unified celestial and terrestrial mechanics.
3. Eötvös Experiment (1889)

Performed by: Loránd Eötvös Objective: To test the equivalence of inertial and gravitational mass.

  • Used a torsion balance to compare the gravitational acceleration of different substances.
  • Verified that acceleration due to gravity is independent of composition.
  • Laid the foundation for Einstein's Equivalence Principle, a cornerstone of General Relativity.
4. Eddington’s Solar Eclipse Observation (1919)

Goal: To test Einstein’s prediction of light bending by gravity.

  • During a total solar eclipse, Eddington photographed stars near the Sun.
  • Found that their apparent positions shifted due to spacetime curvature.
  • Provided the first empirical confirmation of General Relativity.
  • Made Einstein a global scientific icon.
5. Pound–Rebka Experiment (1959)

Location: Harvard University Objective: To measure gravitational redshift.

  • Used gamma rays emitted from iron nuclei in a tower.
  • Observed slight frequency shift due to gravitational potential difference.
  • Confirmed that clocks run slower in stronger gravity, as predicted by GR.
6. Gravity Probe B (2004–2011)

NASA mission testing two predictions of General Relativity:

  • Geodetic effect: warping of spacetime by Earth’s mass.
  • Frame-dragging: Earth “drags” spacetime as it rotates.

Method: Ultra-precise gyroscopes in orbit around Earth. Outcome: Confirmed both effects to within 0.28% (geodetic) and 19% (frame-dragging).

7. LIGO – Detection of Gravitational Waves (2015–Present)

Goal: Detect ripples in spacetime from massive events.

  • First detection in 2015 from merging black holes.
  • Confirmed Einstein’s 1916 prediction.
  • Gravitational waves now provide a new observational window into the universe (multi-messenger astronomy).
Summary

From Galileo’s rolling balls to LIGO’s laser interferometers, gravity experiments have progressively confirmed and extended our understanding of one of nature’s most elusive forces. The Cavendish experiment remains historically significant as the first to directly detect gravitational interaction between ordinary masses.

Each generation of physicists has built upon the previous, using more sensitive instruments and deeper theoretical insights. Today, gravity experiments continue at the frontiers of science, probing not only the structure of space and time but also the quantum nature of the universe itself.