$c$ at the belfry

TL;DR

This paper describes a modern reproduction of Fizeau's 1849 experiment to measure the speed of light, using contemporary equipment and conducted in a historic belfry, with insights into overcoming technical challenges.

Contribution

The paper details the adaptation of Fizeau's classic experiment with modern devices and shares practical solutions for reproducing the measurement in a historic setting.

Findings

Successful measurement of the speed of light using modern equipment

Identification of key technical challenges and solutions

Engagement of the public in scientific measurement

Abstract

In 1849, Hippolyte Fizeau determined the speed of light in a famous experiment. The idea was to measure the time taken for a pulse of light to travel between an intense light source and a mirror about 8 km away. A rotating cogwheel with 720 notches, that could be rotated at a variable speed, was used to chop the light beam and determine the flight time. In 2017, physicists and technicians of the University of Mons in Belgium reproduced the experiment with modern devices to allow members of the public to measure the speed of light themselves. The light source used was a low power laser, and the cogwheel was replaced by an electrically driven chopper, but the general spirit of Fizeau's experiment was preserved. The exhibition was organised in the belfry of Mons, a baroque-style building classified as a UNESCO World Heritage site. The solutions found for the main problems encountered are presented here to help colleagues intending to reproduce the experiment.