Louis Essen 1950s-1960s (precise measurement pioneer)

Cornell researchers 1970s (refined laser-based methods)

CGPM (1983) (defined the current exact value)

Laser Measurements:

The measurements relied on the use of lasers locked on molecular absorption lines, which allowed for very precise measurements of wavelengths. 

Wavelength and Frequency:

The speed of light (c) can be calculated using the formula c = fλ, where f is the frequency and λ is the wavelength of the electromagnetic wave. 

The Value:

The recommended value for c was set to 299,792,458 meters per second. This value is now the standard, and the meter is defined as the distance light travels in a vacuum during 1/299,792,458 of a second. 

Precision and Agreement:

The redefinition reflected the high degree of precision achieved in laser measurements, and the strong agreement between different measurements. 

Initial interferometers, such as the Michelson interferometer, used partially monochromatic or white light sources, with limitations in coherence length and stability.

Detection and alignment technologies evolved from visual fringe evaluation using Vidicon cameras and analog methods in the 1970s to digital CCD detectors and computerised phase measurement in the 1980s and beyond.

Large-scale interferometers like LIGO use advanced Michelson–Fabry–Pérot configurations with Fabry–Pérot cavities to increase effective arm length by multiple reflections, enhancing sensitivity.

The cavity resonance technique, as used by Louis Essen and A.C. Gordon-Smith in the 1940s, employed cavity resonators (wavemeters) to measure radio-wave frequencies with high accuracy.

Earlier acoustic cavity resonators, such as the Helmholtz resonator invented in the 19th century, were simple rigid containers with a neck used to isolate specific sound frequencies, but these were mechanical rather than electromagnetic resonators. They were limited by the precision of cavity construction and the ability to control and measure resonant frequencies accurately.

Rømer was able to calculate the speed of light using the discrepancies in the time between the eclipses of Jupiter's moons.

He measured the time delay of the eclipses, and noticed they were getting longer by up to 16.5 minutes.

He interpreted this as the difference in the times for light to travel from the sun to Jupiter, and back to earth.

Using this discrepancy, and the relative positions of the earth and Jupiter, he was able to calculate the speed of light.

This was extremely accurate for his time.

The speed of light he calculated was 214,000 km/s, which gives him a percentage error of 28.4% under the known value today.

The Wheel had 720 teeth

This experiment was extremely significant for the scientific community, as it was the first real confirmation that the speed of light was finite and measurable

It was the experiment that sparked the scientific community to find more and more accurate values of the speed of light, changing the way we view time and distance forever