Who Discovered the Speed of Light and How was the universal constant measured accurately?

In the late 1800s discovering the true the speed of light had only minor importance because astronomers were the only ones who used this number.

Distances across space are measured in light-years, how far light travels in one year’s time. Since their measurements were only approximations anyway, they could accept a 5 percent (or even 10 percent) error in that value.

Then Albert Einstein created his famed energy-matter equation, E = mc2. Instantly the speed of light, “c,” became critical to a great many calculations. Discovering its true value jumped to the highest priority. Light speed became one of the two most important constants in all physics. A 1 percent error, or even a 0.1 percent error, in “c” was suddenly unacceptably large.

But the problems of discovering the true speed of light, a speed faster than any clock could measure or other machines could detect, were enormous. Albert Michelson invented half a dozen new precision devices and, after 50 years of attempts, was the first human to accurately measure light speed. His discovery earned Michelson the first Nobel Prize to be given to an American physicist.

This was a discovery that was dependent on the invention of new technology and new equipment, just as Galileo’s discovery of moons around other planets was dependent on the invention of the telescope.

In 1928, 74-year-old Albert Michelson struggled to make one last try to accurately measure the speed of light and discover the true value of “c” in Einstein’s famed equation. He had designed, financed, and completed a dozen attempts over the previous 50 years. Michelson was determined this time to measure the speed of light with no more than a 0.001 percent error. That value would finally be accurate enough to support essential nuclear physics calculations.

Four years earlier, Michelson had turned to the famed gyroscope manufacturer, Elmer Sperry, to improve upon the equipment available for his measurements. Now in 1928, the third, and latest, round of equipment improvements was represented by a small octagonal cylinder that had just been driven in a thickly padded crate up the bumpy dirt road to the top of Mt. Baldy in California, Michelson’s test site.

The experiment Michelson designed was simple. He shone a light onto this small mirrored cylinder as it rotated at a high speed, driven by a motor (also invented by Sperry) capable of maintaining an exact speed of rotation. At some point as the mirror turned, it would be perfectly aligned to reflect this light beam toward a stationary, curved mirror at the back of the room. However, the rotating mirror would only reflect light back to that mirror for a very small fraction of a second before it rotated on.

This back wall mirror thus got short pulses of light from each face of the rotating mirror. Each pulse reflected through a focusing lens and out through an opening in the wall 22 miles to Mt. San Antonio. There it bounced off a mirror, through a second focusing lens, and straight back to Mt. Baldy. Here the light pulse once again hit the back wall mirror, and finally reflected back to the rotating cylinder.

Even though each pulse of light would complete this 44-mile journey in less than 1/4000 of a second, the rotating cylinder would have already turned some by the time that light pulse got back from Mt. San Antonio. Returning light would reflect off the rotating mirror and hit a spot on the shed wall. The angle from the cylinder to that spot would tell Michelson how far the mirror had rotated while a pulse of light made the 44-mile round-trip. That would tell him how fast the light had traveled.

While it all sounded simple, it meant years of work to improve the necessary equipment. Sperry created a better light so that it would last through 44 miles of travel. He created a more accurate motor drive so that Michelson would always know exactly how fast the small cylinder was turning.

Sperry designed smoother focusing lenses and a better mirrored cylinder, one that wouldn’t vibrate or distort its mirrored sides under the tremendous forces of high-speed rotation.

Michelson switched on the motor and light. Faster than eyes could see, the light stream shot out to Mt. San Antonio and back. It bounced off the rotating cylinder and onto the far wall.

From the cylinder’s rotational speed and the placement of that mark on the wall, Michelson calculated the speed of light to be 186,284 miles per second, less than 2 mph off of the modern estimate, an error of less than 0.001 percent. With this discovery, scientists in the fields of physics, nuclear physics, and high-energy physics were able to proceed with the calculations that led to nuclear energy and nuclear weapons.

Traveling at light speed, your ship could go from New York to Los Angeles 70 times in less than one second. In that same one second you could make seven and a half trips around the earth at the equator.