The meter is the basic unit of length in the metric system and the International System of Units (SI). You probably use it every day, even if you don’t realize it. For those who grew up with feet and inches, one meter is about 39.37 inches. But the definition itself has changed dramatically over the past two centuries. It was tied to the physical form of the earth, to being anchored to a physical metal bar, and finally becoming a constant of nature that never changes.
In 1791, the French Academy of Sciences proposed a bold idea. They wanted a universal unit of measurement, not limited by the feet of kings or local customs. They defined 1 meter as 1/10 millionth of the distance from the North Pole to the equator, measured along the meridian that passes through Paris. This is a scientific enterprise that makes measurements based on the Earth itself.
This definition took some time, but the Earth was difficult to measure. Errors crept in. Therefore, by 1889 BIPM decided to move away from theoretical definitions towards physical objects. They created an international prototype metre. This was a bar made of 90 percent platinum and 10 percent iridium. A unit of length is defined as the distance between two specific lines marked on that bar. It was stable. It was reproducible. It was also, ultimately, flawed. The rod may be damaged. It could be lost. It is kept in a vault in France, and the rest of the world has to trust their measurements against that single object.
As technology develops, the need for precision increases. By 1960, scientists had learned to measure light waves with incredible precision. They realized they didn’t need the metal rods at all. The metre was redefined based on the atom. Specifically, it has a wavelength of 1,650,763.73, the orange-red light emitted by krypton-86 atoms in a vacuum. This is a big leap forward. It made the standard independent of a physical artifact. The device can now be duplicated in any laboratory with the appropriate equipment.
But scientists aren’t done yet. Laser technology introduced in the 1980s can measure the speed of light in a vacuum with unprecedented precision. The data kept coming back more accurate than ever. It is clear that the speed of light is constant and does not change depending on location or material, and is a fundamental truth of the universe.
Thus, in 1983, the General Conference on Weights and Measures took a decisive turn. They determined that the speed of light is exactly 299,792,458 meters per second. At the same time, they redefined the metre.
One meter is currently defined as the distance light travels in a vacuum in 1/299,792,458 seconds.
The meter is no longer a metal bar in the safe. That’s time multiplied by a fraction of the speed of light.
This may sound like abstract physics, but it is important. This definition enables accurate measurements down to a fraction of a nanometer. GPS systems for driving cars, fiber optic cables for carrying Internet data;
