The speed of light has been measured using various experimental methods throughout history. One of the earliest successful measurements was conducted by Danish astronomer Ole Rømer in the late 17th century. Rømer observed the motion of Jupiter's moon Io and noticed that the time intervals between its eclipses varied depending on Earth's position in its orbit around the Sun. By carefully analyzing these variations, Rømer estimated the speed of light to be finite.
In modern times, several techniques have been employed to measure the speed of light with increasing accuracy. Here are a few commonly used methods:
Time-of-flight method: This method involves measuring the time it takes for light to travel a known distance. A light pulse is emitted, and its time of departure and arrival at different points are measured. By dividing the distance traveled by the time taken, the speed of light can be calculated.
Interferometry: Interferometry techniques use the interference of light waves to measure their speed. This involves splitting a light beam into two separate paths and recombining them to observe interference patterns. By precisely measuring the path length difference and the corresponding interference patterns, the speed of light can be deduced.
Resonance methods: These methods rely on creating standing waves of light in a cavity or resonator. By measuring the resonant frequencies of the cavity and knowing the dimensions, the speed of light can be calculated.
Electromagnetic constants: The speed of light is intimately connected with fundamental constants in electromagnetism, such as the permittivity and permeability of free space. Precise measurements of these constants can be used to derive the speed of light.
As for your second question, regarding how fast we approach the speed of light, it's important to note that objects with mass cannot reach or exceed the speed of light according to our current understanding of physics. As an object with mass approaches the speed of light, its energy and relativistic mass increase while its acceleration decreases. This phenomenon is described by special relativity. Therefore, for massive objects like ourselves, it is impossible to approach the speed of light.