The speed of light is a fundamental constant in physics, denoted by the symbol "c." It is approximately equal to 299,792,458 meters per second in a vacuum. Determining the speed of light is a result of scientific experiments and theoretical calculations. One of the most famous experiments that accurately measured the speed of light was conducted by Danish astronomer Ole Rømer in the late 17th century.
Rømer's experiment involved observing the motion of Jupiter's moon Io. He noticed that the time it took for Io to appear behind Jupiter as it orbited the planet varied depending on the Earth's position in its orbit around the Sun. Rømer correctly deduced that the observed variations were due to the finite speed of light. When the Earth was closer to Jupiter, the apparent motion of Io appeared faster, and when the Earth was farther away, the apparent motion of Io appeared slower. By carefully analyzing these observations, Rømer estimated the speed of light to be about 220,000 kilometers per second, which is close to the actual value.
Since Rømer's experiment, many other methods have been developed to measure the speed of light more accurately. Today, one common technique is to use lasers and highly precise timing instruments. By measuring the time it takes for light to travel a known distance, the speed of light can be calculated. Additionally, the speed of light can be derived from electromagnetic theory and fundamental constants, such as the permittivity and permeability of free space.
As for your question about how fast we approach the speed of light, it is important to note that according to Albert Einstein's theory of relativity, objects with mass cannot reach or exceed the speed of light. As an object with mass accelerates towards the speed of light, its energy and momentum increase while its mass also appears to increase. As the object approaches the speed of light, its mass approaches infinity, making it impossible to reach or exceed the speed of light. This concept is known as relativistic mass increase.
However, it is possible for particles without rest mass, such as photons, to travel at the speed of light. Photons are particles of light and other forms of electromagnetic radiation, and they always travel at this constant speed in a vacuum.