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# How Did Scientists Measure the Speed of Light? UNIVERSE

Speed | The speed of light measured in the vacuum is a physical constant denoted by the letter c and has the value (formula): c = 299,792,458 Km / s. Any electromagnetic radiation (radio waves, infrared radiation, ultraviolet radiation, light radiation) propagates in vacuum at the speed of light – c.

Depending on the environment in which they propagate, electromagnetic waves may have a lower speed than the speed of light but never a higher speed than that. For this reason, the unit of measurement – the meter, was redefined in 1983 as the distance traveled by light in a vacuum over a period of 1 / 299,792,458 of a second.

Before the seventeenth century, it was believed that light was transmitted instantly. This was supported by the observation that there is no apparent gap in the shadow of the Earth on the surface of the Moon, which would be expected if the speed of light would be finite.

Today we know that the speed of light is too high for this delay phenomenon to be visible. Galileo doubted that the light would have an infinite speed and conceived an experiment to measure its speed.

His experiment supposed the arrangement of two lanterns at a distance of several kilometers to observe whether there was a delay of light when it passed the distance between the two lanterns.

We do not know exactly if this experiment was put into practice, and if it was, Galileo realized that the experiment was done on a too small scale and could not have highlighted its value.

The first precise measurement of the speed of light was made by Olaus Roemer in 1676. While studying the movement of one of Jupiter’s satellites, Roemer noticed that, depending on the Earth-Sun-Jupiter geometry, there may be a difference of up to 1000 seconds between the time predicted when the eclipse should occur, and when it was observed. That is, the time elapsed between eclipses varied over a year, corresponding to the periods when the Earth approached Jupiter or moved away.

Roemer’s Speed of Light

Assuming that Jupiter starts at a distance r0 from Earth at the beginning of an eclipse. Jupiter moves away from the earth at speed v. During the next eclipse, Jupiter is at a distance d = vT from Earth, and the light must travel a distance d = vT more than in the first phase.

Knowing the Io period and knowing the relative difference between Earth and Jupiter, Roemer was able to calculate the velocity of the c value. The difference between a single orbital period is small, but Roemer – according to his writings – studied these cumulative changes and noticed that the measurable deviation was 22 minutes.

Roemer came to the conclusion that the amount of time required for light to reach Earth varies because the distance between the two planets also varies. He obtained the value of 214,000 km / s, quite accurately, considering that the distances between the planets were not known precisely.

Over time, many scholars have tried to measure the speed of light as accurately as possible. They have been able to achieve more accurate values with the improvement of experimental methods and devices. Since the 1940s, all measurements made have had a relative measurement error below 0.005%.

In 1973, the National Institute of Standards and Technology (NBS) in Boulder, Colorado, measured the speed of light using a laser using the interferometric method and its value was 299,792.4574 with a relative measurement error of 0.001 km / s.

The speed of light, expressed in units of the International System, is 299,792,458 km / s.

Distances from the earth in light seconds, light minutes and light years

The furthest object known to date The farthest object is at a distance of 13.3 billion light years and is the MACS0647 galaxy, the light beginning its journey at the beginning of the formation of the universe. Light sources The sun is at a distance of 149 million kilometers from Terra. A light beam only takes 8 minutes and 20 seconds to get here. The next closest star is Proxima Centauri, at a distance of 4.2 light years.

Of great importance is perhaps the role of light speed in Albert Einstein’s theory of relativity. It sets the speed of light in the vacuum as the highest possible speed in nature and says that the speed of light towards different observers is the same. The speed of light, c, is an absolute constant – the universal constant in Einstein’s equation, E = mc ^ 2, \! which determines that mass and energy are equivalent.

The paradox of constant light velocity created a great problem for physics, a problem that the American physicist, of German origin, Albert Einstein, ultimately solved in 1905. Einstein suggested that physical theories should not depend on the state of motion of the observer. Instead, he said that the speed of light had to remain constant, and the rest of the physics had to change to observe that. This special theory of relativity has predicted many unexpected physical consequences, all of which have since been observed in nature.

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