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The principle of relativity is one of the two postulates from which Einstein derived the consequences of the theory of invariance, popularly called relativity. The other postulate refers to the speed of light, and it is especially important when comparing observations between two inertial frames of reference, those frames of reference that are at rest or move with a uniform relative speed to each other since we depend mainly from light to make observations.

Einstein concluded in 1905 that: the speed of light has to be exactly the same, it must be a universal constant, for all observers, regardless of whether they move (with constant velocity) with respect to the source of the light. This revolutionary idea became the second postulate of Einstein's special relativity, the principle of constancy of the speed of light

> # Light and all other forms of electromagnetic radiation propagate in empty space with a constant speed c that is independent of the movement of the observer or the emitting body.

# History
According to standard modern physics, all electromagnetic radiation (including visible light) propagates or moves at a constant velocity in a vacuum, commonly known as "velocity of light" (vector magnitude), rather than "velocity of light". light "(scalar magnitude). This is a physical constant denoted as c. The speed c is also the speed of the propagation of gravity in the general theory of relativity. A consequence in the laws of electromagnetism (such as Maxwell's equations) is that the speed c of electromagnetic radiation does not depend on the speed of the object that emits the radiation. Thus, for example, light emitted from a rapidly moving light source would travel at the same speed as light from a stationary source (although the color, frequency, energy, and momentum of the light will change; it is known as Doppler Effect). If this observation is combined with the principle of relativity, it is concluded that all observers will measure the speed of light in the vacuum as one, regardless of the frame of reference of the observer or the speed of the object that emits the light. Because of this, c can be seen as a fundamental physical constant. This fact, then, can be used as a basis for the theory of special relativity. The constant is the speed c, instead of the light itself, which is fundamental for special relativity. Thus, if the light is somehow delayed to travel at a speed less than c, this will not directly affect the theory of special relativity. Observers traveling at high speeds will find that distances and times are distorted according to the Lorentz transformation. However, the transformations distort times and distances so that the speed of light remains constant. A person traveling at a speed near c will also find that the colors of the light in front turn blue and back turn red.

From the seventeenth century, with scientists of old age, it was not considered that the light traveled, for them it was an instantaneous question. This understanding was given from the observation of eclipses. It was only Galileo Galilei who, through the performance of certain experiments, questioned this principle of "instantaneity" of the distance that light travels.

Several experiments were carried out by different scientists, some with luck and others not, nevertheless all these physical studies in this incipient scientific time pursued the objective of measuring the speed of light even with the complications that their instruments and methods were inaccurate and primary. Galileo Galilei was the first to perform an experiment to measure this phenomenon, however, he did not obtain results that help to calculate the time of transmission of light.

Then in the year 1728 James Bradley also studied the speed of light but observing the transformation of the stars, detecting what was the displacement that was given in relation to the movement of the Earth around the Sun, it obtained a value of 301,000 Kilometers per second.

A great variety of methods have been used to improve the accuracy in the measurement, being the case in 1958 of the scientist Froome who arrived at the value of 299,792.5 kilometers per second by means of a microwave interferometer, the most appropriate to the case. The measurement improved qualitatively with the development of laser devices that have greater capacity, great stability and use cesium clocks that improve the accuracy of measurements, this from the year 1970.



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# What is the speed of light?
The speed of light is a measure stipulated by the scientific community, generally used by the fields of the science of physical and astronomical studies. It serves for the understanding of the celestial bodies, astronomical, to know how is their behavior and the transmission of electromagnetic radiation, how light is perceived by the human eye. The theoretical basis of the speed of light is expressed by the relationship that arises between how much is the delay of light in transferring in the vacuum from one point to another and is measured in time. As an example, we can say then that sunlight takes approximately 8 minutes and 19 seconds to reach Earth. It is considered that the speed of light is a universal constant, invariable in physical time and space. It covers 299,792,458 meters per second and 1080 million kilometers per hour. This speed is related to another stipulated measure that is the light year, which refers to the distance traveled by light in the time that lasts a year.

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# Time and space and how they change when we travel at the speed of light.
When we travel at the speed of light or almost at the speed of light, time passes much slower for us than for our friend on Earth. It is not that we notice it because we have seen before that everything remains the same for us, but when we meet him we will see that he has aged much more than us. Besides the distances we see them different, for us, the distances between points become shorter, but this is something that, once again, we can not see because any rule that we carry will also shrink in the same proportion as the rest of distances.

We must remember that this does not mean that we see reality distorted by a failure of our senses, but that when we travel at the speed of light the distance and time really vary. For almost a century that makes Einstein leave us his theory of relativity, hundreds of experiments have confirmed that these effects are real and measurable, not an illusion or a mathematical trick. But of all the effects that occur when we travel at the speed of light, the most curious of all is what happens when our friend and we contemplate the same event.

# Reference
### https://www.thecalculatorsite.com/articles/units/how-fast-is-the-speed-of-light.php
### http://bigthink.com/dr-kakus-universe/what-travels-faster-than-the-speed-of-light
### https://en.wikipedia.org/wiki/Speed_of_light
### https://ktwop.com/2017/12/08/the-magic-which-makes-the-speed-of-light-what-it-is/