Lights are almost everywhere, and they carry information about objects to propagate through space. When we receive light, we can know the information carried by the light, so we can see objects that emit or reflect light. Humanity has long studied not only light sources but also light itself.
Humans have long tried to measure the speed of light. But because the speed of light is so fast, so fast, it is assumed to be infinite. With the advancement of observational and experimental instruments, astronomer Romer measured the speed of light for the first time, and this was also the first time mankind determined that the speed of light is limited. term. Since then, the precision of the experiment has improved, the speed of light has been measured more and more accurately, and its speed is about 300,000 km/s.
The speed of light (more generally, the speed of propagation of electromagnetic radiation) in a vacuum, denoted c, is an important fundamental physical constant in many areas of physics. It has an exact value of 299,792,458 meters per second, because the meter length unit is redefined based on this constant and the standard second. According to special relativity, c is the maximum speed that all energy, matter, and information in the universe can reach. It is the speed at which every massless particle bound to physical fields (including electromagnetic radiation such as light photons) propagates in a vacuum. It is also the propagation speed of gravity (such as gravitational waves) predicted by current theories. Particles and waves travel with velocity c regardless of the motion of the source or of the observer’s inertial frame of reference. After Einstein founded relativity, it became known that the speed of light is indeed very fast, because this is the upper limit of the local speed in the universe, and no other speed can be exceeded. through it. Moreover, regardless of the medium in the frame of reference, the measured speed of light is exactly the same. Based on the principle of the constant speed of light in relativity and the highly accurate measurement of the speed of light, physicists have also determined the speed of light to be 299,792,458 meters/second without affecting the speed of light. affect the current physical quantity. In this way, people no longer have to worry about accurately measuring the speed of light, and can also solve the problem of errors in the equipment used to determine the clock. 
The speed of light propagating in a vacuum is independent of both the motion of the light source as well as of the observer’s inertial frame of reference. The constancy of the speed of light was postulated by Einstein in his 1905 paper on special relativity. Physicists can now only experimentally confirm the speed of light by methods above two Two-way speed of light is frame-independent, because the speed of light cannot be measured in one-way speed of light, ignoring some conventions of uniformity. synchronization between the clock at the source and the clock at the receiver. Whether it’s electron transition, nuclear fusion, nuclear fission, or the annihilation of yin and yang matter, these processes produce light. Once light is created, their speed will immediately reach the speed of light, and they will always travel at this speed. If light were not absorbed by matter, they would not be dissipated in the universe, and the speed would always remain at the speed of light. For example, the first photons created in the universe 13.8 billion years ago are still present today, and they travel through the universe at the speed of light. So, how did light reach the speed of light? How does light maintain the speed of light? What is the driving force that drives the light forward? If it is explained from the point of view of the particle nature of light, then light is composed of photons with no rest mass. According to special relativity, since the static mass of photons is zero, their speed can only be the speed of light. According to the Higgs mechanism of the Standard Model of Particle Physics, when photons without static mass pass through the Higgs field throughout the universe, the photons will not be coupled, so they will not gain static mass, and their speed will not decrease. 
Once the photons are generated, their speed will directly reach the speed of light, and there is no acceleration from zero to the speed of light. Propagation of photons does not require energy to propagate, if they were not absorbed by matter or a black hole, they would always travel aimlessly through a vacuum at the speed of light. On the other hand, if explained from the evaporation of light, the nature of light is electromagnetic wave. Maxwell’s equations of electromagnetism show that electricity and magnetic fields are essentially unified, that a changing electric field excites a magnetic field, and a changing magnetic field excites a repeated electric field, creating out electromagnetic waves. 
Since electromagnetic fields are formed at the speed of light, electromagnetic waves travel at the speed of light. The generation and existence of electromagnetic fields are independent of the medium, so light can travel at the speed of light in a vacuum. Although the spatial structure continues to expand, light propagating through space will appear redshift, but they will not completely disappear in the universe.
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