1905, Einstein (1879-1955) published his special theory of relativity. This theory points out that the only constant in the universe is the speed of light in vacuum, and everything else-speed, length, mass and elapsed time-changes with the observer's frame of reference (specific observation). This theory forms a famous formula: E=MC2. Special relativity holds that time is not absolute (that is, it is fixed). Einstein pointed out that as the speed of linear motion of an object (in the observer's view) increases, time will slow down. Second: When any object moves at the speed of light, its length will be shortened to zero. It is pointed out that time and space are absolute and space and time are completely separated. However, in relativistic mathematics, time and three-dimensional space-length, width and height-together form a four-dimensional space framework, which is called time-space correlation set.

Einstein deduced the equation E=MC2 from his special theory of relativity (where e is energy, m is mass and c is constant speed of light), and he used this equation to explain that mass and energy are equivalent. It is now believed that mass and energy are different forms of the same substance, called mass energy. For example, if the energy of an object is reduced by a certain amount E, its mass is also reduced by an amount equal to MC2. However, whether mass can disappear is only released in another form, which is called radiant energy.

19 15 published his general theory of relativity. He explained why there is no difference between gravity and acceleration. He also explained how gravity is related to the curvature of time and space. Using mathematics, Einstein pointed out that an object bends the surrounding space and time. When an object has a large relative mass (such as a star), this curvature can change the path of any object passing through it, even light. General relativity holds that the bending of space-time will produce gravity. When light passes through some massive celestial bodies, its path is curved, which is due to the curvature of spacetime formed along the massive celestial bodies. Because a black hole is a huge mass concentration, the space-time around it is so curved that even light can't escape.

Newton's classical mechanics has convinced many people for hundreds of years, because it is suitable for low-speed and macroscopic inertial systems. Relativity is applicable to high-speed (near the speed of light) or microscopic quantum states and non-inertial systems, and it is difficult to be confirmed and observed by experiments, so many people can't accept this fact. But relativity can solve this problem well. Therefore, relativity is the cornerstone of modern physics.

References:

http://www.chinamodel.org/news/2003/022004.html