“Each ray of light moves in the coordinate system ‘at rest’ with the definite, constant velocity V independent of whether this ray of light is emitted by a body at rest or a body in motion.” -Albert Einstein, 1905
The more kinetic energy you impart to something, the faster you go. But there’s a limit: the speed of light in a vacuum. In fact, if you have any mass at all, you’ll never reach it; if you have no mass, it’s the only speed you can travel at. But why is it finite instead of infinite? Why does it have the particular value it has? And why couldn’t it be faster or slower under different physical circumstances?
From a theoretical point of view, many of these questions were answered way back in 1865, with the formulation of Maxwell’s equations. But over the past 150 years, measurements and tests have gotten more stringent, new manifestations of the speed of light were found, and new physical consequences have emerged. But the conclusions remain unchanged: the speed of light is the same constant value everywhere and at all times.