The statement that matter has a nonzero speed refers to the fact that ordinary matter, such as objects we encounter in our daily lives, can have varying velocities depending on the reference frame from which they are observed. On the other hand, the speed of light in a vacuum, denoted by the symbol 'c,' is considered to be a fundamental constant of nature and is the same for all observers regardless of their relative motion.
The constancy of the speed of light is a foundational principle of Einstein's theory of relativity. According to this theory, the laws of physics, including the behavior of light and matter, are the same in all inertial reference frames. In other words, the speed of light is invariant and remains constant regardless of the motion of the source or observer. This principle is supported by numerous experimental observations and is considered one of the cornerstones of modern physics.
One of the key pieces of evidence supporting the constancy of the speed of light comes from the famous Michelson-Morley experiment conducted in the late 19th century. This experiment aimed to detect the motion of the Earth through the hypothetical luminiferous ether, a medium once believed to be responsible for propagating light waves. However, the results of the experiment consistently showed that the measured speed of light remained the same in all directions, irrespective of the Earth's motion. This outcome was unexpected and ultimately led to the development of Einstein's theory of special relativity, which explains the constancy of the speed of light.
Since then, numerous experiments and observations have further confirmed the constancy of the speed of light. For example, particle accelerators and experiments in high-energy physics rely on the principles of relativity to make accurate predictions and measurements. Additionally, various astronomical observations, such as the study of distant galaxies and the measurement of cosmic microwave background radiation, align with the predictions of relativity.
In summary, while Einstein's theory of relativity provides a comprehensive framework for understanding the constancy of the speed of light, it is backed by a wealth of experimental evidence gathered over the past century.