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Einstein's argument regarding the constant speed of light in physics, as described in his theory of special relativity, has been widely accepted and extensively tested over the past century. According to this theory, the speed of light in a vacuum, denoted by 'c,' is constant and is the same for all observers regardless of their relative motion.
This principle has been supported by numerous experimental observations and measurements. For example, the famous Michelson-Morley experiment conducted in the late 19th century aimed to detect the Earth's motion through the hypothetical "luminiferous ether." The experiment failed to detect any significant variation in the speed of light, providing evidence for the constancy of the speed of light.
Einstein's argument is based on a logical framework that includes the postulate that the laws of physics should be the same in all inertial reference frames. By incorporating this principle and assuming the constancy of the speed of light, Einstein developed a consistent and mathematically elegant theory of special relativity that resolved several inconsistencies in classical physics.
It is important to note that scientific theories, including Einstein's theory of relativity, are constantly subject to testing and scrutiny. However, to date, the vast majority of experimental evidence and observations support the notion of the constant speed of light. The logical consistency and empirical success of Einstein's arguments have made the constant speed of light a foundational principle in modern physics.