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“In other words, if an observer looks at another frame of reference (like a moving train), he or she will still see that the speed of light is the same even though that frame of reference is moving relative to it. It is because of this symmetry that our standard theory of light, or electromagnetism, cannot accommodate a varying speed of light in empty space. But when a wave of light moves in a different medium, such as glass, Lorentz symmetry is no longer preserved, and light can change speed relative to empty space. This was the essence of Joao's contention. It could be that there is a quantum effect on space-time that fundamentally violates Einstein's cherished Lorentz symmetry, resulting in a variation of the speed of light in the early universe. As it turns out, the shape of extra dimensions in string theory can indeed cause certain constants, including the speed of light, to vary throughout the space-time fabric.”
― Stephon Alexander, The Jazz of Physics: The Secret Link Between Music and the Structure of the Universe