The speed of light was initially experimentally measured in 1849 by Hippolyte Fizeau. It has been established that the speed of light(c) is exactly 299,792,458 m/s. This has been proven countless times and this value, therefore, has also been used to define a metre since 1983. The assumption that the speed of light remains constant was one of the core inputs used by Einstein to form the theory of relativity.
It may then come as a shock that it is probable that light may not always travel at this speed. This is because it is impossible to actually calculate the speed of light in ONE direction. Whenever the speed of light has been measured, it has only been done using reflective surfaces, where light would travel in two directions, from the source to the reflecting surface and back. The average speed is calculated and thus the speed of light is found.
Since the actual speed of light has never been calculated, it is possible that when light travels to a reflective surface and back, the speed of light when it is travelling to the surface is half the value of C and when it comes back to the source, the time taken is instantaneous.
Another way of understanding this is to assume the communication between astronauts on the moon and scientists back on earth. If it takes 5 minutes to send a signal to the astronauts and 5 minutes for the astronauts to send a signal back, it is possible that it would take 10 minutes for the signal to reach the astronauts on the moon but the signal back would be instantaneous. This would mean that the speed of light is faster in one direction and slower in the opposite direction.
Hence, we don’t really know what the speed of light is.
Determining the true speed of light could be the key to connecting general relativity and quantum mechanics. This will only be possible when physics takes the next paradigmatic leap.
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