Unlike in Newtonian mechanics, in the theory of relativity, the coordinate axis is not fixed, and the time axis serves as the background for the observation. In other words, the theory of relativity is background independent (BI). The quantum theory created in the micro-world, in contrast, is background dependent (BD). Efforts to unify both by BI have not yielded much success. In addition, it is recognized that we cannot consider gravity in a quantum theory by BD. This paper focuses on neither the BI method based on the invariant speed of light and the inertial mass observed only in the uniform field nor the method to consider gravity in an existing quantum theory. By using the invariant mass and the speed of light in the frame of reference of the free space of the electromagnetism as well as the observed variable mass and wave speed, a method to link them to the equivalence of light’s momentum was explained. In addition, some cases in which we can observe space from a micro-world by the equivalence principle were examined.
Key words: Background independent, gravity, free space, electromagnetism, invariant mass, variable mass, speed of light, wave speed.
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