This paper proposes a theoretical framework for quantifying subjective experience by synthesizing Einstein's mass-energy equivalence with Fourier analysis. We introduce the concept of "Experiential Energy" ($\mathcal{E}$) and "Mental Mass" ($\mathfrak{m}$), proposing that subjective acceleration into a high-coherence "flow state" follows relativistic principles. Furthermore, we apply the Fourier Transform to map the transition from a discrete, linear time-domain perception (Space-Time Brain) to a continuous, simultaneous frequency-domain perception (Frequency Brain). This model offers a mathematical basis for understanding the relationship between ego-structure, information integration speed, and subjective radiance.