**Category:**

Research Papers

**Sub-Category:**

Quantum Theory / Particle Physics

**Date Published:**

July 7, 2018

**Keywords:**

electromagnetic wave, massless, photon, limit speed, heat, relativity

**Abstract:**

In this article, four issues have reviewed and analyzed: What is the physical nature of energy? Does energy have mass? What was the mass-energy equivalence before relativity? According to the work-energy theorem, what is the physical nature of force, when force is displacing, energy is produced? Finally, the theoretical and empirical approach to the photon mass is compared.

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