By Sebastian Ritter
On November 21, 1905, exactly 114 year ago, Albert Einstein first published the well-known equation E=mc². You might think of nuclear power when seeing E=mc², but here’s something to think about. What does E=mc² really mean and how does it relate to your cup of coffee?
It was June 1905 when the 26-year-old patent commissioner Albert Einstein in Bern, Switzerland, described the basis for the famous equation E=mc² in a publication named “On the electrodynamics of moving bodies.” In it, Einstein developed the “Principle of Relativity”: the laws of physics are “invariant” or “the same” in all inertial systems. Inertia means that a body remains at rest or in unchanging motion unless a force disturbs it. Inertial systems can be described as scenarios where bodies are at rest or move at an unchanging speed. Einstein also introduced a second major principle: the speed of light is unchanging and invariant. Building on these two principles, Einstein published another paper to describe the equivalence of mass and energy. This paper was published in 1905 and was titled “Does the Inertia of an object Depend Upon Its Energy Content?”
What exactly does E=mc^2 mean?
It describes the equivalence of mass and energy in a scenario in which all bodies are at rest or in which the momenta of all bodies sum to zero. The mass m in E=mc^2 is known as the “rest mass” of an object, simply because it is the mass of an object when it is at rest. c is the speed of light. Multiplying the rest mass by the speed of light twice gives the “rest energy” E.
What does equivalence of mass and energy mean?
It means that rest mass and rest energy are simply two names for the same property of a physical body. The only difference is the mathematical factor c^2. But

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