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One of the most profound insights to come about in all of physics has been what’s easily Einstein’s most famous equation: E = mc². Quiet simply, it states that energy is equal to an object’s mass multiplied by the speed of light squared. This simple-seeming mathematical relation holds an enormous amount of physics inside of it, including:

This one equation, E = mc², describes how much energy is inherent to any massive particle at rest, including how much energy it takes to create it and how much energy is released if you destroy it.

But what if your particle isn’t at rest, or what if it doesn’t have any mass at all? In those cases, E = mc² is only half of the meaningful equation. The other half is far more interesting, and is required to make physical sense of what’s going on.

The reason “rest mass” is such an important concept is because motion — the rate of change of an object’s position over time — isn’t an “absolute” physical property in our Universe. Instead, the key lesson from Einstein’s relativity is that irrespective of what your position is or how your position is changing with time, the laws of physics and the constants of nature, including the speed of light, are always going to appear to be the same.

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