Over at Gizmodo:

[Researchers] found instances where moving (but not stationary) atoms spitting out packets of light energy would bring into existence a tiny force that acted like friction, and published research on it earlier this year. A force that exists when an object is moving, but not when it is stationary, violates the core principles of Einstein’s (and Galileo’s) laws of relativity—there isn’t anything special about the laws of physics when something is moving at constant velocity versus when it’s at rest. So, had they accidentally spotted a tiny hole in the most well-accepted theories of physics?

Nah.

But the researchers thought, well, what if they redo all of the physics of this situation, but allow the mass of the atom to change, too?

This, it turns out, resolves the paradox—the moving atom loses a tiny amount of mass through the emission of energy, eliminating the requirement for a velocity-dependent frictional force. Essentially, they came across Einstein’s most famous equation, E=mc^2, demonstrating that energy and mass are proportional using the basic laws of physics.

Einstein: still the undisputed champ at all levels of mass or energy.