What is the essence of Einstein's mass-energy equivalence formula?

The essence of Einstein's mass-energy equivalence formula, expressed as E=mc², highlights the principle that mass and energy are two forms of the same physical entity and can be converted into one another. This means that mass can be transformed into energy and vice versa, illustrating that they are interchangeable.

This principle fundamentally altered our understanding of physics, particularly in the realms of nuclear physics and cosmology, as it demonstrates how even a small amount of mass can yield a tremendous amount of energy. The idea that mass and energy are interconnected has been crucial in explaining various phenomena, including the workings of atomic bombs and the processes that fuel stars.

The other options do not encapsulate the primary concept behind Einstein's formula. While mass may increase with velocity as described in special relativity, this pertains more to relativistic effects rather than the core idea of mass-energy equivalence. The claim that energy cannot change forms contradicts well-established principles in physics, such as the conservation of energy. Lastly, stating that mass is irrelevant in physics overlooks the critical role mass plays in both classical and modern physics contexts.