Mass and energy are entirely separate conserved quantities.

In classical physics, mass and energy were kept in separate accounts.

Mass appeared to be conserved: the amount of matter before and after an ordinary chemical or mechanical process stayed the same. Energy was conserved separately as motion, heat, stored energy, and other forms.

Special relativity connected the two.

Einstein’s famous relation E = mc² says that a body’s rest mass corresponds to an enormous amount of energy. The factor c²—the speed of light squared—is so large that even a tiny change in mass can correspond to a large amount of energy.

The relationship works in the other direction as well. If energy is added to a closed system and stored inside it, the system’s mass increases slightly. If the system releases energy, its mass can decrease.

In ordinary chemical reactions these mass changes are extremely small, so the classical rule of mass conservation remains an excellent approximation.

Nuclear reactions make the effect much easier to measure. The combined mass of the products can differ detectably from the starting nuclei, with the difference appearing as released or absorbed energy.

It is still too loose to say that mass and energy are simply “the same thing.” Modern physics treats mass, energy, and momentum as distinct quantities with precise relationships. What disappeared was the idea that mass and energy are completely separate and independently conserved.