Matter should not be confused with mass, as the two are not quite the same in modern physics. For example, mass is a conserved quantity, which means that its value is unchanging through time, within closed systems. However, matter is not conserved in such systems, although this is not obvious in ordinary conditions on Earth, where matter is approximately conserved. Still, special relativity shows that matter may disappear by conversion into <answer>energy</answer>, even inside closed systems, and it can also be created from <answer>energy</answer>, within such systems. However, because mass (like <answer>energy</answer>) can neither be created nor destroyed, the quantity of mass and the quantity of <answer>energy</answer> remain the same during a transformation of matter (which represents a certain amount of <answer>energy</answer>) into non-material (i.e., non-matter) <answer>energy</answer>. This is also true in the reverse transformation of <answer>energy</answer> into matter.
Special relativity says matter can disappear by conversion into what state?
energy