Matter

Matter is the substance of which all material is made. That means objects which have mass. More specifically, they must have rest mass, which is a form of energy that matter has even when it is not moving (it has no kinetic energy), is extremely cold (it has no thermal energy), etc. Matter is a word that is sometimes used in varying ways in everyday life, whereas mass is a well-defined concept and quantity at least in physics. They are not the same thing, though they are related.^[1]

Ordinary matter is made of tiny particles called atoms. The atoms have spaces between them and they move or vibrate all the time. The particles move faster and move further apart when heated, and then reverse when cooled.

↑ Here we ignore that all matter can have its rest mass converted to other forms of energy (a form of the mass-energy equivalence) by the famous formula E = mc², where E is the energy of a mass m (in this case, rest mass), times c² the speed of light squared. A relatively small amount of matter (i.e. having a small rest mass) may be converted to a large amount other forms of energy that can be released, such as kinetic energy (the energy of motion) and electromagnetic radiation. An example is that positrons and electrons (matter) may transform into photons (non-matter). However, although matter may be created (if other forms of energy are converted to rest mass) or destroyed (if the rest mass is converted to other energy forms) in such processes, the total quantity of energy does not change during the process, although some of it can be released or "escape" from the original location of the matter. English translation of Einstein's paper.

[inertia-1] Here we ignore that all matter can have its rest mass converted to other forms of energy (a form of the mass-energy equivalence) by the famous formula E = mc², where E is the energy of a mass m (in this case, rest mass), times c² the speed of light squared. A relatively small amount of matter (i.e. having a small rest mass) may be converted to a large amount other forms of energy that can be released, such as kinetic energy (the energy of motion) and electromagnetic radiation. An example is that positrons and electrons (matter) may transform into photons (non-matter). However, although matter may be created (if other forms of energy are converted to rest mass) or destroyed (if the rest mass is converted to other energy forms) in such processes, the total quantity of energy does not change during the process, although some of it can be released or "escape" from the original location of the matter. English translation of Einstein's paper.

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