In a wider sense, one can use the word matter simply to refer to fermions. In this sense, matter and antimatter particles (such as an electron and a positron) are identified beforehand. The process inverse to particle annihilation can be called matter creation ; more precisely, we are considering here the process obtained under time reversal of the annihilation process. This process is also known as pair production, and can be described as the conversion of light particles (i.e., photons) into one or more massive particles . The most common and well-studied case is the one where two photons convert into an electron – positron pair.