Abstract

Multiple definitions of time in the case of flat space and curved space are discussed. It is found that Einstein time is not the only definition of time in flat space. Other definitions of time are permissible. On the other hand, the definition of time in General Relativity is too arbitrary. Certain constraints must be imposed to set the coordinate conditions. Therefore, the proper way in spacetime theory should adopt multiple concepts of time, which are related to each other by given rules and have corresponding physical meanings. The definitions of the velocity of light are also briefly discussed since it is related to the definitions of time.