“According to B-theorists, there is no genuine change; rather, there is a permanent sequence of unchanging events, ordered (lined up, if you will) by an earlier-than/later-than relation. For example, World War I (and all the sub-events contained in it) is just as real as the event of your reading this preface, which in turn is just as real as the event of the death of the sun. When we say that World War I is past, we mean that it is earlier than the event of our utterance that World War I is past. When we say that the death of the sun is future, we mean that the death of the sun is later than our utterance that the death of the sun is future. In this sense, B-theorists consider reality to be “untensed”—-events are not intrinsically past, present, or future; rather, they simply exist (out there, somewhere), and ‘past’ and ‘future’ are merely ways of talking about where those events lie relative to the utterance events in which we speak about them. This view may seem counterintuitive, but it is most likely the received view in both physics and philosophy, having been advocated (or said to have been advocated) by figures ranging from Albert Einstein to Bertrand Russell.
According to A-theorists, on the other hand, time is not a frozen sequences of unchanging events. The picture given by A-theorists varies from theorist to theorist, but I will be defending an alternative due to A. N. Prior (and perhaps to Saint Augustine before him) in which, strictly speaking, there is no future and no past “out there” or anywhere. We can say that it will be true that a certain state of affairs (say, the death of the sun) will hold and that it was true that a certain state of affairs (say, World War I) held, but that this does not involve our referring to future and past events or to there being such events for us to refer to. According to this idea (often called presentism), what makes something future or past is how the world stands right now.”