The present is the time that is perceived directly, not as a recollection or a speculation. It is often considered as a hyperplane in space-time, often called now, but it may also be viewed as a duration (see specious presentJames, W. (1893). The principles of psychology. New York: H. Holt and Company. Page 609.).

Overview

Modern physics has not yet been able to explain what we normally understand by 'now'. Furthermore, there is no demonstrable reason why time should move in any one particular direction. This has led to the conclusion that the idea of a present is illusory and does not reflect the true nature of reality. The notion of "now" may be better understood as an unrealistic concept that has evolved in humans and animals to give us an understanding of reality useful only to the extent necessary for survival. For example, if you say "now", the moment you say it, it is in the past.

When comparing time in places separated by great distances, the notion of present becomes unclear. For example, we visually perceive stars to be where they were when the light now reaching our eyes was emitted, because even though light travels at approximately 3 x 10⁸ m/s it takes many years to reach us from distant sources.

When used in the term "Before Present" (BP), as used in expressing ages or dates determined by radiocarbon dating, "Present" is defined as AD 1950.

Philosophical problem

"The present" raises the difficult question: "How is it that all sentient beings experience now at the same time?" There is no logical reason why this should be the case and no easy answer to the question. For example, say somebody named John is experiencing a great deal of pain. John's friend Fred takes pity on John because of John's situation. The problem is: is it logical for Fred to feel bad for John at present, when there is no way to prove that both John and Fred experience the same temporal existence? (See also Solipsism and Philosophy of mind.)

Special Relativity's "present"

It follows from Albert Einstein's Special Theory of Relativity that there is no such thing as absolute simultaneity. Therefore, if we define "present" to be the collection of events that are simultaneous with a given event, then "present" takes different meanings for different observers. Instead, with "present" defined as the events perceived directly, not as a recollection or a speculation, for a given observer "present" takes the form of the observer's past light cone. The light cone of a given event is objectively defined as the collection of events in causal relationship to that event, but each event has a different associated light cone. One has to conclude that in relativistic models of physics there is no place for "the present" as an absolute element of reality. Einstein phrased this as: "People like us, who believe in physics, know that the distinction between past, present, and future is only a stubbornly persistent illusion".