From The Art and Popular Culture Encyclopedia
"The present age [...] prefers the sign to the thing signified, the copy to the original, representation to reality, the appearance to the essence [...]." -- The Essence of Christianity (1841) by Ludwig Feuerbach
The present (or now) is the time that is associated with the events perceived directly and in the first time, not as a recollection (perceived more than once) or a speculation (predicted, hypothesis, uncertain). It is a period of time between the past and the future, and can vary in meaning from being an instant to a day or longer. In radiocarbon dating, the "present" is defined as AD 1950.
It is sometimes represented as a hyperplane in space-time, typically called "now", although modern physics demonstrates that such a hyperplane can not be defined uniquely for observers in relative motion. The present may also be viewed as a duration (see specious present).
The present is contrasted with the past and the future. Modern physics has not yet been able to explain the perceived aspect of 'the present' as 'eliminator of possibilities' that transfers future into past. A complicating factor is that whilst a given observer would describe 'the present' as a spatial structure with zero time lapse, other observers would associate both time and space to this structure and therefore disagree on what constitutes 'the present'.
The direct experience of the present for each human is that it is what is here, now. Direct experience is of course subjective by definition yet, in this case, this same direct experience is true for all humans. For all of us, 'here' means 'where I am' and 'now' means 'when I am'. Thus, the common repeatable experience is that the present is inextricably linked to oneself.
In the time aspect, the conventional concept of 'now' is that it is some tiny point on a continuous timeline which separates past from future. It is not clear, however, that there is a universal timeline or whether, as relativity seems to indicate, the timeline is inextricably linked to the observer. Thus; is 'now' for me the same time as 'now' for you on a universal timeline, assuming a universal timeline exists? Adding to the confusion, in the physics view, there is no demonstrable reason why time should move in any one particular direction.
Adding substance to the supposition that the timeline view of 'now' may not hold the full picture, the qualities of 'now' or the 'present' in the human direct experience are very different to the qualities of past and future available through memory or anticipation. In the human direct experience, 'now' has a certain aliveness, reality and immediacy not present in our concepts of past and future. Indeed, any experience is always happening 'now', even a re-living of some past event. Thus, there is a deep philosophical case for saying that the present moment is all there ever is, from moment to moment.
When comparing time in places separated by great distances, the notion of present becomes more subjective. 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. Thus, light travel time must be taken into account in such time comparisons.
The present in Buddhism
Buddhism and many of its associated paradigms emphasize the importance of living in the present moment — being fully aware of what is happening, and not dwelling on the past or worrying about the future. This does not mean that they encourage hedonism, but merely that constant focus on one's current position in space and time (rather than future considerations, or past reminiscence) will aid one in relieving suffering. They teach that those who live in the present moment are the happiest. A number of meditative techniques aim to help the practiser live in the present moment.
Christianity and eternity
For some Christians God is viewed as being outside of time and from the divine perspective past, present and future are actualised in the now of eternity. This trans-temporal conception of God has been proposed as a solution to the problem of divine foreknowledge (i.e. how can God know what we will do in the future without us being determined to do it) since at least Boethius. Thomas Aquinas offers the metaphor of a watchman, representing God, standing on a height looking down on a valley to a road where past present and future, represented by the individuals and their actions strung out along its length, are all visible simultaneously to God. Therefore, God's knowledge is not tied to any particular date.
Philosophy and science
"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. When care is taken to operationalise "the present", it follows that the events that can be labeled as "simultaneous" with a given event, can not be in direct cause-effect relationship. Such collections of events are perceived differently by different observers. Instead, when focusing on "now" as the events perceived directly, not as a recollection or a speculation, for a given observer "now" 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".