The present is the time that is perceived directly, not as a recollection or a speculation. It is often considered as a point in space-time, often called now, but it may also be viewed as a duration (see specious present). 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 10^8 m/s it takes many years to reach us. 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. 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 t

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