The past is the portion of
timethat has already occurred; [Hegeler, E. C., & Carus, P. (1890). [http://books.google.com/books?id=3KoLAAAAIAAJ The Monist] . La Salle, Ill. [etc.] : Published by Open Court for the Hegeler Institute. page 443. ] it is the opposite of the future.
The past is contrasted with the
present. It is also regarded as the conglomerate of events that happened in a "certain point" in time, within the Space-time continuum. The aforementioned conception is closely related to Albert Einstein's relativity theory. The past is the object of such fields as history, archaeology, archaeoastronomy, chronology, geology, ( historical geology), historical linguistics, law, paleontology, paleobotany, paleoethnobotany, palaeogeography, paleoclimatology, and cosmology.
Humans have recorded the past since
ancienttimes, and to some extent, one of the defining characteristics of human beings is that they are able to record the past, recall it, remember it and confront it with the current state of affairs, thus enabling them to plan accordingly for the future, and to theorise about it as well.
Philosophy and science
According to presentism, the past does not strictly exist; however, the methods of all sciences study the world's past, through the process of evaluating evidence. Presentism is compatible with Galilean relativity, in which time is independent of space but is probably incompatible with Lorentzian/Einsteinian relativity in conjunction with certain other philosophical theses which many find uncontroversial.
classical physicsthe past is just a half of the timeline. In special relativitythe past is considered as absolute pastor the past cone. In Earth's scale the difference between "classical" and "relativist" past is less than 0.05 s, so it can be neglected in most cases.
In the modern
theory of relativity, the conceptual observer is at a geometric point in both space and time at the apex of the ' light cone' which observes events laid out in time as well as space. Different observers can disagree on whether two events at different locations occurred simultaneously depending if the observers are in relative motion (see relativity of simultaneity). This theory depends upon the idea of time as an extended thing and has been confirmed by experiment and has given rise to a philosophical viewpoint known as four dimensionalism. However, although the contents of an observation are time-extended, the conceptual observer, being a geometric point at the origin of the light cone, is not extended in time or space. This analysis contains a paradoxin which the conceptual observer contains nothing, even though any real observer would need to be the extended contents of an observation to exist. This paradox is partially resolved in Relativity theory by defining a ' frame of reference' to encompass the measuring instruments used by an observer. This reduces the time separation between instruments to a set of constant intervals. [ [http://philsci-archive.pitt.edu/archive/00002408/ Petkov 2005] ]
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