String theory

2007-03-15T23:00:31Z

Harrisrj:

String theory, (or super-string theory when coupled with supersymmetry), is a class of models in [[theoretical physics]] which replace zero-dimensional points (particles) in four-dimensional spacetime with one-dimensional strings in an eleven-dimensional spacetime as the fundamental building block of the universe. The elusive goal is to develop a set of equations that unify all known natural forces (gravitational, electromagnetic, weak, and strong). A leading proponent of string theory has been Edward Witten of the [[Institute for Advanced Study]].

There is much debate on whether string theory is truly scientific. Under the [[positivist]] philosophy, models must be falsifiable (i.e. it must be possible to define an experiment which could contradict the theory) for them to qualify as science, and then they must survive these tests. In more general terms, a model is not scientific unless we can use it to make predictions about the world we observe, and it is consistent with those predictions. String theory has yet to yield a particular definitive test, or to produce any models that resemble any known particles or forces.

Some string theorists, such as Leonard Susskind, say that string theory may not be testable but may still provide insights on multiple universes. His recent book is ''The Cosmic Landscape: String Theory and the Illusion of Intelligent Design''.
[[Category:Physics]]

Theory of relativity

2007-03-11T03:43:16Z

Harrisrj:

The Theory of Relativity is a geometrical theory of gravitation, stating that the [[speed of light]] and laws of physics are constant for all (inertial) observers, regardless of their velocities relative to each other.

The theory of relativity was first proposed based on mathematical theory developed by [[Henri Poincaré]] and [[Hendrik Lorentz]]. This theory differs from [[Isaac Newton]]'s theory of gravitation by disposing with the idea of a universal, mutually agreeable scale of time (i.e. a universal clock that all times can refer to) and space (i.e. a universal sheet of "graph paper", which location refers to).

The famous equation attributed to Einstein, ''E=mc<sup>2</sup>'', describes the relationship between energy and the rest mass of a body.

In general terms, relativity predicts that space-time can be curved by massive bodies, so that (for example) near a [[black hole]] the sum of the angles in a triangle is not exactly 180 degrees, time passes more rapidly away from a black hole than near it (for a distant observer) and other apparent violations of [[geometry]] and common sense.

Relativity is important for massive or fast-moving bodies: at low mass and low speed, it can be accurately approximated by [[classical mechanics]] (such as [[Isaac Newton]]'s laws of motion). At the two extremes, modeling the behavior of electrons requires that relativistic effects be taken into account (the chemically significant phenomenon of [[electron spin]] arises from relativity), and the course light passing through a region containing many massive bodies such as galaxies will be distorted ([[classical mechanics]], in which light travels with infinite speed in straight lines, does not predict this). These are both experimentally confirmed ([[electron spin]] was known before relativity arose, and telescopic observations confirm that galactic clusters distort the paths of the light passing through them).

General relativity successfully explains the seemingly anomalous precession of Mercury's perihelion. While other explanations based in Newtonian gravity had been proposed, none were consistent with observation.

British Historian Paul Johnson declares the turning point in 20th century to have been when fellow Briton Sir [[Arthur Eddington]], an esteemed English astronomer, ventured out on a boat off Africa in 1919 with a local Army unit to observe the bending of starlight around the sun during a total eclipse. Upon his return to England declared that his observations proven the theory of relativity. In fact recent analysis of Eddington's work revealed that he was biased in selecting his data, and that overall his data were inconclusive about the theory of relativity. The prediction was later confirmed by more rigorous experiments, such as those performed by the [[Hubble Space Telescope]].

Like other scientific theories, relativity progressed gradually from the status of a "wild surmise," to a theory backed by imperfect experiments whose validity was debated, to a theory whose truth is now almost taken for granted. No particle accelerator built since the 1950s would work unless the mass increase predicted by special relativity not only occurred, but occurred to the precise degree predicted by the theory. In 1972, scientists flew extremely accurate clocks around the world in both directions on commercial airlines, and were directly able to observe the relativistic "twin paradox;" the eastbound clock gained 273 ns and the westbound clock lost 59 ns, matching the predictions of general relativity to within experimental accuracy.<ref>as described in [http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html What is the experimental basis of Special Relativity?], a personal web page, which cites Haefele and Keating (1972), ''Science'' Vol. 177 pp 166-170 as its source</ref><ref><ref>Sullivan, Walter (1972), "Relativity Theory Awaits Affirmation", September 23, 1972, p. 61. Note: Article refers to a different experiment, which Sullivan discusses, saying that if successful it would be "the second time within a year" that relativity had been confirmed, then proceeds to discuss Hafele[sic] and Keating's experiment as the first.</ref>

Relativity has generated a huge following by advocates of moral relativism{{fact}}. The idea of moral relativity may exist independent of (and substantially predates) the theory of relativity, but some moral relativists irrelevantly invoke the theory in attempts to lend legitimacy to this version of morality.

Today some physicists are working on a [[quantum theory of gravity]] (the "theory of everything"), including [[string theory]]. Critics increasingly point out that [[string theory]] is largely untestable and unfalsifiable, and thus potentially unscientific under the principles of science advanced by [[Karl Popper]].

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String theory

Theory of relativity