# Loop quantum gravity Reference Libraries

String Theory -- A string theory is a physical model whose fundamental building blocks are extended objects (strings, membranes and higher-dimensional objects) rather than points. String theories are able to avoid problems, such as infinite energy density, associated with the presence of mathematical points in a physical theory. The term 'string theory' properly refers to both the 26...

Spacetime -- In special relativity and general relativity, time and three-dimensional space are treated together as a single four-dimensional manifold called spacetime (alternatively, space-time; see below). A point in spacetime may be referred to as an event. Each event has four coordinates (t, x, y, z). Just as the x, y, z coordinates of a point depend on the axes one is using, so...

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