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Einstein manifold
In differential geometry and mathematical physics, an Einstein manifold is a Riemannian or pseudo-Riemannian manifold whose Ricci tensor is proportional to the metric. They are named after Albert Einstein because this condition is equivalent to saying that the metric is a solution of the vacuum Einstein equations, although the dimension, as well as the signature, of the metric can be arbitrary, unlike the four-dimensional Lorentzian manifolds usually studied in general relativity.
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Einstein (unit)
An einstein is a unit defined as one mole (6.022×10) of photons, regardless of their frequency. For photons of a given wavelength, einsteins per square metre is a measure of irradiance. The einstein is used in studies of photosynthesis since the light requirement for the production of a given quantity of oxygen is a fixed number of photosynthetically active photons. Photosynthetically active radiation (PAR) is usually reported in microeinsteins per second per square meter (¿E/m/sec).
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Einstein field equations
The Einstein field equations (EFE) or Einstein's equations are a set of 10 equations in Albert Einstein's general theory of relativity which describe the fundamental interaction of gravitation as a result of spacetime being curved by matter and energy. First published by Einstein in 1915 as a tensor equation, the EFE equate spacetime curvature (expressed by the Einstein tensor) with the energy and momentum within that spacetime (expressed by the stress¿energy tensor).
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