Singular they is the use in English of the pronoun they, or its inflected or derivative forms, such as them, their, themself, or themselves, as a gender-neutral pronoun to refer to a single person or an antecedent that is grammatically singular. It typically occurs with an antecedent of indeterminate gender, as in sentences such as: "Somebody left their umbrella in the office. Would they please collect it?" "The patient should be told at the outset how much they will be required to pay." "But a journalist should not be forced to reveal their sources." The singular they had emerged by the 14th century...

In fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two media when the force of gravity or buoyancy tries to restore equilibrium. An example of such an interface is that between the atmosphere and the ocean, which gives rise to wind waves. A gravity wave results when fluid is displaced from a position of equilibrium. The restoration of the fluid to equilibrium will produce a movement of the fluid back and forth, called a wave orbit. Gravity waves on an air–sea interface of the ocean are called surface gravity waves or surface waves, while gravity waves...

Gravitational waves are ripples in the curvature of spacetime that propagate as waves, generated in certain gravitational interactions that propagate outward from their source. The possibility of gravitational waves was discussed in 1893 by Oliver Heaviside using the analogy between the inverse-square law in gravitation and electricity. In 1905 Henri Poincare first predicted gravitational waves (ondes gravifiques) emanating from a body and propagating at the speed of light as being required by the formalism of spacetime. Predicted in 1916 by Albert Einstein on the basis of his theory of general...