<Dd> B = ∇ × A . (\ displaystyle \ mathbf (B) = (\ boldsymbol (\ nabla)) \ times \ mathbf (A).) </Dd> <P> At the end of the 19th century, the electromagnetic field was understood as a collection of two vector fields in space . Nowadays, one recognizes this as a single antisymmetric 2nd - rank tensor field in spacetime . </P> <P> Einstein's theory of gravity, called general relativity, is another example of a field theory . Here the principal field is the metric tensor, a symmetric 2nd - rank tensor field in spacetime . This replaces Newton's law of universal gravitation . </P> <P> Waves can be constructed as physical fields, due to their finite propagation speed and causal nature when a simplified physical model of an isolated closed system is set . They are also subject to the inverse - square law . </P>

Where do fields exist and what do they do