Understanding Nonlocal Quantities in Physics

In physics, a nonlocal quantity is one that depends on the values of field variables at multiple points in space and time, rather than just at a single point. This means that the value of a nonlocal quantity at one location can be influenced by the values of the fields at other locations, even if there is no direct physical connection between them.

One example of a nonlocal quantity is the electromagnetic field itself, which can be described by equations that involve nonlocal operators such as the Laplacian or the Coulomb potential. Another example is the stress-energy tensor, which describes the distribution of energy and momentum in a material, and is a nonlocal quantity because it depends on the values of the matter fields at multiple points in space and time.

Nonlocal quantities are important in many areas of physics, including quantum field theory, condensed matter physics, and gravity. They are often difficult to work with, however, because they involve complex mathematical structures and can be sensitive to the details of the underlying physics.