**Defined in theory: Physical-quantities****Source code: physical-quantities.lisp**

**Documentation:**A zero quantity is one which, when multiplied times any quantity, results in another zero quantity (possibly the same zero). The class of zero quantities includes the number 0, and zero quantities for every physical dimension and order of tensor.

**Subclass-Of:**Physical-quantity

(Zero-Quantity 0) (=> (Zero-Quantity ?X) (Forall (?Q) (=> (Physical-Quantity ?Q) (Zero-Quantity (* ?Q ?X)))))

- Q: Why not make one zero-thing which follows our
intuition?
A: We would have to make exceptions for all of our operators on quantities that depend on physical-dimension or tensor-order.