  Contemporary theories and models for electric power system stability are
predicated on a widely held assumption that the mechanical inertia of the
rotating mass of synchronous generators provides the sole contribution to
stable and synchronized operation of this class of complex networks on
subsecond timescales. Here we formulate the electromagnetic momentum of the
field around the transmission lines that transports energy and present evidence
from a real-world bulk power network that demonstrates its physical
significance. We show the classical stability model for power networks that
overlooks this property, known as the "swing equation", may become inadequate
to analyze systems with high shares of inverter-based resources, commonly known
as "low-inertia power systems". Subsequently, we introduce a plane wave dynamic
model, consistent with the structural properties of emerging power systems with
up to 100% inverter-based resources, which identifies the concept of inertia in
power grids as a time-varying component. We leverage our theory to discuss a
number of open questions in the electric power industry. Most notably, we
postulate that the changing nature of power networks with a preponderance of
variable renewable energy power plants could strengthen power network stability
in the future; a vision which is irreconcilable with the conventional theories.
