Energy as Universal Description of Matter
Einstein and general relativists recognize that all universe content—stars, planets, interstellar gas, living beings—can be described through a single unifying framework rather than treating each as distinct phenomena.
Energy Momentum Tensor as Flux Description
General relativists use the energy-momentum tensor (capital T) to mathematically encode how energy and momentum flow through different directions in spacetime, treating content as fluid-like movements rather than static distributions.
Energy Density as Temporal Flux Component
The first component of the energy-momentum tensor (T⁰⁰) captures what physicists commonly call energy density—the fundamental measure of how much energy crosses a point moving toward the future.
Momentum Density as Spatial Energy Transport
The next tensor components after energy density measure momentum density—equal pairs that quantify how much energy travels through space over time, capturing the spatial motion aspect of energy flows.
Pressure as Spatial Momentum Transfer
The final component of the energy-momentum tensor represents pressure—the most difficult to interpret because it measures how motion through space transmits through space itself, not through time.
From Four to Sixteen Tensor Components
When transitioning from two-dimensional pedagogical examples to four-dimensional spacetime reality, physicists must track the energy-momentum tensor’s expansion from four components to sixteen distinct values.
Energy Flux Relativity and Coordinate Dependence
Einstein’s fundamental insight that motion is relative extends to energy-momentum tensor components—different observers measuring the same physical system obtain different component values depending on their reference frame.
Perfect Fluid Model for Stellar Structure
Astrophysicists modeling stellar interiors approximate stars like the Sun as perfect fluids—matter distributions without viscosity at equilibrium, enabling simplified yet accurate energy-momentum tensor descriptions.