The Weak Equivalence Principle
The weak equivalence principle, also known as the universality of free fall or the Galilean equivalence principle can be stated in many ways. The strong EP includes (astronomic) bodies with gravitational binding energy (e.g., 1.74 solar-mass pulsar PSR J1903+0327, 15.3% of whose separated mass is absent as gravitational binding energy). The weak EP assumes falling bodies are bound by non-gravitational forces only. Either way,
- The trajectory of a point mass in a gravitational field depends only on its initial position and velocity, and is independent of its composition and structure.
- All test particles at the alike spacetime point in a given gravitational field will undergo the same acceleration, independent of their properties, including their rest mass.
- All local centers of mass vacuum free fall along identical (parallel-displaced, same speed) minimum action trajectories independent of all observable properties.
- The vacuum world line of a body immersed in a gravitational field is independent of all observable properties.
- The local effects of motion in a curved space (gravitation) are indistinguishable from those of an accelerated observer in flat space, without exception.
- Mass (measured with a balance) and weight (measured with a scale) are locally in identical ratio for all bodies (the opening page to Newton's Philosophiæ Naturalis Principia Mathematica, 1687).
Locality eliminates measurable tidal forces originating from a radial divergent gravitational field (e.g., the Earth) upon finite sized physical bodies. The "falling" equivalence principle embraces Galileo's, Newton's, and Einstein's conceptualization. Relativistic particles and photons do not fall like local masses. Photons in a gravitational field experience twice the local gravitational acceleration (tracing a null geodesic) - an important test of general relativity versus Newton. Divergent trajectories arising from gravitomagnetism (frame dragging, Lense-Thirring effect), gravitoelectric effects, geodetic effect, Shapiro delay, etc., do not contradict the equivalence principle.
Read more about this topic: Equivalence Principle, Modern Usage
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