Test of the Weak Equivalence Principle (WEP)

 precision astronomy group





Harvard-Smithsonian Center for Astrophysics

Why Test the WEP?


• Central to the present accepted theory of gravity.

-Some theorists argue it is the best arena for finding a breakdown of general relativity.


• The evidence for dark energy may be telling us that we need a new gravity theory.


• Attempts to create a quantum theory of gravity show a failure of the equivalence principle.


• Gravity is the least well tested force.

Newton’s equivalence principle states that the motion of a body solely under the influence of gravity is independent of the composition and internal structure of the body.  Newton’s version is now referred to as the weak equivalence principle (WEP), which includes the universality of free fall (UFF).  With two additional requirements (local Lorentz invariance and local position invariance, which includes temporal invariance), it becomes the Einstein equivalence principle, which “is at the heart of gravitation theory” [Will, C.M., Theory and Experiment in Gravitational Physics, Cambridge University Press, second edition, 1993.  p. 22].

A WEP violation is characterized by η, which is identically zero in any metric theory of gravity, including general relativity [e.g., Fischbach and Talmadge, The Search for Non-Newtonian Gravity, Springer-Verlag, New York, 1999].

where aA is the acceleration of body A and aB is the acceleration of body B, these bodies are moving under the influence of identical gravity fields, and there is no other cause of the acceleration.  The discovery of a WEP violation would have profound implications for physics, astrophysics and cosmology.  Either the discovery of a violation or the confirmation at a deeper level than previous experiments have achieved would inform the development of new theoretical physics, especially the attempts to connect gravity with quantum mechanics and the other forces of nature (weak, electromagnetic, strong).  Further, although tests of the WEP are intended to be sensitive to η, they may also discover previously unknown weak forces. 


What is the WEP?



EP Literature