In particular, a " timelike geodesic congruence " can be interpreted as a family of " free-falling test particles ".
22.
In the language of spacetime, this is equivalent to saying that such test particles move along straight world lines in spacetime.
23.
In the absence of gravity and other external forces, a test particle moves along a straight line at a constant speed.
24.
They are constants of the motion and depend on the initial conditions ( position and velocity ) of the test particle.
25.
While investigating the motion of test particles in these solutions, Einstein and Rosen became convinced that gravitational waves were unstable to collapse.
26.
The equations below pertain to the fields in which the test particles described above move and allow the calculation of those fields.
27.
However, when a test particle is forced to approach an electron, we measure changes in its properties ( charge and mass ).
28.
JBD theories although not changing the geodesic equation for test particles, change the motion of composite bodies to a more complex one.
29.
Also, the effect of gravitational radiation on test particles differs profoundly between scalar theories and tensor theories of gravitation such as general relativity.
30.
Wheeler was intrigued by the possibility that geons could affect test particles much like a massive object, hence " mass without mass ".
