Modern Tests of General Relativity
Recently, these telescopes have measured the deflection of
radio waves by the Sun to extremely high precision,
confirming the amount of deflection predicted by general
relativity aspect to the 0.03% level.
Review of Astronomers' Tools
Light travel time delay testing
Irwin I. Shapiro proposed another test, beyond the classical
tests, which could be performed within the Solar System. It
is sometimes called the fourth "classical" test of general
relativity. He predicted a relativistic time delay (Shapiro
delay) in the round-trip travel time for radar signals
reflecting off other planets.
Observing radar reflections from Mercury and Venus just
before and after it is eclipsed by the Sun agrees with
general relativity theory at the 5% level. More recently,
the Cassini probe has undertaken a similar experiment which
gave agreement with general relativity at the 0.002% level.
The Equivalence Principle
Start at minute 13 to 23:30
The equivalence principle, in its simplest form, asserts
that the trajectories of falling bodies in a gravitational
field should be independent of their mass and internal
structure, provided they are small enough not to disturb the
environment or be affected by tidal forces. This idea has
been tested to extremely high precision by Eötvös torsion
balance experiments, which look for a differential
acceleration between two test masses.
Global Positioning System
The Global Positioning System (GPS) uses accurate, stable
atomic clocks in satellites and on the ground to provide
world-wide position and time determination. These clocks
have gravitational and motional frequency shifts which are
so large that, without carefully accounting for numerous
relativistic effects, the system would not work. This paper
discusses the conceptual basis, founded on special and
general relativity, for navigation using GPS. Relativistic
principles and effects which must be considered include the
constancy of the speed of light, the equivalence principle,
the Sagnac effect, time dilation, gravitational frequency
shifts, and relativity of synchronization.
In general relativity, Lense-Thirring precession or the
Lense-Thirring effect (named after Josef Lense and Hans
Thirring) is a relativistic correction to the precession
of a gyroscope near a large rotating mass such as the
Earth. It is a gravitomagnetic frame-dragging effect.
According to a recent historical analysis by Pfister, the
effect should be renamed as Einstein-Thirring-Lense
effect. It is a prediction of general relativity
consisting of secular precessions of the longitude of the
ascending node and the argument of pericenter of a test
particle freely orbiting a central spinning mass endowed
with angular momentum S.
The difference between de Sitter precession and the
Lense-Thirring effect is that the de Sitter effect is due
simply to the presence of a central mass, whereas the
Lense-Thirring effect is due to the rotation of the
central mass. The total precession is calculated by
combining the de Sitter precession with the Lense-Thirring
Gravity Probe B
Gravity Probe B (GP-B) was a satellite-based mission to
measure spacetime curvature near Earth, and thereby the
stress-energy tensor (which is related to the distribution
and the motion of matter in space) in and near Earth.
By August 2008, the frame-dragging effect had been
confirmed to within 15% of the expected result, and the
December 2008 NASA report indicated that the geodetic
effect was confirmed to better than 0.5%.
Einstein was right - again! (Satellite observations of
Black Holes confirm frame-dragging effect 80 years after
Einstein Online - Spotlights on relativity