Dear VLBI-friends,

  You know this Sunday, 08-SEP-2002, IVS scheduled experiment grav01.
Although all VLBI experiments are important, this experiment is of 
extraordinary importance. The purpose of this experiment is to answer the 
question what is the geometry of our world. The Jupiter will pass in less than 
4 arcmin from the quasar 0839+187. When the radio waves from the quasar 
pass close the gravitational field of the Jupiter their propagation is
retarded as it follows from general relativity. The closer the ray path to
the planet, the more significant retardation. Jupiter like Earth is moving 
on its orbit around the Sun. Retardation caused by its gravitational field
will change with time. The purpose of our VLBI experiment is to detect this 
change. 

  Static retardation predicted by general relativity was detected
in 1960s. VLBI observations confirmed numerically prediction of general 
relativity for stationary gravitational field with precision of 0.02%. But no 
observations of changes of gravitational delay have been done. Changes of 
gravitational retardation of radio waves is sensitive to position of the 
gravitating body. It is also sensitive to the speed of propagation of gravity.
At any given point on the ray path radio photons are affected by the 
gravitation field of the Jupiter not at the moment of time where it is now,
but at the moment of time when the gravitational field started to propagate
in order to reach this point on the trajectory of the photon. Therefore, 
measurement of changes of gravitational retardation allows us to measure the 
speed of propagation of gravity.

  No measurements of propagation of gravity was made so far. General relativity
assumes that gravity propagates with the same speeds as electromagnetic
interaction. This assumption and the assumption that the equality of 
gravitational mass and inertial mass is not a curious coincidence but 
a fundamental law makes field equations simple and beautiful. Equality of 
speed of propagation of electro-magnetic and gravitational interaction has 
a deep meaning: constant c is not the property of electro-magnetic field,
but the property of space-time. That the underlying metric of our world is 
pseudo-euclidean perturbed by the gravitating masses. Is it true? Or the 
concept of a pseudo-euclidian space-time is only an elegant mathematical 
tool applicable for electro-magnetic field, but not applicable for 
gravitational field, and, therefore, its interpretation as the property of the 
world is not warranted? If "c" gravitational is not equal to 
"c" electromagnetic, it means that we live in a quite different world than 
theoreticians think. In which world do we live? This weekend experiment will 
help us to answer this question.

  We asked you to observe in unusual time because the moment of closest
approaching the Jupiter to the quasar is on 15h 55m 36.9s UTC 2002.09.08.
We are not in a position to change the orbit of Jupiter. Events like that 
are very rare. There will no be events like that during next 10 years. So,
if something will go wrong we will lose the opportunity to learn the nature
of your world for long time. 

  I keep my finger crossed that no mistakes will be done and this experiment
will be as successful as many others. Thank you for your consent to observe 
during weekend. 

Sincerely,
Leonid Petrov
06-SEP-2002 09:08:45