  The use of four Lagrange points of the Sun/Earth system for fundamental
physics experiments in space is presented. L1, L2, L4 and L5 rotating rigidly
together with the Earth form a natural reference frame at the scale of the
inner solar system. The idea which is discussed in the paper considers the
possibility of locating four spacecraft in the four cited Lagrange points and
exchanging electromagnetic pulses among them. Including stations on earth,
various closed paths for the pulses are possible. Time of flight measurements
would be performed. The time of flight difference between right- and
left-handed circuits is proportional to the angular momentum of the Sun and the
detection of the effect would reach accuracies better than 1% depending on the
accuracy of the clock. The four points could also be used as "artificial
pulsars" for a relativistic positioning system at the scale of the solar
system. Additional interesting possibilities include detection of a galactic
gravito-magnetic field and also, using a global configuration as a zero area
Sagnac contour, detection of gravitational waves. More opportunities are also
mentioned.
