重力計測による“第5の力”検出の現状と展望

抄録

Recently a somewhat mystifying hypothesis suggests that the inverse-square law of the Newtonian gravitation may not hold good for short mass separations. If this hypothesis is real, the conventional Newtonian gravitational constant behaves as a constant at very large distances but a function of the mass separation in the laboratory distance scale. On the basis of the elementary particle theory, this function is defined as an additional term exponentially decreasing with the mass separation, which is called "Yukawa potential term" on the analogy of meson dynamics in nucleonics. It was really surprizing that the Australian research group detected the Yukawa potential term and determined its parameters by gravity measurements in a 950-m deep mine shaft. Some experimental tests have followed in mine shafts and boreholes. It seems, however, that geological complexity around the test sites interrupted precise determinations of the newtonian constant. A 600-m high tower was used for a vertical gravity gradient determination in North Carolina. The tower survey group once reported that they obtained the parameters, but revised later their conclusion. The other two tower experiments, which were made in Nevada and Colorado, concluded that they gave no solid evidence for non-Newtonian gravitation. A German research group also could not detect any failure in the Newtonian gravitational law by a 22-day long continuous gravity measurement on a 60-m high tower. The above conclusions may imply that the non-Newtonian term will not exist or be too small to be detected even by highly-sensitive gravity measurements for a mass separation ranging from scores to hundreds of meters. The future searching efforts may be concentrated on laboratory tests using torsion-balances or gradiometers but not on field gravity experiments.