Gravitational shielding
The term gravitational shielding refers to a hypothetical process of shielding an object from the influence of a gravitational field. Such processes, if they existed, would have the effect of reducing the weight of an object. The shape of the shielded region would be similar to a shadow from the gravitational shield. For example, the shape of the shielded region above a disk would be conical. The height of the cone's apex above the disk would vary directly with the height of the shielding disk above the earth. Experimental evidence to date indicates that no such effect exists. Gravitational shielding is considered to be a violation of the equivalence principle and therefore inconsistent with both Newtonian theory and general relativity.
The concept of gravity shielding is a common concept in science fiction literature, especially for space travel. One of the first and best known examples is the fictional gravity shielding substance "Cavorite" that appears in H. G. Wells' classic 1901 novel The First Men in the Moon. Wells was promptly criticized for using it by Jules Verne.
As of 2008[update], no experiment was successful in detecting positive shielding results. To quantify the amount of shielding, at the beginning of 20-th century Quirino Majorana suggested an extinction coefficient h that modifies Newton’s gravitational force law as follows:
The best laboratory measurements have established an upper bound limit for shielding of 4.3×10−15 m²/kg. Another recent analysis suggested a lower bound of 0.6×10−15. The best estimate based on the most accurate gravity anomaly data during the 1997 solar eclipse has provided a new constraint on the shielding parameter 6×10−19 m²/kg. However, astronomical observations impose much more stringent limits. Based on lunar observations available in 1908, Poincaré established that h can be no greater than 10−18 m²/kg. Subsequently this bound has been greatly improved. Eckhardt showed that lunar ranging data implies an upper bound of 10−22 m²/kg, and Williams, et al., have improved this to h = (3 ± 5)×10−22 m²/kg. Note that the value is smaller than the uncertainty. The consequence of the negative results of those experiments (which are in good agreement with the predictions of general relativity) is, that every theory which contains shielding effects like Le Sage's theory of gravitation, must reduce those effects to an undetectable level. For a review of the current experimental limits on possible gravitational shielding, see the survey article by Bertolami, et al. Also, for a discussion of recent observations during solar eclipses, see the paper by Unnikrishnan et al.
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