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Another interesting and potentially important propulsion development are the so-called "gravitational shielding" experiments. In these experiments small weights are suspended by extremely sensitive strain gauge devices. Placed closely underneath are rapidly rotating superconducting ceramic discs, which in turn are in a state of levitation due to the superconducting Meissner effect. It has been found in a number of different experiments that a significant fraction of weight is lost from the suspended materials. The effect was first discovered by Tampere University of Technology physicists E. Podkletnov and R. Nieminen, and published in paper entitled "A Possibility of Gravitational Force Shielding by Bulk YBa2Cu307-x Superconductor"(Physica C 203, 1992, pp 441- 444).

Podkletnov later published a second paper with colleague A.D. Levit, "Gravitational Shielding Properties of Composite Bulk YBa2Cu307-x Below 70C Under Electro-magnetic Field"(Tampere University of Technology Report MSU-95 chem, January 1995). This paper provides even stronger evidence for the weight loss effect. In this 2nd experiment, samples of different composition and weight (10-50grams) were placed at distances of 25mm to 1.5 meters from the rotating disk. The resultant mass losses went as high as around 2% -- undeniably a statistically significant amount.

I think a reason NASA is interested in this is that if such an effect is real, then potentially operating it out of a gravity well (e.g. where we send our space probes) means that it might be useful as a propulsion system. That is, something that can appear to reduce weight (it says mass but in reality the samples were weighed) in a zero gravity field might produce "negative" weight - that is, a propulsion effect. As Einstein pointed out, there is no measurable difference between the exposure to a gravitational field and acceleration (the elevator thought experiment.)