is zero mass and spin 2. As yet however there is no experimental or observational evidence that its basic premise is correct. The third effort, which we may call affine loop gravity, recasts the mathematics of general relativity in such a way that the fundamental object is not the metric but a mathematical object called an affine connection, which is analogous to the gauge potentials describing other non-gravitational fields, such as the vector potential of the electromagnetic field. In affine loop gravity areas and volumes are indeed quantized, and the theory has other attractive features. Various authors 6 visualize spacetime as a boiling quantum foam of strange geometries such as virtual black holes and wormholes, or as a dense bundle of 6 dimensional Calabi-Yau manifords, or as a subspace of a more fundamental 10 or 11 dimensional space, or as a lower dimensional holographic projection, or as the eigenvalue space of quantum operators, or as a spin network, or as a woven quantum fabric etc. etc. But despite intense effort over decades none of the many speculative ideas and theories has yet reached a high level of success or general acceptance, and we remain free to consider many possibilities. Perhaps the oddest possibility is that spacetime at the Planck scale is not truly observable and may thus be an extraneous and sterile concept, much as the luminous ether of the nineteenth century proved to be extraneous after the advent of relativity and spacetime — thus obviating decades of theoretical speculation. 3 At present it is certainly not clear what might replace our present concept of spacetime at the Planck scale. This work was partially supported by NASA grant 8-39225 to Gravity Probe B. Thanks go to Robert Wagoner, Francis Everitt, and Alex Silbergleit and members of the Gravity Probe B theory group for useful discussions and to Frederick Martin for patient reading and comments on the manuscript. ACKNOWLEDGEMENTS ∗ Electronic address: Electronic mail: gyroron@gmail.com 1 Plancks comments on his unit system preceded his discovery of the correct black body radi- ation law: M. Planck, “Uber irreversible Strahlungsvorgange,” Sitzungsberichte der Koniglich Preussischen Akademie der Wissenschaften zu Berlin 5, 440 V 480 (1899). 17