Gravitational bar detectors set limits to Planck-scale physics on macroscopic variables

Different approaches to quantum gravity, such as string theory(1,2) and loop quantum gravity, as well as doubly special relativity(3) and gedanken experiments in black-hole physics(4-6), all indicate the existence of a minimal measurable length(7,8) of the order of the Planck length, L-p = root hG/c(3) = 1.6 x 10(-35) m. This observation has motivated the proposal of generalized uncertainty relations, which imply changes in the energy spectrum of quantum systems. As a consequence, quantum gravitational effects could be revealed by experiments able to test deviations from standard quantum mechanics(9-11), such as those recently proposed on macroscopic mechanical oscillators(12). Here we exploit the sub-millikelvin cooling of the normal modes of the ton-scale gravitational wave detector AURIGA, to place an upper limit for possible Planck-scale modifications on the ground-state energy of an oscillator. Our analysis calls for the development of a satisfactory treatment of multi-particle states in the framework of quantum gravity models.