4-dimensional Planck scale is no longer the fundamental scale, which is , where is the number of extra dimensions. This can be seen from the modification of the gravitational potential. For an Einstein–Hilbert gravitational action we haved-dimensional Poisson equation, whose solution is the gravitational potential, d-dimensional, . By contrast, on scales large relative to , where the extra dimensions do not contribute to variations in the potential, behaves like a 4-dimensional potential, i.e., in the extra dimensions, and . This means that the usual Planck scale becomes an effective coupling constant, describing gravity on scales much larger than the extra dimensions, and related to the fundamental scale via the volume of the extra dimensions:
A lower limit on is given by null results in table-top experiments to test for deviations from Newton’s law in 4 dimensions, . These experiments currently  probe sub-millimetre scales, so that[75, 87, 189, 194].
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