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Originally Posted by Tempus
I donīt know much about brane cosmologies but the notion of gravity acting across branes through extra dimensions is already disproved by experience.
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No, that is not the case (see below).
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If such a thing were possible, the GaussīLaw calls for the gravitational flux to spread over those extra dimensions. Since Newton discovered an inverse square law (three dimensions for gravity to act on) instead of inverse N-1 (N dimensions, being N>3), the conclusion is that if those extra dimensions exist, gravity does not act through them. And that holds for brane cosmologies and for whatever cosmology which calls for extra dimensions.
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The inverse N-1 law applies only for dimensions that are larger than the scale gravity is being measured. String theory basically implies that there are 6 additional dimensions that are
small, with the
Planck length a natural length scale for these dimensions. Current measurements only rule out extra
large dimensions, ones with scales of mm's or more. All we know is that there are only three large spatial dimensions (and one time dimension); there is no constraint on the number of dimensions smaller than the mm scale.
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The extra dimensions are not ruled out by this reasoning, just their ability to transmit gravity (or the electromagnetic field).
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Also in reply to
antoniseb, gravity transmits through
all dimensions in String Theory. This is because the graviton cannot be constrained to a brane like other force carriers.