Experiment finds that gravity still works down to 50 micrometers
Gravity is the weakest of the fundamental forces, but it's the only force that operates over very long distances. Hence, planets orbit stars, stars form galaxies, and galaxies cluster. Gravity also operates at the tiniest of scales, too, but its weakness makes it very hard to detect its influence. It's worth trying, though, as violations of the laws of gravity at very small scales would be good evidence for New Physics
. So physicists have been looking, though without much luck so far.
The force due to gravity reduces with the square of the distance. If you double the distance, the force is not halved but reduced to a quarter of its original value. This law, called an inverse-square law, is based purely on geometry: we live in three spatial dimensions, and therefore the inverse-square law holds. However, if the universe has more than three spatial dimensions, the inverse-square law would break.
We know that over long distances-the Earth to the Moon, and the distances between stars-the inverse-square law appears to be correct. At galactic and cosmological scales, the inverse-square law also holds, with the caveat that dark matter and dark energy are required. You might think that what we call dark matter or dark energy would be potential evidence of extra dimensions, but it isn't quite that simple. At these scales, the hidden dimension would have to be both large and unable to influence anything else, like photons. Since we also require consistency, and large hidden dimensions don't appear to offer it at the moment, we are restricted to tiny hidden dimensions and changes to gravity at very small scales.
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