Length Scale

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The length scale shown above is logarithmic, and each tick mark represents a distance 100,000 times longer than the one below it. This span of lengths, from roughly 1030 m on the large end to 10-35 m on the small end, covers all of physics as we know it. It's not possible for something to be larger than the size of the universe, and we're not sure that it even makes sense to talk about a length smaller than the Planck length. We know that general relativity dominates at large scales, where we find galaxy clusters and mountains, and that quantum mechanics dominates at small scales, where we find molecules, atoms, and possibly even strings. The two theories meet somewhere in the middle of the scale, where a theory of quantum gravity is essential. Note that the short-distance tests of gravity described in Unit 3 take place right in the center of the scale, at around 10-4 m. (Unit: 4)