Femtometre (American spelling: femtometer) is an SI measure of length that is equal to 10−15 (femto) of a metre. Its symbol is fm and is commonly used in measuring the diameter of atomic nuclei. The diameter of an atomic nucleus is up to about 15 fm. Neutrons and protons are about 2.5 fm in diameter. In the parlance of particle physicists, a femtometre is often called a fermi (same symbol), after the physicist Enrico Fermi.
CERN physicists investigate the constituents of matter at the subatomic level, where the typical distances are of the order of the femtometre (0.000000000000001 m, or 10
To be able to probe objects of such a small scale, physicists need other tools than your ordinary microscope.
This means, oddly enough, that the smaller the details you want to look at, the larger the machine you will have to build.
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