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The passage around this formula

The instruments above resolve structure at short length scales. A different family of instruments — laser interferometers — pushes precision along a single dimension (typically a length or a phase difference) to an extreme that has little to do with spatial resolution in the microscopy sense. Ground-based gravitational-wave interferometers are the sharpest publicly documented example: designed to detect a fractional change in arm length, expressed as strain, as small as roughly h $\approx$ 5 $\times$ 10^{-22} — a distortion far smaller than a proton’s diameter integrated over a kilometers-long arm [ 10 ] . Reaching that sensitivity is a story about noise budgeting rather than a single dominant…

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