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The detectors must also squelch other vibrations to stabilize the length of each arm to 1 femtometer, the width of a proton.

Pohl and his team were surprised to find a lower value for the radius, pegging it at 0.84 femtometer—well outside the range of potential sizes established by earlier measurements.

The surprise was an answer that was 4 percent smaller, 0.84184 femtometer.

Putting these techniques together gave an answer of about 0.8768 femtometer for the proton’s radius, just less than a quadrillionth of a meter.