A femtometer (fm), also known as a fermi, is an extremely tiny unit of measurement used to express even smaller distances than a picometer. It is equal to one quadrillionth of a meter, which is 0.000000000000001 meters or 1 × 10^-15 meters. The prefix "femto" denotes a factor of 10^-15 in the International System of Units (SI).
Femtometers are typically used in the field of nuclear physics and particle physics to describe the sizes and distances involved in atomic nuclei and subatomic particles. The nucleus of an atom, for example, is on the order of a few femtometers in diameter. In high-energy physics experiments and discussions about fundamental particles like protons, neutrons, and quarks, femtometers are a relevant unit of measurement due to the incredibly small scales involved.
A "handbreadth" is a unit of length that is approximately equal to the width of an adult human hand, excluding the fingers. It is often used in various historical and cultural contexts as a convenient way to estimate small lengths or distances. The actual measurement of a handbreadth can vary somewhat depending on the size of an individual's hand, but it is typically around 3 to 4 inches (about 7.6 to 10.2 centimeters).
In some historical systems of measurement, particularly in the ancient Near East and the Bible, the handbreadth was used as a standardized unit for measuring length. It was one of the smaller units within a larger system of measurement, often used for small-scale applications like measuring cloth, the dimensions of objects, or architectural details.
While the handbreadth is not a commonly used unit of measurement in modern times, it remains a useful and intuitive way to approximate small lengths informally, especially when a quick estimate is needed.