News & Comment

The arrival of a new type of timekeeper heralds the end of the second as we know it, as Helen Margolis explains.

Wonder material graphene makes metrology practical and relaxed, says Andre Geim.

Dimensional analysis is a powerful tool for assessing physical problems, reaffirms Tina Hecksher

High-precision laboratory experiments with neutrons and atoms are converging to a verdict on 'chameleon fields' as a possible explanation of dark energy, explains Tobias Jenke.

Assigning dimensions to physical quantities is not just for practicality. Steven T. Bramwell reflects on the deeper physical connotations of it all.

The path to consistent cgs magnetic units has been long and winding, as is the process of universally adopting SI units. Andreas Trabesinger peeks into the history of the field.

Stephan Schlamminger looks at the origins of the Planck constant and its current role in redefining the kilogram.

Alberto Moscatelli surveys a series of experiments on the electron g-factor that marked the departure from the Dirac equation and contributed to the development of quantum electrodynamics.

Optical-lattice clocks have pushed the limits of frequency measurement — to such an extent that a tiny difference in altitude affects the clock's tick rate, as Hidetoshi Katori elucidates.

Narrowing down the value of the Hubble constant has been problematic — probably a manifestation of the dark-energy mystery, writes Barbara Ryden.

Klaus von Klitzing tells the story of the quantum Hall effect's impact on metrology.

Temperature measurement standards rely on highly reproducible states of matter — including eutectic points, as Jonathan Pearce explains.