News & Comment

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.

Every now and then, an extra second is added to an earthly year — a cause for trouble and debate, as Felicitas Arias has been witnessing.

Nick van Remortel demystifies natural unit systems — and advises what to do when you see a mass expressed in GeV.

The most precise measurements of the atomic masses of the proton and the electron make use of Penning traps, and for the latter, a hydrogen-like ion, as Edmund Myers explains.

The bit is a proper unit of measurement and should be recognized as such, argues Iulia Georgescu.

Michel Van Camp and Olivier de Viron are attracted to the fluctuations in the Earth's gravitational pull.

Some concepts are useful but difficult to quantify, as Philip Ball illustrates with the property of hydrophobicity.

Yuqin Zong sheds light on photometry's fundamental unit.

Enrico Massa and Giovanni Mana expound on the substance of the Avogadro constant.

Vincent Icke paints a portrait of the bewildering cosmological constant.

François Piquemal tells the story of the ampere, which bridges mechanical and electromagnetic units.

Despite intensified efforts, measurements of the gravitational constant continue to fail to converge, as Terry Quinn explains.