The 2019 Eurovision Song Contest is on right now. Above (click to zoom) is a combined word cloud for the songs (or English translations of the songs).

From the point of view of getting into the final, it seems to be bad to sing about Heaven (Montenegro, Portugal), war (Croatia, Finland), cell phones (Belgium, Portugal), or cold (Latvia, Poland, Romania). On the other hand, it’s good to sing about lights (Germany, Norway, Sweden).

Good luck to everyone for the final!


International Nurses Day

Sunday (May 12) is International Nurses Day, a day which marks the contributions that nurses make all around the world. The day is in fact the birthday of Florence Nightingale, who was a pioneer nurse as well as a pioneer of medical statistics. Nurses are multi-talented!

Thank you, all you nurses, for your contributions to the world!

Revising the Metric System

Relationship between the new SI units (image produced using the igraph package of R)

On May 20, a major redefinition of SI (metric) units comes into force. In particular, the second, metre, ampere, mole, kilogram, kelvin, and candela will be defined as follows:

The second (unit of time)

As it is now, the second will be defined using ultra-precise caesium clocks. Specific microwave radiation from caesium atoms is defined to have a frequency of exactly 9.192 631 770 GHz. That is, counting 9,192,631,770 waves will take exactly one second.

The metre (unit of length)

As it is now, the metre will be defined using the speed of light, which is defined to be exactly 299,792,458 metres per second. That is, the metre is the distance travelled by light in one 299,792,458th of a second (where the second is defined as above).

The ampere (unit of electric current)

The definition of the ampere (amp) has been greatly simplified, taking account of the connection between electricity and electrons. The ampere is a coulomb of electric charge flowing past a given point per second, and the charge on a single electron is now defined to be 1.602 176 634 × 10−19 coulombs. Thus an ampere is about 6,241,509,074 billion electrons flowing past a given point in a second.

As a consequence of this new definition, two important natural constants which used to have defined values (the permeability of free space and the permittivity of free space) now have experimentally determined ones. This will require rewriting pretty much every physics and electrical engineering textbook.

The mole (unit of amount of substance)

The mole represents Avogadro’s number of atoms, molecules, or other particles. Previously, Avogadro’s number was defined to be the number of carbon atoms in 12 grams of pure carbon-12. It is now defined to be exactly 6.022 140 76 × 1023.

The kilogram (unit of mass)

Until 2019, the kilogram was defined by the mass of a specific metal cylinder held in Paris. This has been felt to be unsatisfactory for many years. The current definition uses the fact that the energy of a light photon (in joules) is its frequency times Planck’s constant h, which is defined to be exactly 6.626 070 15 × 10−34.

In practice, a Kibble balance will be used to measure weights by balancing them against an electrically produced force. Units derived from the kilogram include:

  • The newton (unit of force): the force needed to accelerate 1 kilogram at a rate of 1 metre per second squared
  • The pascal (unit of pressure): 1 newton of force per square metre
  • The joule (unit of energy): the energy used in applying a force of 1 newton over a distance of 1 metre
  • The watt (unit of power): 1 joule of energy per second
  • The volt (unit of electric potential): the amount of electric potential across a resistance producing 1 watt of heat per ampere of current
  • The ohm (unit of electrical resistance): the resistance which produces 1 ampere of current when 1 volt of electric potential is applied

See also what NIST has to say about the kilogram.

The kelvin (unit of temperature)

Temperature in degrees Celsius was originally measured on a scale with 0 °C being the freezing point of water and 100 °C the boiling point (at standard pressure). The lowest possible temperature turned out to be absolute zero, −273.15 °C. In 1954, the two fixed points on the scale were changed to −273.15 °C (0 kelvins) and the triple point of water, 0.01 °C (273.16 kelvins).

This definition proved unhelpful for calibrating thermometers intended for very high temperatures, and the current definition uses the fact that the average translational kinetic energy (in joules) of a moving atom of a monoatomic ideal gas is (3/2k T, where T is the temperature of the gas in kelvins, and the Boltzmann constant k is defined to be exactly 1.380 649 × 10−23.

The candela (unit of luminous intensity in a given direction)

The definition of the candela remains what it has been, except that it is influenced by the change in definition of the kilogram (and hence the watt). A light source that emits monochromatic yellowish-green light at a frequency of 540 THz (roughly 555 nm wavelength) is taken to emit 683 lumens per watt, and a light source that uniformly radiates 1 candela in all directions has a total luminous flux of 4π lumens (the constant 683 reflects the human ability to perceive light). The lux is a lumen per square metre.

The dream

When the metric system was first introduced, the metre was defined in terms of the world (1/10,000,000 of the distance between the Equator and the North Pole, measured via Paris). Today, the metric system carries that philosophy to its ultimate conclusion, with all units except the candela defined in terms of the universe. Five of the units are defined in terms of fundamental physical constants: the speed of light (first measured by Rømer in 1676), the charge on the electron (first measured directly by Robert A. Millikan in 1909), the Avogadro constant (measured several ways by Jean Perrin around 1910), and the Planck and Boltzmann constants (first defined by Max Planck around 1900).

The redefined metric system is a little difficult to grasp without understanding modern physics, but fortunately most of us will just keep on using exactly the same measurement instruments as we have done for years.

2019 in science so far

This year in science so far (click to zoom). Clockwise from top left:

Hello again, little tree-kangaroo

The Wondiwoi tree-kangaroo (detail of an illustration by Peter Schouten)

National Geographic recently reported an interesting story about the Wondiwoi tree-kangaroo (Dendrolagus mayri). Until recently, this arboreal marsupial was known only from a single specimen collected in the Wondiwoi Peninsula of West Papua in 1928. It was thought to be extinct, and was listed on the “25 most wanted lost species” at But when an amateur expedition visited the dense mountain forests of the Wondiwoi Peninsula, there it was, living happily in the trees. A good-news story from the animal kingdom, for once.

Surprised to find kangaroos living in trees? There are a number of related species that do this, in the rainforests of New Guinea and northern Australia. In fact, members of the kangaroo family live in a range of different habitats (the rock-wallaby would be a less dramatic example).

New Horizons status check

The New Horizons spaceprobe, having given us some lovely pictures of Pluto in 2015, is on its way to the Kuiper belt. But what is the Kuiper belt? Named after Dutch-American astronomer Gerard Kuiper, the Kuiper belt is much like the asteroid belt, but much larger, about 15 times further out from the Sun, and far less well understood.

Initially, New Horizons is headed for the rock, or perhaps pair of rocks, (486958) 2014 MU69, which NASA has nicknamed Ultima Thule. The space probe is due to reach it on January 1st (which will be just short of 13 years after its launch). Currently, New Horizons is 6,360,000,000 km or 5.9 light-hours from Earth, and has recently completed a course-correction manoeuvre.