A kilogram used to be defined by a metal cylinder kept in France. Today, the world's measurement system is anchored instead to exact values of fundamental constants of nature. Since 20 May 2019, the International System of Units, known as SI, has used seven defining constants to establish its units. This change matters whenever scientists compare measurements made in different countries, laboratories or decades. It also raises an interesting question: how can a familiar unit such as a kilogram be connected to quantum physics? The answer lies in the difference between defining a unit and building an instrument that measures it.<\/p>\n
Imagine two laboratories measuring the same sample but using different definitions of mass. Even if their balances were precise, the results could disagree. Common units allow people to compare results and reproduce experiments. They also underpin everyday activities such as selling food, checking medicines, constructing buildings and calibrating electricity meters.<\/p>\n
A unit is an agreed reference for a physical quantity. A measurement compares an object or process with that reference. Precision describes how closely repeated measurements agree, while accuracy concerns how close a result is to the relevant true or reference value. A clear system of units does not guarantee a perfect measurement, but it gives everyone a shared language.<\/p>\n
The base units are the second for time, metre for length, kilogram for mass, ampere for electric current, kelvin for thermodynamic temperature, mole for amount of substance and candela for luminous intensity. Other units can be built from combinations of these.<\/p>\n
For example, speed is measured in metres per second. A newton, the unit of force, is a kilogram metre per second squared. A joule, the unit of energy, is a newton metre. These relationships show why a consistent set of base units is useful: calculations can connect quantities without introducing arbitrary conversion factors at every step.<\/p>\n
For more than a century, the kilogram was associated with the International Prototype of the Kilogram, a carefully manufactured platinum-iridium cylinder<\/a> kept near Paris. Copies were compared with the prototype to maintain a shared mass standard. This system worked remarkably well for its time, but it had an unavoidable weakness: the definition depended on one physical artefact.<\/p>\n Even a protected object can be contaminated, cleaned or changed in subtle ways. Comparisons between prototype masses raised questions about long-term stability. A unit that depends on one object is also difficult to reproduce independently. Scientists therefore sought a definition based on something that would not age or change with handling.<\/p>\n In November 2018, representatives of the international metrology community approved a major revision of SI. The new definitions took effect on 20 May 2019. Four base units\u2014the kilogram, ampere, kelvin and mole\u2014were redefined through fixed numerical values of constants, completing a system in which all seven base units are anchored to fundamental properties of nature.<\/p>\n The change did not make everyday objects suddenly heavier or lighter. A kilogram of flour still has the same practical mass. What changed was the underlying definition and the route by which high-precision laboratories realise it. The aim was continuity for ordinary users and greater long-term stability for advanced science.<\/p>\n The Planck constant, written h, is a fundamental quantity in quantum physics. It connects the energy of a photon with its frequency through the equation E = hf. In the revised SI its numerical value is fixed exactly at 6.62607015 \u00d7 10^-34 joule seconds.<\/p>\n Because a joule can be expressed using kilograms, metres and seconds, fixing h\u2014alongside the definitions of the metre and second\u2014provides a way to define the kilogram. This is a conceptual leap from 'mass equals this metal cylinder' to 'mass is part of a consistent relationship among physical constants'. For related background, Earth Site's explanation of atoms<\/a> introduces the microscopic matter whose behaviour quantum physics describes.<\/p>\n One method uses a Kibble balance, an instrument that compares mechanical power with electrical power. Through quantum electrical standards and carefully controlled measurements, it connects mass to the fixed Planck constant. The engineering is demanding because the apparatus must measure forces, motion and electrical quantities with extraordinary precision.<\/p>\n Another approach uses highly characterised silicon spheres<\/a> and counts atoms indirectly through crystal measurements. Different methods provide independent checks. The point is not that every school needs a Kibble balance; national metrology institutes maintain high-precision standards that can be transferred through calibrated instruments to ordinary workplaces.<\/p>\n The metre is linked to the speed of light in vacuum, which is fixed exactly at 299,792,458 metres per second. With the second already defined, this gives a reproducible basis for length. Scientists can realise the metre using laser interferometry and other optical methods.<\/p>\n The definition does not say that every measuring tape must use a laser. Instead, the calibration chain ultimately connects practical instruments to the agreed reference. The metre's stability matters in everything from precision manufacturing to satellite navigation. The historical shift from physical bars to constants illustrates the same general principle as the kilogram redefinition.<\/p>\n The SI second is based on a specific frequency associated with transitions in caesium-133 atoms. Its defining numerical value is 9,192,631,770 cycles per second. Atomic clocks exploit this reproducible behaviour to maintain highly stable time standards.<\/p>\n Atomic timekeeping supports telecommunications, navigation, financial systems and scientific experiments. A phone does not contain a national atomic clock, but networks can distribute time signals derived from accurate standards. The definition is also independent of the uneven rotation of Earth, which is why modern timekeeping distinguishes atomic time from astronomical measures.<\/p>\n The elementary charge helps define the ampere; the Boltzmann constant helps define the kelvin; the Avogadro constant defines the mole; and a fixed luminous-efficacy value contributes to the definition of the candela. These join the caesium frequency, speed of light and Planck constant to form the seven defining constants.<\/p>\n The terminology can be intimidating, but the principle is straightforward. Instead of asking laboratories to preserve an identical object forever, SI fixes values associated with repeatable physical relationships. The practical techniques vary from electrical quantum standards to thermometry and chemical measurement. Earth Site's Standard Model guide<\/a> offers context for some of the particles and interactions involved.<\/p>\n A mole contains exactly 6.02214076 \u00d7 10^23 specified elementary entities, such as atoms, molecules or ions. The entities must be identified because a mole of water molecules is not the same kind of collection as a mole of oxygen atoms.<\/p>\nWhat changed in May 2019?<\/h2>\n
What is the Planck constant?<\/h2>\n
How can a laboratory realise the kilogram?<\/h2>\n
How is the metre defined?<\/h2>\n
How is the second defined?<\/h2>\n
What are the other defining constants?<\/h2>\n
Does the mole mean a fixed number of atoms?<\/h2>\n