The Planck constant, or Planck's constant, is a fundamental

^{2}⋅s^{−1}, where the

_{J} relates the potential difference ''U'' generated by the Josephson effect at a "Josephson junction" with the frequency ''ν'' of the microwave radiation. The theoretical treatment of Josephson effect suggests very strongly that .
:$K\_\; =\; \backslash frac\; =\; \backslash frac.$
The Josephson constant may be measured by comparing the potential difference generated by an array of Josephson junctions with a potential difference which is known in SI _{J} is related to the Planck constant by
:$h\; =\; \backslash frac.$

_{90}, this gives a measure of the product ''K''_{J}^{2}''R''_{K} in SI units, where ''R''_{K} is the von Klitzing constant which appears in the quantum Hall effect. If the theoretical treatments of the Josephson effect and the quantum Hall effect are valid, and in particular assuming that , the measurement of ''K''_{J}^{2}''R''_{K} is a direct determination of the Planck constant.
:$h\; =\; \backslash frac\; .$

^{−6}. The protons are said to be "shielded" from the applied magnetic field by the electrons in the water molecule, the same effect that gives rise to chemical shift in NMR spectroscopy, and this is indicated by a prime on the symbol for the gyromagnetic ratio, ''γ''′_{p}. The gyromagnetic ratio is related to the shielded proton magnetic moment ''μ''′_{p}, the spin number ''I'' ( for protons) and the reduced Planck constant.
:$\backslash gamma^\_\backslash text\; =\; \backslash frac\; =\; \backslash frac.$
The ratio of the shielded proton magnetic moment ''μ''′_{p} to the electron magnetic moment ''μ''_{e} can be measured separately and to high precision, as the imprecisely known value of the applied magnetic field cancels itself out in taking the ratio. The value of ''μ''_{e} in Bohr magnetons is also known: it is half the electron ''g''-factor ''g''_{e}. Hence
:$\backslash mu^\_\backslash text\; =\; \backslash frac\; \backslash frac$
:$\backslash gamma^\_\backslash text\; =\; \backslash frac\; \backslash frac.$
A further complication is that the measurement of ''γ''′_{p} involves the measurement of an electric current: this is invariably measured in ''conventional'' amperes rather than in SI _{p-90} is used for the measured gyromagnetic ratio using conventional electrical units. In addition, there are two methods of measuring the value, a "low-field" method and a "high-field" method, and the conversion factors are different in the two cases. Only the high-field value Γ′_{p-90}(hi) is of interest in determining the Planck constant.
:$\backslash gamma^\_\backslash text\; =\; \backslash frac\; \backslash Gamma^\_\backslash text(\backslash text)\; =\; \backslash frac\; \backslash Gamma^\_\backslash text(\backslash text).$
Substitution gives the expression for the Planck constant in terms of Γ′_{p-90}(hi):
:$h\; =\; \backslash frac\; \backslash frac\; .$

_{A} multiplied by the elementary charge ''e''. It can be determined by careful _{A} and ''e'' gives the relation to the Planck constant.
:$h\; =\; \backslash frac\; \backslash frac.$

_{A} but as the Avogadro constant is related to the Planck constant it also determines a value for ''h''. The principle behind the method is to determine ''N''_{A} as the ratio between the volume of the unit cell of a _{220}. The molar volume ''V''_{m}(Si) requires a knowledge of the

"The role of the Planck constant in physics" – presentation at 26th CGPM meeting at Versailles, France, November 2018 when voting took place.

{{Authority control Fundamental constants 1900 in science Max Planck

physical constant
A physical constant, sometimes fundamental physical constant or universal constant, is a physical quantity that is generally believed to be both universal in nature and have constant value in time. It is contrasted with a mathematical constant ...

of foundational importance in quantum mechanics
Quantum mechanics is a fundamental theory in physics
Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and f ...

. The constant gives the relationship between the energy of a photon and its frequency
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from '' angular frequency''. Frequency is measured in hertz
The hert ...

, and by the mass-energy equivalence, the relationship between mass and frequency. Specifically, a photon's energy is equal to its frequency multiplied by the Planck constant. The constant is generally denoted by $h$. The reduced Planck constant, or Dirac constant, equal to the constant divided by $2\; \backslash pi$, is denoted by $\backslash hbar$.
In metrology
Metrology is the scientific study of measurement
Measurement is the quantification of attributes of an object or event, which can be used to compare with other objects or events.
In other words, measurement is a process of determinin ...

it is used, together with other constants, to define the kilogram
The kilogram (also kilogramme) is the unit of mass in the International System of Units (SI), having the unit symbol kg. It is a widely used measure in science, engineering and commerce worldwide, and is often simply called a kilo colloquiall ...

, the SI unit of mass. The SI units are defined in such a way that, when the Planck constant is expressed in SI units, it has the exact value
The constant was first postulated by Max Planck
Max Karl Ernst Ludwig Planck (, ; 23 April 1858 – 4 October 1947) was a Germans, German theoretical physicist whose discovery of quantum mechanics, energy quanta won him the Nobel Prize in Physics in 1918.
Planck made many substantial con ...

in 1900 as part of a solution to the ultraviolet catastrophe. At the end of the 19th century, accurate measurements of the spectrum of black body radiation existed, but the distribution of those measurements at higher frequencies diverged significantly from what was predicted by then-existing theories. Planck empirically derived a formula for the observed spectrum. He assumed that a hypothetical electrically charged oscillator in a cavity that contained black-body radiation can only change its energy
In physics
Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department ...

in quantized steps, and that the energies of those steps are proportional to the frequency of the oscillator's associated electromagnetic wave. He was able to calculate the proportionality constant from experimental measurements, and that constant is named in his honor.
In 1905, Albert Einstein
Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born theoretical physicist, widely acknowledged to be one of the greatest and most influential physicists of all time. Einstein is best known for developing the theor ...

determined a "quantum" or minimal element of the energy of the electromagnetic wave itself. The light quantum behaved in some respects as an electrically neutral particle, and was eventually called a photon. Max Planck received the 1918 Nobel Prize in Physics
)
, image = Nobel Prize.png
, alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then " ...

"in recognition of the services he rendered to the advancement of Physics by his discovery of energy quanta".
Origin of the constant

Planck's constant was formulated as part of Max Planck's successful effort to produce a mathematical expression that accurately predicted the observed spectral distribution of thermal radiation from a closed furnace (black-body radiation
Black-body radiation is the thermal electromagnetic radiation
In physics
Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities ...

). This mathematical expression is now known as Planck's law.
In the last years of the 19th century, Max Planck was investigating the problem of black-body radiation first posed by Kirchhoff some 40 years earlier. Every physical body spontaneously and continuously emits electromagnetic radiation
In physics
Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of k ...

. There was no expression or explanation for the overall shape of the observed emission spectrum. At the time, Wien's law fit the data for short wavelengths and high temperatures, but failed for long wavelengths. Also around this time, but unknown to Planck, Lord Rayleigh had derived theoretically a formula, now known as the Rayleigh–Jeans law, that could reasonably predict long wavelengths but failed dramatically at short wavelengths.
Approaching this problem, Planck hypothesized that the equations of motion for light describe a set of harmonic oscillator
In classical mechanics
Classical mechanics is a physical theory describing the motion of macroscopic objects, from projectiles to parts of machinery, and astronomical objects, such as spacecraft
A spacecraft is a vehicle o ...

s, one for each possible frequency. He examined how the entropy
Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodyna ...

of the oscillators varied with the temperature of the body, trying to match Wien's law, and was able to derive an approximate mathematical function for the black-body spectrum,. English translation: ". which gave a simple empirical formula for long wavelengths.
Planck tried to find a mathematical expression that could reproduce Wien's law (for short wavelengths) and the empirical formula (for long wavelengths). This expression included a constant, $h$, which is thought to be for Hilfsgrösse (auxiliary variable), and subsequently became known as the Planck constant. The expression formulated by Planck showed that the spectral radiance of a body for frequency
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from '' angular frequency''. Frequency is measured in hertz
The hert ...

at absolute temperature
Thermodynamic temperature is a quantity defined in thermodynamics as distinct from kinetic theory or statistical mechanics.
Historically, thermodynamic temperature was defined by Kelvin in terms of a macroscopic relation between thermodynami ...

is given by
:$B\_\backslash nu(\backslash nu,\; T)\; =\; \backslash frac\; \backslash frac,$
where $k\_\backslash text$ is the Boltzmann constant
The Boltzmann constant ( or ) is the proportionality factor that relates the average relative kinetic energy of particles in a gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin and the gas con ...

, $h$ is the Planck constant, and $c$ is the speed of light
The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics
Physics is the natural science that studies matter, its fundamental constituents, its motion and b ...

in the medium, whether material or vacuum.
The spectral radiance of a body, $B\_$, describes the amount of energy it emits at different radiation frequencies. It is the power emitted per unit area of the body, per unit solid angle of emission, per unit frequency. The spectral radiance can also be expressed per unit wavelength
In physics
Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of k ...

$\backslash lambda$ instead of per unit frequency. In this case, it is given by
:$B\_\backslash lambda(\backslash lambda,\; T)\; =\backslash frac\backslash frac,$
showing how radiated energy emitted at shorter wavelengths increases more rapidly with temperature than energy emitted at longer wavelengths.
Planck's law may also be expressed in other terms, such as the number of photons emitted at a certain wavelength, or the energy density in a volume of radiation. The SI units of $B\_$ are , while those of $B\_$ are .
Planck soon realized that his solution was not unique. There were several different solutions, each of which gave a different value for the entropy of the oscillators. To save his theory, Planck resorted to using the then-controversial theory of statistical mechanics, which he described as "an act of despair … I was ready to sacrifice any of my previous convictions about physics." One of his new boundary conditions was
With this new condition, Planck had imposed the quantization of the energy of the oscillators, "a purely formal assumption … actually I did not think much about it ..." in his own words, but one that would revolutionize physics. Applying this new approach to Wien's displacement law showed that the "energy element" must be proportional to the frequency of the oscillator, the first version of what is now sometimes termed the " Planck–Einstein relation":
:$E\; =\; hf.$
Planck was able to calculate the value of $h$ from experimental data on black-body radiation: his result, , is within 1.2% of the currently accepted value. He also made the first determination of the Boltzmann constant
The Boltzmann constant ( or ) is the proportionality factor that relates the average relative kinetic energy of particles in a gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin and the gas con ...

$k\_\backslash text$ from the same data and theory.
Development and application

The black-body problem was revisited in 1905, when Lord Rayleigh and James Jeans (on the one hand) andAlbert Einstein
Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born theoretical physicist, widely acknowledged to be one of the greatest and most influential physicists of all time. Einstein is best known for developing the theor ...

(on the other hand) independently proved that classical electromagnetism could ''never'' account for the observed spectrum. These proofs are commonly known as the " ultraviolet catastrophe", a name coined by Paul Ehrenfest in 1911. They contributed greatly (along with Einstein's work on the photoelectric effect) in convincing physicists that Planck's postulate of quantized energy levels was more than a mere mathematical formalism. The first Solvay Conference in 1911 was devoted to "the theory of radiation and quanta".
Photoelectric effect

The photoelectric effect is the emission of electrons (called "photoelectrons") from a surface when light is shone on it. It was first observed by Alexandre Edmond Becquerel in 1839, although credit is usually reserved forHeinrich Hertz
Heinrich Rudolf Hertz ( ; ; 22 February 1857 – 1 January 1894) was a German physicist
A physicist is a scientist
A scientist is a person who conducts scientific research to advance knowledge in an area of the natural scienc ...

,See, e.g., who published the first thorough investigation in 1887. Another particularly thorough investigation was published by Philipp Lenard (Lénárd Fülöp) in 1902. Einstein's 1905 paper discussing the effect in terms of light quanta would earn him the Nobel Prize in 1921, after his predictions had been confirmed by the experimental work of Robert Andrews Millikan. The Nobel committee awarded the prize for his work on the photo-electric effect, rather than relativity, both because of a bias against purely theoretical physics not grounded in discovery or experiment, and dissent amongst its members as to the actual proof that relativity was real.
Before Einstein's paper, electromagnetic radiation such as visible light was considered to behave as a wave: hence the use of the terms "frequency" and "wavelength" to characterize different types of radiation. The energy transferred by a wave in a given time is called its intensity. The light from a theatre spotlight is more ''intense'' than the light from a domestic lightbulb; that is to say that the spotlight gives out more energy per unit time and per unit space (and hence consumes more electricity) than the ordinary bulb, even though the color of the light might be very similar. Other waves, such as sound or the waves crashing against a seafront, also have their intensity. However, the energy account of the photoelectric effect didn't seem to agree with the wave description of light.
The "photoelectrons" emitted as a result of the photoelectric effect have a certain kinetic energy
In physics
Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department o ...

, which can be measured. This kinetic energy (for each photoelectron) is ''independent'' of the intensity of the light, but depends linearly on the frequency; and if the frequency is too low (corresponding to a photon energy that is less than the work function of the material), no photoelectrons are emitted at all, unless a plurality of photons, whose energetic sum is greater than the energy of the photoelectrons, acts virtually simultaneously (multiphoton effect). Assuming the frequency is high enough to cause the photoelectric effect, a rise in intensity of the light source causes more photoelectrons to be emitted with the same kinetic energy, rather than the same number of photoelectrons to be emitted with higher kinetic energy.
Einstein's explanation for these observations was that light itself is quantized; that the energy of light is not transferred continuously as in a classical wave, but only in small "packets" or quanta. The size of these "packets" of energy, which would later be named photons, was to be the same as Planck's "energy element", giving the modern version of the Planck–Einstein relation:
:$E\; =\; hf\; .$
Einstein's postulate was later proven experimentally: the constant of proportionality between the frequency of incident light $f$ and the kinetic energy of photoelectrons $E$ was shown to be equal to the Planck constant $h$.
Atomic structure

It was John William Nicholson in 1912 who introduced h-bar into the theory of the atom which was the first quantum and nuclear atom and the first to quantize angular momentum as ''h''/2. Niels Bohr quoted him in his 1913 paper of the Bohr model of the atom. The influence of the work of Nicholson’s nuclear quantum atomic model on Bohr’s model has been written about by many historians. Niels Bohr introduced the third quantized model of the atom in 1913, in an attempt to overcome a major shortcoming of Rutherford's classical model. The first quantized model of the atom was introduced in 1910 by Arthur Erich Haas and was discussed at the 1911 Solvay conference. In classical electrodynamics, a charge moving in a circle should radiate electromagnetic radiation. If that charge were to be an electron orbiting a nucleus, the radiation would cause it to lose energy and spiral down into the nucleus. Bohr solved this paradox with explicit reference to Planck's work: an electron in a Bohr atom could only have certain defined energies $E\_n$ :$E\_n\; =\; -\backslash frac\; ,$ where $c$ is the speed of light in vacuum, $R\_$ is an experimentally determined constant (the Rydberg constant) and $n\; \backslash in\; \backslash $. Once the electron reached the lowest energy level ($n\; =\; 1$), it could not get any closer to the nucleus (lower energy). This approach also allowed Bohr to account for the Rydberg formula, an empirical description of the atomic spectrum of hydrogen, and to account for the value of the Rydberg constant $R\_$ in terms of other fundamental constants. Bohr also introduced the quantity $\backslash hbar=\backslash frac$, now known as the reduced Planck constant or Dirac constant, as the quantum of angular momentum. At first, Bohr thought that this was the angular momentum of each electron in an atom: this proved incorrect and, despite developments by Sommerfeld and others, an accurate description of the electron angular momentum proved beyond the Bohr model. The correct quantization rules for electrons – in which the energy reduces to the Bohr model equation in the case of the hydrogen atom – were given by Heisenberg's matrix mechanics in 1925 and the Schrödinger wave equation in 1926: the reduced Planck constant remains the fundamental quantum of angular momentum. In modern terms, if $J$ is the total angular momentum of a system with rotational invariance, and $J\_z$ the angular momentum measured along any given direction, these quantities can only take on the values :$\backslash begin\; J^2\; =\; j(j+1)\; \backslash hbar^2,\backslash qquad\; \&\; j\; =\; 0,\; \backslash tfrac,\; 1,\; \backslash tfrac,\; \backslash ldots,\; \backslash \backslash \; J\_z\; =\; m\; \backslash hbar,\; \backslash qquad\backslash qquad\backslash quad\; \&\; m\; =\; -j,\; -j+1,\; \backslash ldots,\; j.\; \backslash end$Uncertainty principle

The Planck constant also occurs in statements of Werner Heisenberg's uncertainty principle. Given numerous particles prepared in the same state, the uncertainty in their position, $\backslash Delta\; x$, and the uncertainty in their momentum, $\backslash Delta\; p\_$, obey :$\backslash Delta\; x\backslash ,\; \backslash Delta\; p\_\; \backslash ge\; \backslash frac\; ,$ where the uncertainty is given as the standard deviation of the measured value from its expected value. There are several other such pairs of physically measurable conjugate variables which obey a similar rule. One example is time vs. energy. The inverse relationship between the uncertainty of the two conjugate variables forces a tradeoff in quantum experiments, as measuring one quantity more precisely results in the other quantity becoming imprecise. In addition to some assumptions underlying the interpretation of certain values in the quantum mechanical formulation, one of the fundamental cornerstones to the entire theory lies in the commutator relationship between the position operator $\backslash hat$ and the momentum operator $\backslash hat$: :$;\; href="/html/ALL/s/hat\_i,\_\backslash hat\_j.html"\; ;"title="hat\_i,\; \backslash hat\_j">hat\_i,\; \backslash hat\_j$ where $\backslash delta\_$ is the Kronecker delta.Photon energy

The Planck relation connects the particular photon energy with its associated wave frequency : :$E\; =\; hf.$ This energy is extremely small in terms of ordinarily perceived everyday objects. Since the frequency ,wavelength
In physics
Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of k ...

, and speed of light
The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics
Physics is the natural science that studies matter, its fundamental constituents, its motion and b ...

are related by $f=\; \backslash frac$, the relation can also be expressed as
:$E\; =\; \backslash frac\; .$
de Broglie wavelength

In 1923, Louis de Broglie generalized the Planck–Einstein relation by postulating that the Planck constant represents the proportionality between the momentum and the quantum wavelength of not just the photon, but the quantum wavelength of any particle. This was confirmed by experiments soon afterward. This holds throughout the quantum theory, including electrodynamics. The de Broglie wavelength of the particle is given by :$\backslash lambda\; =\; \backslash frac,$ where denotes the linear momentum of a particle, such as a photon, or any other elementary particle. The energy of a photon with angular frequency is given by :$E\; =\; \backslash hbar\; \backslash omega\; ,$ while its linear momentum relates to :$p\; =\; \backslash hbar\; k\; ,$ where is an angular wavenumber. These two relations are the temporal and spatial parts of the special relativistic expression using 4-vectors. :$P^\backslash mu\; =\; \backslash left(\backslash frac,\; \backslash vec\backslash right)\; =\; \backslash hbar\; K^\backslash mu\; =\; \backslash hbar\backslash left(\backslash frac,\; \backslash vec\backslash right).$Statistical mechanics

Classical statistical mechanics requires the existence of (but does not define its value). Eventually, following upon Planck's discovery, it was speculated that physical action could not take on an arbitrary value, but instead was restricted to integer multiples of a very small quantity, the " lementary quantum of action", now called the ''Planck constant''. This was a significant conceptual part of the so-called " old quantum theory" developed by physicists including Bohr, Sommerfeld, and Ishiwara, in which particle trajectories exist but are hidden, but quantum laws constrain them based on their action. This view has been replaced by fully modern quantum theory, in which definite trajectories of motion do not even exist; rather, the particle is represented by a wavefunction spread out in space and in time. Thus there is no value of the action as classically defined. Related to this is the concept of energy quantization which existed in old quantum theory and also exists in altered form in modern quantum physics. Classical physics cannot explain either quantization of energy or the lack of classical particle motion. In many cases, such as for monochromatic light or for atoms, quantization of energy also implies that only certain energy levels are allowed, and values in between are forbidden.Reduced Planck constant

Implicit in the dimensions of the Planck constant is the fact that the SI unit of frequency, thehertz
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that o ...

, represents one complete cycle, 360 degrees or radians, per second.
In applications where it is natural to use the angular frequency (i.e. where the frequency is expressed in terms of radians per second instead of cycles per second or hertz
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that o ...

) it is often useful to absorb a factor of into the Planck constant. The resulting constant is called the reduced Planck constant or Dirac constant. It is equal to the Planck constant divided by , and is denoted by $\backslash hbar$ (pronounced "h-bar"):
:$\backslash hbar\; =\; \backslash frac\; .$
Value

The Planck constant has dimensions of angular momentum. In SI units, the Planck constant is expressed with the unit joule perhertz
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that o ...

(J⋅Hz) or joule-second (J⋅s).
:$h\; =\; \backslash mathrm$
:$\backslash hbar=\; =\; 1.054\backslash \; 571\backslash \; 817...\backslash times\; 10^\backslash \; \backslash text\backslash text\; =\; 6.582\backslash \; 119\backslash \; 569...\backslash times\; 10^\backslash \; \backslash text\backslash text.$
The above values have been adopted as fixed in the 2019 redefinition of the SI base units
In 2019, four of the seven SI base unit
The SI base units are the standard units of measurement
A unit of measurement is a definite magnitude of a quantity, defined and adopted by convention or by law, that is used as a standa ...

.
Understanding the 'fixing' of the value of ''h''

Since 2019, the numerical value of the Planck constant has been fixed, with a finite decimal representation. Under the present definition of thekilogram
The kilogram (also kilogramme) is the unit of mass in the International System of Units (SI), having the unit symbol kg. It is a widely used measure in science, engineering and commerce worldwide, and is often simply called a kilo colloquiall ...

, which states that "The kilogram ..is defined by taking the fixed numerical value of to be when expressed in the unit J⋅s, which is equal to kg⋅mmetre
The metre ( British spelling) or meter ( American spelling; see spelling differences) (from the French unit , from the Greek noun , "measure"), symbol m, is the primary unit of length in the International System of Units (SI), though its ...

and the second
The second (symbol: s) is the unit of time in the International System of Units (SI), historically defined as of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds ...

are defined in terms of speed of light
The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics
Physics is the natural science that studies matter, its fundamental constituents, its motion and b ...

and duration of hyperfine transition of the ground state
The ground state of a quantum-mechanical system is its stationary state of lowest energy; the energy of the ground state is known as the zero-point energy of the system. An excited state
In quantum mechanics
Quantum mechanics is ...

of an unperturbed caesium-133 atom ." This implies that mass metrology
Metrology is the scientific study of measurement
Measurement is the quantification of attributes of an object or event, which can be used to compare with other objects or events.
In other words, measurement is a process of determinin ...

aims to find the value of one kilogram, and the kilogram is ''compensating''. Every experiment aiming to measure the kilogram (such as the Kibble balance and the X-ray crystal density method), will essentially refine the value of a kilogram.
As an illustration of this, suppose the decision of making to be exact was taken in 2010, when its measured value was , thus the present definition of kilogram was also enforced. In the future, the value of one kilogram must be refined to times the mass of the International Prototype of the Kilogram (IPK).
Significance of the value

The Planck constant is related to the quantization of light and matter. It can be seen as a subatomic-scale constant. In a unit system adapted to subatomic scales, the electronvolt is the appropriate unit of energy and the petahertz the appropriate unit of frequency. Atomic unit systems are based (in part) on the Planck constant. The physical meaning of the Planck constant could suggest some basic features of our physical world. The Planck constant is one of the smallest constants used in physics. This reflects the fact that on a scale adapted to humans, where energies are typical of the order of kilojoules and times are typical of the order of seconds or minutes, the Planck constant is very small. One can regard the Planck constant to be only relevant to the microscopic scale instead of the macroscopic scale in our everyday experience. Equivalently, the order of the Planck constant reflects the fact that everyday objects and systems are made of a ''large'' number of microscopic particles. For example, green light with awavelength
In physics
Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of k ...

of 555 nanometre
330px, Different lengths as in respect to the molecular scale.
The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm) or nanometer (American and British English spelling differences#-re ...

s (a wavelength that can be perceived by the human eye to be green
Green is the color between cyan
Cyan () is the color between green and blue on the visible spectrum of light. It is evoked by light with a predominant wavelength between 490 and 520 nm, between the wavelengths of green and blue. ...

) has a frequency of (). Each photon has an energy . That is a very small amount of energy in terms of everyday experience, but everyday experience is not concerned with individual photons any more than with individual atoms or molecules. An amount of light more typical in everyday experience (though much larger than the smallest amount perceivable by the human eye) is the energy of one mole of photons; its energy can be computed by multiplying the photon energy by the Avogadro constant
The Avogadro constant, commonly denoted or , is the proportionality factor that relates the number of constituent particles (usually molecule
A molecule is a group of two or more atom
Every atom is composed of a nucleus and o ...

, , with the result of , about the food energy in three apples.
Determination

In principle, the Planck constant can be determined by examining the spectrum of a black-body radiator or the kinetic energy of photoelectrons, and this is how its value was first calculated in the early twentieth century. In practice, these are no longer the most accurate methods. Since the value of the Planck constant is fixed now, it is no longer determined or calculated in laboratories. Some of the practices given below to determine the Planck constant are now used to determine the mass of the kilogram. All of the methods given below ''except'' the X-ray crystal density method rely on the theoretical basis of the Josephson effect and the quantum Hall effect.Josephson constant

The Josephson constant ''K''volt
The volt (symbol: V) is the unit of electric potential, electric potential difference ( voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745–1827).
D ...

s. The measurement of the potential difference in SI units is done by allowing an electrostatic force to cancel out a measurable gravitational force, in a Kibble balance. Assuming the validity of the theoretical treatment of the Josephson effect, ''K''Kibble balance

A Kibble balance (formerly known as a watt balance) is an instrument for comparing two powers, one of which is measured in SIwatt
The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after Jame ...

s and the other of which is measured in conventional electrical units. From the definition of the ''conventional'' watt ''W''Magnetic resonance

The gyromagnetic ratio ''γ'' of an object is the ratio of its magnetic moment to its angular momentum, which is directly related to the constant of proportionality between the frequency ''ν'' ofnuclear magnetic resonance
Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, ...

(or electron paramagnetic resonance for electrons) and the applied magnetic field ''B'': . It is difficult to measure gyromagnetic ratios precisely because of the difficulties in precisely measuring ''B'', but the value for proton
A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge
Electric charge is the physical property of matter that causes charged matter to experience a force
In physics
Physics is the na ...

s in water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts a ...

at is known to an uncertainty of better than 10ampere
The ampere (, ; symbol: A), often shortened to amp,SI supports only the use of symbols and deprecates the use of abbreviations for units. is the unit of electric current in the International System of Units (SI). One ampere is equal to ele ...

s, so a conversion factor is required. The symbol Γ′Faraday constant

The Faraday constant ''F'' is the charge of one mole of electrons, equal to the Avogadro constant ''N''electrolysis
In chemistry
Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, ...

experiments, measuring the amount of silver
Silver is a chemical element
A chemical element is a species of atoms that have a given number of protons in their nuclei, including the pure substance consisting only of that species. Unlike chemical compounds, chemical elements ...

dissolved from an electrode in a given time and for a given electric current. Substituting the definitions of ''N''X-ray crystal density

The X-ray crystal density method is primarily a method for determining the Avogadro constant ''N''crystal
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, ma ...

, measured by X-ray crystallography
X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a ...

, and the molar volume of the substance. Crystals of silicon
Silicon is a chemical element
A chemical element is a species of atoms that have a given number of protons in their nuclei, including the pure substance consisting only of that species. Unlike chemical compounds, chemical elements ...

are used, as they are available in high quality and purity by the technology developed for the semiconductor
A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way ...

industry. The unit cell volume is calculated from the spacing between two crystal planes referred to as ''d''density
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematic ...

of the crystal and the atomic weight of the silicon used. The Planck constant is given by
:$h\; =\; \backslash frac\; \backslash frac\; .$
Particle accelerator

The experimental measurement of the Planck constant in the Large Hadron Collider laboratory was carried out in 2011.See also

* CODATA 2018 *International System of Units
The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system
The metric system is a system of measurement that ...

* Introduction to quantum mechanics
* Planck units
* Wave–particle duality
Notes

References

Citations

Sources

*External links

"The role of the Planck constant in physics" – presentation at 26th CGPM meeting at Versailles, France, November 2018 when voting took place.

{{Authority control Fundamental constants 1900 in science Max Planck