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A commemoration plaque for Max Planck on his discovery of the Planck constant, in front of Humboldt University, Berlin. English translation: "Max Planck, discoverer of the elementary quantum of action h, taught in this building from 1889 to 1928."

The Planck constant (denoted $h\,$) is a physical constant used to describe the sizes of quanta. It plays a central part in the theory of quantum mechanics, and is named after Max Planck, one of the founders of quantum theory. The Planck constant divided by 2π is called the reduced Planck constant (also known as the Dirac constant) and is denoted $\hbar \,$, pronounced "h-bar" or "h-cross".

The Planck constant is the proportionality constant between the energy E of a photon and its frequency ν. The relation between energy and frequency is known as Planck's relation:

$E=h\nu \,$

The reduced Planck constant is used when the frequency is expressed in radians per second (rad/s) instead of cycles per seconds (Hz). A frequency expressed in radians per second is often referred to as an angular frequency ω, where ω = 2πν:

$E=\hbar \omega \,$

The Planck constant and the reduced Planck constant are related to quantization. Quantum behavior differs from classical behaviour because $h\,$ is not equal to 0. The non-zero value of the Planck constant is the reason phenomena occurring in quantum physics display discrete behavior (e.g. spectral lines ) rather than assuming a continuous range of possible values.

Significance of the size of the Planck constant

Expressed in the SI units of joule seconds (J·s), the Planck constant is one of the smallest constants used in physics. The significance of this is that it reflects the extremely small scales at which quantum mechanical effects are observed, and hence why we are not familiar with quantum physics in our everyday lives in the way that we are with classical physics. Indeed, classical physics can essentially be defined as the limit of quantum mechanics as the Planck constant tends to zero.

In natural units, the reduced Planck constant is taken as 1 (i.e., the Planck constant is 2π), as is convenient for describing physics at the atomic scale dominated by quantum effects.

The Planck constant has dimensions of energy multiplied by time, which are also the dimensions of action. In SI units, the Planck constant is expressed in joule seconds (J·s). The dimensions may also be written as momentum times distance (N·m·s), which are also the dimensions of angular momentum. The value of the Planck constant is:

$h=\,\,\,6.626\ 068\ 96(33)\times 10^{-34}\ {\mbox{J}}\cdot {\mbox{s}}\,\,\,=\,\,\,4.135\ 667\ 33(10)\times 10^{-15}\ {\mbox{eV}}\cdot {\mbox{s}}.$

The two digits between the parentheses denote the standard uncertainty in the last two digits of the value.

The value of the reduced Planck constant is:

$\hbar \ ={\frac {h}{2\pi }}=\,\,\,1.054\ 571\ 628(53)\times 10^{-34}\ {\mbox{J}}\cdot {\mbox{s}}\,\,\,=\,\,\,6.582\ 118\ 99(16)\times 10^{-16}\ {\mbox{eV}}\cdot {\mbox{s}}$

The figures cited here are the 2006 CODATA-recommended values for the constants and their uncertainties. The 2006 CODATA results were made available in March 2007 and represent the best-known, internationally-accepted values for these constants, based on all data available as of 31 December 2006. New CODATA figures are scheduled to be published approximately every four years.

Unicode reserves codepoints U+210E (ℎ) for the Planck constant, and U+210F (ℏ) for the Dirac constant.

Recent values published after CODATA 2006

In October 2005, the National Physical Laboratory (NPL) reported initial measurements of the Planck constant using a newly improved watt balance. They report a value of:

$h=\,\,\,6.626\ 070\ 95(44)\times 10^{-34}\ {\mbox{J}}\cdot {\mbox{s}}\,\,\,$

which is significantly different (statistically) from the 2006 CODATA value above. The NPL value was published after 2006 CODATA, and along with other future measurements will be taken into account in the next CODATA published value.

Origins of Planck's constant

The Planck constant, $h\$, was proposed in reference to the problem of black-body radiation. The underlying assumption to Planck's law of black body radiation was that the electromagnetic radiation emitted by a black body could be modeled as a set of harmonic oscillators with quantized energy of the form:

$E=h\nu =h\omega /(2\pi )=\hbar \omega \$

$E\$ is the quantized energy of the photons of radiation having frequency (Hz) of $\nu \$ (nu) or angular frequency (rad/s) of $\omega \$ (omega).

This model proved extremely accurate, but it provided an intellectual stumbling block for theoreticians who did not understand where the quantization of energy arose — Planck himself only considered it "a purely formal assumption". This line of questioning helped lead to the formation of quantum mechanics.

In addition to some assumptions underlying the interpretation of certain values in the quantum mechanical formulation, one of the fundamental corner-stones to the entire theory lies in the commutator relationship between the position operator ${\hat {x}}$ and the momentum operator ${\hat {p}}$:

$[{\hat {p}}_{i},{\hat {x}}_{j}]=-i\hbar \delta _{ij}$

where $\delta _{ij}\,$ is the Kronecker delta. For more information, see the mathematical formulation of quantum mechanics.

Usage

The Planck constant is used to describe quantization. For instance, the energy (E) carried by a beam of light with constant frequency ($\nu \,$) can only take on the values

$E=nh\nu \,,\quad n\in \mathbb {N} .$

It is sometimes more convenient to use the angular frequency $\omega =2\pi \,\nu$, which gives

$E=n\hbar \omega \,,\quad n\in \mathbb {N} .$

Many such "quantization conditions" exist. A particularly interesting condition governs the quantization of angular momentum. Let J be 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

${\begin{aligned}J^{2}=j(j+1)\hbar ^{2},\quad &j=0,1/2,1,3/2,\ldots ,\\J_{z}=m\hbar ,\qquad \quad \quad &m=-j,-j+1,\ldots ,j.\end{aligned}}$

Thus, $\hbar$ may be said to be the "quantum of angular momentum".

The Planck constant also occurs in statements of Heisenberg's uncertainty principle. Given a large number of particles prepared in the same state, the uncertainty in their position, $\Delta x$, and the uncertainty in their momentum (in the same direction), $\Delta p$, obey

$\Delta x\,\Delta p\geq {\begin{matrix}{\frac {1}{2}}\end{matrix}}\hbar$

where the uncertainty is given as the standard deviation of the measured value from its expected value.

There are a number of other such pairs of physically measurable values which obey a similar rule.

Reduced Planck constant

The reduced Planck constant $\hbar ={\frac {h}{2\pi }}\$, differs only from the Planck constant by a factor of $2\pi$. The Planck constant is stated in SI units of measurement, joules per hertz, or joules per (cycle per second), while the reduced Planck constant is the same value stated in joules per (radian per second).

In essence, the reduced Planck constant is a conversion factor between phase (in radians) and action (in joule-seconds) as seen in the Schrödinger equation. The Planck constant is similarly a conversion factor between phase (in cycles) and action. All other uses of the Planck constant and the reduced Planck constant follow from that relationship.

See also

• Electromagnetic radiation
• Natural units
• Planck units
• Quantum Hall effect
• Schrödinger equation
• Wave–particle duality

References

• Barrow, John D. (2002). The Constants of Nature; From Alpha to Omega - The Numbers that Encode the Deepest Secrets of the Universe. Pantheon Books. ISBN 0-375-42221-8.

External links

• Planck's original 1901 paper