Planck's constant

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A commemoration plaque for Max Planck on his discovery of Planck's 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."

Planck's constant (denoted h) is a physical constant that is used to describe the sizes of quanta. It plays a central role in the theory of quantum mechanics, and is named after Max Planck, one of the founders of quantum theory. A closely-related quantity is the reduced Planck constant (also known as Dirac's constant and denoted $\hbar$ , pronounced "h-bar"). Planck's constant is also used in measuring energy emitted by light photons, such as in the equation E=h $ν$ , where E is energy, h is Planck's constant, and $ν$ (Greek letter nu) is frequency.

Planck's constant and the reduced Planck's constant are used to describe quantization, a phenomenon occurring in subatomic particles such as electrons and photons in which certain physical properties occur in fixed amounts rather than assuming a continuous range of possible values.

1 Units, value and symbols
2 Origins of Planck's constant
3 Usage
4 Dirac's constant
5 Significance of the size of Planck's constant
6 See also
7 References
8 External links

[edit] Units, value and symbols

Planck's constant has dimensions of energy multiplied by time, which are also the dimensions of action. In SI units Planck's constant is expressed in joule-seconds. The dimensions may also be written as momentum times distance (N·m·s), which are also the dimensions of angular momentum. Often the unit of choice is eV·s, because of the small energies that are often encountered in quantum physics.

The value of Planck's constant is:

$h =\,\,\, 6.626\ 0693 (11) \times10^{-34}\ \mbox{J}\cdot\mbox{s} \,\,\, = \,\,\, 4.135\ 667\ 43(35) \times10^{-15}\ \mbox{eV}\cdot\mbox{s}$

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

The value of Dirac's constant is:

$\hbar\ \equiv \frac{h}{2\pi} = \,\,\, 1.054\ 571\ 68(18)\times10^{-34}\ \mbox{J}\cdot\mbox{s} \,\,\, = \,\,\, 6.582\ 119\ 15(56) \times10^{-16}\ \mbox{eV}\cdot\mbox{s}$

The figures cited here are the 2002 CODATA-recommended values for the constants and their uncertainties. The 2002 CODATA results were made available in December 2003 and represent the best-known, internationally-accepted values for these constants, based on all data available as of 31 December 2002. New CODATA figures are scheduled to be published approximately every four years.

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

[edit] Origins of Planck's constant

Planck's 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 $δ i j$ is the Kronecker delta. For more information, see the mathematical formulation of quantum mechanics.

[edit] Usage

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

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

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

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

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

Planck's constant also occurs in statements of Heisenberg's uncertainty principle. The uncertainty (more precisely: the standard deviation) in any position measurement, $Δ x$ , and the uncertainty in a momentum measurement along the same direction, $Δ p$ , obeys

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

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

[edit] Dirac's constant

Dirac's constant or the "reduced Planck's constant", $\hbar = \frac{h}{2 \pi} \$ , differs only from Planck's constant by a factor of $2π$ . Planck's constant is stated in SI units of measurement, joules per hertz, or joules per (cycle per second), while Dirac's constant is the same value stated in joules per (radian per second). Both constants are conversion factors between energy units and frequency units.

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

[edit] Significance of the size of Planck's constant

Expressed in the S.I. units of J·s, Planck's 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 Planck's constant tends to zero. However, in the natural units describing physics at the atomic scale, Planck's constant becomes equal to 1, reflecting the fact that physics at the atomic scale is dominated by quantum effects.

[edit] See also

[edit] References

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