# Contents

## Idea

Frequency is a number of occurences of a periodic event in time.

For waves: number of wave crests at a given space point per time.

## Definition

For $p \in \mathbb{N}$ and $\mathbb{R}^{p,1} \simeq_{\mathbb{R}} \mathbb{R}^{p+1}$ the Cartesian space $\mathbb{R}^{p+1}$ regarded as Minkowski spacetime, then the frequency of a plane wave

$x \mapsto e^{ - 2 \pi i k_\mu x^\mu}$

with respect to the chosen coordinate system is the 0-component of the given wave vector $k$:

$\nu \coloneqq k_0 \,.$

The combination

$\omega \coloneqq 2 \pi \nu = 2\pi k_0$

is called the angular frequency.

plane waves on Minkowski spacetime

$\array{ \mathbb{R}^{p,1} &\overset{\psi_k}{\longrightarrow}& \mathbb{C} \\ x &\mapsto& \exp\left( \, i k_\mu x^\mu \, \right) \\ (\vec x, x^0) &\mapsto& \exp\left( \, i \vec k \cdot \vec x + i k_0 x^0 \, \right) \\ (\vec x, c t) &\mapsto& \exp\left( \, i \vec k \cdot \vec x - i \omega t \, \right) }$
symbolname
$c$speed of light

$\hbar$Planck's constant

$\,$$\,$

$m$mass

$\frac{\hbar}{m c}$Compton wavelength

$\,$$\,$

$k$, $\vec k$wave vector

$\lambda = 2\pi/{\vert \vec k \vert}$wave length

${\vert \vec k \vert} = 2\pi/\lambda$wave number

$\omega \coloneqq k^0 c = -k_0 c = 2\pi \nu$angular frequency

$\nu = \omega / 2 \pi$frequency

$p = \hbar k$, $\vec p = \hbar \vec k$momentum

$E = \hbar \omega$energy

$\omega(\vec k) = c \sqrt{ \vec k^2 + \left(\frac{m c}{\hbar}\right)^2 }$Klein-Gordon dispersion relation

$E(\vec p) = \sqrt{ c^2 \vec p^2 + (m c^2)^2 }$energy-momentum relation

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Last revised on November 7, 2017 at 08:46:56. See the history of this page for a list of all contributions to it.