The
phase velocity of a
waveIn physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass...
is the rate at which the
phasePhase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
of the wave propagates in space. This is the
speedIn kinematics, the speed of an object is the magnitude of its velocity ; it is thus a scalar quantity. The average speed of an object in an interval of time is the distance traveled by the object divided by the duration of the interval; the instantaneous speed is the limit of the average speed as...
at which the phase of any one
frequencyFrequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
component of the wave travels. For such a component, any given phase of the wave (for example, the crest) will appear to travel at the phase velocity. The phase velocity is given in terms of the
wavelengthIn physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
λ (lambda) and period
T as
Or, equivalently, in terms of the wave's
angular frequencyIn physics, angular frequency ω is a scalar measure of rotation rate. Angular frequency is the magnitude of the vector quantity angular velocity...
ω, which is the number of oscillations per unit of time, and
wavenumberIn the physical sciences, the wavenumber is a property of a wave, its spatial frequency, that is proportional to the reciprocal of the wavelength. It is also the magnitude of the wave vector...
k, which is the number of oscillations per unit of space, by
To understand where it comes from, imagine a basic sine wave,
A cos (
kx−
ωt). Given time
t, the source produces
ωt oscillations. At the same time, the initial wave front propagates away from the source through the space to the distance
x to fit the same amount of oscillations,
kx =
ωt. So that the propagation speed
v is
v =
x/
t =
ω/
k. The wave propagates faster when higher frequency oscillations are distributed less densely in space. Formally,
Φ =
kx−
ωt is the phase. Since
ω = −d
Φ/d
t and
k = +d
Φ/d
x, the wave speed is
v = d
x/d
t =
ω/
k.
Relation to group velocity, refractive index and transmission speed
Since pure sine wave cannot convey any information, some change in amplitude or frequency, known as
modulationIn electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
, is required. By combining two sines with slightly different frequencies and wavelengths,
the amplitude becomes a sinusoid with phase speed of
vg = Δ
ω/Δ
k. It is this modulation that represents the signal content. Since each amplitude
envelope contains a group of internal waves, this speed is usually called the
group velocityThe group velocity of a wave is the velocity with which the overall shape of the wave's amplitudes — known as the modulation or envelope of the wave — propagates through space....
.
In reality, the
vp =
ω/
k and
vg = d
ω/d
k ratios are determined by the media. The relation between phase speed,
vp, and speed of light,
c, is known as
refractive indexIn optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
,
n =
c/
vp =
ck/
ω. Taking the derivative of
ω =
ck/
n, we get the group speed,
Noting that
c/
n =
vp, this shows that group speed is equal to phase speed only when the wave number is a constant: d
k/d
n = 0. Otherwise, when the phase velocity varies with frequency, velocities differ and the medium is called
dispersiveIn optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency, or alternatively when the group velocity depends on the frequency.Media having such a property are termed dispersive media...
.
The phase velocity of
electromagnetic radiationElectromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
may – under certain circumstances (for example anomalous dispersion) – exceed the
speed of lightThe speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
in a vacuum, but this does not indicate any superluminal information or energy transfer. It was theoretically described by physicists such as
Arnold SommerfeldArnold Johannes Wilhelm Sommerfeld was a German theoretical physicist who pioneered developments in atomic and quantum physics, and also educated and groomed a large number of students for the new era of theoretical physics...
and
Léon BrillouinLéon Nicolas Brillouin was a French physicist. He made contributions to quantum mechanics, radio wave propagation in the atmosphere, solid state physics, and information theory.-Early life:...
. See
dispersionIn optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency, or alternatively when the group velocity depends on the frequency.Media having such a property are termed dispersive media...
for a full discussion of wave velocities.
Matter wave phase
In
quantum mechanicsQuantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
, particles also behave as waves with
complexA complex number is a number consisting of a real part and an imaginary part. Complex numbers extend the idea of the one-dimensional number line to the two-dimensional complex plane by using the number line for the real part and adding a vertical axis to plot the imaginary part...
phases. By the de Broglie hypothesis, we see that
Using
relativisticSpecial relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...
relations for energy and momentum, we have
where
E is the total energy of the particle (i.e. rest energy plus
kinetic energyThe kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
in kinematic sense),
p the
momentumIn classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...
,
the
Lorentz factorThe Lorentz factor or Lorentz term appears in several equations in special relativity, including time dilation, length contraction, and the relativistic mass formula. Because of its ubiquity, physicists generally represent it with the shorthand symbol γ . It gets its name from its earlier...
,
c the
speed of lightThe speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
, and β the speed as a fraction of
c. The variable
v can either be taken to be the speed of the particle or the group velocity of the corresponding matter wave. Since the particle speed
for any particle that has mass (according to
special relativitySpecial relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...
), the phase velocity of matter waves always exceeds
c, i.e.
and as we can see, it approaches
c when the particle speed is in the relativistic range. The superluminal phase velocity does not violate special relativity, as it carries no information. See the article on
signal velocityThe signal velocity is the speed at which a wave carries information. It describes how quickly a message can be communicated between two separated parties...
for details.
Slow phase velocity and superluminality
Contrarily, a subluminal phase velocity may be corresponding to a velocity faster than light, i.e.
because the product is constant
. For example, the phase velocity of commercial electric power (frequency 60Hz) in
copperCopper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
is 3 m/s according to the original definition
. Thus, the velocity
should be 10
8 c on account of
The value equals energy flow velocity
of the field given by classical electrodynamics (
is
Poynting vectorIn physics, the Poynting vector can be thought of as representing the directional energy flux density of an electromagnetic field. It is named after its inventor John Henry Poynting. Oliver Heaviside and Nikolay Umov independently co-invented the Poynting vector...
and
is
energy densityEnergy density is a term used for the amount of energy stored in a given system or region of space per unit volume. Often only the useful or extractable energy is quantified, which is to say that chemically inaccessible energy such as rest mass energy is ignored...
).
See also
- Wave propagation
Wave propagation is any of the ways in which waves travel.With respect to the direction of the oscillation relative to the propagation direction, we can distinguish between longitudinal wave and transverse waves....
- Group velocity
The group velocity of a wave is the velocity with which the overall shape of the wave's amplitudes — known as the modulation or envelope of the wave — propagates through space....
- Propagation delay
Propagation delay is a technical term that can have a different meaning depending on the context. It can relate to networking, electronics or physics...
- Wave propagation speed
- Dispersion (optics)
In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency, or alternatively when the group velocity depends on the frequency.Media having such a property are termed dispersive media...
- Shear wave splitting
Shear wave splitting, also called seismic birefringence, is the phenomenon that occurs when a polarized shear wave enters an anisotropic media . The incident shear wave splits into two polarized shear waves . Shear wave splitting are typically used as a tool for testing the anisotropy of an area...
External links