- Lorentz factor
The Lorentz factor or Lorentz term appears in several equations in
special relativity , includingtime dilation ,length contraction , and therelativistic mass formula. Because of its ubiquity, physicists generally represent it with the shorthand symbol "γ". It gets its name from its earlier appearance in Lorentzian electrodynamics. The Lorentz factor is named after the Dutch physicistHendrik Lorentz . [ [http://www.nap.edu/html/oneuniverse/motion_knowledge_concept_12.html One universe] , byNeil deGrasse Tyson , Charles Tsun-Chu Liu, and Robert Irion.]It is defined as:
:
where:
: is the velocity in terms of the
speed of light ,: "u" is the velocity as observed in the reference frame where time "t" is measured: "τ" is theproper time , and: "c" is the "speed of light".Approximations
The Lorentz factor has a Maclaurin series of:
:
The approximation γ ≈ 1 + 1/2 β2 is occasionally used to calculate relativistic effects at low speeds. It holds to within 1% error for v < 0.4 c (v < 120,000 km/s), and to within 0.1% error for v < 0.22 c (v < 66,000 km/s).
The truncated versions of this series also allow physicists to prove that
special relativity reduces toNewtonian mechanics at low speeds. For example, in special relativity, the following two equations hold:::
For γ ≈ 1 and γ ≈ 1 + 1/2 β2, respectively, these reduce to their Newtonian equivalents:
::
The Lorentz factor equation can also be inverted to yield:
:
This has an asymptotic form of:
:
The first two terms are occasionally used to quickly calculate velocities from large γ values. The approximation β ≈ 1 - 1/2 γ-2 holds to within 1% tolerance for γ > 2, and to within 0.1% tolerance for γ > 3.5.
Values
Rapidity
Note that if
tanh "r" = "β", then "γ" = cosh "r". Here thehyperbolic angle "r" is known as the rapidity [ [http://pdg.lbl.gov/2005/reviews/kinemarpp.pdf Kinematics] , byJ.D. Jackson , See page 7 for definition of rapidity. ] . Inparticle physics , rapidity is usually defined as (For example, see [Introduction to High-Energy Heavy-Ion Collisions, byCheuk-Yin Wong , See page 17 for definition of rapidity. ] )::Using the property ofLorentz transformation , it can be shown that rapidity is additive, a useful property that velocity does not have. Thus the rapidity parameter forms aone-parameter group , a foundation for physical models.Sometimes (especially in discussion ofsuperluminal motion ) γ is written as "Γ" (uppercase-gamma) rather than "γ" (lowercase-gamma).The Lorentz factor applies to
time dilation ,length contraction andrelativistic mass relative to rest mass in Special Relativity. An object moving with respect to an observer will be seen to move in slow motion given by multiplying its actual elapsed time by gamma. Its length is measured shorter as though its local length were divided by γ.γ may also (less often) refer to . This may make the symbol γ ambiguous, so many authors prefer to avoid possible confusion by writing out the Lorentz term in full.
Derivation
One of the fundamental postulates of Einstein's
special theory of relativity is that all inertial observers will measure the same speed of light in vacuum regardless of their relative motion with respect to each other or the source. Imagine two observers: the first, observer , traveling at a constant speed with respect to a secondinertial reference frame in which observer is stationary. points a laser “upward” (perpendicular to the direction of travel). From 's perspective, the light is traveling at an angle. After a period of time , has traveled (from 's perspective) a distance ; the light had traveled (also from perspective) a distance at an angle. The upward component of the path of the light can be solved by thePythagorean theorem .:
Factoring out gives,
:
The distance that sees the light travel is and equating this with calculated from reference frame gives,
:
which simplifies to
:
ee also
*
Special relativity
*Lorentz transformation
*Pseudorapidity External links
[http://www.calctool.org/CALC/phys/relativity/gamma Web-interfaced calculator to convert between speed and gamma values.]
References
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