Conjugacy class

2013-08-13T15:41:15Z

24.212.218.19: /* Example */

{{Use dmy dates|date=July 2013}}
{{Quantum field theory}}
In [[physics]], the '''fine-structure constant''', commonly denoted ''[[Alpha|α]],'' is a [[fundamental physical constant]], namely the [[coupling constant]] characterizing the strength of the [[electromagnetic interaction]]. Being a [[dimensionless quantity]], it has the same numerical value in all [[system of units|systems of units]]. [[Arnold Sommerfeld]] introduced the fine-structure constant in 1916.

The currently accepted value of ''α'' is {{physconst|alpha|round=auto|after=.}}

==Definition==
Three equivalent definitions of ''α'' in terms of other [[fundamental physical constant]]s are:

: <math>\alpha = \frac{k_\mathrm{e} e^2}{\hbar c} = \frac{1}{(4 \pi \varepsilon_0)} \frac{e^2}{\hbar c} = \frac{e^2 c \mu_0}{2 h}</math>

where:
* ''e'' is the [[elementary charge]];
* ''ħ'' = ''h''/2π is the reduced [[Planck constant]];
* ''c'' is the [[speed of light in vacuum]];
* ''ε''0 is the [[electric constant]] or permittivity of free space;
** or µ0 is the [[magnetic constant]] or permeability of free space;
** or ''k''e is the [[Coulomb constant]].

In electrostatic [[cgs]] units, the unit of [[electric charge]], the [[statcoulomb]], is defined so that the [[Coulomb's law|Coulomb constant]], ''k''e, or the [[electric constant|permittivity factor]], 4π''ε''0, is 1 and [[dimensionless]]. Then the expression of the fine-structure constant becomes the abbreviated
: <math>\alpha = \frac{e^2}{\hbar c} </math>
which is an expression commonly appearing in physics literature.

In [[natural units]], commonly used in high energy physics, where ''ε''0 = ''c'' = ''ħ'' = 1, the value of the fine-structure constant is<ref>Peskin, M.; Schroeder, D. (1995). An Introduction to Quantum Field Theory. Westview Press. ISBN 0-201-50397-2. p. 125.</ref>
: <math> \alpha = \frac{e^2}{4 \pi}. </math>
As such, the fine-structure constant is just an alternative expression of the [[elementary charge]]; <math> \scriptstyle e \ = \ \sqrt{4 \pi \alpha} \ \approx \ </math> 0.30282212 in terms of the natural unit of charge.

==Measurement==
[[Image:EighthOrderMagMoment.svg|frame|right|Two example eighth-[[Perturbation theory|order]] [[Feynman diagrams]] that contribute to the electron self-interaction. The horizontal line with an arrow represents the electron while the wavy-lines are virtual photons, and the circles represent virtual electron-positron pairs.]]
The 2010 [[CODATA]] recommended value of ''α'' is<ref name="CODATA 2010">P.J. Mohr, B.N. Taylor, and D.B. Newell (2011), "The 2010 CODATA Recommended Values of the Fundamental Physical Constants" (Web Version 6.0). This database was developed by J. Baker, M. Douma, and S. Kotochigova. Available: http://physics.nist.gov/constants [Thursday, 02-Jun-2011 21:00:12 EDT]. National Institute of Standards and Technology, Gaithersburg, MD 20899.</ref>
:<math> \alpha = \frac{e^2}{(4 \pi \varepsilon_0) \hbar c} = 7.297\,352\,5698(24) \times 10^{-3}.</math>
This has a relative standard uncertainty of 0.32 [[parts per billion]].<ref name="CODATA 2010" />
For reasons of convenience, historically the value of the [[Multiplicative inverse|reciprocal]] of the fine-structure constant is often specified. The 2010 CODATA recommended value is given by<ref name="CODATA 2010" />
:<math> \alpha^{-1} = 137.035\,999\,074(44).</math>

While the value of ''α'' can be ''estimated'' from the values of the constants appearing in any of its definitions, the theory of [[quantum electrodynamics]] (QED) provides a way to measure ''α'' directly using the [[quantum Hall effect]] or the [[anomalous magnetic moment]] of the [[electron]]. The theory of QED predicts a relationship between the [[gyromagnetic ratio|dimensionless magnetic moment]] of the [[electron]] and the fine-structure constant ''α'' (the magnetic moment of the electron is also referred to as "[[Landé g-factor|Landé ''g''-factor]]" and symbolized as ''g''). The most precise value of ''α'' obtained experimentally (as of 2012) is based on a measurement of ''g'' using a one-electron so-called "quantum cyclotron" apparatus, together with a calculation via the theory of QED that involved 12,672 tenth-order [[Feynman diagrams]]:<ref>
{{Cite journal
|author=Tatsumi Aoyama, Masashi Hayakawa, Toichiro Kinoshita, Makiko Nio
|year=2012
|title=Tenth-Order QED Contribution to the Electron g-2 and an Improved Value of the Fine Structure Constant
|journal=[[Physical Review Letters]]
|volume=109
|issue=11 |page=111807
|doi=10.1103/PhysRevLett.109.111807
|url=http://link.aps.org/doi/10.1103/PhysRevLett.109.111807
|arxiv = 1205.5368v2 |bibcode = 2012PhRvL.109k1807A }}</ref>
: <math>\alpha^{-1} = 137.035\,999\,173(35).</math>

This measurement of ''α'' has a precision of 0.25 [[parts per billion]]. This value and uncertainty are about the same as the latest experimental results.<ref>{{cite journal|author1=Rym Bouchendira|author2=Pierre Cladé|author3=Saïda Guellati-Khélifa|author4=François Nez|author5=François Biraben|title=New determination of the fine-structure constant and test of the quantum electrodynamics|year=2010|doi=10.1103/PhysRevLett.106.080801|journal=Physical Review Letters|volume=106|issue=8|arxiv=1012.3627|bibcode = 2011PhRvL.106h0801B }}</ref>

==Physical interpretations==
The fine-structure constant, ''α'', has several physical interpretations. ''α'' is:
* The square of the ratio of the [[elementary charge]] to the [[Planck charge]]
:: <math>\alpha = \left( \frac{e}{q_\mathrm{P}} \right)^2.</math>
* The ratio of two energies: (i) the energy needed to overcome the [[electrostatic repulsion]] between two electrons a distance of ''d'' apart, and (ii) the energy of a single [[photon]] of wavelength <math>\lambda = 2\pi d</math> (from a modern perspective, of angular wavelength ''r'' = ''d'' ; see [[Planck relation]]):
:: <math>\alpha = \frac{e^2}{4 \pi \varepsilon_0 d} \times \frac{\lambda}{h c} = \frac{e^2}{4 \pi \varepsilon_0 d} \times {\frac{2 \pi d}{h c}} = (\frac{e^2}{4 \pi \varepsilon_0 r} \times {\frac{r}{\hbar c}}) = \frac{e^2}{4 \pi \varepsilon_0 \hbar c}.</math>
* The ratio of the velocity of the electron in the [[Bohr model of the atom]] to the [[speed of light]]. Hence the square of α is the ratio between the [[Hartree energy]] ({{nowrap|27.2 eV {{=}} twice the [[Rydberg constant|Rydberg energy]]}}) and the [[electron]] [[rest mass]] (511 keV).
* The ratio of three characteristic lengths: the [[classical electron radius]] <math>r_e</math>, the [[Bohr radius]] <math>a_0</math> and the [[Compton wavelength]] of the electron <math>\lambda_e</math>:
:: <math>r_e = {\alpha \lambda_e \over 2\pi} = \alpha^2 a_0</math>
* In [[quantum electrodynamics]], ''α'' is the [[coupling constant]] determining the strength of the interaction between [[electron]]s and [[photon]]s. The theory does not predict its value. Therefore ''α'' must be determined experimentally. In fact, ''α'' is one of the about 20 empirical [[Standard Model#Theoretical aspects|parameters in the Standard Model]] of [[particle physics]], whose value is not determined within the Standard Model.
* In the [[electroweak theory]] unifying the [[weak interaction]] with [[electromagnetism]], ''α'' is absorbed into two other [[coupling constant]]s associated with the electroweak [[gauge theory|gauge fields]]. In this theory, the [[electromagnetic interaction]] is treated as a mixture of interactions associated with the electroweak fields. The strength of the [[electromagnetic interaction]] varies with the strength of the [[energy]] field.
* Given two hypothetical point particles each of Planck mass and elementary charge, separated by any distance, α is the ratio of their electrostatic repulsive force to their gravitational attractive force.
* In the fields of [[electrical engineering]] and [[solid-state physics]], the fine-structure constant is one fourth the product of the characteristic [[impedance of free space]], ''Z''0 = 1/(''c'' ''ε''0), and the [[conductance quantum]], ''G''0 = ''2e''2/''h'':
:: <math>\alpha = \frac{1}{4}\,Z_0\,G_0</math>.

When [[perturbation theory (quantum mechanics)|perturbation theory]] is applied to [[quantum electrodynamics]], the resulting [[perturbative]] expansions for physical results are expressed as sets of [[power series]] in ''α''. Because ''α'' is much less than one, higher powers of ''α'' are soon unimportant, making the perturbation theory extremely practical in this case. On the other hand, the large value of the corresponding factors in [[quantum chromodynamics]] makes calculations involving the [[strong nuclear force]] extremely difficult.

According to the theory of the [[renormalization group]], the value of the fine-structure constant (the strength of the electromagnetic interaction) grows [[logarithm]]ically as the [[energy scale]] is increased. The observed value of ''α'' is associated with the energy scale of the [[electron mass]]; the electron is a lower bound for this energy scale because it (and the [[positron]]) is the lightest charged object whose [[quantum loop]]s can contribute to the running. Therefore 1/137.036 is the value of the fine-structure constant at zero energy. Moreover, as the energy scale increases, the strength of the electromagnetic interaction approaches that of the other two [[fundamental interaction]]s, a fact important for [[grand unification]] theories. If quantum electrodynamics were an exact theory, the fine-structure constant would actually diverge at an energy known as the [[Landau pole]]. This fact makes quantum electrodynamics inconsistent beyond the [[perturbative]] expansions.

===Classical considerations===
The fine-structure constant, ''α'', expresses the strength of the electrodynamic interaction. It can already be determined with mainly classical considerations. If the electron is described in a classical model by the circulation of a massless charge on a circular track with radius ''r'' it is embedded in its own synchrotron radiation field
.<ref name="GP">
{{cite arxiv |author=G. Poelz |title=On the Wave Character of the Electron |eprint=1206.0620 |class=physics.class-ph |year=2012 }}
</ref>
The interaction with photons result in an angular momentum <math>L=1\hbar</math>. Then with general arguments one obtains the chain
: <math>1\hbar=L=r\cdot p=r\cdot E/c=T \cdot E/(2\pi)=\lambda E/(2 \pi c)</math>,

with momentum ''p'', total energy ''E'', circulation period ''T'', and ''λ'' the Compton wavelength which is the wavelength of the fundamental mode of the synchrotron radiation.

The power of the radiation is <math>P=E/T</math> and leads to <math>2\pi \hbar=E^2/P</math>.

The power of the synchrotron radiation is for <math>\beta=1</math>
<ref>
{{cite book |last1=Iwanenko |first1=D. |last2=Sokolov |first2=A. |title=Klassische Feldtheorie |publisher=Akademie-Verlag |location=Berlin |year=1953 |chapter=39ff }}
</ref>

: <math>P=\frac{e^2 c}{4\pi\varepsilon_0 r^2}\cdot Sum_n</math>

with <math>Sum_n</math> the sum over integrals of the contributing modes n. With <math>P=1.01\cdot 10^{7}[W]</math> which corresponds to <math>Sum_n=22</math> and which is close to numerical calculations in the model, one gets the values for the elementary charge ''e'' and the fine structure constant ''α''.

More convincing is the use of arguments from quantum mechanics: The interaction of the charge with its field leads to oscillations and replaces the point like charge by a charge distribution. The energy of this distribution is <math>E=\frac{e^2}{4\pi\varepsilon_0 R}</math> where R describes the radius of this distribution. The comparison with the mass of the lepton <math>m c^2=\hbar \omega=\hbar c/R</math> yields <math>E/(m c^2)= \alpha</math>.
<ref name="GP"/>

==History==
[[Arnold Sommerfeld]] introduced the fine-structure constant in 1916, as part of his theory of the [[special relativity|relativistic]] deviations of [[atom]]ic [[spectral line]]s from the predictions of the [[Bohr model]]. The first physical interpretation of the fine-structure constant ''α'' was as the ratio of the velocity of the electron in the first circular orbit of the relativistic [[Bohr atom]] to the [[speed of light]] in the vacuum.<ref>
{{Cite web
|author=
|title=Introduction to the Constants for Nonexperts – Current Advances: The Fine-Structure Constant and Quantum Hall Effect
|url=http://physics.nist.gov/cuu/Constants/alpha.html
|work=The NIST Reference on Constants, Units, and Uncertainty
|publisher=[[NIST]]
|accessdate=11 April 2009
}}</ref> Equivalently, it was the quotient between the minimum [[angular momentum]] allowed by relativity for a closed orbit, and the minimum angular momentum allowed for it by quantum mechanics. It appears naturally in Sommerfeld's analysis, and determines the size of the splitting or [[fine structure|fine-structure]] of the hydrogenic [[Lyman series|spectral lines]].

The fine-structure constant so intrigued physicist [[Wolfgang Pauli]] that he collaborated with psychiatrist [[Carl Jung]] in an extraordinary quest to understand its significance.<ref>
{{Cite journal
|author=P. Varlaki, L. Nadai, J. Bokor
|year=2008
|title=Number Archetypes and Background Control Theory Concerning the Fine Structure Constant
|url=http://uni-obuda.hu/journal/Varlaki_Nadai_Bokor_14.pdf
|journal=[[Acta Polytechnica Hungarica]]
|volume=5 |issue=2 |pages=71
|doi=
}}</ref>

==Is the fine-structure constant actually constant?==
While the fine-structure constant is known to approach 1/128 at interaction energies above 80 GeV,<ref>
{{Cite web
| author=P.J. Mohr (NIST)
| year=2010
| title=Physical Constants
| url=http://pdg.lbl.gov/2011/reviews/rpp2011-rev-phys-constants.pdf
| publisher=[[Particle Data Group]]
| accessdate=21 December 2011
}}</ref>
physicists have pondered for many years whether the fine-structure constant is in fact constant, i.e., whether or not its value differs by location and over time. Specifically, a varying ''α'' has been proposed as a way of solving problems in [[cosmology]] and [[astrophysics]].<ref>
{{Cite book
|author=E.A. Milne
|year=1935
|title=Relativity, Gravitation and World Structure
|publisher=[[Clarendon Press]]
|pages=
|isbn=
|lccn=
|oclc=
}}</ref><ref>
{{Cite journal
|author=P.A.M. Dirac
|year=1937
|title=The Cosmological Constants
|journal=[[Nature (journal)|Nature]]
|volume=139 |page=323
|doi=10.1038/139323a0
|bibcode = 1937Natur.139..323D
|issue=3512}}</ref><ref>
{{Cite journal
|author=G. Gamow
|year=1967
|title=Electricity, Gravity, and Cosmology
|journal=[[Physical Review Letters]]
|volume=19 |page=759
|doi=10.1103/PhysRevLett.19.759
|bibcode=1967PhRvL..19..759G
|issue=13
}}</ref><ref>
{{Cite journal
|author=G. Gamow
|year=1967
|title=Variability of Elementary Charge and Quasistellar Objects
|journal=[[Physical Review Letters]]
|volume=19 |page=913
|doi=10.1103/PhysRevLett.19.913
|bibcode=1967PhRvL..19..913G
|issue=16
}}</ref> More recently, theoretical interest in varying constants (not just ''α'') has been motivated by [[string theory]] and other such proposals for going beyond the [[Standard Model]] of particle physics. The first experimental tests of this question examined the [[spectral line]]s of distant astronomical objects, and the products of [[radioactive decay]] in the [[Oklo]] [[natural nuclear fission reactor]]. The findings were consistent with no change.<ref>
{{Cite journal
|author=J.-P. Uzan
|year=2003
|title=The Fundamental Constants and Their Variation: Observational Status and Theoretical Motivations
|journal=[[Reviews of Modern Physics]]
|volume=75 | pages = 403–455
|doi=10.1103/RevModPhys.75.403
|arxiv=hep-ph/0205340
|bibcode=2003RvMP...75..403U
|issue=2
}}</ref><ref>
{{cite arXiv
|author=J.-P. Uzan
|year=2004
|title=Variation of the Constants in the Late and Early Universe
|class=astro-ph
|eprint=astro-ph/0409424
}}</ref><ref>
{{Cite journal
|author=K. Olive, Y.-Z. Qian
|year=2003
|title=Were Fundamental Constants Different in the Past?
|journal=[[Physics Today]]
|volume=57 |issue=10 |pages=40–45
|doi=10.1063/1.1825267
|bibcode = 2004PhT....57j..40O }}</ref><ref>
{{Cite book
|author=J.D. Barrow
|year=2002
|title=The Constants of Nature: From Alpha to Omega—the Numbers That Encode the Deepest Secrets of the Universe
|publisher=[[Random House|Vintage]]
|isbn = 0-09-928647-5
}}</ref><ref>
{{Cite book
|author=J.-P. Uzan, B. Leclercq
|year=2008
|title=The Natural Laws of the Universe: Understanding Fundamental Constants
|publisher=[[Springer Science+Business Media|Springer Praxis]]
|isbn=978-0-387-73454-5
}}</ref><ref>
{{Cite book
|author=F. Yasunori
|year=2004
|chapter=Oklo Constraint on the Time-Variability of the Fine-Structure Constant
|chapterurl=http://www.springerlink.com/content/20dt5p8t8ene319q/
|title=Astrophysics, Clocks and Fundamental Constants
|url=http://www.springerlink.com/content/yvjtrlw7grcn/
|pages=167–185
|series=Lecture Notes in Physics
|publisher=[[Springer Berlin]]
|isbn=978-3-540-21967-5
}}</ref>

More recently, improved technology has made it possible to probe the value of ''α'' at much larger distances and to a much greater accuracy. In 1999, a team led by [[John K. Webb]] of the [[University of New South Wales]] claimed the first detection of a variation in ''α''.<ref>
{{Cite journal
|author = J.K. Webb ''et al''.
|year = 1999
|title = Search for Time Variation of the Fine Structure Constant
|journal = [[Physical Review Letters]]
|volume = 82 | issue = 5 | pages = 884–887
|doi = 10.1103/PhysRevLett.82.884
|arxiv = astro-ph/9803165
|bibcode=1999PhRvL..82..884W
}}</ref><ref>
{{Cite journal
|author = M.T. Murphy ''et al''.
|year = 2001
|title =Possible evidence for a variable fine-structure constant from QSO absorption lines: motivations, analysis and results
|journal = [[Monthly Notices of the Royal Astronomical Society]]
|volume = 327 | page = 1208
|doi = 10.1046/j.1365-8711.2001.04840.x
|arxiv = astro-ph/0012419 |bibcode = 2001MNRAS.327.1208M
|issue = 4 }}</ref><ref>
{{Cite journal
|author = J.K. Webb ''et al''.
|year = 2001
|title = Further Evidence for Cosmological Evolution of the Fine Structure Constant
|journal = [[Physical Review Letters]]
|volume = 87 | issue = 9 | pages = 091301
|doi = 10.1103/PhysRevLett.87.091301
|arxiv = astro-ph/0012539
|pmid = 11531558
|bibcode=2001PhRvL..87i1301W
}}</ref><ref>
{{Cite journal
| author = M.T. Murphy, J.K. Webb, V.V. Flambaum
| year = 2003
| title = Further Evidence for a Variable Fine-Structure Constant from Keck/HIRES QSO Absorption Spectra
| journal = [[Monthly Notices of the Royal Astronomical Society]]
| volume = 345 | page = 609
| doi =10.1046/j.1365-8711.2003.06970.x
|arxiv = astro-ph/0306483 |bibcode = 2003MNRAS.345..609M
| issue = 2 }}</ref> Using the [[Keck telescopes]] and a data set of 128 [[quasars]] at [[redshifts]] 0.5 < ''z'' < 3, Webb ''et al.'' found that their spectra were consistent with a slight increase in ''α'' over the last 10–12 billion years. Specifically, they found that
:<math>\frac{\Delta \alpha}{\alpha} \ \stackrel{\mathrm{def}}{=}\ \frac{\alpha _\mathrm{prev}-\alpha _\mathrm{now}}{\alpha_\mathrm{now}} = \left(-5.7\pm 1.0 \right) \times 10^{-6}.</math>

In 2004, a smaller study of 23 absorption systems by Chand ''et al.'', using the [[Very Large Telescope]], found no measureable variation:<ref>
{{Cite journal
|author=H. Chand ''et al''.
|year=2004
|title=Probing the Cosmological Variation of the Fine-Structure Constant: Results Based on VLT-UVES Sample
|journal=[[Astronomy & Astrophysics]]
|volume=417 |page=853
|doi=10.1051/0004-6361:20035701
|bibcode=2004A&A...417..853C
|arxiv = astro-ph/0401094
|issue=3 }}</ref><ref>
{{Cite journal
|author=R. Srianand ''et al''.
|year=2004
|title=Limits on the Time Variation of the Electromagnetic Fine-Structure Constant in the Low Energy Limit from Absorption Lines in the Spectra of Distant Quasars
|journal=[[Physical Review Letters]]
|volume=92
|issue=12 |page=121302
|doi=10.1103/PhysRevLett.92.121302
|pmid=15089663
|bibcode=2004PhRvL..92l1302S
|arxiv = astro-ph/0402177 }}</ref>
:<math> \frac{\Delta \alpha}{\alpha_\mathrm{em}}= \left(-0.6\pm 0.6\right) \times 10^{-6}.</math>

However, in 2007 simple flaws were identified in the analysis method of Chand ''et al.'', discrediting those results.<ref>
{{Cite journal
|author=M.T. Murphy, J.K. Webb, V.V. Flambaum
|year=2007
|title=Comment on "Limits on the Time Variation of the Electromagnetic Fine-Structure Constant in the Low Energy Limit from Absorption Lines in the Spectra of Distant Quasars"
|journal=[[Physical Review Letters]]
|volume=99 |page=239001
|doi=10.1103/PhysRevLett.99.239001
|bibcode=2007PhRvL..99w9001M
|issue=23
|arxiv = 0708.3677 }}</ref><ref>
{{Cite journal
|author=M.T. Murphy, J.K. Webb, V.V. Flambaum
|year=2008
|title=Revision of VLT/UVES Constraints on a Varying Fine-Structure Constant
|journal=[[Monthly Notices of the Royal Astronomical Society]]
|volume=384 |page=1053
|doi=10.1111/j.1365-2966.2007.12695.x
|bibcode=2008MNRAS.384.1053M
|arxiv = astro-ph/0612407
|issue=3 }}</ref> Nevertheless, systematic uncertainties are difficult to quantify and so the Webb ''et al.'' results still need to be checked by independent analysis, using quasar spectra from different telescopes.{{citation needed|date=May 2012}}

King ''et al.'' have used [[Markov Chain Monte Carlo]] methods to investigate the algorithm used by the UNSW group to determine <math>\Delta\alpha/\alpha</math> from the quasar spectra, and have found that the algorithm appears to produce correct uncertainties and maximum likelihood estimates for <math>\Delta\alpha/\alpha</math> for particular models.<ref>
{{cite arXiv
|author=J. King, D. Mortlock, J. Webb, M. Murphy
|year=2009
|title=Markov Chain Monte Carlo methods applied to measuring the fine-structure constant from quasar spectroscopy
|class=astro-ph
|eprint=0910.2699
}}</ref> This suggests that the statistical uncertainties and best estimate for <math>\Delta\alpha/\alpha</math> stated by Webb ''et al.'' and Murphy ''et al.'' are robust.

Lamoreaux and Torgerson analyzed data from the [[Oklo]] [[natural nuclear fission reactor]] in 2004, and concluded that ''α'' has changed in the past 2 billion years by 4.5 parts in {{val|e=8}}. They claimed that this finding was "probably accurate to within 20%." Accuracy is dependent on estimates of impurities and temperature in the natural reactor. These conclusions have to be verified.<ref>
{{Cite book
| author = R. Kurzweil
| year = 2005
| title = [[The Singularity Is Near]]
| publisher = [[Penguin Group|Viking Penguin]]
| pages = 139–140
| isbn = 0-670-03384-7
}}</ref><ref>
{{Cite journal
| author = S.K. Lamoreaux, J.R. Torgerson
| year = 2004
| title = Neutron Moderation in the Oklo Natural Reactor and the Time Variation of Alpha
| journal = [[Physical Review D]]
| volume = 69 |pages=
| doi= 10.1103/PhysRevD.69.121701
| arxiv = nucl-th/0309048
|bibcode = 2004PhRvD..69l1701L
| issue = 12 }}</ref><ref>
{{Cite web
| author=E.S. Reich
| date=30 June 2004
| title=Speed of Light May Have Changed Recently
| url=http://www.newscientist.com/article/dn6092-speed-of-light-may-have-changed-recently.html
| publisher=[[New Scientist]]

| accessdate=30 January 2009
}}</ref><ref>
{{Cite web
| author=
| date=12 May 2005
| title=Scientists Discover One Of The Constants Of The Universe Might Not Be Constant
| url=http://www.sciencedaily.com/releases/2005/05/050512120842.htm
| publisher=[[ScienceDaily]]
| accessdate=30 January 2009
}}</ref>

In 2007, Khatri and Wandelt of the University of Illinois at Urbana-Champaign realized that the [[21 cm line|21 cm hyperfine transition in neutral hydrogen]] of the early Universe leaves a unique absorption line imprint in the [[cosmic microwave background radiation]].<ref name=Khatri>
{{Cite journal
| author = R. Khatri, B.D. Wandelt
| year = 2007
| title = 21-cm Radiation: A New Probe of Variation in the Fine-Structure Constant
| journal = [[Physical Review Letters]]
| volume = 98
| issue = 11 | pages = 111301
| doi = 10.1103/PhysRevLett.98.111301
| arxiv = astro-ph/0701752
| pmid=17501040
| bibcode=2007PhRvL..98k1301K
}}</ref> They proposed using this effect to measure the value of ''α'' during the epoch before the formation of the first stars. In principle, this technique provides enough information to measure a variation of 1 part in {{val|e=9}} (4 orders of magnitude better than the current quasar constraints). However, the constraint which can be placed on ''α'' is strongly dependent upon effective integration time, going as ''t''−1/2. The European [[LOFAR]] [[radio telescope]] would only be able to constrain Δ''α''/''α'' to about 0.3%.<ref name=Khatri/> The collecting area required to constrain Δ''α''/''α'' to the current level of quasar constraints is on the order of 100 square kilometers, which is economically impracticable at the present time.

In 2008, Rosenband ''et al.''<ref>
{{Cite journal
| author = T. Rosenband ''et al''.
| year = 2008
| title = Frequency Ratio of Al+ and Hg+ Single-Ion Optical Clocks; Metrology at the 17th Decimal Place
| journal = [[Science (journal)|Science]]
| volume = 319 | issue = 5871 | pages = 1808–12
| doi = 10.1126/science.1154622
| pmid = 18323415
|bibcode = 2008Sci...319.1808R }}</ref> used the frequency ratio of {{element|aluminium|charge=+}} and {{element|mercury|charge=+}} in single-ion optical atomic clocks to place a very stringent constraint on the present time variation of ''α'', namely {{nowrap begin}}Δ''α̇''/''α'' = {{val|-1.6|2.3|e=-17}}{{nowrap end}} per year. Note that any present day null constraint on the time variation of alpha does not necessarily rule out time variation in the past. Indeed, some theories<ref>
{{Cite journal
| author = J.D. Barrow, H.B. Sandvik, J. Magueijo
| year = 2001
| title = The Behaviour of Varying-Alpha Cosmologies
| journal = Phys. Rev. D
| arxiv = astro-ph/0109414 |bibcode = 2002PhRvD..65f3504B |doi = 10.1103/PhysRevD.65.063504
| volume = 65
| issue = 6 }}</ref> that predict a variable fine-structure constant also predict that the value of the fine-structure constant should become practically fixed in its value once the universe enters its current [[dark energy]]-dominated epoch.

=== Australian dipole ===
In September 2010 researchers from Australia said they had identified a dipole-like structure in the fine-structure constant across the observable universe. They used data on [[quasar]]s obtained by the [[Very Large Telescope]], combined with the previous data obtained by Webb at the [[Keck telescopes]]. The fine-structure constant appears to have been larger by one part in 100,000 in the direction of the southern hemisphere constellation [[Ara (constellation)|Ara]], 10 billion years ago. Similarly, the constant appeared to have been smaller by a similar fraction in the northern direction, billions of years ago.<ref>
{{Cite web
|author=H. Johnston
|date=2 September 2010
|url=http://physicsworld.com/cws/article/news/43657
|title=Changes spotted in fundamental constant
|work=[[Physics World]]
|accessdate=11 September 2010
}}</ref><ref>
{{Cite journal
| author=J.K.Webb ''et al''.
| date= 23 August 2010
| arxiv=1008.3907
| title = Evidence for spatial variation of the fine-structure constant
| journal = Physical Review Letters
|bibcode = 2011PhRvL.107s1101W
| author2=King, J. A.
| author3=Murphy, M. T.
| author4=Flambaum, V. V.
| author5=Carswell, R. F.
| author6=Bainbridge, M. B.
| doi = 10.1103/PhysRevLett.107.191101
| volume=107
| issue=19 }}</ref><ref>
{{Cite thesis
| degree=PhD
| title=Searching for variations in the fine-structure constant and the proton-to-electron mass ratio using quasar absorption lines
| author=J. A. King
| year=2010
| publisher=University of New South Wales
| url=http://handle.unsw.edu.au/1959.4/50886
}}</ref>

In September and October 2010, after Webb's released research, physicists [[Chad Orzel]] and [[Sean M. Carroll]] suggested different approaches of how Webb's observations may be wrong. Orzel argues that the study may contain wrong data due to subtle differences in the two telescopes, in which one of the telescopes the data set was slightly high and on the other slightly low, so that they cancel each other out when they overlapped. He finds it suspicious that the triangles in the plotted graph of the quasars are so well-aligned (triangles, being the 3-omega of the data). Carroll suggested a totally different approach; he looks at the fine-structure constant as a scalar field and claims that if the telescopes are correct and the fine-structure constant varies smoothly over the universe, then the scalar field must have a very small mass. However, previous research has shown that the mass is not likely to be extremely small. Both of these scientists' early criticisms point to the fact that different techniques are needed to confirm or contradict the results, as Webb, et al., also concluded in their study.<ref>
{{Cite web
|author=Chad Orzel
|date=14 September
|url=http://scienceblogs.com/principles/2010/09/_httpksjtrackermitedu20100907e.php
|title=Why I'm skeptical about changing Fine-Structure constant
}}</ref><ref>
{{Cite web
|author=Sean Corroll
|date=18 October
|url=http://blogs.discovermagazine.com/cosmicvariance/2010/10/18/the-fine-structure-constant-is-probably-constant/
|title=The fine-structure constant is probably constant
}}</ref>

In October 2011, Webb ''et al.'' reported<ref>
{{Cite journal
|author=J. K. Webb, J. A. King, M. T. Murphy, V. V. Flambaum, R. F. Carswell, and M. B. Bainbridge
|year=2011
|title=Indications of a Spatial Variation of the Fine Structure Constant
|journal=[[Physical Review Letters]]
|volume=107
|issue=19
|doi=10.1103/PhysRevLett.107.191101
|bibcode=2011PhRvL.107s1101W
|arxiv = 1008.3907 }}</ref>
a variation in α dependent on both redshift and spatial direction. They report "the combined data set fits a spatial dipole" with an increase in α with redshift in one direction and a decrease in the other. "[I]ndependent VLT and Keck samples give consistent dipole directions and amplitudes...."

==Anthropic explanation==
The [[anthropic principle]] is a controversial argument of why the fine-structure constant has the value it does: stable matter, and therefore life and intelligent beings, could not exist if its value were much different. For instance, were ''α'' to change by 4%, stellar [[Nuclear fusion|fusion]] would not produce [[carbon]], so that carbon-based life would be impossible. If ''α'' were > 0.1, stellar fusion would be impossible and no place in the universe would be warm enough for life as we know it.<ref>
{{Cite journal
| author = J.D. Barrow
| year = 2001
| title = Cosmology, Life, and the Anthropic Principle
| journal = [[Annals of the New York Academy of Sciences]]
| volume = 950 | issue = 1 | pages = 139–153
| doi = 10.1111/j.1749-6632.2001.tb02133.x
|bibcode = 2001NYASA.950..139B }}</ref>

==Numerological explanations==
As a dimensionless constant which does not seem to be directly related to any [[mathematical constant]], the fine-structure constant has long fascinated physicists. [[Richard Feynman]], one of the originators and early developers of the theory of [[quantum electrodynamics]] (QED), referred to the fine-structure constant in these terms:

{{quote|There is a most profound and beautiful question associated with the observed coupling constant, e - the amplitude for a real electron to emit or absorb a real photon. It is a simple number that has been experimentally determined to be close to 0.08542455. (My physicist friends won't recognize this number, because they like to remember it as the inverse of its square: about 137.03597 with about an uncertainty of about 2 in the last decimal place. It has been a mystery ever since it was discovered more than fifty years ago, and all good theoretical physicists put this number up on their wall and worry about it.) Immediately you would like to know where this number for a coupling comes from: is it related to pi or perhaps to the base of natural logarithms? Nobody knows. It's one of the greatest damn mysteries of physics: a magic number that comes to us with no understanding by man. You might say the "hand of God" wrote that number, and "we don't know how He pushed his pencil." We know what kind of a dance to do experimentally to measure this number very accurately, but we don't know what kind of dance to do on the computer to make this number come out, without putting it in secretly!|[[Richard Feynman]]|{{Cite book
|author=Richard P. Feynman
|year=1985
|title=[[QED: The Strange Theory of Light and Matter]]
|publisher=[[Princeton University Press]]
|page=129
|isbn=0-691-08388-6
|postscript=
}}}}

Regarding the occurrence of numerological explanations [[I. J. Good]] maintains:

{{quote|There have been a few examples of numerology that have led to theories that transformed society: see the mention of Kirchhoff and Balmer in Good (1962, p. 316) ... and one can well include Kepler on account of his third law. It would be fair enough to say that numerology was the origin of the theories of electromagnetism, quantum mechanics, gravitation.... So I intend no disparagement when I describe a formula as numerological.

When a numerological formula is proposed, then we may ask whether it is correct. ... I think an appropriate definition of correctness is that the formula has a good explanation, in a Platonic sense, that is, the explanation could be based on a good theory that is not yet known but ‘exists’ in the universe of possible reasonable ideas.
|[[I. J. Good]]|{{Cite book
|author=I. J. Good
|year=1990
|title="A Quantal Hypothesis for Hadrons and the Judging of Physical Numerology." in G. R. Grimmett (Editor), D. J. A. Welsh (Editor). ''Disorder in Physical Systems''
|publisher=[[Oxford University Press]]
|page=141
|isbn=978-0198532156
|postscript=
}}}}

[[Arthur Eddington]] argued that the value could be "obtained by pure deduction" and he related it to the [[Eddington number]], his estimate of the number of protons in the Universe.<ref>
{{Cite book
|author=A.S Eddington
|year=1956
|chapter=The Constants of Nature
|editor=J.R. Newman
|title=The World of Mathematics
|volume=2 |pages=1074–1093
|publisher=[[Simon & Schuster]]
|isbn=
|oclc=
|lccn=
}}</ref> This led him in 1929 to conjecture that its reciprocal was precisely the [[integer]] [[137 (number)|137]]. Other physicists neither adopted this conjecture nor accepted his arguments but by the 1940s experimental values for 1/α deviated sufficiently from 137 to refute Eddington's argument.<ref>
{{Cite journal
|author=H. Kragh
|year=2003
|title=Magic Number: A Partial History of the Fine-Structure Constant
|journal=[[Archive for History of Exact Sciences]]
|volume=57 |issue=5 |pages=395
|doi=10.1007/s00407-002-0065-7
}}</ref> Attempts to find a mathematical basis for this dimensionless constant have continued up to the present time. For example, the mathematician [http://www.fine-structure-constant.org James Gilson] suggested (earliest [[archive.org]] entry dated December 2006 [http://web.archive.org/web/20070104030245/http://www.maths.qmul.ac.uk/~jgg/page5.html]), that the fine-structure constant has the value:

:<math> \alpha = \frac{\cos \left(\pi/137 \right)}{137} \ \frac{\tan \left(\pi/(137 \cdot 29) \right)}{\pi/(137 \cdot 29)} \approx \frac{1}{137.0359997867}, </math>

29 and 137 being the 10th and 33rd [[prime number]]s. In 2007, the difference between the [[CODATA]] value for α and this theoretical value was about {{val|3|e=-11}}, about 6 times the standard error for the measured value, but as of 2010 CODATA correction the difference became much greater.

==Quotes==
{{quote|The mystery about ''α'' is actually a double mystery. The first mystery – the origin of its numerical value ''α'' ≈ 1/137 has been recognized and discussed for decades. The second mystery – the range of its domain – is generally unrecognized.|Malcolm H. Mac Gregor|{{Cite book
|author = M.H. MacGregor
|year = 2007
|title = The Power of Alpha
|page= 69
|publisher = [[World Scientific]]
|isbn = 978-981-256-961-5
|postscript = 
}}}}

{{quote|If alpha [the fine-structure constant] were bigger than it really is, we should not be able to distinguish matter from ether [the vacuum, nothingness], and our task to disentangle the natural laws would be hopelessly difficult. The fact however that alpha has just its value 1/137 is certainly no chance but itself a law of nature. It is clear that the explanation of this number must be the central problem of natural philosophy.
|Max Born|{{Cite book
|author = A.I. Miller
|year = 2009
|title = Deciphering the Cosmic Number: The Strange Friendship of Wolfgang Pauli and Carl Jung
|page= 253
|publisher = [[W.W. Norton & Co.|W.W. Norton & Co]]
|isbn = 978-0-393-06532-9
|postscript = 
}}}}

==See also==
* [[Electric constant]]
* [[Gravitational coupling constant]]
* [[Planck's constant]]
* [[Speed of light]]

==References==
{{Reflist|2}}

==External links==
* [[Stephen L. Adler]], [http://lss.fnal.gov/archive/1972/pub/Pub-72-059-T.pdf "Theories of the Fine Structure Constant α"] FERMILAB-PUB-72/059-T
* [http://physics.nist.gov/cuu/Constants/alpha.html "Introduction to the constants for nonexperts"], adapted from the ''[[Encyclopædia Britannica]]'', 15th ed. Disseminated by the [[NIST]] web page.
* [http://physics.nist.gov/cuu/Constants/codata.pdf CODATA recommended value of α], as of 2006.
* [http://scienceworld.wolfram.com/physics/FineStructureConstant.html "Fine Structure Constant"], Eric Weisstein's World of Physics website.
* [[John D. Barrow]], and John K. Webb, [http://www.sciam.com/article.cfm?articleID=0005BFE6-2965-128A-A96583414B7F0000&ref=sciam "Inconstant Constants"], ''[[Scientific American]]'', June 2005.
* {{cite web|last=Eaves|first=Laurence|title=The Fine Structure Constant|url=http://www.sixtysymbols.com/videos/finestructure.htm|work=Sixty Symbols|publisher=[[Brady Haran]] for the [[University of Nottingham]]|authorlink=Laurence Eaves|year=2009}}

{{DEFAULTSORT:Fine-Structure Constant}}
[[Category:Electromagnetism]]
[[Category:Concepts in physics]]
[[Category:Fundamental constants]]
[[Category:Dimensionless numbers]]

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