Ashtekar variables: Difference between revisions
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{{Quantum mechanics}} | |||
In [[theoretical physics]], '''quantum geometry''' is the set of new mathematical concepts generalizing the concepts of [[geometry]] whose understanding is necessary to describe the physical phenomena at very short distance scales (comparable to [[Planck length]]). At these distances, [[quantum mechanics]] has a profound effect on physics. | |||
==Quantum gravity== | |||
{{Main|quantum gravity}} | |||
Each theory of [[quantum gravity]] uses the term "quantum geometry" in a slightly different fashion. [[String theory]], a leading candidate for a quantum theory of gravity, uses the term quantum geometry to describe exotic phenomena such as [[T-duality]] and other geometric dualities, [[mirror symmetry (string theory)|mirror symmetry]], [[topology]]-changing transitions, minimal possible distance scale, and other effects that challenge our usual geometrical intuition. More technically, quantum geometry refers to the shape of the spacetime manifold as seen by [[D-branes]] which includes the quantum corrections to the [[metric tensor]], such as the worldsheet [[instanton]]s. For example, the quantum volume of a cycle is computed from the mass of a [[Membrane (M-theory)|brane]] wrapped on this cycle. As another example, a distance between two quantum mechanics particles can be expressed in terms of the [[Lukaszyk–Karmowski metric]].<ref>[http://www.springerlink.com/content/y4fbdb0m0r12701p A new concept of probability metric and its applications in approximation of scattered data sets], Łukaszyk Szymon, Computational Mechanics Volume 33, Number 4, 299–304, Springer-Verlag 2003 {{doi|10.1007/s00466-003-0532-2}}</ref> | |||
In an alternative approach to quantum gravity called [[loop quantum gravity]] (LQG), the phrase "quantum geometry" usually refers to the [[Scientific formalism|formalism]] within LQG where the observables that capture the information about the geometry are now well defined operators on a [[Hilbert space]]. In particular, certain physical [[observable]]s, such as the area, have a [[discrete spectrum]]. It has also been shown that the loop quantum geometry is [[non-commutative geometry|non-commutative]]. | |||
It is possible (but considered unlikely) that this strictly quantized understanding of geometry will be consistent with the quantum picture of geometry arising from string theory. | |||
Another, quite successful, approach, which tries to reconstruct the geometry of space-time from "first principles" is [[Discrete Lorentzian quantum gravity]]. | |||
==Quantum states as differential forms== | |||
{{main|Wavefunction}} | |||
{{see also|Differential forms}} | |||
[[Differential forms]] are used to express [[quantum states]], using the [[wedge product]]:<ref>''The Road to Reality'', Roger Penrose, Vintage books, 2007, ISBN 0-679-77631-1</ref> | |||
:<math>|\psi\rangle = \int \psi(\mathbf{x},t) |\mathbf{x},t\rangle \mathrm{d}^3\mathbf{x} </math> | |||
where the [[position vector]] is | |||
:<math>\mathbf{x} = (x^1,x^2,x^3) </math> | |||
the differential [[volume element]] is | |||
:<math>\mathrm{d}^3\mathbf{x}=\mathrm{d}x^1\wedge\mathrm{d}x^2\wedge\mathrm{d}x^3 </math> | |||
and ''x''<sup>1</sup>, ''x''<sup>2</sup>, ''x''<sup>3</sup> are an arbitrary set of coordinates, the upper [[Index notation|indices]] indicate [[Covariance and contravariance of vectors|contravariance]], lower indices indicate [[Covariance and contravariance of vectors|covariance]], so explicitly the quantum state in differential form is: | |||
:<math>|\psi\rangle = \int \psi(x^1,x^2,x^3,t) |x^1,x^2,x^3,t\rangle \mathrm{d}x^1\wedge\mathrm{d}x^2\wedge\mathrm{d}x^3 </math> | |||
The overlap integral is given by: | |||
:<math>\langle\chi|\psi\rangle = \int\chi^*\psi \mathrm{d}^3\mathbf{x}</math> | |||
in differential form this is | |||
:<math>\langle\chi|\psi\rangle = \int\chi^*\psi \mathrm{d}x^1\wedge\mathrm{d}x^2\wedge\mathrm{d}x^3 </math> | |||
The probability of finding the particle in some region of space ''R'' is given by the integral over that region: | |||
:<math>\langle\psi|\psi\rangle = \int_R\psi^*\psi \mathrm{d}x^1\wedge\mathrm{d}x^2\wedge\mathrm{d}x^3 </math> | |||
provided the wave function is [[Wave function|normalized]]. When ''R'' is all of 3d position space, the integral must be 1 if the particle exists. | |||
Differential forms are an approach for describing the geometry of [[Mathematical curves|curves]] and [[surfaces]] in a coordinate independent way. In [[quantum mechanics]], idealized situations occur in rectangular [[Cartesian coordinates]], such as the [[potential well]], [[particle in a box]], [[quantum harmonic oscillator]], and more realistic approximations in [[spherical polar coordinates]] such as [[electrons]] in [[atoms]] and [[molecules]]. For generality, a formalism which can be used in any coordinate system is useful. | |||
==See also== | |||
* [[Noncommutative geometry]] | |||
==References== | |||
{{reflist}} | |||
==Further reading== | |||
* ''Supersymmetry'', Demystified, P. Labelle, McGraw-Hill (USA), 2010, ISBN 978-0-07-163641-4 | |||
* ''Quantum Mechanics'', E. Abers, Pearson Ed., Addison Wesley, Prentice Hall Inc, 2004, ISBN 9780131461000 | |||
* ''Quantum Mechanics Demystified'', D. McMahon, Mc Graw Hill (USA), 2006, ISBN(10-) 0-07-145546 9 | |||
* ''Quantum Field Theory'', D. McMahon, Mc Graw Hill (USA), 2008, ISBN 978-0-07-154382-8 | |||
==External links== | |||
*[http://cgpg.gravity.psu.edu/people/Ashtekar/articles/spaceandtime.pdf Space and Time: From Antiquity to Einstein and Beyond] | |||
*[http://cgpg.gravity.psu.edu/people/Ashtekar/articles/qgfinal.pdf Quantum Geometry and its Applications] | |||
*[http://hypercomplex.xpsweb.com/articles/221/en/pdf/main-01e.pdf Hypercomplex Numbers in Geometry and Physics] | |||
{{Physics-footer}} | |||
[[Category:Quantum gravity]] | |||
[[Category:Quantum mechanics]] | |||
[[Category:Mathematical physics]] |
Revision as of 23:34, 20 November 2013
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In theoretical physics, quantum geometry is the set of new mathematical concepts generalizing the concepts of geometry whose understanding is necessary to describe the physical phenomena at very short distance scales (comparable to Planck length). At these distances, quantum mechanics has a profound effect on physics.
Quantum gravity
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Each theory of quantum gravity uses the term "quantum geometry" in a slightly different fashion. String theory, a leading candidate for a quantum theory of gravity, uses the term quantum geometry to describe exotic phenomena such as T-duality and other geometric dualities, mirror symmetry, topology-changing transitions, minimal possible distance scale, and other effects that challenge our usual geometrical intuition. More technically, quantum geometry refers to the shape of the spacetime manifold as seen by D-branes which includes the quantum corrections to the metric tensor, such as the worldsheet instantons. For example, the quantum volume of a cycle is computed from the mass of a brane wrapped on this cycle. As another example, a distance between two quantum mechanics particles can be expressed in terms of the Lukaszyk–Karmowski metric.[1]
In an alternative approach to quantum gravity called loop quantum gravity (LQG), the phrase "quantum geometry" usually refers to the formalism within LQG where the observables that capture the information about the geometry are now well defined operators on a Hilbert space. In particular, certain physical observables, such as the area, have a discrete spectrum. It has also been shown that the loop quantum geometry is non-commutative.
It is possible (but considered unlikely) that this strictly quantized understanding of geometry will be consistent with the quantum picture of geometry arising from string theory.
Another, quite successful, approach, which tries to reconstruct the geometry of space-time from "first principles" is Discrete Lorentzian quantum gravity.
Quantum states as differential forms
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Differential forms are used to express quantum states, using the wedge product:[2]
where the position vector is
the differential volume element is
and x1, x2, x3 are an arbitrary set of coordinates, the upper indices indicate contravariance, lower indices indicate covariance, so explicitly the quantum state in differential form is:
The overlap integral is given by:
in differential form this is
The probability of finding the particle in some region of space R is given by the integral over that region:
provided the wave function is normalized. When R is all of 3d position space, the integral must be 1 if the particle exists.
Differential forms are an approach for describing the geometry of curves and surfaces in a coordinate independent way. In quantum mechanics, idealized situations occur in rectangular Cartesian coordinates, such as the potential well, particle in a box, quantum harmonic oscillator, and more realistic approximations in spherical polar coordinates such as electrons in atoms and molecules. For generality, a formalism which can be used in any coordinate system is useful.
See also
References
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Further reading
- Supersymmetry, Demystified, P. Labelle, McGraw-Hill (USA), 2010, ISBN 978-0-07-163641-4
- Quantum Mechanics, E. Abers, Pearson Ed., Addison Wesley, Prentice Hall Inc, 2004, ISBN 9780131461000
- Quantum Mechanics Demystified, D. McMahon, Mc Graw Hill (USA), 2006, ISBN(10-) 0-07-145546 9
- Quantum Field Theory, D. McMahon, Mc Graw Hill (USA), 2008, ISBN 978-0-07-154382-8
External links
- Space and Time: From Antiquity to Einstein and Beyond
- Quantum Geometry and its Applications
- Hypercomplex Numbers in Geometry and Physics
- ↑ A new concept of probability metric and its applications in approximation of scattered data sets, Łukaszyk Szymon, Computational Mechanics Volume 33, Number 4, 299–304, Springer-Verlag 2003 21 year-old Glazier James Grippo from Edam, enjoys hang gliding, industrial property developers in singapore developers in singapore and camping. Finds the entire world an motivating place we have spent 4 months at Alejandro de Humboldt National Park.
- ↑ The Road to Reality, Roger Penrose, Vintage books, 2007, ISBN 0-679-77631-1