Register to reply 
Dirac Equation Covariance 
Share this thread: 
#1
Nov2112, 11:09 PM

P: 69

Is the Dirac Equation generally covariant and if not, what is the accepted version that is?
For general coordinate changes beyond just Lorentz, how do spinous transform? 


#2
Nov2212, 04:05 AM

P: 1,020

Covariance of dirac eqn can be proved,see here from 14 to 22
http://www.mathpages.com/home/kmath654/kmath654.htm Infinitesimal lorentz transformation as it acts on Diracspinors is S=1(i/4)Δω^{μv}σ_{μv},General case is much complicated. 


#3
Nov2212, 05:35 AM

Sci Advisor
Thanks
P: 4,160

Lorentz covariance of the Dirac Equation is easy, but for general covariance you do have to modify the equation slightly, and in particular you need to use a set of basis vectors (a tetrad or vierbein) at each point. See Wikipedia's page on "The Dirac Equation", and in particular the section on that page, "Dirac Equation in Curved Spacetime". (Similar remarks apply whether the spacetime is curved or just the coordinate system.)



#4
Nov2212, 08:48 AM

Sci Advisor
HW Helper
P: 11,894

Dirac Equation Covariance
Chapter 13 of Wald's book maybe ?



#5
Nov2312, 02:05 AM

P: 1,020

General theory of spinors is treated in book'the theory of spinors' by cartan.



#6
Nov2312, 02:34 AM

Sci Advisor
P: 4,569

The confusing thing is that Lorentz transformation is a special case of a general coordinate transformation, but we know that spinors do transform under Lorentz transformations (as spinors, i.e., "square roots" of vectors). Isnt't it in contradiction with the claim that spinors transform as scalars under GENERAL coordinate tranformations, which includes Lorentz transformations? The answer is that the transformation of spinors is not a fact of nature, but is a matter of a mathematical convenience. (After all, spinor fields cannot be observed.) In other words it is a matter of DEFINITION, and there are two different definitions of spinor transformation. In one definition they transform as spinors under Lorentz transformations, while in ANOTHER definition they transform as scalars under general coordinate transformations. The important fact is that the transformation of PHYSICAL quantities does not depend on the definition. For example, the Dirac current transforms as a vector in BOTH definitions. In one definition it is due to the transformation of spinors, while in another definition it is due to the transformation of the gamma matrix. For more details see also Sec. 8.3.3.3 of http://arxiv.org/abs/1205.1992 


#7
Nov2312, 05:57 AM

Sci Advisor
Thanks
P: 4,160




#8
Nov2312, 06:09 AM

Sci Advisor
P: 4,569

Thus, to understand spinors in curved spacetime, to a certain extent one must first UNLEARN what one learned about spinors in flat spacetime. Otherwise, one may remain confused. 


#9
Nov2412, 12:23 AM

Sci Advisor
P: 888

Let [itex]G[/itex] be a group of coordinate transformation. Assume that for all [itex]g \in G[/itex], we have the following action on the spin group [itex]SL(2,\mathbb{C})[/itex]:
[tex] G( \psi_{\alpha}(x)) \equiv \bar{\psi}_{\alpha}(\bar{x}) = S_{\alpha}{}^{\beta}(g) \psi_{\beta}(x) [/tex] If we take [itex]g = \Lambda \in SO(1,3)[/itex], then the relation [itex]SO(1,3) \simeq SL(2, \mathbb{C})[/itex] means that [itex]S(\Lambda)[/itex] forms an irreducible nonunitary matrix representation of [itex]SL(2, \mathbb{C})[/itex]. Now if we take [itex]g = \frac{\partial \bar{x}}{\partial x} \in GL(4, \mathbb{R})[/itex], then a theorem by Cartan with very long proof shows that [itex]S^{\beta}{}_{\alpha}(g) = \delta^{\beta}_{\alpha}[/itex]. This means that the group of general coordinate transformations (the manifold mapping group), which is bigger that [itex]GL(4, \mathbb{R})[/itex], acts trivially on spinors. Sam 


#10
Nov2412, 03:58 AM

Sci Advisor
P: 888

So in that sense I don't really see the confusion; it's a matter of distinguishing between flat and curved indices. 


#11
Nov2412, 03:59 AM

Sci Advisor
P: 888

[tex] e_{\mu}{}^A = \delta_{\mu}{}^A [/tex] and thus don't need to distinguish between flat and curved indices anymore. This amounts to saying that the tangents space at a point in Minkowski space can be chosen as Minkowski space itself. 


#12
Nov2412, 04:02 AM

Sci Advisor
P: 888

[tex] \gamma^{\mu} \equiv e^{\mu}{}_A \gamma^A [/tex] in order to make the Dirac equation gctinvariant. Second, you need to define covariant derivatives on spinors. As spinors only exist in the tangent space, you need to define a covariant derivative D there, which is accomplished with the spin connection. So you should also replace the partial derivative on psi with the covariant one. Because spinors don't have curved indices, they transform as scalars under gct's. This shows that in dealing with spinors in curved spacetime one really needs the vielbein instead of the metric. 


#13
Nov2412, 09:39 AM

Sci Advisor
P: 4,569




#14
Nov2412, 10:20 AM

Sci Advisor
HW Helper
P: 11,894

I've never seen a treatment of spinors in flat spacetime (especially for the purpose of QFT) using the methods of modern differential geometry (the theory of fiber bundles). Even Wald brings in the 'heavy artillery' only for curved spacetime, but at the price of keeping spinors as classical objects, which is, of course, nonsensical.



#15
Nov2612, 02:47 AM

Sci Advisor
P: 4,569

Dextercioby, does it mean that you would agree with my proposal in #13?



#16
Nov2612, 01:16 PM

Sci Advisor
HW Helper
P: 11,894

Yes, the formalism of tetrads is useful for general spinor theory, but unfortunately unnecessary for the purpose of QFT which uses only the algebraic + functional analytic properties of spinor fields (operatorvalued distributions) and spinor fields are derived from the representation theory of the restricted Poincaré group.
Anyways, there are no spinor(ial field)s in a classical theory, either SR or GR, so the geometric theory of spinors is necessary in SUGRA or other theories of quantum gravity, for example. 


#17
Nov2712, 03:38 AM

Sci Advisor
P: 4,569

Such a view of spinors may also be used to demystify the "fact" that spinors change sign when rotated by 2 pi. With such an approach spinors are spacetime scalars, so they actually don't change at all when rotated by any angle. They have nontrivial transformation properties (including the change of sign for a rotation by 2 pi) only with respect to the ABSTRACT INTERNAL "space" carrying a complex 2dimensional representation of the group SO(1,3).



Register to reply 
Related Discussions  
Demonstration of Dirac equation covariance  Quantum Physics  2  
Confusion about How Dirac discovered Dirac's Equation  High Energy, Nuclear, Particle Physics  2  
Question about lorentzcovariance of Dirac equation  Quantum Physics  2  
Dirac equation and general covariance  Quantum Physics  17 