- #1
jdrudmin
- 3
- 0
Differential geometry (which includes general relativity) often introduces the length differential, expressed as ds2=gabdxadxb, to introduce the covariant form of the metric tensor gab. However, this formulation scales ds2 incorrectly. The appearance of an index as a superscript, as in dxa, indicates that under a dilation, dxa scales as a contravariant tensor. That is, if we double the length of our measuring rod, then the value of dxa falls in half. For a covariant tensor, such as a gradient, the index appears as a subscript, as in dxa. If we double the length of our measuring rod, then the value of the of dxa doubles. Rotations and other lorentz transformations do NOT affect covariant and contravarient vectors differently because the projection of one unit vector on another is reflexive. The common representation of a lorentz transformation as [tex]\gamma[/tex]ab displays invariance of a lorentz transformation under dilation. If ds2=gabdxadxb, then ds2 is invarient under dilation. Then ds does NOT describe the length of a differential displacement, which should scale contravariantly, like dxa, because in another coordinate system, ds=dx'a. Writing ds2=dxadxa for Cartesian coordinates gives ds proper scaling for a differential displacement. But, to describe a Minkowski space, the sign on dr2 must differ from that on dt2. So one of them must have an explicit factor of i. This sign appears in the metric if we adopt David Hestenes' formulation gab=[tex]\gamma[/tex]a[tex]\gamma[/tex]b in terms of the basis vectors [tex]\gamma[/tex]a. This formulation leaves the metric in a familiar form. If the negative sign attaches to dt2, then [tex]\gamma[/tex]a[tex]\gamma[/tex]a=3-1=2 instead of 4. Then if [tex]\gamma[/tex]a[tex]\gamma[/tex]b=g(ab)+g[ab] operates on [tex]\gamma[/tex]b, then 2[tex]\gamma[/tex]a=[tex]\gamma[/tex]a+g[ab][tex]\gamma[/tex]b, or [tex]\gamma[/tex]a=g[ab][tex]\gamma[/tex]b, so that the antisymmetric part g[ab] transforms tensors without dilation beyond that in the basis vectors, like the symmetric part g(ab). This symmetry might say something about why we observe variation in only one time dimension rather than the expected three. If [tex]\gamma[/tex]a[tex]\gamma[/tex]a=1-3=-2, then -[tex]\gamma[/tex]a=g(ab)[tex]\gamma[/tex]b and -[tex]\gamma[/tex]a=g[ab][tex]\gamma[/tex]b, to preserve the absence of further dilation.