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ELFT HÍRADÓ

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**From**: "Szabados,L." <lbszab AT rmki.kfki.hu>**To**: fizinfo AT lists.kfki.hu**Cc**: rmkiusers AT lists.kfki.hu**Subject**: [Fizinfo] szeminariumok**Date**: Thu, 6 Apr 2006 14:46:24 +0200 (CEST)**List-archive**: <http://sunserv.kfki.hu/pipermail/fizinfo>**List-id**: ELFT HÍRADÓ <fizinfo.lists.kfki.hu>

RELATIVITÁSELMÉLETI SZEMINÁRIUM

Elõadó: Prof James M Nester

National Central University, Chungli, Taiwan

Az elõadás címe: On the zeros of spinor fields and

orthonormal frames

t: 2006 április 13 (csütörtök), 14.00

(x,y,z): KFKI RMKI III. épület, tanácsterem

Kivonat:

Some time ago (as a key ingredient in a positive energy

proof [1]) we proposed certain rotational gauge conditions for an

orthonormal frame on a Riemannian manifold [2]. However, except for

nearly flat spaces, it was not clear if the conditions could be

satisfied. Then Dimakis and Muller-Hoissen [3] showed how to

construct such frames from solutions of the Dirac spinor equation

(which generally exist)---as long as the spinor field had no zero

points. A later "proof" that zeros cannot happen [4] is not

convincing. Based on our recent examination of specific examples

described here we know that zeros can happen, but we argue that they

are not generic. Consequently the "special orthonormal frame" gauge

conditions can be used essentially without reservations.

References:

[1] J.M. Nester, Int. J. Mod. Phys. A 4 (1989) 1755

Phys. Lett A 139 (1989) 112,

Class. Quant. Grav. 8 (1991) L19.

[2] J.M. Nester, J. Math. Phys. 30 (1989) 624.

[3] A. Dimakis and F. Muller-Hoissen, Phys. Lett. A 142 (1989) 73.

[4] V. Pelykh, J. Math Phys. 41 (2000) 5550.

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RELATIVITÁSELMÉLETI SZEMINÁRIUM

Elõadó: Prof James M Nester

National Central University, Chungli, Taiwan

Az elõadás címe: On gravitational energy in a small region

t: 2006 április 18 (kedd), 14.00

(x,y,z): KFKI RMKI III. epulet, tanacsterem

Kivonat:

The (quasi-)localization of energy for gravitating systems

remains an outstanding issue. One of the desiderata (related to

positive energy) is that the small region vacuum limit should be

proportional the Bel-Robinson tensor with a positive coefficient.

Here we report on our investigation of this limit for certain

energy-momentum expressions. Specifically we find that none of the

traditional pseudotensors satisfy this critera, however there are

certain linear combinations of them which do. Moreover we can

construct a large number of new pseudotensors which do have this

desired Bel-Robinson vacuum limit. In contrast to these rather

artificial constructs Moller's tetrad energy-momentum "tensor"

naturally has this property. This is also true for one of our four

proposed covariant Hamiltonian boundary term quasi-local

energy-momentum expressions. Thus the small region vacuum limit is

a fairly strong selection condition.

**[Fizinfo] szeminariumok**,*Szabados,L., 04/06/2006*

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