Τετάρτη, 8 Αυγούστου 2007

Διάφορα links περί Σχετικότητας, Βαρύτητας και Gödel

Σκέφτηκα να βάλω κάποια πράγματα που διαβάζω αυτές τις μέρες. Ίσως κάποιος τα βρει ενδιαφέροντα.

Για αρχή, ένα ενδιαφέρων paper από τον φίλο Θωμά (όχι τον Ακινάτη), που είναι και πιο κοντά στα χωράφια μου

Theory of gravitation theories: a no-progress report
by Thomas P Sotiriou, Valerio Faraoni, Stefano Liberati
Already in the 1970s there where attempts to present a set of ground rules, sometimes referred to as a theory of gravitation theories, which theories of gravity should satisfy in order to be considered viable in principle and, therefore, interesting enough to deserve further investigation. From this perspective, an alternative title of the present paper could be ``why are we still unable to write a guide on how to propose viable alternatives to general relativity?''. Attempting to answer this question, it is argued here that earlier efforts to turn qualitative statements, such as the Einstein Equivalence Principle, into quantitative ones, such as the metric postulates, stand on rather shaky grounds -- probably contrary to popular belief -- as they appear to depend strongly on particular representations of the theory. This includes ambiguities in the identification of matter and gravitational fields, dependence of frequently used definitions, such as those of the stress-energy tensor or classical vacuum, on the choice of variables, etc. Various examples are discussed and possible approaches to this problem are pointed out. In the course of this study, several common misconceptions related to the various forms of the Equivalence Principle, the use of conformal frames and equivalence between theories are clarified.
arXiv:0707.2748v1 [gr-qc]

Σήμερα είδα και αυτό το paper στο arxiv, το οποίο με μια πρώτη ματιά φαίνεται να είναι καλό

Relativity without tears
by Z. K. Silagadze
Abstract: Special relativity is no more a new revolutionary theory but firmly established cornerstone of modern physics. The teaching of special relativity, however, still follows its presentation as it was unfolded historically, trying to convince subjects of this teaching that Newtonian physics is natural but incorrect and special relativity is its paradoxical but correct amendment. I argue in this article in favor of logical instead of historical trend in teaching of relativity and that special relativity is neither paradoxical nor correct (in the absolute sense of nineteen century) but the most natural expected description of real space-time around us valid for all practical purposes. This last circumstance constitutes a profound mystery of modern physics better known as the cosmological constant
problem.
arXiv:0708.0929v1 [physics.ed-ph]


Με αφορμή τη συζήτηση στο blog του κ. Σαμαρτζή, στο θέμα της κριτικής του βιβλίου του Dawkins (καλή συζήτηση γίνεται και εδώ), και την επιμέρους συζήτηση για τον Gödel, είπα να βάλω και ένα άρθρο από το arxiv που συζητά την επαφή (και την επίδραση) που είχε ο Gödel με (στην) Φυσική

Goedel and Physics
by John D. Barrow
We introduce some early considerations of physical and mathematical impossibility as preludes to the Goedel incompleteness theorems. We consider some informal aspects of these theorems and their underlying assumptions and discuss some the responses to these theorems by those seeking to draw conclusions from them about the completability of theories of physics. We argue that there is no reason to expect Goedel incompleteness to handicap the search for a description of the laws of Nature, but we do expect it to limit what we can predict about the outcomes of those laws, and examples are given. We discuss the Goedel universe and the role it played in exposing the full spectrum of possibilities that a global understanding of space-time would reveal. Finally,we show how recent studies of supertasks have shown how global space-time structure determines the ultimate capability of computational devices within them.
arXiv:physics/0612253v2 [physics.hist-ph]

Αξίζει όποιος ενδιαφέρεται, να ψάξει και τις αναφορές του άρθρου.

Τέλος από το site του ESO μία πάρα πολύ όμορφη φωτογραφία που απεικονίζει τον Γαλαξία μας.


Image of the night sky above Paranal on 21 July 2007, taken by ESO astronomer Yuri Beletsky. A wide band of stars and dust clouds, spanning more than 100 degrees on the sky, is seen. This is the Milky Way, the Galaxy we belong to. At the centre of the image, two bright objects are visible. The brightest is the planet Jupiter, while the other is the star Antares. Three of the four 8.2-m telescopes forming ESO's VLT are seen, with a laser beaming out from Yepun, Unit Telescope number 4. The laser points directly at the Galactic Centre. Also visible are three of the 1.8-m Auxiliary Telescopes used for interferometry. They show small light beams which are diodes located on the domes. The exposure time is 5 minutes and because the tracking was made on the stars, the telescopes are slightly blurred.


"The images are not composite", emphasises Yuri Beletsky. "The camera was being tracked on the stars, which can be easily noticed if you look at the telescope domes on the image (they look a little fuzzy). The colour of the laser beam on the first image actually looks pretty close to what one can see on the sky with the unaided eye."...

At the time the pictures were taken, astronomers were indeed using the SINFONI instrument (see ESO 21/04) to study the Galactic Centre, having a close look at the supermassive black hole that lurks in there.


Αυτά...