Subject: The timelessordering
Date: Thu, 09 Dec 2004 04:55:33 +0200 From: Dimi Chakalov <dimi@chakalov.net> To: D Oriti <d.oriti@damtp.cam.ac.uk> CC: elivine@perimeterinstitute.ca, fotini@perimeterinstitute.ca, c.isham@imperial.ac.uk, sorkin@physics.syr.edu Dear Dr. Oriti, May I comment on your recent paper [Ref. 1]. Your feedback will be highly appreciated. I'm afraid the key phrase "depend on an external time parameter as is the case in ordinary quantum mechanics" is utterly unclear, since we don't know how to embed the events produced by QM measurements in Minkowski's cone, http://Goddoesnotplaydice.net/points.html#cloud Thus, your next key statement, "(T)he very concept of
time is not
I find your proper time formalism and proper time variable very interesting, and invite you to look at my speculations on 'clearly identified notion of causality' in backgroundindependent theories, http://Goddoesnotplaydice.net/York.html#emergence Kindest regards, Dimi Chakalov
[Ref. 1] D. Oriti, The Feynman propagator
for quantum gravity: spin foams, proper time, orientation, causality and
timelessordering,
"In quantum gravity, because of background independence
and
"(T)he very concept of time is not fundamantal at all
and should instead emerge only in a semiclassical approximation [3]."
"(O)ne can argue that the notion of causality is actually
more primitive than that of time, more fundamental, being already present
in the notion of ordering between events, of a fundamental directionality
in spacetime, and that as such can be present, as a *seed* of what will
then be, in a semiclassical approximation, (...). We agree with this latter
point of view.
"The answer [1, 2] is that all current spin foam models
do not depend, in their amplitudes, on the orientation of the underlying
2complex, i.e. they do not depend on the orientation data we identified
above. The way this is achieved is quite simple in all models: in the expression
for the amplitudes for spin foams the terms that can be understood as contributions
from opposite orientations are summed simmetrically thus erasing the dependence
on the orientation itself.
"This orientation independence leads to interpreting the
current spin
"The proper time formalism we have developed could find
interesting
"....... in absence of an external time coordinate, but
with a clearly
Note: See also D. Oriti's Ph. D. Thesis "Spin Foam Models of Quantum Spacetime", grqc/0311066, particularly Sec. 1.2.1, "Background independence and relationality", and the problem of time in canonical quantum gravity (the freezing of the donkian Hamiltonian), There is also a very nice quote from Groucho Marx in D. Oriti's grqc/0311066: "Those are my principles, and if you don't like them... well, I have others." Keeping this piece of wisdom in your mind, see my email to Erich Schoedl below. D. Chakalov Richard Feynman, Character Of Physical Law, p. 8 ============ Subject: The
shape of space
In your online article "What Exactly Is Curved?", 02/09/2003, http://www.physicspost.com/articles.php?articleId=101 you wrote: "A tensor is typically a linear function designed to take a certain combination of input data, and convert this data to a specific set of output data. The perfect example of a tensor is the Riemann curvature tensor. This tensor takes three inputs, the 4velocity of test particle along a certain geodesic, a small separation to a neighboring geodesic, and the 4velocity of the same test particle along this neighboring geodesic." I wonder if you know Graham Nerlich's "The shape of space", http://Goddoesnotplaydice.net/Nerlich.html#3 "(W)ithout the affine structure there is nothing to determine how the [free] particle trajectory should lie. It has no antennae to tell it where other objects are, even if there were other objects (...). It *is because spacetime has a certain shape that world lines lie as they do*." I've been trying to link these "antennae" to the Christoffel symbols, which are 3rd rank tensors, but nothing came out from this exercise. Why Christoffel symbols? Prof. G. 't Hooft was very kind to explain the issue (private communication, 20 January 2002): "There should not be such a thing as a 'gravitational field stressenergy' at the r.h.s. of Einstein's equations. In a comoving frame there is no gravitational field, hence no grav. stressenergy! In any other frame where you do have effective grav. fields, you automatically get the correct grav. contribution to the stressenergy from the Christoffel symbols in the covariant derivatives." I believe geodesics are made in a way similar to the collective movement of a shoal of fish swinging along a coral reef, http://Goddoesnotplaydice.net/LHC.html#Holon It seems to me that there is something similar in GR, in the way a test particle would "anticipate" its neighbors from the "shoal of fish", and would pinpoint its correct tangent vector "online", onepointatatime, thus making a geodesic. See also "Beyond space and time", by Robert Matthews, New Scientist, 17 May 1997, http://Goddoesnotplaydice.net/Kiefer.html#note Perhaps you and your colleagues can help. Kindest regards, Dimi Chakalov
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