Subject: Brief comments on C. Rovelli's writings
Date: Tue, 21 Aug 2007 19:30:55 +0300
From: Dimi Chakalov <>
To: Chris Isham <>
Cc: <>,

Dear Chris,

I posted some brief comments on the ideas of your former student at

If you or someone else happen to endorse those "partial observables", please drop me a line.

Best regards,



Subject: Scrupulous intellectual honesty: The nature of time
Date: Fri, 22 Feb 2002 23:18:17 +0200
From: "Dimiter G. Chakalov" <>
To: Carlo Rovelli <>,
BCC: [snip]

Dear Dr. Rovelli,

In MATTERS OF GRAVITY, gr-qc/0109034, you've said the following:

Carlo Rovelli: "But success, I think, can only be granted by scrupulous intellectual honesty. (...) The hardest part to digest is not the relational nature of space; it is the relational nature of time."

I believe the puzzle of "relational nature" of time, as explained by John Baez five years ago,

is an extremely subtle issue, and we all should approach it with scrupulous intellectual honesty.

You define 'partial observables' in gr-qc/0110035 as "a physical quantity to which we can associate a (measuring) procedure leading to a number" [Ref. 1]. Philosophically, this is nothing but a 'shadow on Plato's cave', as explained with John's jackets parable,

Regrettably, you chose not to reply to my email notes sent in the past two years.

In your completely revised version of gr-qc/0111037 from 22 February 2002 [Ref. 2] and in its follow-up gr-qc/0202079 [Ref. 3], you declared that will start "from scratch".

Regrettably, you didn't acknowledge the Ithaca Interpretation of QM [Ref. 4] and the work by many researches advocating the 'relational reality' interpretation of QM [Ref. 5 and Ref. 6].

In my email from Wed, 07 Nov 2001 12:44:17 +0100, subject: gr-qc/0111016, I asked a simple question:

"Can you compute something related to QFT and gravity that other people can't?"

Regrettably, you chose again to keep silent.

Please bear in mind that the whole idea of 'relational reality' or 'correlations, not correlata' [Ref. 4] means one simple thing: the quantum reality does NOT exist in the physical time, as read with a clock,

Hence the nature of time boils down to the challenge of the *geometrical* presentation of the infinitesimal,

Do you believe that those bubbles in loop quantum gravity might do the job, provided we get "a much better understanding of the theory of (intersecting) knots in 3 dimensions"  ?

I will be happy to hear from you. I'm sure you will reply with scrupulous intellectual honesty.

You can read this email at

Sincerely yours,

Dimiter G. Chakalov
(last update 22.02.2002)


[Ref. 1] Carlo Rovelli. Partial observables. Sat, 6 Oct 2001 10:50:25 GMT,

"The distinction between partial and complete observables was discussed in [C. Rovelli, "Is there incompatibility between the ways time is treated in general relativity and in quantum mechanics?", in "Conceptual problems of quantum gravity", A. Ashtekar and J. Stachel, eds., Birkhauser, New York, 1991]. The distinction is often implicitly used, but we are not aware of any other detailed discussion on it in the literature.

"Partial observables and complete observables

"Let us start from the following two definitions.

"Partial observable: a physical quantity to which we can associate a (measuring) procedure leading to a number.

"Complete observable: a quantity whose value can be predicted by the theory (in classical theory); or whose probability distribution can be predicted by the theory (in quantum theory).

[Ref. 2] Carlo Rovelli. A note on the foundation of relativistic mechanics. I: Relativistic observables and relativistic states. Date (revised v2): Thu, 21 Feb 2002 16:11:49 GMT,
Comments: 7 pages, no figures. Completely revised version

Abstract: "Is there a version of the notions of "state" and "observable" wide enough to apply naturally and in a covariant manner to relativistic systems? I discuss here a tentative answer."

[Sec. III B] "The usual nonrelativistic definition of state refers to the properties of a system at a certain moment of time. Denote this conventional notion of state as the "instantaneous state". The space of the instantaneous states is the usual nonrelativistic phase space.

[Sec. IV] "The fundamental ingredient is once more the extended configuation space of the partial observables  C .  Quantum theory gives probabilities of observing a certain correlation given that a certain other correlation was (has? - D.C.) been observed.

[Sec. V] "Mechanics can be seen as the theory of the evolution of the physical variables in time only in the nonrelativistic limit. In a fully relativistic context, mechanics is a theory of correlations between partial observables, or the theory of the relative evolution of partial observables with respect to each other."

[Ref. 3] Carlo Rovelli. A note on the foundation of relativistic mechanics. II: Covariant hamiltonian general relativity. Thu, 21 Feb 2002 16:20:22 GMT,
Comments: 7 pages, no figures, 2nd part of gr-qc/0111037

"In particular: what are "states" and "observables" in a theory without background spacetime, without external time and without an asymptotic region?

"The very foundation of general covariant physics is the idea that the notion of a simultaneity surface all over the universe is devoid of physical meaning.

"A generally covariant notion of instantaneous state, or evolution of states and observables in time, make little physical sense. In a general gravitational field we cannot assume that there exists a suitable asymptotic region, and therefore we do not have a notion of scattering amplitude and S matrix. In this context, it is not clear what we can take as states and observables of the theory, and what is the meaning of dynamics. (...) I have argued that mechanics can be seen as the theory of the evolution in time only in the nonrelativistic limit. In general, mechanics is a theory of relative evolution of partial observables with respect to each other. More precisely, it is a theory of correlations between partial observables. Given a state, classical mechanics determines which correlations are observable and quantum mechanics gives the probability amplitude (or probability density) for each correlation.

"It simply means that the general relativistic spacetime coordinates are not directly related to observations. The theory does not describe the dependence of the field components on x^m, but only the relative dependence of the partial observables of  C  on each other."

[Ref. 4] N. David Mermin. The Ithaca Interpretation of Quantum Mechanics.

Sec. IV: "Central to it is the doctrine that *the only proper subject of physics are correlations among different parts of the physical world*. (...) There is no absolute state of being; there are only correlations between sybsystems."

[Ref. 5] Jeeva S. Anandan. Causality, Symmetries, and Quantum Mechanics.

[Ref. 6] Rodolfo Gambini, Rafael A. Porto. Relational Reality in Relativistic Quantum Mechanics.

Subject: Partial (non)observables
Date: Tue, 22 Jan 2002 13:34:10 +0200
From: "Dimiter G. Chakalov" <>
CC: John Stachel <>,
     Abhay Ashtekar <>,
     Jonathan Halliwell <>,
     Bill Unruh <>
BCC: [snip]

RE: Carlo Rovelli. Partial observables. Date (revised v3): Mon, 21 Jan 2002 13:22:53 GMT,

"But how can a correlation between two non-observable quantities be observable?"

Dear Professor Rovelli,

A correlation may be prepared in the putative global time mode, i.e., without an explicit time parameter [Ref. 1], but it is *not* observable,

See also


I will appreciate your comments and insights, as well as those of all recipients of this email.

Sincerely yours,

Dimiter G. Chakalov
(last update 22 January 2002)

[Ref. 1] J.J. Halliwell, J. Thorwart. Life in an Energy Eigenstate: Decoherent Histories Analysis of a Model Timeless Universe. Mon, 21 Jan 2002 18:02:12GMT,

"The most significant feature of this equation is that it contains absolutely no reference to time whatsoever. It is usually argued that "time", or more precisely, the physical systems that we use to measure time, are containedalready in the gravitational and matter fields [3,4,5]. While this is veryplausible it leaves us with the question as to how to extract interesting physical predictions from this wave function, given the absence of the time coordinate that plays such a central role in standard quantum theory."

Subject: gr-qc/0111016
Date: Wed, 07 Nov 2001 12:44:17 +0100
From: "Dimiter G. Chakalov" <>
BCC: [snip]

Dear Professor Rovelli,

May I ask two questions regarding your gr-qc/0111016 [Ref. 1], particularly the extended configuration space and the inevitable "retrodictions" from it, as advocated by Jim Hartle (gr-qc/9304006).

1. Can you compute something related to QFT and gravity that other people can't?

2. Specifically, can you resolve the problem of gravity and QED? The discrepancy is quite huge, 120 orders of magnitude. If you're on the right track, I believe you should be able to say something, from your perspective, and solve the puzzle of the energy density of the vacuum,


Dimiter G. Chakalov
(last update 4 November 2001)


[Ref. 1] Michael Reisenberger, Carlo Rovelli. Spacetime states and covariant quantum theory. Tue, 6 Nov 2001 18:16:53 GMT,

"We observe that the preferred role of time in quantum theory is the consequence of an idealization: that measurements are instantaneous. Canonical quantum theory can be given a covariant form by dropping this idealization. States prepared by non-instantaneous measurements are described by "spacetime smeared states".  The theory can be formulated in terms of these states,without making any reference to a special time variable.

"Our aim is to support the view that general covariant quantum mechanics is consistent.  That is, we can consistently use quantum theory without abandoning the covariant treatment of spacetime which is characteristic of general relativity.  More precisely, we can define a quantum theorycorresponding to a covariant system without necessarily having to single out a preferred time variable.  In our opinion, the often alleged profound inconsistency between the way time is treated in general relativity and in quantum mechanics is not real. It is not more real than the inconsistency between general relativity and *classical* hamiltonian mechanics: we expectthat quantum mechanics admits a fully general covariant formulation, just as classical hamiltonian mechanics does.

"This formulation clarifies some issues in the formulation of the quantum theory of a single relativistic particle, and in relation to the time of arrival problem, and it helps us to give a consistent interpretation to quantum cosmological models.

"We shall call the normalizable state  |f>  labeled by the spacetime function f(X,T)  a "spacetime-smeared quantum state", or simply a  "spacetime state". It is important to notice that this denomination refers simply to the representation that the state is given. It is a perfectly ordinary normalizable quantum state in the ordinary Hilbert state of the theory.

"It is clear that the spacetime-smeared representation is highly redundant -- many different spacetime functions  $f$  give rise to the same state -- while the instantaneous representation is unique. This situation is a bit reminiscent of that found in electromagnetism, where the gauge freedom can be essentially eliminated in the Coulomb gauge, but at the cost of making the formalism non-covariant.

"We need an interaction hamiltonian  H_int , representing the interaction that gives rise to the measurement.  H_int  must have the following properties. First, it must cause the transition  |0> --> |1> . Second, the particle should interact only at or around the spacetime position  X=0, T=0 . Thus the interaction hamiltonian must be time dependent, and vanish for late and early times. We have to concentrate the interaction around  T=0 . However, we cannot have a perfectly instantaneous interaction because this would require infinite force. We must therefore assume that the interaction is non vanishing for a finite period of time. Putting these requirements together, and requiring also that the hamiltonian is self-adjoint, we arrive at an interaction hamiltonian of the form (...).

"Thus we can conclude that if we prepare a state concentrated around  X=0, T=0  by means of a physical measurement procedure like the one described, we necessarily obtain a spacetime-smeared state defined by a function  f(X,T) with support in a finite region  R  around  X=0, T=0 . The size of  R  is determined by the accuracy of the measuring apparatus in resolving distances*and time intervals*.


"A (well known) crucial observation is that most interesting physical systems, and in particular all gravitational systems, such as full general relativity with or without matter, cosmological models ...  are not given in terms of a hamiltonian: they are given directly in the covariant  formulation. Therefore not only the covariant formulation of mechanics appears to be more generalthan the hamiltonian one, but such a wider generality is required for the theories that better describe our world.

 "One can try to "deparametrize" these theories by picking one of the configuration space space variables and identifying it as the time variable. It is sometimes claimed that such a deparametrization is necessary in order to understand the quantum properties of the these systems. But such a deparametrization adds an element of arbitrariness which is certainly not part of the classical dynamics. Since the classical dynamics of these systemsdoes not select any preferred independent "time" variable  T , we think that their quantum mechanics should not select a preferred time variable either.

"To understand their quantum dynamics, we must therefore have a formulation of quantum theory in which time plays no special  role. This is the motivation for the definition of covariant quantum theory that we give in this section.


"This completes the definition of general covariant quantum theory.

"Finally, consider the case in which  R^prime  is in the past of  R . Strictly speaking, this case refers to a situation which has no meaning in conventional quantum mechanics: it refers to a situation in which the measurement is made at an earlier time than the preparation. Therefore the general theory given in Section {general} gives more predictions than the ones usually considered in conventional quantum theory. The additional predictions can be more accurately denoted "retrodictions", since they are statements about a time which is in the past with respect to the time at we assume to have information about the state.

"Jim Hartle has long argued that such retrodictions can be added to standard predictions of quantum theory, and in fact, that they have to be added, if we want to make sense of any statement about the past deriving from our knowledge of the present. Either we give up the possibility of making *any* statementabout the past, or we take retrodictions as these statements. We refer to Hartle's paper \cite{}{Hartle}  for a detailed discussion.


"Here we are more interested in the interpretation of the theory once the propagator is given, than in the actual construction of the propagator. Let us nevertheless say something on the derivation of  W(x,y)  itself. There is a number of ways of constructing this object starting from the classical theory. For instance,  W(x,y)  may be defined as a sum over classical histories \cite{}{Hartle} .

"The technical ingredient to be added to the quantum formalism is the notion of spacetime-smeared quantum state. This is a state generated by a measurements that is not instantaneous. In particular, localization measurements can be naturally described in terms of states associated to spacetime regions, or, more in general, regions in the extended configuration space. The key element of the theory, from this point of view, is the propagator  W(x,y) . This quantity is a two point function on configuration space, it is strictly related (but not identical!) to the "probabilityamplitude for the system to be detected in  x  if it was detected in  y ".

"The remaining conceptual difficulty regards the possibility of associating probabilities to sequences of measurements. We see two possible solutions to this difficulty. The first by reducing any such sequence to a single measurement or, equivalently, to sets of commuting measurements, by including the apparatus in the theory. The second by introducing the notion of time ordering of the observer."

Note: Carlo Rovelli posted today a very interesting dialog between a grad student, Sal, and a professor in theoretical physics, Simp. My comments labeled with D.C. are inserted with red.

Carlo Rovelli, A dialog on quantum gravity, hep-th/0310077 v2, October 14, 2003


Simp - Is loop gravity unitary?
Sal - No, as far as I understand.
Simp - This is devastating.
Sal - Why?
Simp - Because unitarity is needed for consistency.
Sal - Why?
Simp - Because without unitarity probability is not conserved.
Sal - Conserved in what?
Simp - In time.
Sal - Which time?
Simp - What do you mean "which time?". Time.
Sal - There isn’t a unique notion of time in GR.
Simp - There is no coordinate t?
Sal - There is, but any observable is invariant under change of t, therefore everything is
constant in this t just by gauge invariance.
D.C. - Hi guys, aren't you aware that t is not an observable? See John Baez in my White Paper and ponder on the question of the intrinsic "time" of the gauge invariance recipes.

Simp - And if there is no background time, there cannot be unitary evolution, right?
Sal - Yes.
D.C. If you can think of the background time only, then I'd agree. But you've been tacitly using a different kind of time. See above.
Simp - I am not sure I can digest a theory where there is no space and no time to start with, and without unitarity...
Sal - I suppose this is why there is so much resistance to loop gravity ... Again, everybody searches background independence, but when you see it, it is sort of scary... Anyway, we can all believe what we like, until experiments will prove somebody right and somebody wrong, and for the moment no experiment is talking to us ... Future will tell ... But my point is that the absence of unitarity does not imply that the theory is inconsistent. Only that the notion of time is intertwined with dynamics. It is similar to the fact that there is no conserved energy in a closed universe ...

Simp - Alright, I accept this. But we have been digressing... can we try to wrap up?

Sal - Alright. I suppose your conclusion is that loop gravity is (a) too different from usual QFT, (b) not completed and (c) not yet able to recover low energy physics...

D.C. - Did you say "not yet"? The hardest thing of all is to find a black cat in a dark room, especially if there is no cat, says Confucius.

Simp - And your conclusion is that string theory (a) does not describe the real world in which we live, (b) is not predictive because it can agree with any experimental outcome, (c) it requires an immense baggage of new phenomenology like supersymmetry, and extradimensions, which we do not see, and (d) it has not lead to a true conceptual merge of QM with the GR’s notions of space time...

D.C. - I think loop gravity and string theory are just Barbies. My daughter would say "I want this Barbie, and that's it."

By the way, Sal is still looking for a job...

D.C. You mean job, not a Barbie? Finding a real job may be difficult, there are far too many Barbies around...

Many thanks to Ted Newman, Gary Horowitz, Abhay Ashtekar, Daniele Oriti, Lee Smolin, Warren Siegel, Simone Speziale and Juan Maldacena for corrections and suggestions.

D.C. Hey, you missed me! Will check out your third, utterly revised version of hep-th/0310077.  A great mathematical physicist and visionary, Carlo Rovelli, said the following: "But success, I think, can only be granted by scrupulous intellectual honesty." Read all about it above.

Dimi Chakalov
Institute for Applied Metaphysics (IAM)
October 14, 2003