What is the significance of the Sahlmann algebra in loop quantum gravity?

[marcus]

diffeo covariance of the representation/page 18

selfAdjoint,
there is a nice easy parsing job to be done on page 18---just interpreting how a diffeomorphism φ:Σ --> Σ
acts on cylinder functions! and on the vectorfields X.

Remember that cylinder functions C:A --> Complexneumbers
(I'm making an exception and writing Salmann's notation C instead of the uppercase Psi that O-L use)
So cylinder finctions are not even defined on the basic 3D manifold! How (naive question) are they to be acted on by a diffeomorphism? You could work this out just by following the customs, without being told, but O-L says how anyway.

C is constructed from an edge set {e1,...,eN} and an N-fold group-eater c. So we just let the diffeo act on the edges!
The new edge-set is {φ(e1),...,φ(eN)}
the new cylinderfunction φC is what you get using that, and the same N-fold group-eater c as before.

O-L writes it out:
(φC)[A] = c[A(φ(e1)),...,A(φ(eN))]

What is left to do? The Sahlmann algebra is made of two things, cylinder functions and X_S,ƒ operators each of which is constructed using a 2D surface S and a testfunction ƒ:S --> G'

The X does some differential fiddling with an edge where it punctures the surface. So if we are going to move the edge set (using the diffeomorphism) we have to move the whole kit and kaboodle along with it----the surface S --> φS

HERE I BELIEVE THEY HAVE A TYPO see what you think. You see what they have on page 18 defining the new testfunction ƒ-tilde

What they should have, I think (correct me if I'm mistaken) is
ƒ-tilde: φS --> G' defined by ƒ(φ^-1).

It is just a composition of the two functions---you are on φS and you first go by φ^-1 which takes you to S and then you go by ƒ and get to G'.

Anyway instead of writing S-tilde and ƒ-tilde the way they do I will just write (unless you find an error) φS and ƒ(φ^-1)

Then φ applied to E(S,ƒ) = E(φS, ƒ(φ^-1)

Now to finish saying how φ acts on the Sahlmann we have to say what φX is for any of those derivations X. That means take a cylinder function C and say what the new derivation φX does to it. Well move C over to φC and work on it:

(φX) C =

(φX) C = X_{φS, ƒ(φ^-1)} φC

I had better post this before a computer glitch loses it.

 

[marcus]

Equation 4.9 on page 18 of O-L

Now we know how a diffeo φ acts on a cylinder function C
to give a new cylinder function φC

The cylinder functions act on themselves by multiplication. For any C we have
Mult_C Cyl^oo --> Cyl^oo , which just sends C' into CC'.

What equation 4.9, the first part, says is that
φ Mult_C φ^-1 = Mult_φC

Let's check that by applying both sides to a cylinder function C'.
Imagine that the diffeo φ maps you from Ohio to Kentucky and that C' is defined using a graph somewhere in Kentucky. The left hands side is

φ (Mult_C (φ^-1 C'))

which means "take C' back to Ohio and multiply it by C and ship it out to Kentucky again"

The right hand side is Mult_φC C',
which means "ship C out to Kentucky and multiply C' by it on site".
So both have the same effect. That is how diffeomorphism covariance is supposed to be.

The other part of equation 4.9 is about derivations X
and says if you take a cylinder function C and bring it back to Ohio and do the (S,ƒ) derivation to it and then return it to Kentucky this has the same effect as taking the whole operation over to Kentucky and doing the derivation on site using (φS, ƒ(φ^-1)

So recall that the Sahlmann algebra SAHL is an algebra of linear operators Cyl^oo --> Cyl^oo that is generated by the Mults and the X's. What we now have is a DIFFEOMORPHISM ACTION on SAHL which we have looked at working on the generators in a bit of detail.

φ started out as Σ --> Σ and we jacked it up to a linear map φ: Cyl^oo --> Cyl^oo

And now we can conjugate elements a of SAHL by it

φ a φ^-1 is going to be a new element of SAHL which means it is a new linear map on Cyl^oo which we have looked at in the cases of the two types of generators.

Now. what does it mean for a REPRESENTATION of the Sahlmann algebra to be covariant?

It is going to turn out that the representation based on the Ashtekar-Lewandowski measure is covariant and also conversely that any representation of SAHL if it meets certain criteria including diffeo-covariance will be equivalent to one based on that measure. So we should get an idea of what a covariant rep of the algebra is.

 

[selfAdjoint]

Originally posted by marcus
selfAdjoint,
there is a nice easy parsing job to be done on page 18---just interpreting how a diffeomorphism φ:Σ --> Σ
acts on cylinder functions! and on the vectorfields X.

...


O-L writes it out:
(φC)[A] = c[A(φ(e1)),...,A(φ(eN))]

What is left to do? The Sahlmann algebra is made of two things, cylinder functions and X_S,ƒ operators each of which is constructed using a 2D surface S and a testfunction ƒ:S --> G'

The X does some differential fiddling with an edge where it punctures the surface. So if we are going to move the edge set (using the diffeomorphism) we have to move the whole kit and kaboodle along with it----the surface S --> φS

HERE I BELIEVE THEY HAVE A TYPO see what you think. You see what they have on page 18 defining the new testfunction ƒ-tilde

What they should have, I think (correct me if I'm mistaken) is
ƒ-tilde: φS --> G' defined by ƒ(φ^-1).

It is just a composition of the two functions---you are on φS and you first go by φ^-1 which takes you to S and then you go by ƒ and get to G'.

Anyway instead of writing S-tilde and ƒ-tilde the way they do I will just write (unless you find an error) φS and ƒ(φ^-1)

Then φ applied to E(S,ƒ) = E(φS, ƒ(φ^-1)

Now to finish saying how φ acts on the Sahlmann we have to say what φX is for any of those derivations X. That means take a cylinder function C and say what the new derivation φX does to it. Well move C over to φC and work on it:

(φX) C =

(φX) C = X_{φS, ƒ(φ^-1)} φC

I had better post this before a computer glitch loses it.

I went around and around on this, and finally came down on your side. Your explanation is so clear, natural, and simple, and I couldn't make any dual space pullback work (that's what their notation would suggest). I am still troubled because it isn't what you would expect from a typo, and I can only suppose that in an early draft of this paper they were working from a dual perspective, a la Sahlmann, and then decided to convert to direct vector spaces, and this was overlooked in the conversion.

 

[marcus]

Originally posted by selfAdjoint
I went around and around on this, and finally came down on your side. Your explanation is so clear, natural, and simple, and I couldn't make any dual space pullback work (that's what their notation would suggest). I am still troubled because it isn't what you would expect from a typo, and I can only suppose that in an early draft of this paper they were working from a dual perspective, a la Sahlmann, and then decided to convert to direct vector spaces, and this was overlooked in the conversion.

I took another look at (the third line of) equation (4.7) on page 18

it looks to me as if what was meant was ƒ o φ^-1, that is, ƒ( φ^-1)----the only thing that is defined on the right domain---but what got written down was

(φ^-1|_S)* ƒ

BTW I believe that might parse better if one simply added a tilde to the S----tilde S was their notation for φ(S) the image of the surface as mapped by the diffeomorphism.

then one would have

(φ^-1|_φ(S))*

and φ^-1 would be restricted to the new
surface φ(S). That seems now to make good sense and
I'm surprised I didnt think of it earlier.

It would have the same effect if one took a couple of the symbols in a different order, first restricting φ to the domain S and then taking inverse map


((φ|_S)^-1)*

Then one could appeal to a possible meaning of the much over-used * and say that with any diffeomorphism ζ the operation
ζ* applied to a function ƒ MEANS to take ƒ(ζ).

In fact that is just how they used the (much overused!) asterisk
five lines further down the page in a slightly different context where they say "...diffeomorphisms act naturally on connections considered as functions defined on edges by
(φ* A) (e) = (A o φ) (e) = A(φ(e))..."
or words to that effect
it is one or two lines down from equation (4.8)

I feel all right about that. If you feel confident of the interpretation then it is just an omitted tilde, or a couple of transposed symbols, and we don't have to make an issue with Okolow.

My esteem for O-L continues unabated, while my enthusiasm for rovelli's draft textbook is no longer quite up to what it was earlier

 

[selfAdjoint]

Let me push on with O-L. I have been totally distracted by home events the last few days, but I had printed off your comments and was studying the paper with their aid. I have to resume that.

After making the old college try on Rovelli, I have concluded that I can postpone it. If I need more depth on the Ashtekar variables I can go to Ashtekar himself, he discusses them in several papers. But I think these current papers are giving us a better school than a textbook right now.

 

[marcus]

Originally posted by selfAdjoint
...I think these current papers are giving us a better school than a textbook right now.

I agree!

 

[marcus]

Hello again S.A.,

just to say I'm still around and reading in that group
of recent preprints. Despite the fact that I much prefer
Lewandowski's style (or more exactly that of O-L)
They never get to the point of exponentiating the
X's.

Today I was looking in arxiv for a followup paper---they reference an "in progress" four-way collaboration O+L+S+T but that has not appeared, as far as I could tell.

In a couple of the Sahlmann-Thiemann papers they exponentiate
the X operators and get an analog of the Weyl algebra. If I get
too impatient for the next Lewandowski paper I will have
another look at one or the other of the S-T papers. I am wondering if you looked at either and got much of an impression.
I find Thiemann's writing style a bit hard to take for some reason.

Was also looking today at some papers by Daniele Oriti (cambr.) and Etera Livine (marseilles)---both postdoctorate fellows. don't know if they are known to you or not

 

[selfAdjoint]

I've spent half a day meditating on this sentence.

"..tilde_E is a vector density of weight 1 which takes values in the space G^*' dual to the Lie Algebra G'."

I know what all the words mean, but I can't wrap my head around the concept of a vector density which takes values in a dual L.A.

The coordinate representation, which we went over earlier is not much clearer.

tilde_E = tilde_E^a_iτ^*i(X)∂_a

Thus the space in which tilde_E is defined is spanned by the n x d tensor products τ^*i(X)∂_a.

The τ^*i, as he says span the dual L.A. and the ∂_a span the covectors in Σ.

But what image does this call up in your mind?

 

[marcus]

Originally posted by selfAdjoint
I've spent half a day meditating on this sentence.

"..tilde_E is a vector density of weight 1 which takes values in the space G^*' dual to the Lie Algebra G'."

I know what all the words mean, but I can't wrap my head around the concept of a vector density which takes values in a dual L.A.

The coordinate representation, which we went over earlier is not much clearer.

tilde_E = tilde_E^a_iτ^*i(X)∂_a

Thus the space in which tilde_E is defined is spanned by the n x d tensor products τ^*i(X)∂_a.

The τ^*i, as he says span the dual L.A. and the ∂_a span the covectors in Σ.

But what image does this call up in your mind?

This will be wordy and not, I fear, well directed to you as reader, selfAdjoint. I will probably say things you've already figured out or read elsewhere.

the "ashekar new variables" are a pair (A,E)
what does the E represent intuitively and geometrically?
in the literature the E is called "electric field" and also
"inverse densitized triad"
the E is approached from 2 separate directions

approach #1, assume a classical situation with a metric and say with a foliation into constant time 3D manifolds---or, it does not matter how, we somehow have a spatial (3D) manifold with the metric restricted to it.
the inverse triad is like the square root of the metric
here's a typical equation, g is the determinant of the metric:

g g^ab = tildE^a_itildE^b_i

a,b are spatial indices and i,j are "internal" referring to a basis chosen for the Lie algebra.

approach #2, E^a_i is introduced as a (distributional) functional derivative with respect to Aⁱ_a which must be integrated against test functions ("suitably smeared") in order to be well-defined

E^a_i(x) = - i hbar G δ/(δAⁱ_a(x))

"The functional derivative with respect to the connection 1-form A(x) is a vector density of weight one, or equivalently, a 2-form. Contracting the vector density E^a with the Levi-Civita density gives the dual of the triad, which is a 2-form
E = ε_abc E^adx^b dx^c.
Hence they may be identified. Since 2-forms are naturally integrated against 2-surfaces..."

Im quoting page 20 of a 1999 paper by Gaul/Rovelli but I've seen the same thing over and over. I will post this without completing the idea, which goes something like this:

the question is mental pictures surrounding the E's
the essence of the gravitational field can be seen in various things----the metric, and the E's are the squareroot of the metric---and in the "locally inertial frame" of which the triad is a kind of 3D proxy----and in the connection, with respect to which the E turns out to be a functional derivative.

and because of nature is such a tease we don't get to have the E's until they are smeared out as FLUXES thru sample 2D surfaces. We have to use 2D surfaces to catch the E-flux, which is a pain in the neck. But it is the only way we get to know them---by integrating them

a tensor density is just something that tranforms with an extra factor of the jacobian when you change coordinates (as you know well) and a vector density of "weight one" takes the jacobian to the power one---so it is really begging to be made into a 2-form

now some people just identify things that are the same information in a slapdash fashion but O-L dot their eyes and cross their tees, our job is to figure out why they have the values be in the Lie algebra dual!

I am ashamed to say so far my post is mere preamble!

When a basis is chosen, as it has been with the L.A., then a vectorspace and its dual look virtually the same to most people. People like Rovelli---practical types---identify them without even noticing that they are doing so. Others may say "this is not really the same but by *abuse of notation* we will use the same symbol...". And still others will painstakingly use distinct symbols.
I must stop typing and have a look

 

[marcus]

Gradually by observing the obvious it gets clearer.
Relativists are accustomed to having a metric, so they can raise and lower indices with complete innocence whenever they feel the urge.

But if you have a frame of vectors in V the functions which correspond to expressing something IN THAT BASIS live in V*
and the bigger the vees are the smaller, in some sense, are the vee-star functions that tell you the coefficients to use in expressing something in that basis.

So when they talk about the "inverse triad" it has to be in the dual of whatever the triad is in.

Also since I get to explain some stuff that a random passerby might wonder about (even tho you know it), one thing
the tensor product (X) does is give you a way to say "with values in".
If you have some real-valued linear functions W and you want to make them have values in the plane R² then you say
R² (X) W-----which is a new vectorspace consisting of the same linear functions except now they have values in the plane. So some of what we were writing before,
quoting from O-L, is for instance just a compact way of writing A explicitly as a 1-form with values in the Lie algebra.

A = Aⁱ_aτ_i(X)dx^a

Then there is the bundlebusiness, which turns out to be easier than it seemed at first. It only enters early on in some optional streamlining of definitions

You can always construct a manifold as a principal bundle with some group and then make various associated bundles where the fiber can be any homogeneous space of the group---anything the group acts vigorously enough on. This means that arranging for the LIE ALGEBRA to be the fiber is a natural. Because the group acts on it by adjoint!
there is an irrestistible mathematical urge to do this

once you have a manifold, think of a good structure group for it, and make the (P,G,M) the principal bundle----a frames bundle will do----and once you have that you can hardly refuse to call up the "associated" bundle which has G' as fiber

so as novices we may well walk in and be amazed to see all these G'--valued functions and G'* valued functions and no one will think to explain. Or, if you ask, they will just say oh that is just how it is, we have all these Lie algebra valued things. It must be connections---infinitesimal rotations as you take an infinitesimal step in some direction---oh yes! it must be that, or some other physical reason. But they don't explain that to a large extent it is the compelling naturalness of the construction

 

[marcus]

I've been looking at papers by the current postdocs, often they will be by two or more postdocs, or several plus a senior person.
People age so fast in mathematical physics research already
Ashtekar, Rovelli, even Baez?, Lewandowski, Thiemann are old
and they have to bring up a new generation.

Daniele Oriti (a guy, Italians spell Daniel that way)
Hanno Sahlmann
Martin Bojowald
Etera Richard Livine (dont know if he or she)
Robert Oeckl
Florian Conrady (new name, Heidelberg and Berlin)
Andrzej Okolow (only the one paper that i know of)
Sergei Alexandrov

Funny thing, when you see an online photo of someone like
Rovelli he looks really young. Baez too.

I do not think the list is complete or representative but I notice some things: like there seem to be a lot of people from
Cambridge, Potsdam (MPI and U. Berlin), Marseilles, Lyon, Warsaw. People from Madrid and Mexico City also come up, though as it happens I haven't listed them.

May be a thought here, maybe not. The fact that Rovelli seems to have moved back from Pittsburgh etc to Marseilles. Maybe the weather is nicer.

 

[marcus]

what mental image comes to mind?

In a previous post you asked what mental image comes to mind regarding, I think, the "electric field" or "inverse densitized triad" E.

It is a good kind of question to consider. As you say "how to get one's mind around" the Ashtekar new variables (A, E)

It is interesting that A gets integrated along curves and E gets integrated over 2D surfaces so that what one ends up dealing with are not (A,E) but (holonomies, fluxes).

Notations like h_e[A] for integrate the connection A along the edge e.

And like E_{S, ƒ}[E] for flux of E thru surface S sampled by function ƒ.

So in the end it is cylinder functions (holonomies) and derivations (made using fluxes)

This generates an urge to turn around and go back to the roots.
What is the physical meaning of A,E?
Connections are pretty conceptual and graphic---not much trouble visualizing a tangent vector veering as it is moved about.
But how to imagine E?
What kind of feel do you get about the manifold from knowing E, what kind of grip on it do you get.
the fact that E is essentially an inverse triad seems important.
something that will take a vector and write it out for you in a special basis or frame
and that it remembers the metric
well the events of the day are tugging, must go

 

[marcus]

more intuition about A, E

from Ashtekar's not-too-technical 2002 paper "Q. Geom. in Action"
http://arxiv.org/math-ph/0202008
page 5

"Let me now turn to specifics. It is perhaps simplest to begin with a Hamiltonian or symplectic description of general relativity. The phase space is the cotangent bundle. The configuration variable is a connection, A on a fixed 3-manifold Σ representing 'space' and (as in gauge theories) the momenta are the 'electric field' 2-forms E, both of which take values in the Lie-algebra of SU(2)."

Notice he has them be 2-forms valued in G' from the start (others might begin with vector densities valued in G'* and then take the dual and get 2-forms to integrate over surfaces, but he is just a little more direct) Then he refers to the "orthonormal triad" interpretation, to give a little geometric intuition about them.

"In the present gravitational context, the momenta acquire a geometrical significance: their Hodge-duals _*E can be naturally interpreted as orthonormal triads (with density weight 1) and determine the dynamical, Riemannian geometry of Σ. Thus, (in contrast to Wheeler's geometrodynamics)
the Riemannian structures on Σ are now built from momentum variables.

The basic kinematic objects are holonomies of A, which dictate how spinors are parallel transported along curves, and the 2-forms E, which determine the Riemannian metric of Σ.

(Matter couplings to gravity have also been studied extensively [2, 1].)..."

 

[marcus]

E describes "the flux lines of area"

Continuing the quote from Ashtekar on page 5 of the same paper, there is another bit of intution about E.

"...In the quantum theory, the fundamental excitations of geometry are most conveniently expressed in terms of holonomies [3, 4]. They are thus one-dimensional, polymer-like and, in analogy with gauge theories, can be thought of as 'flux lines of the electric field'. More precisely, they turn out to be flux lines of areas: an elementary flux line deposits a quantum of area on any 2-surface S it intersects. Thus, if quantum geometry were to be excited along just a few flux lines, most surfaces would have zero area and the quantum state would not at all resemble a classical geometry.

Semi-classical geometries can result only if a huge number of these elementary excitations are superposed in suitably dense configurations [13, 14]. The state of quantum geometry around you, for example, must have so many elementary excitations that 10⁶⁸ of them intersect the sheet of paper you are reading, to endow it an area of 100 cm².

Even in such states, the geometry is still distributional, concentrated on the underlying elementary flux lines; but if suitably coarse-grained, it can be approximated by a smooth metric. Thus, the continuum picture is only an approximation that arises from coarse graining of semi-classical states..."

 

[selfAdjoint]

Thanks Marcus, for all these explanations. I am going to carefully go over the Ashtekar 2002 paper you linked to. 2-forms with values in G' at first sound more reasonable than vector densities with values in the dual space of G', but in fact - what is a 2-form? It's something times dx/\dy (being cheesy about the coordinates). Is he saying that for every such dx/\dy we have an associated map into G' (linear? preserving product?)

Or consider that tensor product from O-L, τ^* {X} ∂ (suppressing indexes). Can we look on that as a (dual) 2-form?

BTW I think the relation of a vector density to a 2-form is via that completely antisymmetric Levi-Civitta density. A vector density of weight one changes sign on reflection (because the determinant of the Jacobian of a reflection is -1).

 

[marcus]

Originally posted by selfAdjoint

BTW I think the relation of a vector density to a 2-form is via that completely antisymmetric Levi-Civitta density. A vector density of weight one changes sign on reflection (because the determinant of the Jacobian of a reflection is -1).

I agree, Levi-Civita usually written with epsilon

I just happened to have the Sahlmann/Thiemann paper (gr-qc/0303074) open to page 3 where it says:

E_S,ƒ = ∫_S E^a_i ƒⁱε_abcdx^bdx^c

 

[marcus]

selfAdjoint, your question "what mental image" related to E or tilde-E has gotten me focussed on getting some intuition. After lunch I was thinking one thing space is doing all the time is expanding----the metric, distances between pairs of stationary objects like galaxies, is increasing all the time...so does this show up in E?

What happens to the electric field or the "densitized inverse triads" or the fluxes thru sample surfaces?

All this stuff at some level is fairly straightforward and I think there is a straightforward answer----E gets bigger.

The area and volume operators---selfadjoint operators on the hilbert space, observables---they are calculated from the E's

There is a derivation, we did not go thru it here yet but it is
in several papers.

So as the E's get bigger all the areas and volumes of things will get bigger. because E is at the root of those observables.

E is something, on the tongue-tip, it is an idea---cant quite think of the word. But it is understandable, with some intuitive content. Maybe something will occur to you

 

[marcus]

Something has happened to Thomas Thiemann's expository
style. His October 2002 "Lectures on Loop Quantum Gravity"
is loaded with intuitive perceptions about the basic
elements of the theory with even some almost cartoon-like sketches and is considerably more readable than his
October 2001 "Introduction to Modern Canonical Quantum General Relativity" (written for LivingReviews).

One might try to dispose of the difference by saying he's targeting
a different audience but I doubt that explains even half
of it

Here's the link to "Lectures" in case anyone wants to check it out
http://arxiv.org/gr-qc/0210094

 

[selfAdjoint]

It's not just the audience but the occasion. The lectures are, well, _lectures_. To what looks lkike a bunch of bright grad students. The living reviews piece is a high-falutin' non-working, get it right or rue it megillah.

BTW I found a quick and dirty intro to these matters in Week 7 of Baez's This week's finds.

 

[marcus]

Originally posted by selfAdjoint
It's not just the audience but the occasion. The lectures are, well, _lectures_. To what looks like a bunch of bright grad students. The living reviews piece is a high-falutin' non-working, get it right or rue it megillah.

BTW I found a quick and dirty intro to these matters in Week 7 of Baez's This week's finds.

Glad you mentioned Week 7 of Baez finds. I had looked at it some time ago but got more out of it when I went back to it yesterday

I will prefer any day Thiemann hobnobbing with the grad students to Thiemann making pronouncements ex cathedra like he is the pope (in an ecumenical spirit I have broadened the ethnic scope----stuffed shirts are universal---but must concede the metaphor of the megillah is hard to beat)