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marcus

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## Main Question or Discussion Point

I believe the job of the Loops05 conference at Potsdam in October is to forge an alliance of research programs into a field called "Nonperturbative Quantum Gravity" so that there can be faculty positions in NQG in physics departments at various major universities.

well that sounds serious and dignified, and even historical, but everybody knows the stakes are high and in its own way it means there is going to be a circus maximus

like Fay Dowker for whom it is an axiom that spacetime is discrete (a causal set is about as discrete as you can get) is going to collide head-on with Renate Loll, who has found no evidence of spacetime discreteness, or of a minimal length scale, in her computer experiments with CDT.

Personally, I think that CDT and Causets are similar: a causal triangulated manifold is actually an excellent example of a certain kind of causal set----the set of simplexes causally ordered by the way they are glued. And the only difference I can see is that Renate goes to the limit as the simplexes get small. You might think that was just a LITTLE difference

The difference is Fay does not go to the limit. She has all these, like, beads or beans of spacetime with each one being exactly equal to the planck volume, arranged in a partial ordering (causality) with a certain finiteness condition (which is automatically satisfied in Renate's triangulated manifolds). So it is like she was going to do Renate's thing but at a certain size or scale of simplex, at a certain point, she STOPPED MAKING IT SMALLER.

So even tho the two approaches Causet and CDT are extremely similar and almost indistinguishable in some sense, all hell is liable to break loose when you put them together. Because for one it is an axiom that spacetime is made of discrete beans and in the other approach not.

I think we should try to understand this issue. Why is it so important to so many Quantum Gravitists that spacetime be in some fashion or other discrete? What is at the root of this urge or drive or intuition or feeling that it ought to be that way?

If you go surf the website of Loop05, you will see that Renate Loll is WAY OUTNUMBERED by QG people who have espoused this discretness idea in one way or another. Loop, for instance, is CONSTRUCTED on a continuum but it gets discrete volume and area spectra as a result. (they are not however simple wholenumber multiples of planck units, as you might expect from Fay's picture!) so Rovelli and Ashtekar and Smolin all include the idea of discretness in their statements about Loop even though it is not so cut-and-dried full frontal discretness as with Fay. Fay is radically discrete and they are sort of politely and modestly discrete.

Like a spin network in LQG is a GRAPH so that is not so discrete as a heap of beans. But still it is rather more discrete than a chunk of continuum out of differential geometry.

how shall we understand this conference? Is it a series of headon collisions?

the stakes are very high (I feel) but I can't quite say what they are. Now is the time (I feel) for this fragmentary bunch of programs to coalesce into a kind of Nonperturbative Alliance.

But my feeling about academics is they dont come together easily, when they are intellectually at odds. This discreteness issue is a FAULT LINE. (or so I'm thinking)

I wonder if there are other irreconcilable differences.

Well, in case anyone else is interested, I will get some links:

here is the Loops05 website ("programme" has a list of speakers like Fay and Renate)

http://loops05.aei.mpg.de/

here is a Fay Dowker manifesto

www.dpf2003.org/xx/qg/dowker.pdf[/URL]

here is a Raphael Sorkin manifesto

[url]http://www.arxiv.org/abs/gr-qc/9706002[/url]

here is a nontechnical statement from Loll about CDT

[url]http://www.phys.uu.nl/~loll/Web/research/research.html[/url]

Here is the opening shot from Loll, page 2 paragraph 2 of

[PLAIN]http://arxiv.org/hep-th/0505113 [Broken]

Slow progress in the quest for quantum gravity has not hindered speculation on what kind of mechanism may be responsible for resolving the short-distance singularities.

...

... We have recently begun an analysis of the microscopic properties of these quantum spacetimes. As in previous work, their geometry can be probed in a rather direct manner through Monte Carlo simulations and measurements. At small scales, it exhibits

Actually didn't Newton and Leibniz have it out over some issue or other? this is in the good old academic tradition. It's how things happen, how WORK gets done.

Now here is Renate citing Fay on page 2, paragraph 2 of ANOTHER paper

http://arxiv.org/hep-th/0507012 [Broken]

A time-honoured part of this discussion is the question of whether a sum over different spacetime topologies should be included in the gravitational path integral. The absence to date of a viable theory of quantum gravity in four dimensions has not hindered speculation on the potential physical significance of processes involving topology change [CITES HOROWITZ, CITES DOWKER]. Because such processes necessarily violate causality, they are usually considered in a Euclidean setting where the issue does not arise. Even if one believes that Euclidean quantum gravity without a sum over topologies exists nonperturbatively as a fundamental theory of nature – something for which there is currently little evidence –, ...

well that sounds serious and dignified, and even historical, but everybody knows the stakes are high and in its own way it means there is going to be a circus maximus

like Fay Dowker for whom it is an axiom that spacetime is discrete (a causal set is about as discrete as you can get) is going to collide head-on with Renate Loll, who has found no evidence of spacetime discreteness, or of a minimal length scale, in her computer experiments with CDT.

Personally, I think that CDT and Causets are similar: a causal triangulated manifold is actually an excellent example of a certain kind of causal set----the set of simplexes causally ordered by the way they are glued. And the only difference I can see is that Renate goes to the limit as the simplexes get small. You might think that was just a LITTLE difference

The difference is Fay does not go to the limit. She has all these, like, beads or beans of spacetime with each one being exactly equal to the planck volume, arranged in a partial ordering (causality) with a certain finiteness condition (which is automatically satisfied in Renate's triangulated manifolds). So it is like she was going to do Renate's thing but at a certain size or scale of simplex, at a certain point, she STOPPED MAKING IT SMALLER.

So even tho the two approaches Causet and CDT are extremely similar and almost indistinguishable in some sense, all hell is liable to break loose when you put them together. Because for one it is an axiom that spacetime is made of discrete beans and in the other approach not.

I think we should try to understand this issue. Why is it so important to so many Quantum Gravitists that spacetime be in some fashion or other discrete? What is at the root of this urge or drive or intuition or feeling that it ought to be that way?

If you go surf the website of Loop05, you will see that Renate Loll is WAY OUTNUMBERED by QG people who have espoused this discretness idea in one way or another. Loop, for instance, is CONSTRUCTED on a continuum but it gets discrete volume and area spectra as a result. (they are not however simple wholenumber multiples of planck units, as you might expect from Fay's picture!) so Rovelli and Ashtekar and Smolin all include the idea of discretness in their statements about Loop even though it is not so cut-and-dried full frontal discretness as with Fay. Fay is radically discrete and they are sort of politely and modestly discrete.

Like a spin network in LQG is a GRAPH so that is not so discrete as a heap of beans. But still it is rather more discrete than a chunk of continuum out of differential geometry.

how shall we understand this conference? Is it a series of headon collisions?

the stakes are very high (I feel) but I can't quite say what they are. Now is the time (I feel) for this fragmentary bunch of programs to coalesce into a kind of Nonperturbative Alliance.

But my feeling about academics is they dont come together easily, when they are intellectually at odds. This discreteness issue is a FAULT LINE. (or so I'm thinking)

I wonder if there are other irreconcilable differences.

Well, in case anyone else is interested, I will get some links:

here is the Loops05 website ("programme" has a list of speakers like Fay and Renate)

http://loops05.aei.mpg.de/

here is a Fay Dowker manifesto

www.dpf2003.org/xx/qg/dowker.pdf[/URL]

here is a Raphael Sorkin manifesto

[url]http://www.arxiv.org/abs/gr-qc/9706002[/url]

here is a nontechnical statement from Loll about CDT

[url]http://www.phys.uu.nl/~loll/Web/research/research.html[/url]

Here is the opening shot from Loll, page 2 paragraph 2 of

[PLAIN]http://arxiv.org/hep-th/0505113 [Broken]

Slow progress in the quest for quantum gravity has not hindered speculation on what kind of mechanism may be responsible for resolving the short-distance singularities.

**A recurrent idea is the existence of a minimal length scale, often in terms of a characteristic Planck-scale unit of length in scenarios where the spacetime at short distances is fundamentally discrete**. An example is that of so-called loop quantum gravity, where the discrete spectra of geometric operators measuring areas and volumes on a kinematical Hilbert space are often taken as evidence for fundamental discreteness in nature [CITES ASHTEKAR AND SMOLIN]. 1 Other quantization programs for gravity, such as the ambitious causal set approach [CITES SORKIN], postulate fundamental discreteness at the outset....

... We have recently begun an analysis of the microscopic properties of these quantum spacetimes. As in previous work, their geometry can be probed in a rather direct manner through Monte Carlo simulations and measurements. At small scales, it exhibits

**neither fundamental discreteness nor indication of a minimal length scale**. Instead, we have found evidence of a fractal structure (see [7], which also contains a detailed technical account of the numerical set-up). What we report on in this letter...Actually didn't Newton and Leibniz have it out over some issue or other? this is in the good old academic tradition. It's how things happen, how WORK gets done.

Now here is Renate citing Fay on page 2, paragraph 2 of ANOTHER paper

http://arxiv.org/hep-th/0507012 [Broken]

A time-honoured part of this discussion is the question of whether a sum over different spacetime topologies should be included in the gravitational path integral. The absence to date of a viable theory of quantum gravity in four dimensions has not hindered speculation on the potential physical significance of processes involving topology change [CITES HOROWITZ, CITES DOWKER]. Because such processes necessarily violate causality, they are usually considered in a Euclidean setting where the issue does not arise. Even if one believes that Euclidean quantum gravity without a sum over topologies exists nonperturbatively as a fundamental theory of nature – something for which there is currently little evidence –, ...

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