The Issue of the Beginning (new paper by Ashtekar)

[marcus]

watching worldview change bit by bit is like watching grass grow.
In a preprint posted today on arxiv, Abhay Ashtekar provided a bit more support to the view that black hole bounce may bridge to another big bang

this paper is a write-up of a talk he gave last year. a large part is aimed at nonspecialists and is accessible to general audience.


http://arxiv.org/abs/physics/0605078
The Issue of the Beginning in Quantum Gravity
Abhay Ashtekar
15 pages, 2 figures. History and Philosophy of Physics. Based on an invited talk at the 7th International Conference on the History of General Relativity (HGR7), "Einstein and the Changing World View of Physics, 1905-2005", held at Tenerife, Canary Islands in 2005

"The goal of this report is to provide an up to date account of results on the quantum nature of the big bang, obtained in loop quantum cosmology. They suggest a radical modification of the paradigm provided by general relativity for the issue of the Beginning. The article is addressed primarily to historians and philosophers of science."

 

[marcus]

sample quote from page 13 of the new Ashtekar paper

"...A qualitative picture that emerges is that the non-perturbative quantum geometry corrections are ‘repulsive’. While they are negligible under normal conditions, they dominate when curvature approaches the Planck scale and halt the collapse that would classically have lead to a singularity. In this respect, there is a curious similarity with the situation in the stellar collapse where a new repulsive force comes into play when the core approaches a critical density, halting further collapse and leading to stable white dwarfs and neutron stars. This force, with its origin in the Fermi-Dirac statistics, is associated with the quantum nature of matter. However, if the total mass of the star is larger than, say, 5 solar masses, classical gravity overwhelms this force. The suggestion from LQC is that a repulsive force associated with the quantum nature of geometry may come into play and could be strong enough to counter the classical, gravitational attraction, preventing the formation of singularities. Since this force is negligible until one enters the Planck regime, predictions of classical relativity on the formation of trapped surfaces, dynamical and isolated horizons would still hold. But assumptions of the standard singularity theorems would be violated. There would be no singularities, no abrupt end to space-time where physics stops. Non-perturbative, background independent quantum physics would continue.

Returning to the issue of the Beginning, the big-bang appears to be an artifact of the assumption that the continuum, classical space-time of general relativity should hold at all scales. LQC strongly suggests that this approximation breaks down when the matter reaches Planck density. One might have at first thought that, since this is a tiny portion of space-time, whatever quantum effects, they would have negligible effect on global properties of space-time and hence almost no bearing on the issue of The Beginning. However, detailed LQC calculations have shown that this intuition may be too naive. The ‘tiny portion’ may actually be a quantum bridge to another large universe. The physical, quantum space-time of could be significantly larger than what general relativity had us believe. The outstanding open issue is whether this scenario persists when inhomogeneities are adequately incorporated in the analysis. If it does, we would have a brand new answer to the age old question of the Beginning. "his italics, which I blued for additional emphasis. my bolding to highlight what I think is a key point about the "tiny portion" serving as a "bridge"

 

[wolram]

Mathatics, the universal, mend all, it can fill any gap, or put some fill into the gap, or prove the gap is none existent, in other words maths can be the cement that holds your house together or the rendering that covers any
imperfection, it is usefull and useless, maths is absolutly useless in describing our universe.

 

[hellfire]

The outstanding open issue is whether this scenario persists when inhomogeneities are adequately incorporated in the analysis. If it does, we would have a brand new answer to the age old question of the Beginning.

Is it really possible to incorporate inhomogeneities in the scenario? Inhomogeneities need of infinite degrees of freedom. I guess they could be only added to the isotropic and homogeneous models if the back-reaction of matter on space-time is neglected. Otherwise one must go back and start with the full theory of quantum gravity to analize them. And as far as I know (please correct me) there is currently no known methods for how to proceed in such a case.

 

[marcus]

Otherwise one must go back and start with the full theory of quantum gravity to analize them...

it appears to be not so, hellfire.
a simple example is where gravitational collapse (BH) is studied in the spherically symmetric case

the distribution of matter can be bumpy, but it depends only on R

so it is not the full theory, but it is not the simplest case either.

I should get you some links. A fair amount of work has been done recently on cases that are intermediate between the simplest case and the full theory. According to what you said this ought to be impossible but it appears not to be.

As far as I can see, Bojowald's program is to gradually work his way up the ladder, in understanding quantum gravitational collapse----gradually dealing with more complicated cases.

there is a team at Penn State---several people involved---some numerical work using the computer, as well as analytical

General Relativity has always been difficult to use in full generality, as a rule all the known classical solutions involve some simplifying assumptions, some symmetry. It is not "all or nothing" but rather a progression of hairier and hairier cases. So since classical is like that, the quantum gravity research is in some sense in familiar territory.
We have "been here before".

An interesting difference is that the full quantum theory is still incomplete so EVEN IF ONE WERE ready to address the problem of collapse in full generality one could not yet do it! But as a practical matter that does not seem to matter very much to the people actually doing the research. they have their plate full just handling simpler but increasingly more complicated cases.

To make it easy for myself why don't I just give you one link to Bojo papers since 2001 and you can scan and find various different cases of increasing complexity being considered. His papers are not the only ones but they are in a way representative.

http://arxiv.org/find/grp_physics/1/au:+Bojowald/0/1/0/2001,2002,2003,2004,2005,2006/0/1

Looking over this list reminds me that one of the things they are doing is PUTTING MATTER INTO THE PICTURE and they have to gradually deal with cases where the distribution of matter is increasingly complicated.

 

[marcus]

some people other than Bojowald engaged in same sort of research

[21] A. Ashtekar, T. Pawlowski and P. Singh, Quantum Nature of the Big Bang, Phys. Rev. Lett. 96, 141301 (2006) arXiv:gr-qc/0602086.

[22] A. Ashtekar, T. Pawlowski and P. Singh, Quantum Nature of the Big Bang: An analytical and numerical Investigation I, arXiv:gr-qc/0604013.

[23] A. Ashtekar, T. Pawlowski and P. Singh, Quantum Nature of the Big Bang: An analytical and numerical Investigation II (IGPG Pre-print).

this is just a sample but it illustrates that the investigation is NUMERICAL, that is they use the computer to calculate the quantum wavefunction and plot the evolution

Oh, it also illustrates that the work is not about black hole collapse---it includes work on the big bang (or rather "bounce") as well.

 

[marcus]

Mathatics, the universal, mend all, it can fill any gap, or put some fill into the gap, or prove the gap is none existent, in other words maths can be the cement that holds your house together or the rendering that covers any
imperfection, it is usefull and useless, maths is absolutly useless in describing our universe.

my contractor friend used to refer to plaster as "mud"
and his answer when there was a crack or some imperfection was "put some mud in it"
very handy stuff, plaster.

In the case of Gen Rel singularities, it has proven rather difficult. Efforts go back to 1960 or 1970. It was always expected that quantizing would remove the classical singularities (as had happened in other fields like the quantum theory of the atom---which eventually gave us transistors) but in the case of Gen Rel for a long time it didnt work.

People kept trying to quantize Gen Rel and fix its singularity problems. they worked for over 40 years and couldn't find a "plaster" that would "stick"

finally in 2001 Bojowald quantized a simple cosmological model in a way that worked and the singularity vanished.

But Ashtekar (who is a weighty figure in science) was still understandably cautious. What he has started saying now, he could have gone out on a limb and risked saying back in 2001.

He has waited for 5 years to see if the limb is strong enough to bear his weight. And he has evidently also encouraged his postdocs to test it out for him (P. Singh and T. Pawlowski).

Probably during those 5 years from 2001 to present, Ashtekar was asking himself similar questions to what you ask, wolram.
"Is this fixing of the problem just an ILLUSION? The problem (the breakdown of Gen Rel) is certainly real, but is the solution the right one? Will the plaster stick? or will it go away?"

Ashtekar has a major reputation at stake and he is the opposite of a gambler. So I have been watching what he says, which has evolved gradually over the past couple of years, with some care.

The choreography is quite curious. In some sense Smolin (who is not averse to risk) was 10 years ahead of the present, back in 1995 and 1996 with what he said about the connectability of BH and BB. And John Arch. Wheeler was even more ahead of Smolin but he is legendary. But the community did not COUNT what Smolin said in 1995 because the collective mind is more cautious and abhors speculation.

the collective view will only move just about as fast as Ashtekar moves.

 

[hellfire]

it appears to be not so, hellfire.
a simple example is where gravitational collapse (BH) is studied in the spherically symmetric case

You should check section 4.3.1 of Bojowald's http://arxiv.org/gr-qc/0601085 . I think my comment is in line with this. The first bullet in that section refers to the incorporation of the inhomogeneities in a preexisting homogeneous and isotropic cosmological model, that can be only done by ignoring back-reactions. The second and third ones refer to working out inhomogeneities from the full theory. This is my reading, but of course I may be missing something. By the way, I don't know anything about inhomogeneities in models of gravitational collapse, are these relevant here?

 

[marcus]

You should check section 4.3.1 of Bojowald's http://arxiv.org/gr-qc/0601085 .

the first and third bullets refer to work which has been done IIRC. I will try to locate it.
the thing you cite is a good thing to quote! I will paste it in

====bojo LQC section 4.3.1 exerpt===
4.3.1 Available approximations For the analysis of inhomogeneous situations there are several different approximation schemes:

• Use only isotropic quantum geometry and in particular its effective description, but couple to inhomogeneous matter fields. Problems in this approach are that back-reaction effects are ignored (which is also the case in most classical treatments) and that there is no direct way how to check modifications used in particular for gradient terms of the matter Hamiltonian. So far, this approach has led to a few indications of possible effects.

• Start with the full constraint operator, write it as the homogeneous one plus correction terms from inhomogeneities, and derive effective classical equations. This approach is more ambitious since contact to the full theory is realized. So far, there are not many results since a suitable perturbation scheme has to be developed.

• There are inhomogeneous symmetric models, such as the spherically symmetric one or Einstein–Rosen waves, which have infinitely many kinematical degrees of freedom but can be treated explicitly. Also here, contact to the full theory is present through the symmetry reduction procedure of Sec. 6. This procedure itself can be tested by studying those models between homogeneous ones and the full theory, but results can also be used for physical applications involving inhomogeneities. Many issues which are of importance in the full theory, such as the anomaly problem, also arise here and can thus be studied more explicitly.
===endquote===

 

[marcus]

PHYSORG.com picked up on the Ashtekar research and did a popular article:

http://www.physorg.com/news66660003.html

nice to see quantum gravity getting some media play.
It's not Newsweek but maybe that will be coming along as well one of these days.