The Issue of the Beginning in Quantum Gravity

[Martin]

a new preprint from Abhay Ashtekar

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."

[Martin]

sample quote from page 13 of Ashtekar's 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. "

[Martin]

he is being very careful not to overinterpret the theoretical results obtained in QG so far, the wording is cautious.

 

what I believe he is hinting at is the possibility that the quantum bounce at the center of a black hole----a "tiny portion of spacetime"---can be a bridge to a new largescale region: can, in effect, initiate another branch of the big bang.

 

A lot of the article is written for non-tech general audience. but there are pages of technical stuff too---something for everybody. Some here might find it of interest.

[Ragib]

Yes, quite interesting. I find it amazing that Eiensteins work is still quite cutting edge. How long was he on the cutting edge of physics, 20, 30 years?

[Royston]

The problem I'm interested in concerning the 'beginning', is if our universe inflated from the centre of a black hole, then our universe was a product of a previous universe and so on. I guess I'm missing something.

 

There was also talk in the article (I think...I'm nowhere near the level to understand the article fully) of curvature on planck size scales. I'm particularly interested in this, as this is my next area of study...beginning at the end of this month. Though I more than likely misread this, because surely that would be a breakthrough.

[Martin]

Yes, quite interesting. I find it amazing that Einsteins work is still quite cutting edge. How long was he on the cutting edge of physics, 20, 30 years?

 

 

that is a good point, the man himself was only at peak creativity for a few decades. From 1905 to something like (the 20 or 30 years you suggest) 1925 or 1935.

 

I can't say exactly, not being familiar with those biographical details.

 

But his work is still not completely assimiliated in the sense that the Standard Model of particle physicists (which has not yet been replaced) is based on a form of Quantum Field Theory (QFT) which conforms only to 1905 special relativity. today's QFT does not conform to 1915 general relativity.

 

the goal of a general relativistic quantum physics (which Rovelli's book Quantum Gravity states exactly that way) is still a goal, not a reality.

 

It is an interesting situation.

 

the blood on your head bothers me

[Martin]

The problem I'm interested in concerning the 'beginning'' date=' is if our universe inflated from the centre of a black hole, then our universe was a product of a previous universe and so on. I guess I'm missing something.

 

There was also talk in the article (I think...I'm nowhere near the level to understand the article fully) of curvature on planck size scales. I'm particularly interested in this, as this is my next area of study...beginning at the end of this month. Though I more than likely misread this, because surely that would be a breakthrough.[/quote']

 

I don't think you are missing anything. the ideas people have about the "beginning" are unproven hypotheses for now. But they won't always be so. I feel sure that some of the models will eventually be tested and some will fail and be chucked out. And others will pass the tests of prediction and observation and will gradually gain some credibility.

 

You never finally arrive at the truth, you can just keep testing and replacing the models that fail.

 

Fortunately the serious cosmological models are beginning to make predictions about what new instruments will observe. If observations (in particular of gammaray bursts, GRB, and finer measurement of CMB) do not square with what the theories predict then that will falsify the theories and they can be chucked.

 

there are some "theories" that are not predictive enough to be falsifiable, but I am not treating them as real scientific theories and as part of the discussion

 

Snail, notice that Ashtekar is INTENTIONALLY VAGUE. he has to be because he is talking about theoretical work in progress that still has to be put to observational test. everything he says tends to be heavily qualified. So if you dont get a definite picture, you are not missing anything. There ISN'T a definite picture that a responsible scientist can give you, without a lot of qualifying "ifs and buts"

[bascule]

Martin, what's your take on Lee Smolin's idea that "the world we are imbedded in is nothing but an ever evolving network of relationships"

 

Still reading through this paper...

[Martin]

Martin' date=' what's your take on Lee Smolin's idea that "the world we are imbedded in is nothing but an ever evolving network of relationships"

 

Still reading through this paper...[/quote']

 

Bascule, always nice to hear from you! I was away from this thread for several days and just now saw your question.

 

I think the best thing smolin has written in a long time is this survey paper called

"Generic Predictions"

http://arxiv.org/abs/hep-th/0605052

 

You ask what I think. I think that people need a triangle made up of

 

VISUALIZATIONS OF SPACETIMEMATTER
=
=
=
=
=
corresponding verbal metaphors==========equations with predictive content

 

 

You are presenting me with a verbal clue, but I dont know what equations it refers to, I can guess that he is referring to the way you can model spacetime and matter with a model called "spin networks" but he may mean something more general.

 

Spin networks are just labeled graphs----the edges are labeled with some extra information rather as if they were colored.

 

There are a bunch of alternative rules people are trying out by which these networks evolve. You can read about this in the article "Generic Predictions" if you want.

 

Each different model can potentially make different predictions, the game is to find the predictions and test them so you can proceed by elimination---by getting rid of models.

 

Smolin says there are some GENERIC predictions that all or a large number of these "causal spin network" models predict. He lists some interesting new phenomena that astronomers should be able to look for and detect, which if they dont detect those things it DISCREDITS THE WHOLE CLASS of models.

 

this is playing the game. theories are supposed to be built so they can be tested either ASAP or at least within a reasonable time whatever the community thinks is reasonable. theorists should make a good faith effort to provide theories that are testable etc .

 

anyway it is a survey paper which covers a range of models and indicates that he is thinking about deriving testable predictions from them

You might want to check it out.

 

What do I think? I think that we want to find a beautiful new visualization of what makes up spacetimematter at microscopic level and what the dynamics of it are at microscopic level.

that visulization should have equations, the equations should be predictive in nature, and the predictions should be testable.

 

I think smolin is as close as anyone to a fruitful visualization that will have good equations. but we can't know for sure.

What he SAYS about his mental images and his equations is of secondary importance. I dont know where your quote or paraphrase comes from but it sounds fine to me as far as it goes. on a verbal level what else could space be besides the web of spatial relationships?

 

and smolin recently introduced a way in which twists and knots in the web of space can represent matter----a suprising scheme that reproduces much or all of the standard model list of particles---he got it from a young Australian particle theorist and adapted it to his networks-----the tangling and untangling of the knots imitates the interactions and decays of the known particles. It could just be a bizarre coincidence.

 

anyway the idea of a constantly transforming web of spatial relations which can embody matter and the reactions of matter

(and the interaction between matter and geometry we call gravity)

seems like a promising thing to try out, and that is all he is doing:

he and his friends are trying a bunch of causal spin networks out, these days.

 

we cant jump to conclusions and say he is right or wrong. it is just a good thing to try, I think.

 

the main thing i think in answer to what you ask is that people need to get

 

visualization

 

words=========equations

 

and they need the whole triangle, and the equations have to test out empirically

and the only thing to do is work towards this, which Smolin is doing

(the words are just a temporary clue to what he and his buddies are working on at present)

[bascule]

You ask what I think. I think that people need a triangle made up of

 

Code:

VISUALIZATIONS OF SPACETIMEMATTER

=

=

=

=

=

corresponding verbal metaphors==========equations with predictive content

 

You are presenting me with a verbal clue, but I dont know what equations it refers to, I can guess that he is referring to the way you can model spacetime and matter with a model called "spin networks" but he may mean something more general.

 

Yeah, I'm aware that verbal explanations are completely unsatisfactory in converying underlying details, and thus without a mathematical understanding there's little I really can know. I hope it isn't too annoying when laymen like me attempt to extract the most general understanding verbally...

 

I tried reading this paper. The first few pages were incredibly compelling, then it starts getting into math and weird guy's names I've never heard of and ugh. I think I'll wait until after I can link my brain to the Internet before I try to understand this stuff.

 

Smolin says there are some GENERIC predictions that all or a large number of these "causal spin network" models predict. He lists some interesting new phenomena that astronomers should be able to look for and detect, which if they dont detect those things it DISCREDITS THE WHOLE CLASS of models.

 

Awesome, I had no idea LQG was making testable predictions.

 

What do I think? I think that we want to find a beautiful new visualization of what makes up spacetimematter at microscopic level and what the dynamics of it are at microscopic level.

that visulization should have equations, the equations should be predictive in nature, and the predictions should be testable.

 

This paper reiterated to me one of the ideas I got out of Brian Greene's books, namely that at the Planck Length space seems to be a lot less smooth and uniform than the way we perceive it on the macroscale. Seems to me that figuring out the geometry of space on the Planck scale is what the quantum gravitational theories are all trying to do. As I don't understand the math I have no way of satisfactorally visualising what these sorts of geometries might actually be like, but it's alluded to (by descriptions of M-theory I've seen, and as far as I can tell, this paper) as being some kind of web structure. I don't really get the spin connections you and this paper were talking about... all I really know of is the Aspect experiment demonstrating quantum entanglement affecting spin. I'm not sure if what you're describing is anything like the graph structure of interactions I envision.

 

Spin networks are just labeled graphs----the edges are labeled with some extra information rather as if they were colored.

 

I'm sure that means something very different to you than it does to me, guess it gets back to your triangle of physics knowledge...

 

and smolin recently introduced a way in which twists and knots in the web of space can represent matter----a suprising scheme that reproduces much or all of the standard model list of particles---he got it from a young Australian particle theorist and adapted it to his networks-----the tangling and untangling of the knots imitates the interactions and decays of the known particles. It could just be a bizarre coincidence.

 

In one of my baseless thought experiments I've envisioned something which I think seems like that, although, again, the triangle: what I read, and what really is are two very different things...

 

I think smolin is as close as anyone to a fruitful visualization that will have good equations. but we can't know for sure.

What he SAYS about his mental images and his equations is of secondary importance.

 

Sadly, for me, it's of primary importance, because the rest of what he does is completely inaccessible to me. Of course, I'm useless in working towards a solution to the problem, so I guess it doesn't really matter.

 

I dont know where your quote or paraphrase comes from but it sounds fine to me as far as it goes.

 

The quote comes from his response to the EDGE 2006 question which you linked!

 

I was assuming the quote (and his entire response) was in regards to his fecund universe theory, which is the main aspect of his theories that I've been interested in.

[Martin]

thoughtful comments as usual

it is not quite as bad as you say on one point. I think you do both the verbal and the visualization---you just leave out the equations part.

 

(moreso than I do, for instance, that part is relative---nobody reads them all, i think)

 

I forget links, I lost touch with that 2006 Edge question, and his response.

 

I dont know, now, what he was referring to but I'd guess that it is about the network or relational way to visualize space. It uses graphs (or sometimes triangle-like building blocks) to present an evolving geometry. Several currently studied approaches go that way.

 

Daniele Oriti has a new book in the works at Cambridge Press with a subtitle something like "towards a new understanding of space time and matter". I like the title. It is a collection of different people's work so it illustrates alternative approaches to the same goal (quantum model of spacetime, quantum gravity) and lets one glimpse what the various approaches have in common. as well as how they differ.

 

Monday (15 May) is Kepler Day.

His life shows that sometimes, when you need them enough, you can get really new ideas. In his writing he sometimes sounds a bit coo-coo but that may just be the Middle Ages talking (it was still only around 1600).

And he provided Newton with what to take off from.

[bascule]

thoughtful comments as usual

it is not quite as bad as you say on one point. I think you do both the verbal and the visualization---you just leave out the equations part.

 

(moreso than I do' date=' for instance, that part is relative---nobody reads them all, i think)

 

I forget links, I lost touch with that 2006 Edge question, and his response.

 

I dont know, now, what he was referring to but I'd guess that it is about the network or relational way to visualize space. It uses graphs (or sometimes triangle-like building blocks) to present an evolving geometry. Several currently studied approaches go that way.[/quote']

 

I always assumed that fecund universes were the main thrust of what he was getting at, however the idea seems anything but testable, at least for now:

 

http://en.wikipedia.org/wiki/Fecund_universes

 

The fecund universes theory of cosmology advanced by Lee Smolin suggests that the rules of biology apply on the grandest scales. (It is also often, somewhat inaptly, referred to as "cosmological natural selection".) It is summarized in his book aimed at a lay audience, The Life of the Cosmos (ISBN 019510837X).

 

In this view, a collapsing black hole causes the emergence of a new universe on the "other side", whose fundamental constant parameters (speed of light, Planck length and so forth) may differ slightly from those of the universe where the black hole collapsed. Each universe therefore gives rise to as many new universes as it has black holes. (Thus the theory contains the evolutionary ideas of "reproduction" and "mutation" of universes, but has no analogue of natural selection.)

 

Daniele Oriti has a new book in the works at Cambridge Press with a subtitle something like "towards a new understanding of space time and matter". I like the title. It is a collection of different people's work so it illustrates alternative approaches to the same goal (quantum model of spacetime, quantum gravity) and lets one glimpse what the various approaches have in common. as well as how they differ.

 

I've been reading Three Roads to Quantum Gravity, although I'd really like to read Life of the Cosmos now... books like that, and the ones Brian Greene has written, may gloss over all the important details but I really think they do a good job of at least conveying the core of the ideas that physicists are working with.

 

Monday (15 May) is Kepler Day.

His life shows that sometimes, when you need them enough, you can get really new ideas. In his writing he sometimes sounds a bit coo-coo but that may just be the Middle Ages talking (it was still only around 1600).

And he provided Newton with what to take off from.

 

Well, Kepler wasn't half as nuts as Tycho Brahe, but probably wouldn't have ever figured out the laws of planetary motion without his data.