Questions on LQG and LQC

[Royston]

With regards to the recent findings by Ashtekar and Bojowald I have a couple of questions which maybe someone could answer.

 

If a black hole represents the beginings of a universe, and the inflation theory holds, in relation to our universe where do these 'other' universes reside ?

 

Given that we can observe black holes in our universe, does this mean they have not yet reached the stage of inflation (obviously we can't observe this) or are they actually 'gateways' (for want of a better word) into these other universes.

 

What I don't understand is if there are multiple universes then black holes can only eventually inflate outside the boundaries of our universe (which doesn't seem right to me) or they reside in another dimension that we don't experience.

 

Given that for every black hole there is another universe, and black holes in those universes and so on and so on, then the amount of dimensions needed for these universes to exist must be phenomenal. For each universe to exist in relation to the other, and the fact they all follow the same laws of inflation then each one would have to have extra dimensions so they can all exist alongside each other...hope this makes sense, I am struggling to get my thought process into words here.

 

Any help explaining where these universes exist in relation to ours would be greatly appreciated. I understand that there's nothing proven with LQG (as yet) but I remember thinking of literally infinite universes when I was very young so I have soft spot (if you like) for this theory...despite my very limited understanding of the subject.

[Royston]

What I don't understand is if there are multiple universes then black holes can only eventually inflate outside the boundaries of our universe

 

I'm using the term 'boundaries' very loosely indeed, it's purely to illustrate that there maybe multiple universes clustered as opposed to entwined with each other.

 

If these quetions are impossible to answer without getting exceptionally technical then please say, or if they're just impossible to answer, full stop. (period)

[Martin]

Snail, the possibility which you envisage, in your post, but then said "doesnt seem right" was, in fact, right after all! I colored blue, in your post, what I think is the right idea. I made bold text some things I thought were heading the wrong direction in your post. There is no need to imagine other dimensions.

 

With regards to the recent findings by Ashtekar and Bojowald I have a couple of questions which maybe someone could answer.

 

If a black hole represents the beginings of a universe' date=' and the inflation theory holds, in relation to our universe [b']where[/b] do these 'other' universes reside ?

 

Given that we can observe black holes in our universe, does this mean they have not yet reached the stage of inflation (obviously we can't observe this) or are they actually 'gateways' (for want of a better word) into these other universes.

 

What I don't understand is if there are multiple universes then black holes can only eventually inflate outside the boundaries of our universe (which doesn't seem right to me) or they reside in another dimension that we don't experience.

 

Given that for every black hole there is another universe, and black holes in those universes and so on and so on, then the amount of dimensions needed for these universes to exist must be phenomenal. For each universe to exist in relation to the other, and the fact they all follow the same laws of inflation then each one would have to have extra dimensions so they can all exist alongside each other...hope this makes sense, I am struggling to get my thought process into words here.

 

Any help explaining where these universes exist in relation to ours would be greatly appreciated. I understand that there's nothing proven with LQG (as yet) but I remember thinking of literally infinite universes when I was very young so I have soft spot (if you like) for this theory...despite my very limited understanding of the subject.

 

It seems to me that you may not have considered that spacetime does not need any larger encompassing space in which to exist. I suppose you could express it by saying that spacetime doesnt need a "where" in which to reside because it IS where.

 

I bolded the word "where" in your post, as an indicator of where I think the problem is.

 

This is not anything especially new. It goes back to classic vintage-1915 Einstein relativity. In that model, spacetime is a 4D continuum and it is NOT INSIDE any higherdimensional continuum. A common version of that model has space EXPAND in the sense that distances are time-dependent and get larger with time. I'm talking about the usual cosmology model. That spacetime does not have to "reside" in any larger venue, it does not expand INTO anything, it doesnt have a location----because it is location itself.

 

So nothing that Ashtekar and Bojowald envisage requires extra dimensions. A new tract can continue outwards from the pit of a black hole WITHOUT BUMPING US.

 

this is the really hard thing to understand, there is no denying that it IS difficult to picture----the new tract made by continuing onwards from the pit of a black hole DOES NOT BUMP OUR UNIVERSE because it is a continuation with its own time direction. And we don't share any larger encompassing arena with it---there is no "absolute space container" containing both our universes in which they can bump.

 

The bugger actually branches off from us and evolves on its own without interfering---it doesnt impact back on us. this is, I think, what many people find very difficult, if not impossible, to grasp.

 

There is more to say about this, and I expect I will post again, and other people may join in too. but this is all I wanted to say right now.

 

BTW Snail thank you for being careful to mention that LQG and LQC is work-in-progress and not a proved theory. Ashtekar and Bojowald say this too. What we are talking about is conjecture----theoretical possibilities that they are now probing----not finished work ready for testing.

 

they have quite a ways to go before they can say something like "if our big bang resulted from a black hole-like gravitational collapse then the CMB (cosmic microwave background) should exhibit such and such features, and if you look at next year's CMB data we predict you will see this and this feature.

 

It is going to take considerable patience. but at least things seem to be progressing

[Royston]

Thankyou very much Martin, this is precisely the kind of answer I was looking for.

I reiterated 'boundaries' as I understand the basic principles of relativity but sometimes have to go back on myself when trying to visualize it's effects...exceptionally hard when you have other universes to contend with...this is now a lot clearer, and has opened many other questions...especially with regard to thermal dynamics.

 

Needless to say I will be picking your brains (or anyone else adept in the subject ) as this theories implications has been using a lot of my thinking time (if you don't mind.) In fact the last few questions in the OP have been plaguing me for quite some time.

 

I am, relative to the experts, very new to the subject and it's only recently that I've developed a passion for understanding physics, but you've made my day, thankyou

[Royston]

A new tract can continue outwards from the pit of a black hole WITHOUT BUMPING US.

 

this is the really hard thing to understand' date=' there is no denying that it IS difficult to picture----the new tract made by continuing onwards from the pit of a black hole DOES NOT BUMP OUR UNIVERSE[/b'] because it is a continuation with its own time direction. And we don't share any larger encompassing arena with it---there is no "absolute space container" containing both our universes in which they can bump.

 

I have a few more questions (apologies if this all seems obvious to most)...and paradoxes that are bugging me.

 

With the quote above we can safely assume that thermal dynamics (with regards to the 2nd law) holds inside a black hole, and it's eventual inflation. Space-time breaks down, but as the 'arrow of time' continues (despite getting skewed relative to it's source) then (this may be way off the mark) the universe generated is out of sync with our 'arrow of time'. I'm figuring the dimension of time is fundamental to these sister universes not 'bumping' ours. (again correct me if I'm going wrong here.)

 

With regards to the first law does there have to be a 'mother' universe, where matter is essentially being borrowed to form the other universes. The one thing I can't get my head round is if black holes are formed by a star that's big enough for there to be gravitational collapse then there would have to be a uniformed 'mother' universe for black holes to be generated in the first place. So how does an initial collapse happen to form the start of this 'mother' universe ?

 

The only way I can see round this...(so it's probably wrong) is if the amount of matter needed to create a black hole is relative to other objects in space-time. So supposing (in our universe) the calculations that constitute when a star collapses is relative to the uniformed objects that are already present, then in the beginnings of this 'mother' universe is it possible to have collapse on a much smaller scale. I've already started a thread with this idea, and was predominantly ignored (so I guess it's incorrect) but how can you have this initial collapse to start the universe when there's nothing big enough around to constitute a collapse ? Am I delving into something that really requires a hell of a lot of maths behind it, or can this be explained so I can start to build a mental picture.

 

Just for the record I've heavily considered that space-time is 'where', and I think I was slipping up by removing myself from the model of the universe in order to build a mental picture, so creating 'boundaries'. I think it's a case of getting over old habits when visualizing models, and why I keep on having to go back on myself when applying relativity to these models.

[Royston]

With the quote above we can safely assume that thermal dynamics (with regards to the 2nd law) holds inside a black hole' date=' and it's eventual inflation. Space-time breaks down, but as the 'arrow of time' continues (despite getting skewed relative to it's source) then (this may be way off the mark) the universe generated is out of sync with our 'arrow of time'[/b']. I'm figuring the dimension of time is fundamental to these sister universes not 'bumping' ours. (again correct me if I'm going wrong here.)

 

Ok, the last bit of the paragraph (italic) is almost predisposing that there needs to be a factor so that a universe can exist relative to ours, and as these universes don't exist in a 'larger arena', this isn't necessary...and not really what I was getting at, but can see this statement being misinterpreted. The first part of the paragraph (bold) I'm going to stick with unless someone can find fault with it.

 

Maybe I'm just clutching at straws with the last post. However if anyone can explain how a collapse of space-time can happen before inflation, without the need for a large enough mass to be present (to obviously form a black hole) then this will explain the paradox I'm struggling with...any help please.

 

If my questions are too confusing or just plain daft, then I'm more than happy to take it on the chin.

[Martin]

...

 

With regards to the first law does there have to be a 'mother' universe' date=' where matter is essentially being borrowed to form the other universes. ...

 

The only way I can see round this...(so it's probably wrong) is if the amount of matter needed to create a black hole is relative to other objects in space-time. ....[/quote']

 

Hello Snail, I was distracted elsewhere and did not see your post. I thought the thread stopped with your post #4 and I did not realize you had more questions.

 

I cannot answer all your questions.

 

I can see that you are making an heroic effort to apply the FIRST LAW but it only applies to things in an isolated system----where you can imagine putting a giant baggie around the apparatus or whatever it is, insulating it from rogue events like big bangs and inflation scenarios.

 

Alan Guth of MIT who was the American usually credited with inventing the inflation scenario (there was also one or more russians) has often said that "The Universe may be the Ultimate Free Lunch" because during his inflation process (right after the big bang) ENERGY IS NOT CONSERVED. Or so it would appear. In fact during Guth's inflation huge amounts of energy are created by the expansion of space------most of the energy in the universe was not there at the moment expansion began but was (incredibly enough) created.

 

for me, I practically choke on this. I hate to have to appeal to authority, but have no alternative in this case.

 

I just now googled "Guth free lunch" and got these two Wikipedia entries:

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

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

 

To me it takes half the fun out of physics to have to appeal to authority, but that is how it is in this case, like it or not.

The authorities tell us that in General Relativity THERE IS NO GLOBAL CONSERVATION LAW. energy conservation for the Universe as a whole HAS NEVER BEEN PROVED.

 

energy conservation is only obeyed as a law in a localized region (like our galaxy or cluster of galaxies and its surroundings), where spacetime is either approximately flat or at least not expanding very fast so that you can imagine putting a giant baggie around it.

 

I am so sorry to have to tell you this Snail!

 

So a star say of 10 solar masses could collapse to a BH and a universe of TRILLIONS OF TRILLIONS OF SOLAR MASSES could come out from the pit of that hole. because of the Damn Alan Guth inflation mechanism!

 

Alan Guth has sometimes tried to justify the apparent violations by pointing to a special catch. the catch is that gravitational energy cancels the matter-energy----at the same time as energy in the form of light and matter-particles to eventually form stars is being CREATED the huge mass of matter-energy is sinking deeper and deeper into a gravity WELL of its own making and so energy is being GIVEN UP AS FAST AS IT IS BEING MADE. this is how Alan Guth has explained it. Actually I shouldn't try to describe this. I am very dubious of it.

 

I googled a link to Alan Guth giving a talk on this one time. Maybe I will find it and if I do I will put it here.

 

Anyway, in extreme situations like a gravitational collapse and a big bang is looks like nature gets a special permit to disobey her own energy-law. so not to worry about this. I see in the meantime you DID worry and you came up with a way to get around the problem, but you didnt have to because it actually is not a problem.

[Martin]

BTW Snail, have you heard of Dark Energy as described by the cosmological constant.

 

this is a clear case of what I said. Dark energy is typically represented by a constant energy density. So and so much per cubic kilometer.

 

Take some otherwise empty space and watch it expand. After one lightyear has stretched out to ten. and one cubic lightyear has expanded to a volume which is 1000 cubic lightyears, there is now a THOUSAND TIMES as much energy.

 

because the density, the amount per unit volume, stays the same----that is what Einstein's "cosmological constant" is about: it's constant-----so if a volume increases the amount of energy in it increases proportionately.

 

I know it is puzzling. It is routine in cosmology. But they don't talk about it in Freshman Physics. you cant put the baggie you get in Freshman Physics around an expanding space. but fortunately all that stuff is only evident at very large scales. On OUR scale, space expansion and dark energy are not even noticeable. And energy conservation is virtually perfect.

[Royston]

 

I am so sorry to have to tell you this Snail!

 

So a star say of 10 solar masses could collapse to a BH and a universe of TRILLIONS OF TRILLIONS OF SOLAR MASSES could come out from the pit of that hole. because of the Damn Alan Guth inflation mechanism!

 

Alan Guth has sometimes tried to justify the apparent violations by pointing to a special catch. the catch is that gravitational energy cancels the matter-energy----at the same time as energy in the form of light and matter-particles to eventually form stars is being CREATED the huge mass of matter-energy is sinking deeper and deeper into a gravity WELL of its own making and so energy is being GIVEN UP AS FAST AS IT IS BEING MADE. this is how Alan Guth has explained it. Actually I shouldn't try to describe this. I am very dubious of it.

 

Thanks again Martin, and corrections to my ideas are more than welcome (I'm yet to study physics seriously, so a clearer picture of what I'm dealing with is very beneficial.)

 

I guess I was slipping up with the laws of thermal dynamics, as the 2nd law has been used to describe entropy, even within the frame of a black hole, so I assumed the 1st law had to hold when dealing with collapse and inflation of space-time.

 

From what I've read so far, energy can be created through the volume of space-time increasing as it expands (I use space-'time' as the distance between two points increases as well.) But there dosn't seem to be an indication that energy is destroyed...I had a scan through the links ( I'll study them more closely tomorrow.) It does appear that the creation of energy and gravity are very closely linked...anyway, I certainly have a lot to mull over for now, and I'll be posting more on this subject in due course.

[Royston]

One question, what is the force that drives expansion. So far, with expansion comes more gravitational waves...an increase in volume creates more mass, on a universal scale. I'm not talking about a closed system where thermal dynamics holds. It seems gravity doesn't explain anything about the creation of energy.

 

The creation of energy seems to govern the strength of gravity.which poses a problem, any thoughts ?

[Royston]

One question' date=' what is the force that drives expansion. So far, with expansion comes more gravitational waves...an increase in volume creates more mass, on a universal scale. I'm not talking about a closed system where thermal dynamics holds. It seems gravity doesn't explain anything about the creation of energy.

 

The creation of energy seems to govern the strength of gravity.which poses a problem, any thoughts ?[/quote']

 

Well now I've read a few articles on dark energy, and it's repulsive force (so-called anti-gravity) I'll retract my last question (also I had a few JD's that evening...and it shows.)

 

I am slightly confused about the existence/ non-existence of dark energy / matter. I thought Neutrinos made up a small percentage of dark matter, and I remember hearing that 'shadows' of dark matter were found through observations of the CMB ?? Is dark energy / matter still speculation, or has there been more evidence (recently) to confirm that it's present ?

 

I'm going to plough my way through a few related articles, and I'm sure I'll post again with (I promise) a more sensible question.

[Martin]

...

 

I'm going to plough my way through a few related articles' date=' and I'm sure I'll post again with (I promise) a more sensible question.[/quote']

 

Your questions are fine. they are real good as far as I can see. they just go beyond what I feel able to answer properly, at least for now. maybe someone else will help out. or i will take another try later after some mulling over

[Royston]

Well I'm very pushed for time today, but found this article that deals with the 'emphasis on the cosmological constant as the possible choice for the dark energy'...I'd like to add, as the article explains, there is no evidence for variation of dark energy density with red shift.

 

So with my previous post on 'shadows' detected through the CMB observations...we can assume that these observations are not conclusive.

 

http://www.citebase.org/cgi-bin/fulltext?format=application/pdf&identifier=oai:arXiv.org:astro-ph/0510492

 

Link to the 'shadow' article...

 

http://www.sdss.org/news/releases/20030721.darkenergy.html

[Royston]

Another article that 'probes' dark energy...

 

http://arxiv.org/PS_cache/hep-th/pdf/0404/0404170.pdf

 

I found this section particularly interesting...

 

'So far sharpest experiments could probe gravity upto 0.1 mm and, in the matter sector, probe could be possible upto electroweak scale Mew = 100GeV, having wavelength as short as - 1.97 x 10-16 cm. It means that it is difficult to realise 4-dimensional gravity for length scales less than 0.1 mm. So, it is reasonable to think higher-dimensional gravity for scales smaller than this scale. The idea is parallel to the brane-world gravity, where it is assumed that gravity is stronger in higher dimensional space-time, called bulk and only a small part of it is realized in the observable universe.

 

What's the position of LQG on 'small scale' gravity...if it's too technical please say.

 

Another interesting point in the article is that 'dark energy is depleting through expansion...so creating dark matter.' So this creation is forcing the expansion of our universe. If energy increases when going into a 'gravity well' (black hole) as it's 'work' is, excuse the pun 'redundant' and gravity takes over then what's stopping dark energy creation in a black hole.

 

If this is the case then gravity holds as an attracting force only...more dark energy is created than went in to a black hole, and the creation of dark matter through the inflation period increases the volume of space-time. There's a hell of a lot more 'dark energy converting to dark matter' to take up space in our universe than observable matter...if this creation is happening at present then surely this would describe the 'acceleration of expansion'. This is really off of the top of my head...as always feel free to correct my feeble knowledge of this subject.

[Martin]

Another article that 'probes' dark energy...

 

http://arxiv.org/PS_cache/hep-th/pdf/0404/0404170.pdf

'So far sharpest experiments could probe gravity upto 0.1 mm and' date=' in the ...

 

What's the position of LQG on 'small scale' gravity...if it's too technical please say. [/quote']

 

It is not too technical. It is a good question. Again bear in mind that I can't speak with authority but only on the basis of what I've read. I have so far NEVER MET WITH AN LQG PREDICTION of different strength gravity showing up at the 0.1 mm scale. I wish I had!

 

I think if that were found (and a group in Seattle Washington is looking at that scale) it could be more interesting to string theorists. But there were rumors that the Washington group had found something, back in July, and they have not been confirmed. So nothing as yet. [if someone knows of more recent results that are positive, please let us know!]

 

So far LQG has made very few testable predictions. the main one concerns an energy-dependence of the speed of light predicted to show up in measurements of Gammaray bursts by an orbiting telescope (GLAST) scheduled for launch in 2007.

 

To my knowledge there are no LQG predictions about doable smallscale gravity experiments. (effects are predicted down at planck scale but that is not practical to reach)

 

Snail, your other questions do not seem to have to do specifically with LQG. You have some interesting ideas about dark energy, dark matter, and black holes. I am glad you are reading widely, including arxiv.org articles, and exercising imagination. I will try to respond but am not sure I can.

 

(I dont know what dark matter actually is, and can't imagine how dark energy could be converted into d.m. But it MIGHT be, so it is fine that you follow out the consequences.

Also I dont see how dark energy is depleted by expansion. Since its energy density is usually assumed to be constant. But these things MIGHT be, and I think you are developing some interesting lines of thought by concentrating on these possibilities. It is not standard cosmology as I know it, but that is not necessarily bad. A lot of people venture out in new directions.)

Another interesting point in the article is that 'dark energy is depleting through expansion...so creating dark matter.' So this creation is forcing the expansion of our universe. If energy increases when going into a 'gravity well' (black hole) as it's 'work' is, excuse the pun 'redundant' and gravity takes over then what's stopping dark energy creation in a black hole.

 

If this is the case then gravity holds as an attracting force only...more dark energy is created than went in to a black hole, and the creation of dark matter through the inflation period increases the volume of space-time. There's a hell of a lot more 'dark energy converting to dark matter' to take up space in our universe than observable matter...if this creation is happening at present then surely this would describe the 'acceleration of expansion'.

 

BTW Snail, the article you found by Srivastava is not actual LQG even though it says "loop" in the title. it is hard for me to place. could be a bit on the fringe, or could be connected to some part of string theorizing. I am no knocking it either! It has a lot of stimulating ideas and is not a bad thing to have found, but it is not like any LQG I'm familiar with so I am not likely to be of much help reading it!

 

================

EDIT. Afterthought. snail, if you want more mainstream quantum gravity reading suggestions ask me. I can point you to highlights of the most recent QG conference, areas where progress is being made, maybe some survey articles you could read

 

this is not necessarily however as going off into the wilderness by oneself without any direction and seeing what one encounters. what you are doing on your own is fine!

(but there is a more conventional way that might work too, if it didnt bore you)

[Royston]

To my knowledge there are no LQG predictions about doable smallscale gravity experiments. (effects are predicted down at planck scale but that is not practical to reach)

Snail' date=' your other questions do not seem to have to do specifically with LQG. You have some interesting ideas about dark energy, dark matter, and black holes. I am glad you are reading widely, including arxiv.org articles, and exercising imagination. I will try to respond but am not sure I can.

 

(I dont know what dark matter actually is, and can't imagine how dark energy could be converted into d.m. But it MIGHT be, so it is fine that you follow out the consequences.

Also I dont see how dark energy is depleted by expansion. Since its energy density is usually assumed to be constant. But these things MIGHT be, and I think you are developing some interesting lines of thought by concentrating on these possibilities. It is not standard cosmology as I know it, but that is not necessarily bad. A lot of people venture out in new directions.)![/quote']

 

Hi Martin, thanks again for a helpful response. I was just thinking about a model around a few things mentioned in the article...dark energy / matter are very much theoretical and there are many views on how it propogates and behaves, as I'm sure you're aware. I'm really toying with a few ideas that could maybe link collapse and inflation with creation of energy.

 

The flaw with my idea is that it would only hold with the early state of the universe...not when it's reached a smooth uniform state. More creation of dark matter + repelling force (anti-gravity) would mean an acceleration of expansion. I'm not sure that purely the increase of volume in an already uniformed universe would be enough for the expansion to be accelerating.

 

I agree I've totally swayed off LQG, so I'll need to a bit more background reading on it's specifics.

 

BTW Snail, the article you found by Srivastava is not actual LQG even though it says "loop" in the title. it is hard for me to place. could be a bit on the fringe, or could be connected to some part of string theorizing. I am no knocking it either! It has a lot of stimulating ideas and is not a bad thing to have found, but it is not like any LQG I'm familiar with so I am not likely to be of much help reading it!

 

lol, I realise this (albeit it's under a thread titled 'questions on LQG & LQC) this was why I was asking what stance LQG had on some of the contents in the article, but no offense taken.

 

================

EDIT. Afterthought. snail' date=' if you want more mainstream quantum gravity reading suggestions ask me. I can point you to highlights of the most recent QG conference, areas where progress is being made, maybe some survey articles you could read

 

this is not necessarily however as going off into the wilderness by oneself without any direction and seeing what one encounters. what you are doing on your own is fine!

(but there is a more conventional way that might work too, if it didnt bore you)[/quote']

 

I would very much appreciate some more mainstream material. It certainly wouldn't bore me...and I really need to get my head round quantum mechanics...beyond it's concept and history. Please bear in mind that I can't read equations, (so trying to understand quantum mechanics properly will pose as very problematic) it took me a long time to understand how E=MC2 fitted into the 'concept' of relativity...the latter came to me very quickly indeed, and I appreciated the models it threw out a great deal. It's amazing how quickly your view of reality can change in a few seconds !

 

As I've already stated in other threads I am going to study the subject seriously next year, until then, concepts are all I have.

[Martin]

I would very much appreciate some more mainstream material. It certainly wouldn't bore me...and I really need to get my head round quantum mechanics...beyond it's concept and history. Please bear in mind that I can't read equations' date=' ...

 

As I've already stated in other threads I am going to study the subject seriously next year, until then, concepts are all I have. [/quote']

 

I will suggest a few papers for you to try-----this is a way of learning what you can and like to read.

 

I will suggest one, you tell me "I can't read that" or "I do not like that", and then I will suggest a different one-----until I either run out of ideas or you find one that you like.

 

they may all have some equations, but sometimes one can still get something out of a paper even if one SKIPS the equations. with any paper be sure you read the introductory section and also the conclusions at the end, if there are conclusions.

 

the short paragraph on page 1 is usually the "ABSTRACT" which gives a concentrated summary---often confusing because it is so condensed.

 

the all-important introduction section usually starts on page 2.

 

if an article is a REVIEW or survey article or if it written for NONSPECIALISTS it will usually be easier to understand than a straight research paper. but a research paper may still have stuff in the introduction and conclusions that you can get.

 

here is something you should know about if you are interested in quantum gravity:

 

Loll et al.

The Universe from Scratch

http://arxiv.org/abs/hep-th/0509010

 

click on PDF to download the whole 30-page article

 

Please see if you can get something out of it.

 

also check out the author's website

http://www.phys.uu.nl/~loll/Web/title/title.html

 

on the homepage there you will see a menu with "Research" and "Press" among other things. If you select Research you get this:

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

 

this also gives a brief not-too-technical overview, Press also has some stuff in English but it is mostly in foreign languages and not so good.

I think the best is the 30-page survey for non-specialists "Universe from Scratch"

 

Loll CDT is temporarily making the most progress, I think, of any of the QG approaches. It is attracting more attention than LQG for the moment. So it would be good to know about it even if later you go back and read more about LQG and LQC.

[bascule]

So... if the formation of a black hole instigates a new "universe" (or at least one which seems causally disconnected from ours), wouldn't Hawking radiation allow some state information to be passed back from it somehow? (meaning the causal disconnectedness isn't total)

[Martin]

So... if the formation of a black hole instigates a new "universe" (or at least one which seems causally disconnected from ours), wouldn't Hawking radiation allow some state information to be passed back from it somehow? (meaning the causal disconnectedness isn't total)

 

the black hole information paradox has still not been resolved despite the highly publicized statement by Stephen Hawking at the Dublin 2004 conference.

 

The leading edge of this, as far as I can see, is the Ashtekar Bojowald paper and they make a big point of holding back from giving conclusive answers. the issues you raise are very much being hashed out and personally I dont expect a clear resolution any time soon like this year or next.

 

Bascule, I personally SUSPECT for what its worth and that's not much, that if spacetime extends in a new future direction from the pit so that spacetime CONTINUES in a new tract (which A and B are offering as a serious possibility) then information can NOT get back because it would be traveling backwards in time.

 

this means that I suspect that some information falling into the hole is actually lost from our branch of spacetime, never to reappear, and some of the information will somehow re-emerge as the black hole evaporates.

 

But as far as I can see it still is an open question and anybody's guess.

[Royston]

Loll CDT is temporarily making the most progress' date=' I think, of any of the QG approaches. It is attracting more attention than LQG for the moment. So it would be good to know about it even if later you go back and read more about LQG and LQC.[/quote']

 

I'll bear that in mind, thanks a lot for the links Martin, and for being so responsive with my 'BIG' questions.

[Martin]

Snail, the situation has improved as regards the availability of information about LQG and LQC, because the lecture notes and VIDEO from the October Loops conference is now available for download

 

here is the full program

http://loops05.aei.mpg.de/index_files/Programme.html

 

to my mind the best talks to download and hear are:

Ashtekar

http://loops05.aei.mpg.de/index_files/abstract_ashtekar.html

Rovelli

http://loops05.aei.mpg.de/index_files/abstract_rovelli.html

Smolin

http://loops05.aei.mpg.de/index_files/abstract_smolin.html

Reuter

http://loops05.aei.mpg.de/index_files/abstract_reuter.html

 

The Ashtekar talk is about LQC, what replaces the cosmological singularity, and the initial efforts being made to investigate conditions before the BB.

 

when you click on the Ashtekar link, you will see two links. One to the transparencies (which serve as lecture notes to read while listening) and one to the recorded talk.

 

It is a good idea to download BOTH the transparencies and the talk.

the talk will take a fairly long time to download. but I find that as you listen to one talk you can be downloading the next, so it is not so time-consuming

 

The Rovelli talk gives an overview of LQG and current developments.

 

There is also a talk by Renate Loll about CDT, but I found that the audio recording was broken up so that I sometimes could not hear what she was saying. Her research is making great progress and is exciting, but the talk is frustrating because no lecture notes and patchy audio.

[Royston]

Hi Martin, sorry for the late reply (have been very busy recently) and I'll be sure to have a look at the links you've kindly provided.

 

I managed to read some of the CDT article you provided in an earlier post, and I must admit I had never really appreciated curvature on small scales...I've always applied curvature in a more cosmological sense, and it's really opened my eyes to the complexity of correlating the small with the large...albeit this just goes back to GR (again) but still worth considering the constraints of quantifying small scale space-time and how it differs to the constraints of equating large scale space-time...obvious, but worth appreciating.

 

Unfortunately I've been so busy I havn't even been able to give much thought, or have had time to read and digest the article and others recently...but I have a few spare evenings ahead of me where I'll give them some proper attention. So this is more of a thankyou note than anything else.

[Martin]

Hi Martin' date=' sorry for the late reply (have been very busy recently) and I'll be sure to have a look at the links you've kindly provided.

...[/quote']

 

glad you had time to look at some CDT, Snail. About hurry I dont like to rush either. it is good to read something and then set it aside for a few days and let it soak in to the subconscious, and then come back to it if you feel like it in a day or two, and then let it mature some more, and so on.

 

right now I am periodically re-reading Smolins lecture notes from his October 10 talk----it is so interesting it pushing most other things out of mind----but then I take a break and do other things that I have to do, and when i come back I understand it better the next time.

 

what i am gradually digesting might not be good for you BUT IN CASE YOU ARE CURIOUS AND WANT TO TAKE A LOOK here are the links:

 

Here is the abstract with links to both the slides and the recorded talk

http://loops05.aei.mpg.de/index_files/abstract_smolin.html

 

Here is the lecture notes slides. I keep going back to the slides from around #35 to around #70. Not sure about the exact numbers. The slides are quick to download.

http://loops05.aei.mpg.de/index_files/PDF_Files/smolin.ppt

 

Here is the recorded talk. Allow at least a quarter of an hour for it to download---while you do something else

http://loops05.aei.mpg.de/index_files/Video/smolin.wmv

 

this Smolin talk is apt to be very confusing but IMO it is the most interesting thing in QG I have seen in some months-------CDT is also, but this Smolin thing is LQG and almost more maybe more interesting.

[tholan]

How is it made sure that the minisuperspace models that result from constraining the degrees of freedom of the geometry can lead to a sensible quantum cosmology if the quantized general relativity is basically a timeless theory?

[Martin]

... if the quantized general relativity is basically a timeless theory?

 

the symmetry-reduced models belong to LQC, which is not a timeless theory

 

in LQC the Hamiltonian turns out to be a time-evolution difference equation

 

originally LQC stemmed from a canonical version of the full theory, but it is it's own independent quantum cosmology and makes its own predictions! What it originally split off of is a historical issue.

 

The way to judge the validity of LQC is to derive predictions from it that can be tested. Two people who have done some work on that are Parampreet Singh (Penn State) and Roy Maartens (Portsmouth).

 

ultimately, one would expect to relate the simplified LQC theory to one of the FULL THEORIES OF QUANTUM GRAVITY but which? None of the full theories are completely worked out yet and one can only try to guess which of the approaches are considered most promising by seeing which ones the leaders in the field are working on.

 

If you ask string theorists what is LQG you may get their picture and it may have little to do with what is actually being worked on. The best way to get an idea of the field is to look at the program of the recent Loops '05 conference.

http://loops05.aei.mpg.de/index_files/Programme.html

The talks and slides are recorded and in most cases can be dowloaded---just click on the speaker's name to get the abstract and the links.

Especially check out the MORNING talks because they are the invited presentations by recognized leaders in the field.

For a sample of what research lines are active, look at the talks by Smolin, Rovelli, Baez, Loll, Reuter, Gambini, Freidel.

IIRC none of these speakers are concerned CANONICAL formulations involving a Hamiltonian constraint. So "timelessness" is not a feature. What the most people seem to be working on are PATH INTEGRAL or "sum-over-histories" approaches.

 

For example look at Carlo Rovelli, a core mainstream LQG figure. His paper is all following a spinfoam (path integral) approach. A few years ago he was focusing more on a canonical approach which you might have called "timeless". Actually it was not exactly timeless because it had time observables---it treated time in a relational way.

 

But now Rovelli's work is with spinfoams, which are FOUR dimensional----they are states of spacetime geometry. Time is obviously present in a very explicit way!

 

So how should I respond to your question?

... if the quantized general relativity is basically a timeless theory?

 

I guess the only thing to say is that it ISN'T "basically a timeless theory?"

Socalled "canonical" quantized Gen Rel can be called timeless because the Hamiltonian is a constraint, and then (as people did in the 1990s) the "problem of time" was addressed various ways and people found various ways to reintroduce time (again mostly in the 1990s). But IN CONTEMPORARY WORK the approach to quantize Gen Rel is not timeless.

 

Usually the people who have that misconception, in my experience, are string theorists. So I urge anyone who is interested to get some direct experience of the research lines represented at this year's Loops '05 conference.

 

Also tholan, I dont want to say that the canonical approach is bad, some people still work on it! Notably a group at Beijing and also Thomas Thiemann at AEI. And they may turn out to be right after all! But for the moment at least the pendulum has swung way over in the path-integral direction---that is where most of the progress of the past 2 or 3 years is appearing.