What is the relationship between loop quantum gravity and relational spacetime?

[Hoku]

I'm a lay person reading Three Roads to Quantum Gravity. I like the book very much but do have trouble understanding parts of it. Hopefully I can find clarification on a point of confusion for me.

I thought loop quantum gravity was based on relational spacetime - that spacetime wasn't actually a "thing" but a phenomena experienced from the relative events in the universe. Yet, in the postscript, it is said that there is a "possibility of observing the atomic structure of space itself". But, from a relational view, there IS no "space itself", right? Isn't "space itself" the language of an absolute view?

 

[Naty1]

Hello Hoku, welcome to the forums,

There is a LOT to digest in THREE ROADS...I been through parts of it two or three or four times...and still don't understand it all...If the author really understood any of the three roads, he'd know whixch one was right or that all were wrong...so don't feel bad you don't get it all...nobody does...

Have you read the whole book??...some questions I had during my first reading were addressed later in the book...

and the answer to your post is there are different perspectives on just what space and time are...

in a nutshell, nobody knows...nor what energy,mass, nor force are either...all seemed to have "popped" out at the start of the universe ala the big bang, or have always been around via an acordian like mechanism, branes expanding and contracting and bouncing off each other in finite sized bangs...

If you look at page 9, you'll see:

...three broad roads to quantum gravity...relativity gave birth to string theory, quantum theory to loop quantum gravity..

and he goes on right there to discuss Planck scale...
so you see right up front he's thinking something it appears you can measure. But oops then on page 134 he has Penrose spin networks, relational not physical..or are they?? he says they are at Planck scale, but real or imagined?

Turns out further research will reveal these spin networks are solutions to the Wheeler Dewitt quantum gravity equations utilizing Wilson and Polykov quantized loops extended to form these Penrose spin Networks...what the dickens does that all mean?...I don't know the math but what it means to me is that different mathematical approaches, based on different theories, seem related, just like the different versions of string theory are related via M theory...yet a lot remains uncertain...lots of ambiguity...
.
for a further extension of the spin network ideas, check out page 184...
turns out Penrose spin networks have been under consideration for maybe 30 years...and have not led as far as hoped...like string theory...so there seems to be more math than real understanding at this point..

It's been a year or two since I read the book so I'm not really conversant with many details, but if you'd like to discucss other things, might as well post them here and we can share viewpoints...maybe others will have the book or specific knowledge as well...a few have also read it...

 

[Hoku]

Thanks for the reply Naty1! After more than 80-views without comment, I was worried that my question was so painfully stupid that it wasn't worth response. I know that nobody really knows what spacetime is but I think we're all trying to figure it out and it's a topic of interest for me so I wanted to start a discussion.

One of the greatest things I liked and have gotten from this book was the idea of relational spacetime but, after reading more of the book, I'm not sure I can agree with it. If theorists are describing space as a discrete lattice from a relational point of view, why couldn't it have already existed as a discrete lattice before the big bang from an absolute view?

On page 18, Smolin says, "It is even absurd to speak of a space with only one thing in it, for there would be no relationships to define where that one thing is". But, if spacetime is absolute, then it would be the "stage" onto which the big bang exploded and its lattice would be the defining points for where that one thing is. Does that make sense? There is no "absurdity" from this point of view.

Here's another point that I may not be clear on. On pages 111-112, Smolin suggests that space is made up of the lines of force holding complimentary quarks together. What I'm unclear on is why those lines are in space when the quarks are right next to each other in protrons, electrons, etc. He says that if you pull the quarks apart, their lines of force stretch indefinitely, but most quarks are not being stretched, they are happily confined in particles. So why are their lines of force in space? (Is this another stupid question?)

Another issue I'm having is found on page 106. From pages 100-105, he is desribing one of the reasons they believe space is discrete in the first place. He says that a discrete space preserves the second law of thermodynamics as it relates to black holes. But on page 106, he says that if you half a volume of discrete space it creates two new regions that, together, give you more volume than you started with. Well, wait a minute, isn't this contradicting the FIRST law of thermodynamics?? This appears to be an issue. Or am I missing something? My first assumption is that I'm missing something, but I can't figure out what it is (perhaps a page I haven't gotten to yet?).

Finally, on page 183, he says that string theorists biggest problem is making the theory background independant. But, since we still don't know what spacetime is - and it could well be an absolute, defining structure for our universe in lattice form that the big bang fills into - then I don't see where the problem is. Maybe background dependence is the rule. Or, maybe, there is no quantum gravity at all! Either way, I'm an avid supporter of the search.

I'm really curious to know what other people think on this issue and hope that others will pipe in.

I have not read all of the book because it is difficult to read. I don't attribute this to my lack of formal science training or inability to understand. I can actually pinpoint the places that I have trouble reading past and describe the reasons why they don't work for me. I think they are valid reasons. So I try to skip around until I find something that makes sense again. Thanks for your page suggestions!

 

[Naty1]

First of all, as you read more and more you'll realize how little we understand but how much we can describe..."We know much we understand little".

If theorists are describing space as a discrete lattice from a relational point of view, why couldn't it have already existed as a discrete lattice before the big bang from an absolute view?

Theorists have no idea what initiated the big bang...nobody knows if space is continuous (as in relativity) or discrete as in quantum mechanics...all big bang theories have a start for unknown reasons and the THEORY is right after it starts. Roger Penrose does not even agree inflation solves the problems that need to be solved.

On page 18, Smolin says, "It is even absurd to speak of a space with only one thing in it, for there would be no relationships to define where that one thing is". Does that make sense?

Smolin I think refers to the fact to that speed, for example, is relative. You can obeserve a plane from reference (frame) of Earth and see it move a t one speed; from the plane it seems like it moves not at all; ever sit in a train at a station and watch another train go by..except for feeling of bumps, you can't tell who is moving...

But, if spacetime is absolute, then it would be the "stage" onto which the big bang exploded and its lattice would be the defining points for where that one thing is.

So far scientists believe that if the big bang is correct, nothing existed before it started...

Depends just exactly what you mean. Absolute spacetime was proven incorrect by Einstein since mass,energy,pressure curves/bends it and speed makes in contract...there not even absolute energy..kinetic and potential energy depends on the observer...

So why are their lines of force in space? (Is this another stupid question?)

Electromagnetic and gravitational forces all extend to infinity while decreasing in strength...gravity and electromagnetic vary as 1/R²...for example...I believe the strong force rises as quarks are separated then stays steady and really strong to infinity..that's why we can't find quarks floating around individually.

Another issue I'm having is found on page 106. From pages 100-105, he is desribing one of the reasons they believe space is discrete in the first place. He says that a discrete space preserves the second law of thermodynamics as it relates to black holes.

he is probably referring to the holographic principle here and definitely is on page 102 when he says "a law of physics that allows information to be converted into geometry..." and that gre from Beckenstein's bound which even Hawking did not believe at first...

So another theory is that we are all about information, NOT atoms nor lattices...

But on page 106, he says that if you half a volume of discrete space it creates two new regions that, together, give you more volume than you started with. Well, wait a minute, isn't this contradicting the FIRST law of thermodynamics?? This appears to be an issue. Or am I missing something? My first assumption is that I'm missing something, but I can't figure out what it is (perhaps a page I haven't gotten to yet?).

I highlighted that and had a question mark...I don't know. He is referring to Placnk scale geometries, about 10^-33 cm...and an aspect of loop quntum gravity...If you are interested, I'd pose that as a separate question in "Beyond the standard model"...He describes it a bit on the top of page 107..."As with volume, the theory limits the possible areas a surface can have to a finite set of values...jumps between areas.." so apparently some solutions of loop quantum gravity appear as something like 1/N, where N is any integer and you don't get values inbetween...(just a crude analogy)

Finally, on page 183, he says that string theorists biggest problem is making the theory background independant.

I'll see if I can find a current discussion in the forums on that issue...one thing you can search for is "emergent spacetime"...

But, since we still don't know what spacetime is - and it could well be an absolute, defining structure for our universe in lattice form that the big bang fills into - then I don't see where the problem is. Maybe background dependence is the rule. Or, maybe, there is no quantum gravity at all! Either way, I'm an avid supporter of the search.

I don't know of any major theorist who believes space is an everlasting absolute structure...at least I have no read that yet; but also remember "everybody" has been wrong in physics over and over and over.

I have not read all of the book because it is difficult to read.

I have read about 20 such texts and each has it obstacles to understanding...in part because the authors can only go so far in their own theoretical understanding...but each adds some new understanding and perspective...this stuff is VERY detailed and it took me about two years of reading on my own to begin to tie different concepts together...

Perhaps you should give Smolin's book THE TROUBLE WITH PHYSICS a try...it's also excellent and discusses what Smolin believes physicts don't understand or don't agree on among themselves. The ELEGANT UNIVERSE by Grian Greene and THE BLACK HOLE WARS by Leonard Susskind and BLACK HOLES AND TIME WARPS by Kip Thorne are all excellent.
I have read all once and most twice or more and am now repeating the last one again...

 

[Naty1]

Causal dynamic triangulation is another view of how spacetime might emerge from "nothing"...it's a viewpoint completely different from our discussions but related to spin foam networks and maybe even aspects of Penrose twistor theory...a search will turn up some discussions here in the forums...

We seem to have more theories than we do dimensions of space, that's for sure...

 

[Hoku]

Thanks again Naty1! You helped put my mind at ease. One of the greatest things I learned is that relational spacetime, although it's been an idea for some time, was first accepted on a mass scale from relativity. I thought LQG was the first to try and make use of the idea. Now I have a better platform from which to move forward, and some good book ideas, too! I'll definitely check them out.

 

[Naty1]

Who was actually first, and who gets credit for being first are often different!

For example, apparently Einstein was unaware of the groundbreaking work of Poincaire and others who almost, but not quite, discovered special relativity before he...Einstein later paid homage to those who would have beat him by maybe 10 or 20 years but just could not quite put all the pieces together...in fact Lorentz and Fitzgerald (of transformfame) could have also done it had they suspended their belief in "ether" and the variable speed of light...but they could not break free from the "old" concepts they had "learned"...

So here on the forums one has to be careful that because an "elegant" answer is provided, based on current theory, in believing its really accurate (correct). I have had long debates where several others have said "space and time are continuous" (because it is assumed that way in relativity) while I argued they are discrete, quantized, (because of ample theoretical but unproven concepts) and I stilll have an open mind...who knows??... since nobody knows what either space or time is, nor their precise origin and relationship...