Exploring String Theory & LQG: A Beginner's Guide

[mcgucken]

Could Someone Please Explain Exactly What String Theory and LQG Are Trying To Do?

I have recently had the opportunity to ask this question to a couple prominent physicists, and they could not answer it.

What is the purpsoe of these theories? What do they explain that is not explained by SR, GR and QM?

What are the postulates of String Theory? Are there any? Is there any premise as simple or beautiful as Einstein saying "The speed of light is constant in all frames," or "energy is quantized," or Bohr stating, "The electron orbits the nucleus at only discrete energy levels."

Does String Theory or LQG have any simple forumlas associated with them, such as E=mc^2, E=hv, or something like Shroedinger's Equation?

Do they account for non-locality as demonstarted in EPR, Bell's Theorem, and Aspect's experiments?

Does either theory unify Rel. and QM?

Finally, why should we have to unify relativity and QM?

Must we also unify Hiphop and Classical Music in some new musical genere? Or can we realize that each works in a given venue, and that music and the human heart underly both?

 

[selfAdjoint]

Could Someone Please Explain Exactly What String Theory and LQG Are Trying To Do?

I have recently had the opportunity to ask this question to a couple prominent physicists, and they could not answer it.

What is the purpsoe of these theories? What do they explain that is not explained by SR, GR and QM?

GR and QM are apparently not able to combine into a unified theory. One basic purpose of both strings and quantum gravity programs is to remedy this by achieving a theory that spans both and reduces to each as special cases. In addition string theory is motivated the belief that it does describe basic nature better that the previous theory, the Standard Model of Particle Physics. The Standard Model has about 19 undefined parameters in it, and another goal of physicists is to find explanations for these. It is one of the boasts of string theory that it does not have ANY undefined parameters. These claims of string theory are not belied by its present multivacuum difficulties.

What are the postulates of String Theory? Are there any? Is there any premise as simple or beautiful as Einstein saying "The speed of light is constant in all frames," or "energy is quantized," or Bohr stating, "The electron orbits the nucleus at only discrete energy levels."

The premise of string theory is contained in the Nambu-Goto Lagrangian; The local action of a string worldsheet is proportional to its change in area.

Does String Theory or LQG have any simple forumlas associated with them, such as E=mc^2, E=hv, or something like Shroedinger's Equation?

See above.

Do they account for non-locality as demonstarted in EPR, Bell's Theorem, and Aspect's experiments?

QM and all the research you site does not assert nonlocality as local realist theories would have it. It shows that local realist theories are false.

Does either theory unify Rel. and QM?

String theory claims its graviton reproduces Einstein's GR physics in flat spacetime. The quantum gravity workers are all trying to couple quantum matter to their various quantum gravities.

Finally, why should we have to unify relativity and QM?

From the time of Galileo science has been monist, looking for unified explanations. It bothers physicists that the two best theories around are so contradictory to each other.

Must we also unify Hiphop and Classical Music in some new musical genere? Or can we realize that each works in a given venue, and that music and the human heart underly both?

This confuses an artistic field where there's no one right answer with a rational one where you do seek for THE answer. BTW all through my lifetime classical composers have been trying, with variable success, to unify their music with the everchanging manifestations of popular music, starting with rag time and jazz.

 

[mcgucken]

The need for complete unification across disciplines (TOE) is that it would unify science, theology, and philosophy . . . a prerequisite for ameliorating religious and secular fundamentalism . . . without which tolerance and sustainability are but words.

Thanks for the answers!

There is only one science of the heart, and that is art. :)

I'm not sure that any physical theory will ever unify poetry and physics, without oversimplifying poetry and leading physics astray.

Has string theory had any successes in anything its attempted?

Has it unified quantum mechanics and relativity?

 

[Pengwuino]

There is only one science of the heart, and that is art. :)

Incorrect. It is called cardiology

 

[Hurkyl]

The basic programme of String Theory, as I understand it, is to take the wildly successful Standard Model, and tweak in a way that is likely to preserve the successful features of the Standard Model, yet also includes a graviton from which General Relativity can emerge. Also, it would like to provide a way to derive the fundamental constants of the Standard Model.


The basic programme of Quantum Gravity, as I understand it, is the modest goal of simply trying to figure out how to quantize General Relativity. The hope is that once this task is accomplished, it will be more clear how to merge Quantum Gravity with the Standard Model.

Loop Quantum Gravity is the branch of Quantum Gravity that postulates that basic geometric excitations take the form of loops.

 

[mcgucken]

When you say "LQG wants to quantize gravity," what exactly do you mean?

Why must gravity be quantized?

How will it be quantized?

Did quantum mechanics arise because somoene wanted to quantize photons/particles, or did quantum mechanics arise because photons/particles are quantized?

 

[Pengwuino]

Its not because they WANT to quantize photons and particles, its because it is quantized. We do things to find out how things truly are. Gravity doesn't have to be quantized but people do these things and study things and do various experiments to find out IF it is. Also, if we knew if it were quantized, we will know how its quantized and we can't just tell it how to be quantized.

 

[mcgucken]

Instead of qunatizing gravity, couldn't we just quantize time, space, or space-time?

This would explain the quantum behavior of small particles in and short distances, while allowing for the classical behavior of large objects and long distances.

 

[Pengwuino]

Well I am far as hell from an expert but that woudlnt explain quantized energy levels of say, electrons. Someone else probably has a better explanation.

 

[Hurkyl]

There have been experiments supporting the idea of quantum gravity:


The experiments essentially bounce ultra-cold neutrons off of a reflective surface, and then check what heights the neutrons could attain.

http://physicsweb.org/articles/news/6/1/9
http://www.newscientist.com/channel/fundamentals/quantum-world/dn1801

The end result was that the height of detected neutrons was quantized, thus strongly suggesting that gravitational potential energy must be quantized.

 

[Hurkyl]

Instead of qunatizing gravity, couldn't we just quantize time, space, or space-time?

Don't forget that, according to GR, gravity is space-time.


LQG has achieved a quantized space-time: the basis states are "spin networks" -- graphs whose nodes represent a "chunk" of space-time, and whose edges represent the boundary between neighboring chunks. The area and volume operators of LQG have quantized spectra.

 

[Hurkyl]

By the way, the quantum behavior of bound particles in the Standard Model is not suggestive of quantized space-time. Furthermore, this neglects free particles for which things like energy do not have discrete spectra.

(Incidentally, I do not know of a bound system for which the position operator has a discrete spectrum, but then again I don't know a vast array of examples)

AFAIK, people have tried to directly put QFT onto a discrete lattice, with little success.

 

[Chronos]

... Linear Quadratic Gaussian (LQG) theory and Brane theory, as variants of string theory...

LQG is [at least here] usually taken to mean Loop Quantum Gravity. I'm not aware of any association between Linear Quadratic Gaussian theory and string theory.

 

[vanesch]

Gravity doesn't have to be quantized but people do these things and study things and do various experiments to find out IF it is.

It is not so much that we have a choice, and the "unification of gravity and quantum theory" is not a matter of "what if" ? If gravity and quantum theory could exist peacefully together, that would disturb nobody, but they don't: both theories (general relativity and the standard model) give wildly incompatible predictions (even inconsistencies) when you push them into domains where both are relevant.

It is a bit analogous to the following story. On a faraway planet there's a civilisation of beings, the size of an ant and two geniuses have established 2 theories: Euclidianus has a theory of the surface on which they live (2-dim flat surface), and this works well for road and field work and coastal navigation. Newtonibus has a theory of planets being points and this works well to predict the motions in the sky. But both theories (flat Earth vs. points in space) are INCOMPATIBLE, however, if you speculate in domains which are far beyond the experimental reach of these creatures, like "what happens when I make a very very long road, how come that I cannot reach another planet" and so on.
So comes a new theory: planets are spheres !
This unifies both previous theories: in small enough areas, the surface of the sphere approaches the flat surface of Euclidianus, and at far away distances, the spheres look like points as in the theory of Newtonibus.
That doesn't change the fact that for all practical purposes, the theories of Euclidianus and Newtonibus are sufficient to explain about all phenomena in the sky and on the land, and that it will be damn difficult for these creatures to TEST the new theory.

cheers,
Patrick.

 

[Hans de Vries]

Could Someone Please Explain Exactly What String Theory and LQG Are Trying To Do?

Every major String Theory revolution seems to be aimed at making it more
difficult to falsify it... Job security?


In the seventies so many nice theories ended up in the waste bin because
of non-renormizability. The first String Theory revolution in the eighties then
made it easy to renormalize about anything. One could develop theories
without fear again.

But it gave an explosive number of theories. Surely from N different theories
there must be N-1 wrong? So who's right and who is wrong? Infighting
and conflict was the result. Then, the second String Theory revolution
in the nineties unified them all in 11 dimensions and peace returned.

Nevertheless impatience grew because it predicted no SM parameters.
Then, luckily, the latest String Theory revolution adequately solved this
nuisance with the Landscape model which tells the world there's no need
to predict anything since it's just all "random". And so, it relieved the
community from the outside pressures.

Two days ago there was a panel discussion about what the next String
Theory revolution could be:

http://www.fields.utoronto.ca/programs/scientific/04-05/string-theory/strings2005/panel.html

Why should you want yet another one when you've already achieved that
String Theory can't be falsified in a thousand years? Making it THE theory for
the next Millennium...


Regards, Hans

 

[mcgucken]

Every major String Theory revolution seems to be aimed at making it more
difficult to falsify it... Job security?


In the seventies so many nice theories ended up in the waste bin because
of non-renormizability. The first String Theory revolution in the eighties then
made it easy to renormalize about anything. One could develop theories
without fear again.

But it gave an explosive number of theories. Surely from N different theories
there must be N-1 wrong? So who's right and who is wrong? Infighting
and conflict was the result. Then, the second String Theory revolution
in the nineties unified them all in 11 dimensions and peace returned.

Nevertheless impatience grew because it predicted no SM parameters.
Then, luckily, the latest String Theory revolution adequately solved this
nuisance with the Landscape model which tells the world there's no need
to predict anything since it's just all "random". And so, it relieved the
community from the outside pressures.

Two days ago there was a panel discussion about what the next String
Theory revolution could be:

http://www.fields.utoronto.ca/programs/scientific/04-05/string-theory/strings2005/panel.html

Why should you want yet another one when you've already achieved that
String Theory can't be falsified in a thousand years? Making it THE theory for
the next Millennium...


Regards, Hans

Hello Hans,

I pretty much figured this as String Theory has not achieved anything in the realm of physics.

It's sad that it's given so much prominence.

 

[mcgucken]

Who are the top young String Theorists? What are their definitive papers?

Einstein wrote a few definitive papers. Debroglie wrote a definitive paper. Maxwell and Planck wrote definitive papers.

Have any string theorists written any definitive papers? I'd love to read the definitive papers, although I think I already have.

Who are the big names in string theory these days? Who are the young guns?

What's the latest of the latest? What's the buzz, hype, and new new thing?

 

[Hurkyl]

You could try searching for them. Marcus, for example, manages to bring up an amazing wealth of papers from people working on Quantum Gravity, and shares his findings in this very forum. I can't imagine it would be that hard to find papers on String Theory, if that's what you would prefer to see.

www.arxiv.org would probably be a good place to look.

 

[mcgucken]

But with Relativity and Quantum Mechanics, we know the definitive papers without having to google them.

And even if I found the greatest papers on String Theory, how would I know?

What are the leading journals for String Theory?

I think I know, but I might not. Thanks!

 

[Hurkyl]

A very brief amount of searching brings up this historical timeline:

http://superstringtheory.com/history/history4.html

I would surmise that if you find the papers behind those events, you would have found for what you're looking.

 

[quantumdude]

But with Relativity and Quantum Mechanics, we know the definitive papers without having to google them.

That's because over the years they have trickled down to the textbook level, so they are common knowledge to anyone who has studied physics in the last several decades. To find string theory papers you have to search for them. But as has been noted, it's not that hard.

 

[mcgucken]

But how come you can't name any top papers?

How come nobody can name any, and yet we can supposedly go find them without too much effort?

String theory is 30+ years old.

It didn't take Einstein's, Bohrs, and Feynman's definitive papers 30 years to "trickle" down.

So again I ask, what are String Theory's definitive papers?

If they don't exist, that's fine--it wouldn't surprise me.

 

[quantumdude]

But how come you can't name any top papers?

Because I'm not into string theory. Look, what is your objective here? To actually find these papers, or to thumb your nose at string theorists? If it's the former, then follow the lead Hurkyl gave you. But if it's the latter, then you will have to find another website on which to conduct your crusade.

 

[mcgucken]

I'm just wondering why nobody can name any leading papers on String Theory in a forum devoted to String Theory.

If someone could point out some major papers on String Theory, then perhaps we would have something to discuss in this forum.

 

[quantumdude]

I'm just wondering why nobody can name any leading papers on String Theory in a forum devoted to String Theory.

I thought you were wondering what the top papers are? If that is the case, then why don't you look for them? That was, after all, your supposed purpose for starting this thread. Or was that just a pretense?

If someone could point out some major papers on String Theory, then perhaps we would have something to discuss in this forum.

If you have nothing to discuss here, then by all means don't.

 

[mcgucken]

Because I'm not into string theory. Look, what is your objective here? To actually find these papers, or to thumb your nose at string theorists? If it's the former, then follow the lead Hurkyl gave you. But if it's the latter, then you will have to find another website on which to conduct your crusade.

The site Hurkyl gave me has absolutely no links to papers.

http://superstringtheory.com/

The site's a joke--no equations, no papers, no postulates, no fundamental principles--just the typical hand-waving and cool pictures.

 

[quantumdude]

The site Hurkyl gave me has absolutely no links to papers.

He gave you 2 sites. The other one is the arXiv. I know for a fact that string papers have been uploaded to that site.

 

[Severian596]

If someone could point out some major papers on String Theory, then perhaps we would have something to discuss in this forum.

If they don't exist, that's fine--it wouldn't surprise me.

Troll alert! LoL, forum personas make me laugh. This guy is looking for trouble.

 

[mcgucken]

Troll alert! LoL, forum personas make me laugh. This guy is looking for trouble.

I'm not looking for trouble.

I'm just looking for someone to direct me to a couple of the leading papers on String Theory.

Then we can pick one and perhaps discuss it.

Thanks in advance!

 

[quantumdude]

I've merged this thread with the thread, "Could Someone Please Explain Exactly What String Theory and LQG Are Trying To Do?" The content of the two was not sufficiently different to warrant 2 separate threads.

 

[marcus]

this was post #6 in the original thread
relating to the question "what string theory and LQG are trying to do"
and shed some light. want to keep tabs on this one and few others

The basic programme of String Theory, as I understand it, is to take the wildly successful Standard Model, and tweak in a way that is likely to preserve the successful features of the Standard Model, yet also includes a graviton from which General Relativity can emerge. Also, it would like to provide a way to derive the fundamental constants of the Standard Model.


The basic programme of Quantum Gravity, as I understand it, is the modest goal of simply trying to figure out how to quantize General Relativity. The hope is that once this task is accomplished, it will be more clear how to merge Quantum Gravity with the Standard Model.

Loop Quantum Gravity is the branch of Quantum Gravity that postulates that basic geometric excitations take the form of loops.

 

[marcus]

this is post #15 of orig.
another possibly illuminating one. Patrick quoted Pengwuino question and then replied.

Gravity doesn't have to be quantized but people do these things and study things and do various experiments to find out IF it is.

It is not so much that we have a choice, and the "unification of gravity and quantum theory" is not a matter of "what if" ? If gravity and quantum theory could exist peacefully together, that would disturb nobody, but they don't: both theories (general relativity and the standard model) give wildly incompatible predictions (even inconsistencies) when you push them into domains where both are relevant.

It is a bit analogous to the following story. On a faraway planet there's a civilisation of beings, the size of an ant and two geniuses have established 2 theories: Euclidianus has a theory of the surface on which they live (2-dim flat surface), and this works well for road and field work and coastal navigation. Newtonibus has a theory of planets being points and this works well to predict the motions in the sky. But both theories (flat Earth vs. points in space) are INCOMPATIBLE, however, if you speculate in domains which are far beyond the experimental reach of these creatures, like "what happens when I make a very very long road, how come that I cannot reach another planet" and so on.
So comes a new theory: planets are spheres !
This unifies both previous theories: in small enough areas, the surface of the sphere approaches the flat surface of Euclidianus, and at far away distances, the spheres look like points as in the theory of Newtonibus.
That doesn't change the fact that for all practical purposes, the theories of Euclidianus and Newtonibus are sufficient to explain about all phenomena in the sky and on the land, and that it will be damn difficult for these creatures to TEST the new theory.

cheers,
Patrick.

 

[marcus]

this was post #16, Hans responding directly to the part of the orig. question about what string research tries to accomplish.

Every major String Theory revolution seems to be aimed at making it more
difficult to falsify it... Job security?


In the seventies so many nice theories ended up in the waste bin because
of non-renormizability. The first String Theory revolution in the eighties then
made it easy to renormalize about anything. One could develop theories
without fear again.

But it gave an explosive number of theories. Surely from N different theories
there must be N-1 wrong? So who's right and who is wrong? Infighting
and conflict was the result. Then, the second String Theory revolution
in the nineties unified them all in 11 dimensions and peace returned.

Nevertheless impatience grew because it predicted no SM parameters.
Then, luckily, the latest String Theory revolution adequately solved this
nuisance with the Landscape model which tells the world there's no need
to predict anything since it's just all "random". And so, it relieved the
community from the outside pressures.

Two days ago there was a panel discussion about what the next String
Theory revolution could be:

http://www.fields.utoronto.ca/programs/scientific/04-05/string-theory/strings2005/panel.html

Why should you want yet another one when you've already achieved that
String Theory can't be falsified in a thousand years? Making it THE theory for
the next Millennium...


Regards, Hans

 

[marcus]

trying to paraphrase vanesch and hurkyl, connecting with earlier part of thread

Vanesch (Patrick wlcm back long time no see) says QM and GR need to be unified in a single theory (a quantized Gen Rel I guess) because in situations where they both are supposed to be applicable they DISAGREE!
It is disturbing to have models of nature that you believe describe things right which say different things. nature can't really be that way, can it?

So then Hurkyl says that String program and QG program are two very different ways of responding to the problem.
One (QG) simply tries to quantize Gen Rel. But Gen Rel is a geometric theory of gravity that says "gravity=geometry", gravity effects arise from the shape of the universe, from dynamic spacetime geometry. So that means that QG has to somehow get a handle on all the possible geometries of the universe. So in some way QG is modest and focused on one goal---quantizing Gen Rel. But that involves a somewhat dauntingly ambitious project of coping with all these shapes of spacetime.

The other programme (String) says Hurkyl, "is to take the wildly successful Standard Model, and tweak in a way that is likely to preserve the successful features of the Standard Model, yet also includes a graviton from which General Relativity can emerge. Also, it would like to provide a way to derive the fundamental constants of the Standard Model."
That is, you don't try to quantize the geometries, you take a static geometric stage on which things happen and you quantize a particle that carries the gravitational force or mediates gravitational interaction, just like photon mediates EM interaction. the nice part is you don't have to worry about a myriad changing uncertain spacetime geometry, you usually just take one smooth fixed geometry and let particles run around in it---that makes things easier. but there is also the hard, ambitious part, of wanting to include the whole standard model, all the usual particles, and add gravity in, and get a single picture.

so the first thing one notices is that the two programs have radically different aims. even if String program could be a smashing success (no clear evidence that is happening) even then it wouldn't satisfy the QG people. Because there would be a smooth fixed, usually flat, geometric background, in which the particles run around. Just seeing gravitons swimming in a static background wouldn't make them happy. they have to see expanding space, big bangs, collapsing stars, colliding black holes and all that very dynamic actionpacked highly curved stuff. they want all those possible geometries quantized in a blurry cloud of uncertainty. and geometric operators reading off information from it.

And even if the QG program overwhelmingly succeeded it would still not satisfy the String people because it would only quantize Gen Rel. It would not include all the colors of quark and the families of neutrinos and all that Standard Model business. It would just provide a quantum spacetime for things to happen in.

 

[Severian596]

I'm not looking for trouble.

I'm just looking for someone to direct me to a couple of the leading papers on String Theory.

Then we can pick one and perhaps discuss it.

Thanks in advance!

Hopefully I'm not repeating someone else, but have you looked at this forum topic yet?

https://www.physicsforums.com/showthread.php?t=8493