What Do String Theory and Loop Quantum Gravity Aim to Explain?

  • #31
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
Hurkyl said:
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.
 
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  • #32
this is post #15 of orig.
another possibly illuminating one. Patrick quoted Pengwuino question and then replied.
vanesch said:
Pengwuino said:
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.
 
  • #33
this was post #16, Hans responding directly to the part of the orig. question about what string research tries to accomplish.

Hans de Vries said:
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... :smile:


Regards, Hans
 
  • #34
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.
 
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  • #35
mcgucken said:
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
 
  • #36
Thanks for all the feedback here, but all I'm looking for is one or two papers we could discuss.

If nobody else suggests a couple papers, or if nobody knows of any, then I will introduce a couple papers to discuss, which I believe are the leading papers/definitive papers on String Theory.

But it seems very strange to me that nobody can name a definitive paper.

When we discuss Relativity, be it SR or GR, there are a couple of very definitive papers, written by an Individual--Einstein.

When we discuss QM, there are perhpas more definitive papers, but there are definitive papers, written by Bohr, Planck, Einstein, Debroglie, Heisenberg, Dirac, Pauli, Shrodenger.

But somehow String Theory seems to get by without any definitive papers or individuals. I know there's Brian Greene and Edward Witten, and I've read some of their scholarly work, but I wouldn't call it definitive, as it doesn't add up--it's more of a promise and a request for faith.

Thanks again for all the feedback, and I hope I'm not stepping out of bounds, but to have a proper discussion of string theory, it seems we would need to start with string theory's central postualtes, which would be in a definitive paper somewhere.

Thanks!
 
  • #37
I move that we discuss Witten's foundational paper which includes Witten's postulates of String Theory.

After that I propose that we discuss Brian Greene's revolutionary paper on String Thoery and the Laws of String Theory he lays out.

I suppose we can continue the discussion in this thread. I have the papers--can anyone find any online copies or online descriptions of the central postulates of String Theory?

Feel free to post away concerning Witten's and Greene's postulates.

Here's the wikipedia page on Witten:

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

Here's the wikipedia page on Brian Greene:

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

Unfortunately neither of these pages describe their postulates of String Theory, nor link to any of Greene's nor Witten's definitive papers. Perhaps we could add that information--wikipedia is "open source."
 
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  • #38
I apologize if I have done anything to inhibit this thread.

I agree that we should only discuss theories here, and not personalities. I am sorry for bringing up Brian Greene and Edward Witten. From hereon out let us discuss only their postulates and laws, but not their names, quotes, tv shows, assertions, musings, opinions, promises, nor personalities. Physics, after all, must be based in physics.

Let us only discuss String Theory's leading postulates, laws, and predictions in the perfect vacuum it deserves.

Thank you.

Please feel free to post a postulate, law, or prediction of String Theory, but nothing else.
 
  • #39
In defense of a Poetry/physics link

mcgucken said:
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?

You are probably correct that a physical theory, as such are now defined, could ever unify physics and poetry. However, such would be a requirement if there were to be a TOE. TOE, plausible or not, has attracted some serious attention.

TOE must define, in addition to the phenomena of physics, both life and consciousness in physical terms. The definition of life is not now clearly defined across all disciplines; and less is known of its origin; however, many scientists are attempting an understanding of both.

Consciousness is equally difficult, if not more so, to define. A definition might possibly be accomplished by reducing consciousness to a form of exceptionally rapid, complex, analog feedback.

Poetry is a result of consciousness (possibly, a feedback response from a person’s fundamental physical origins; much like a physical massage). If consciousness/life were physical manifestations, it would seem to follow that physics and poetry might have a similar origin. Such is the quest of TOE.

I agree ENTIRELY with your various assessments of string theory throughout many posts.

However:

String theory, simply put: tries to explain, mathematically, fundamental physical phenomena in such a manner that the phenomena is unified and reconciles with observation.

String theory incorporates some accurate ideas; that are observable on a macro scale (compared to the scale of many/most strings); such as: seminal energy (“dark” energy) that is manifested as vibrating (actually, complex oscillating) strings.

String theory errs when it incorporates, or attempts to explain, the irreconcilable theories of conventional physics’ standard models, which have proven to be incorrect except under specific conditions or parameters. The standard models in their present form are contrived; and no fundamental theory, as string theory claims to be, can be expected to integrate them.

Strings theory attempts to incorporate physics’ conventional, contrived forces that are ill-defined in a manner such that most knowledgeable persons trained in philosophical logic would think that physics relies upon metaphysics.

String theory is also incomplete in that it does not precisely, and reconcilably, define, mathematically, the internal structure of its strings or their motion, which motion can be construed as seminal motion.

String theory also is silent concerning the etiology of inertial forces as observed in nature as demonstrated by accelerating galactic recession.

String theory is correct in assuming that an infinite source of energy manifests as complex oscillations and emanates from an undetermined source (and, also . . . so returns to this source).

These oscillations, which are complex amalgams of slide, swing, and vibration, must be defined mathematically so that they incorporate all the observed properties of nature, which are properties that must also be mathematically expressed. Simply: sinusoidal and elliptical equations must be related in a relativistic manner at the macro and micro levels.

A starting point, leading to new physical paradigms should not be too difficult; these new paradigms must consider the geometry and source that would, together, describe the genesis of these complex, seminal oscillations and their etiology as they morph to mass.

Loop Quantum Gravity (LQG) theory and Brane theory, as variants of string theory, are subject to the same above limitations; they are imperfect mathematical tools working at irreconciled limits that are imposed by conventional physics.

A purpose of these theories is to unite SR GR and QM in such a way that natural phenomena can be explained with a single and/or a few fundamental concepts. Their prospects of success without a “new physics” as proposed by Weinberg the philosopher/physicist is most unlikely.

A unification of physics and poetry is as dependent upon the natural origins of number theory as it is on defining the natural origins of “action-at-a distance.”

The most simple formulas associated with fundamental number theory are: 1.) “epsilon equals one,” which has to do with the proof of one and a most unusual quality of all ellipses; and, 2.) “the natural function, x^2 – x,” which mathematically, heuristically, represents a soliton that is a wave function found in all natural phenomena.

If TOE should ever be found, physicists must lead the way, as philosophers and theologians are not equipped to recognize the proofs.

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.

This diatribe is all because I feel a need to defend a physics/poetry link. :)
 
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  • #40
Could someone please post a postulate, law, or prediction of String Theory?

We would all very much like to discuss it!

Thanks!
 
  • #41
mcgucken said:
Could someone please post a postulate, law, or prediction of String Theory?

One important basic postulate of String Theory, is that
LQG won't work

this is known as the Motl Axiom and is invoked to prove an important theorem, the One Best Hope Theorem:

String is our one best hope of a theory unifying QM and GR

Your next statement, Mcgucking, is quite possibly mistaken, unless ironically intended

We would all very much like to discuss it!

[the above post is kidding]
 
  • #42
Well I just caught on to the fact that Dr. McGucken has a similar format to PF site

http://physicsmathforums.com/index.php?

http://physicsmathforums.com/

and it evidently has parts that haven't been posted in yet
and the maximum visitors was in March 2005
so probably is asking for some aggressive promoting

so now I think i understand better why we have some commotion and
occasional argumentativeness

Mcgucken I believe you would like to attract some of us (not me because i am basically too stodgy but some of the faster crowd) to come over to your Forums site! I think that is fine and perfectly legitimate and i hardly blame you since there is a fine bunch of people here! You would naturally like some to come and be members. That is all well and good.

However it means that you personally are not someone whom we can assimilate to the PF culture and all that, such as it is (and it is pretty nice actually) because you HAVE YOUR OWN OUTSIDE THING.

So I think I will not try to pursue this thread and discuss what LQG and other approaches to quantum gravity are trying to do---which is pretty hard any way. quantum gravity people ARE trying to achieve something, but it really is not easy to talk about IMHO.
 
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  • #43
Hello Marcus,

I am only trying to discuss the postulates of String Theory, wherever I can on the internet.

I'm assuming that the postulates will be the same, assuming that all posters are in inertial frames. :)
 
  • #44
The postulates of ST are assumptions

mcgucken said:
Could someone please post a postulate, law, or prediction of String Theory?

We would all very much like to discuss it!

Thanks!
As I understand ST, its current postulates are assumptions.

ST assumes that fundamental energy evolves and manifests in the form of strings of varying or unknown definition, in an unknown manner, from an unknown source.

These strings are usually said to vibrate; however, the better string theorists realize that they oscillate; though, I am not aware of anyone that has described the geometry or relativity of said oscillation in detail. From what I understand the only validity of ST is the assumption of oscillating energy. If the internal structure of this energy was understood and all else thrown out, ST might have some redeeming merit.

Then, ST attempts, with the “vibrating” strings to explain the etiology of light and gravity from the coalescence, or whatever, of said string phenomena. Thus, light and gravity would have a common foundation; and thus, be unified. At present ST depends on conventional theory to explain the process of “whatever” and that which is being unified.

Assuming that the problems of paragraph one and two can be resolved (they should require little more than a desktop computer, philosophical logic, and a day or so to reconcile with current observation.); ST still can not achieve its goal. You cannot reconcile ill-defined forces that are currently defined such that they are little more than metaphysical (requiring much faith to accept) concepts.

Understanding nature cannot be accomplished from the “top down.” A good beginning would be to agree on a most fundamental concept and then determine its source and evolution. I know of nothing that “exists” that does not have motion; so motion of “nothing” would be a good starting point. Next its source, geometry, and “nothing” must be pinned down. Once this is done, the other parts of the jig-saw puzzle should quickly fall into place, as we have observation to guide the procedure.

One must be careful not to go astray because of the influence of current academic theory.

And, of course, as McGucken/Astro is aware, the puzzle’s solution should not be rushed, as it will detrimentally influence many grants and consequent sinecures.

Imagine, if metaphysical gravity waves were shown to be a hoax how many jobs would be lost. Caltech, alone, is approaching a billion dollars on a single gravity wave seeking experiment.
 
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