Alternatives to QFT

waterfall

I'm not asking if strings background is 11 or 25 dimensions. What I'm asking is whether gravitons are kinda like an effective field theory. Meaning if we would say use nonperturbative models, "gravitons" are no longer needed or don't have to exist? For example. If reality is ultimately not defined by strings or LQG but by some actual AsD/CFT scenerio where the actual things are in some distant surface or holographic. Then there is no gravitons although we can still use the analysis of flat spacetime + gravitons = curved spacetime maybe as exercise in a physics class for large scale limit or as dual?

marcus

WF you are conversing very well with Atyy and may get more from your Q&A with him. I watch various lines of QG research and get especially interested when one seems to be making strides. I don't have beliefs about what "reality ultimately is".

I do not suspect "there is some actual AdS/CFT scenario with things on a distant surface". Maybe Atyy has thought more about that and can discuss it with you.

There is a saying "It's not what Nature IS, it's how it responds to measurements." Most of the time that is what I have in mind when I think of physical models. The experimenter defines a state by measuring/establishing initial conditions, then he predicts future measurements, probabilities, expectations consequent on that, and checks. What we experience is a network of related events. That goes for geometric relations as well as other quantum fields that live on or in the geometry.

waterfall

So Craig Hogan Holometer idea is not supported by mainstream. Wonder how he got the million dollar funding for it. Anyway.

Beckenstein has this interesting article about the holographic principle:

http://www.phys.huji.ac.il/~bekenste...aphic_Univ.pdf

"CAN WE APPLY the holographic principle to the universe at large? The real universe is a 4-D system: it has volume and extends in time. If the physics of our universe is holographic, there would be an alternative set of physical laws, operating on a 3-D boundary of spacetime somewhere, that would be equivalent to our known 4-D physics. We do not yet know of any such 3-D theory that works in that way. Indeed, what surface should we use as the boundary of the universe? One step toward realizing these ideas is to study models that are simpler than our real universe."

So he is not entirely discounting that there is an actual AsD/CFT counterpart in our universe. Hope Hogan has the results soon so we can discount it or confirm it (if anyone has the results, then update us anytime in the future).

When you build a house. Would you build one with volume or just a wall if they both serve the same purpose. A wall would be fine and one can live in the wall. Lol...

marcus

Is there something wrong with the experiment? I don't understand your comment.

I do not suspect that low energy SUSY is right, but I certainly do not begrudge the money and time to test for it at the LHC!
A lot of people are skeptics about SUSY (and extra spatial dimensions) but I don't remember hearing them complaining about resources devoted to testing.

You probably know more about the Hogan experiment than I do, haven't followed that lately. so if there is something you think is wrong why not explain?

If you can't maybe someone else?

waterfall

Isn't it we were discussing it in this thread the other day

http://www.physicsforums.com/showthread.php?t=441577

I became aware of Hogan Holo-meter because it is the cover in this month Scientific American:

http://www.scientificamerican.com/ar...-space-digital

I interpret it as saying he is building the holo-meter to actually test if our universe is some kind of hologram something akin to AsD/CFT! If Sci-Am just exaggerate it to get audience. Pls. let us know the true purpose of the holo-meter.

marcus

I can't read the article. I heard about this in 2008, and posted in that thread in 2010. I have no fresh information.

As I say I do not suspect that the universe is a noisy hologram, or any kind of hologram. But I don't know any reason to object to the experiment. Do you? I can't say much because I don't know the details about the actual experiment.

waterfall

The experiment is just an long extended MMX like apparatus. See:

http://www.symmetrymagazine.org/brea...phic-universe/

There are many videos about superstrings but none about LQG. Why don't they make one?
About Superstrings. I wonder if you agree with the following site being labelled the official string theory web site.

http://superstringtheory.com/blackh/blackh4.html

Some interesting bits:

"Is spacetime fundamental?
Note that there is a complication in the relationship between strings and spacetime. String theory does not predict that the Einstein equations are obeyed exactly. String theory adds an infinite series of corrections to the theory of gravity. Under normal circumstances, if we only look at distance scales much larger than a string, then these corrections are not measurable. But as the distance scale gets smaller, these corrections become larger until the Einstein equation no longer adequately describes the result.
In fact, when these correction terms become large, there is no spacetime geometry that is guaranteed to describe the result. The equations for determining the spacetime geometry become impossible to solve except under very strict symmetry conditions, such as unbroken supersymmetry, where the large correction terms can be made to vanish or cancel each other out.
This is a hint that perhaps spacetime geometry is not something fundamental in string theory, but something that emerges in the theory at large distance scales or weak coupling. This is an idea with enormous philosophical implications. "

I wonder if you or Atyy has paper related to it. Aren't there other string theorists or enthusiasts here?

Mike H

I have to ask - do you have numbers backing up the claim of "billions of dollars have been invested in String theory", or are you pulling that out of thin air? Given that the National Science Foundation only has about $6 to 7 billion per year to work with for everything as of late, I'm not sure I can buy that estimate. While certainly there are other funding sources, it doesn't pass my order-of-magnitude sniff test.

Going back to lurking.....

marcus

Good point Mike H. Welcome and hope you post more. Theorists are not costly to support. Experimental physics is much more costly. I'm glad that string theorists have been handsomely supported for the past several decades so long as they don't abuse the privilege. It is unfortunate only in cases where they dismiss, discredit, and try to shut out other rival programs. Or hype their wares in such a way that it raises unrealistic expectations.

"Billions" sounds ridiculous to me, as I guess it does to you as well. I actually hadn't thought about it and don't feel confident I could make a useful estimate. Would you say that in the USA investment in string research has been perhaps 100 times the investment in LQG? (which is certainly not much!) or 200? or is it more like 500? Hard to say. Investment in string researchers seems to be declining though, judging by the declining rate of first-time faculty hires. Things may eventually come into balance.

Demystifier

Here is my (very very rough) estimate.
Assume that there are 1000 scientists in the world working on string theory. If each costs 100.000 $ per year, this gives 100 millions $ per year. Applying this number to the last 20 years gives 2 billions $. If half of that money is payed by USA, then it is 1 billion $ in last 20 years payed by USA.

bhobba

Some progress has been made in doing QFT non perturbatively and even in developing a completely mathematically rigorous version similar to what Von Neumann did for QM - but the mathematical difficulty is very formidable. In such a formulation it may be possible to solve stuff non perturbatively. That is not to say QFT is wrong - its just that mathematicians and physicists have different standard of rigour.

In normal quantum mechanics time and space are treated differently - time is a parameter - spaces is an observable. In a relativistic theory you really need to treat them on equal footing. QFT makes position a parameter so you deal with fields - the other approach of making time an observable evidently was tried - and failed - even though a textbook I have says it worked - people on this forum who know more than I do said it in fact failed.

To the best of my knowledge QFT is not faulty.

Thanks
Bill

waterfall

Your post reminds me of this unanswered distinction between time as parameter in non-relativistic QM vs coordinate thing in relativistic QFT and others treating parameter and coordinate as having same meanign so I wrote a thread in the relativity forum for this unresolved question http://www.physicsforums.com/showthr...52#post3777052

waterfall

I'm trying to find the connection between Renormalization Group and the Final Theory that can explain the RG being based on effective field theory. The above doesn't mention about Loop Quantum Gravity, just string theory and Asymptotic Safety. If Loop Quantum Gravity were proven to approximate classical GR. Won't it explain or complete why the Renormalization Group is only an effective field theory.. I wonder why you didn't include LQG above.

Haelfix

I realize that certain people on this forum have a tendency to get ahead of themselves, but I really don't think its ok to throw technical words together willy nilly if you don't understand what they mean.
The renormalization group is not an 'effective field theory'. It's not really a group at all! Its a set of partial differential equations (technically 'flow' equations) that explains the scaling behaviour of certain quantities in quantum field theory.

More to the point.. Before you can understand advanced topics like string theory, quantum gravity, and so forth, it really behooves posters to first learn some modicum of basic physics first!
I assure you, none of the advanced material can possibly make sense unless you get the logic, ideas and preferably the mathematics of the introductory material first.

suprised

Absolutely so, I was about saying this too. And I mean real textbooks, written by actual scientists, not books like Not Even Wrong. I see from the kind of questions being asked here, that some minds some completely corrupted by this kind of books, probably confused beyond repair! Sorry to say that.

marcus

Waterfall, I'm glad to see your friend Bill Hobba has joined us. He seems experienced careful and well-informed. Belated welcome, Bill!

I think I see what you are driving at (the unaccustomed use of some technical terms doesn't bother me in this case as long as the intuition comes thru.) I think there is a kernel of insight.

The RG-based approach (Asym. Safety) might be limited in its ability to resolve certain classical singularities and nevertheless it might be nearly right---effectively right within certain limits.

Let's imagine, just for the sake of illustration, that AS works as long as the underlying manifold which it requires is not going to develop singularities or defects---a topological condition. AS requires you to set out some prior metric on the smooth manifold you plan to be working with, for starters, so that scale can be defined in the first place. then it has some key numbers change with scale and run to a happy conclusion. But in its present form AS seems to be having trouble resolving the big bang singularity.

We can't use the word "effective" because that word is owned by people who do conventional perturbation theory--a type of math where you have a long series of numbers describing a blip on a flat background, and stuff like that. Each number is calculated according to its own elaborate formula and a theory is "effective" if you can just consider the low energy terms and it works OK.

We don't want to offend these gentlemen, so we need a new word like, say, "quasi-excellent" to describe what Asymptotic Safety might achieve. It might be effectively successful as a basis for quantizing gravity EXCEPT for not resolving the big bang singularity.

Because of the breakdown of conventional topology itself or some damn reason like that, so what's a poor theory supposed to do? if it's defined on a smooth manifold model continuum. It is effectively right except it doesnt quite make it where the basic topological or else smoothness assumption breaks down. So we call it "quasi-excellent"

I'm only half serious here, trying to imagine what you are driving at, by attempting a speculative illustration of what might be.

So then you say (to generalize a bit) suppose SOME quantum theory of geometry, Loop or some other, turns out to reproduce Gen Rel.

Then (I hear you reasoning) since Gen Rel is asymptotically safe, then that QG theory, Loop say, must be asymptotically safe. So it would be not only quasi-excellent, it would also resolve the singularity, so it would be fully excellent. It would complete the picture, geometry-wise.

And then you'd have to see if you could build satisfactory matter-fields on it.

It could be very convenient if Loop or some such QG turned out to underly and complete AS, then one could use AS, which is continuum-based and has a conventional manifold, all the way back in time to very near start of expansion and then seamlessly shift theoretical gears and continue on. But that's just speculation. People are only just getting started implementing RG-type stuff in Loop. Maybe some other related QG (like Oriti GFT or Livine's approach) is farther along. I dont have a complete picture, by far.

One extremely nice thing is the recent Cai Easson paper indicating that AS could give inflation "for free" just by the running of the couplings and without a made-up "inflaton" field having to be added on and finetuned. This is the nicest thing I've seen this year. Maybe someone will tell me why it doesn't work.
To me this makes it seem almost imperative that Loop should embrace and encompass AS, to acquire that yummy feature.

Anyway waterfall, I see sense in your post, rebounding off of the Atyy post you copied. IMO there's a valuable kernel of insight.

waterfall

Lol.. of course I know that. My post is in the context of the thread we were discussing in
http://www.physicsforums.com/showthr...=579379&page=2 where science advisor atyy (in message #20) replied:

"Renormalization has nothing to do with infinities. QED is renormalizable and it has a cut-off - it is not a true theory valide at all energies, it is only an effective theory like gravity, valid below the Planck scale. Once you have a cut-off, there are no infinities. Sometimes you are lucky and you get a theory where you can remove the cut-off, like QCD. But in QED, as far as we know, the cut-off probably cannot be removed."