Answer your string questions for a nickle!

[BenTheMan]

I started this thread for general stringy questions, if anyone is interested. Discussion is carried over from http://www.scienceforums.net/forum/showthread.php?t=28281

 

I get that we must be smaller to observe them. Like a mite walking around the ant that is walking around the rope right? But why 10? Why not 1000 or an infinite number?

 

Wormwood---

 

Good question.

 

This is actually one of the great successes of string theory---the fact that it predicts a number of dimensions is a success that no other approach to quantum gravity can claim, yet. The dimensions are predicted from the theory, by the requirement that the theory be consistent with quantum mechanics.

 

In order to quantize a theory consistently, we have to introduce an idea called gauge invariance. This basically says that physics cannot depend on the way that we describe nature. In string theory, a one dimensional string moves through space, sweeping out a two dimensional world-sheet. Gauge invariance for the world sheet means that we can assign coordinates in any manner that we wish. In order to actually do a calculation, though, you have to PICK a set of coordinates---this is called choosing a gauge.

 

The idea to use gauge symmetries to describe nature is not a new one, and is something that you would learn about in the second semester of a good quantum field theory class This is something that Glashow, Weinberg and Salam received the nobel prize for in 1979.

 

Anyway, when you choose a gauge, you find that the theory has a bunch of unphysical stuff---stuff that would NORMALLY wreck a theory. But this stuff depends on the gauge that you choose, so we know that the stuff is truly unphysical.

 

Sorry if this is confusing. Think of it like ths... Maybe you remember solving projectile motion problems in phsyics class. These are problems like, ``you fire a cannon with such and such an initial velocity and such and such a trajectory. Calculate how long it takes for the cannon ball to hit the ground.'' Anyway, you would solve the quadratic equation and get answers like t = 0 and t = 10 sec. You KNOW the t = 0 answer isn't right, so you toss it out and keep the other one.

 

In string theory, though, you find out that consistently tossing the unphysical stuff out REQUIRES that we live in ten dimensions. This is a definite (but useless!) prediction of string theory---we live in 10 dimensions.

 

I stress, this is a prediction of the theory. One can do general relativity in an arbitrary number of dimensions, for example. This is one thing that (I think) bugged Einstein---he couldn't explain why we lived in 4 dimensions. And, as far as I know, no other approach to gravity derives the number of space-time dimensions.

[YT2095]

neat idea!

and I have a few I`de like to ask:

 

1) what are strings made of?

2) can you break/Cut these?

3) where did they come from?

 

and if you can keep it in plain English with little to no math, like you were trying to explain this to a 4 year old, Thanks

[BenTheMan]

Hmmm. Hopefully I won't make any mistakes, but if ajb is floating around here, maybe he can correct them for me

 

Well, these questions are pretty good, and some of them don't have answers.

 

1.) As far as we know, strings are made of...strings. IF string theory describes our universe, then a string is truly fundamental. This is a bit like asking, ``What are electrons made of?'' Well...electrons.

 

Now, you could try to get a bit fancy and claim that electrons are made of energy, in which case you would be happy to accept that strings are also made out of energy.

 

2.) You can break a string. This is how you preform scattering experiments.

 

 

This is also one of the triumphs of string theory---it gives general relativity in the low energy limit. So, suppose you compute two closed strings going to two closed strings, as above. The one loop corrections look like:

 

 

You'll notice there are close analogues with particle physics.

 

Now, once you compute these scattering amplitude, you find exactly general relativity! This is a BIT unexpected, as the theory was constructed from general quantum field theory considerations. So we didn't ENGINEER GR, we got it for free---it fell in our laps, so to speak. I can't describe this in any detail because I've never actually done the calculation, but I have it on good authority that this is the case

 

In other approaches to quantum gravity, they start with GR. But then they have to ENGINEER particle physics. In some sense, we started with particle physics (quantum field theory), and got GR for free.

 

3.) Where did strings come from?

 

This is a bit more philosophical. I could say ``The Big Bang'' and be done with it. Again, this question is a bit like asking where electrons come from. And it may be that this question doesn't really have an answer.

 

Sorry if this is less than what you expected! But there are many questions that may never be answered, and you've stumbled on a few of them.

[Martin]

I started this thread for general stringy questions, if anyone is interested. .

 

Good idea!

My question is about the SIZE of the stringy research community. For me, bigness does not necessarily equal effectiveness or quality and I would like to know your opinion about this:

if the US string community is in process of DOWNSIZING, could this translate into more significant research, tackling the persistent problems, more real progress?

 

I'd like to know your viewpoint on this (if fewer but better quality papers, from your personal perspective, were being produced each month you would have less reading to do just to keep up and more time for original work, I would guess---can you personally see advantages to belonging to a smaller research community?)

 

Just to round out the question, very briefly (10 words or less?) what are the persistent fundamental problems with the stringy framework?

[BenTheMan]

My question is about the SIZE of the stringy research community. For me, bigness does not necessarily equal effectiveness or quality and I would like to know your opinion about this:

if the US string community is in process of DOWNSIZING, could this translate into more significant research, tackling the persistent problems, more real progress?

 

Maritn---I can't speak about the size of the community, specifically. It has always been tough to find faculty positions in the US in string theory, and you can check the Rumor Mill---the numbers of string faculty hired for the past several years has been more or less constant.

 

The HEPAP report recently suggested expanding the amount of money available for string grad students and post docs, so if anything I think there will be a slight upturn. The flip-side is that big, multi-national collaborations like ILC, LHC, and Planck will draw many workers into the field of phenomenology.

 

The graduate students that I know are mostly accepting of the fact that they'll never get post docs, much less faculty positions. Many of them are just fine with going into industry and starting with six figure salaries. There are lots of positions in China, India, and Europe, so the people who really want to do physics generally don't mind going to these places.

 

Just to round out the question, very briefly (10 words or less?) what are the persistent fundamental problems with the stringy framework?

 

One can certainly say more than ten words here.

 

The biggest problem with string theory is that it's a hard problem. I think that moduli stabilization (i.e. controlling the size of the six extra circles) is probably pretty poorly understood. For example, the Type IIA people (like Cvetic, Blumenhagem, et al) who build d-brane models can stabilize their moduli using fluxes, but can't get realistic phenomenology. Worse, there is no unique way to turn the fluxes on, so the moduli problem is restated as a ``flux turning-on'' problem. This aside, the fact that the moduli CAN be stabilized is a pretty big success.

 

But the type IIA models don't have very much to do with reality---they have poor low energy phenomenology, and are fine-tuned. Conversely, our models (which have excellent low energy phenomenology) have no way to stabilize the moduli.

[YT2095]

Thanks! but I have More questions if that`s ok?

 

1) are there different Types of strings, like different Charges and such so Like strings repel etc...

 

2) How do Photons fit into this plan?

 

3) how would I go about controlling some of these strings?

 

4) what makes you think this is the correct theory?

[Martin]

if the US string community is in process of DOWNSIZING, could this translate into more significant research, tackling the persistent problems, more real progress?

 

thanks for responding to the other part of my question. You didn't answer this part though.

 

HEPAP reported a planned 20 percent cutback in stringy theory faculty (grantsupported at universities) and we've been seeing the publication numbers going down since 2002 or so. You envisage many grad students going into industry (finance is what i hear most often). so there is considerable evidence of an ongoing downsize trend. So what I am asking is, IF that is in fact going on, do you see it as GOOD FOR THE THEORY ITSELF or not?

 

What advantages if any, do you see to having a smaller (but possibly more select) community publishing fewer (but possibly more significant) research papers?

 

I am distinguishing between the people and the science. Obviously one can't simply equate the personal welfare of individuals in the community with the interests of scientific theory development.

 

You say something very interesting

The graduate students that I know are mostly accepting of the fact that they'll never get post docs, much less faculty positions. Many of them are just fine with going into industry and starting with six figure salaries. There are lots of positions in China, India, and Europe, so the people who really want to do physics generally don't mind going to these places.

 

So if what you say accurately reflects the situation, then the individual welfare issues are quite OK by your standards, and not to worry. But what about the effect of string downsizing on the quality and possible success of the research itself? That's what I was originally asking about. Any views?

[BenTheMan]

HEPAP reported a planned 20 percent cutback in stringy theory faculty (grantsupported at universities) and we've been seeing the publication numbers going down since 2002 or so.

 

The decrease was in faculty, but the panel reccomended an INCREASE for graduate students and post-docs, I believe. If I am wrong, then I am wrong, but I remember reading on several physics blogs things to this effect.

 

so there is considerable evidence of an ongoing downsize trend. So what I am asking is, IF that is in fact going on, do you see it as GOOD FOR THE THEORY ITSELF or not?

 

I don't know if there is evidence for a downsizing trend. More graduate students are studying string theory now because Elegant Universe came out when they were all in high school.

 

As far as being good for the theory, who's to say? Obviously if there are more people working on a problem, progress is more likely to be quick.

 

Martin---I'd really rather stick to the science, as opposed to the sociology.

 

YT---

 

I will respond to your questions in a bit. I must go in to work now:)

[Martin]

More graduate students are studying string theory now because Elegant Universe came out when they were all in high school.

HEPAP report page 47 "..continuously declining ability to recruit students and to support them effectively once they are engaged...". But I am asking about the SCIENCE and your point about grad student support is more about the sociology,

so I will not say more.

 

Martin---I'd really rather stick to the science, as opposed to the sociology.

 

Yes! I too!. That is what I am asking you about----the effect on the science. Whether what is going on now could actually be good for the THEORY (if a falsifiable fully scientific theory develops, or if not, the proto-theory).

Present trends in the community might turn out to be a blessing-in-disguise-type HELP for the development of a fully predictive scientific theory. The question I'm asking is about science itself (as opposed to people-interest).

 

It's a vital question. If you prefer we could set up a special thread just for this question. Like "Could downsizing in the string community help progress towards a falsifiable theory?"

[BenTheMan]

That is what I am asking you about----the effect on the science. Whether what is going on now could actually be good for the THEORY (if a falsifiable fully scientific theory develops, or if not, the proto-theory).

Present trends in the community might turn out to be a blessing-in-disguise-type HELP for the development of a fully predictive scientific theory. The question I'm asking is about science itself (as opposed to people-interest).

 

I thought the context was clear. I wish to answer question about science, not about long term goals of the field.

 

Either way, it probably isn't good for any field to be downsized. When money stops going in to a field, when jobs become harder to find, more smart people would rather get a real job than worry about having to find the next post doc. A good friend of mine left strings for this reason, and now pulls down about ten times my graduate student salary in Chicago (before bonuses). And getting a post doc for him (and probably even a faculty position) would have been cake. He has over 150 citations of his work (http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+A+FRIESS&FORMAT=wwwcitesummary&SEQUENCE=), which is HUGE for a grad student.

 

When more people work on a problem, progress is faster. There will always be people like Witten and Polchinski in the field who set trends and solve hard problems, but there are tons of small problems that a graduate student of average intelligence (enter: me) can solve. And all of these little problems have to be solved to have any progress whatsoever.

 

The major thrust in the next decade will be explaining LHC/Planck data (more the former than the latter), and trying to get international support for the ILC, unless the LHC sees split SUSY or extra dimensions. Barring these unlikely events, more students will go into phenomenology because there will be tons of jobs for them (at the grad student level).

 

Outside of high energy, recent advances in biology tell me that biophysics is the next big thing. And of all the grad students admitted to OSU with me, only two others (of 25 or so) were even remotely interested in high energy. Mostly people were interested in biophysics and condensed matter (OSU has a great AMO program).

 

So the field I think is stagnant, and if the US institutions are downsizing, then China, India and Europe are on an upswing---there will be jobs in String Theory in the next ten years, but probably not as many in the US. This, again, is not necessarily a bad thing. The focus in the US will probably be towards experiment in the next 20 years, especially if the ILC is built here. Even longer term, if the VLHC is delayed, string theory will probably take another upturn, as it did after the SSC was killed during the Clinton administration.

[Wormwood]

In order to quantize a theory consistently, we have to introduce an idea called gauge invariance. This basically says that physics cannot depend on the way that we describe nature. In string theory, a one dimensional string moves through space, sweeping out a two dimensional world-sheet. Gauge invariance for the world sheet means that we can assign coordinates in any manner that we wish. In order to actually do a calculation, though, you have to PICK a set of coordinates---this is called choosing a gauge.

One dimensional? How is that even possible? A two dimensional world sheet sounds strange enough, like a universe with no depth, but one dimensional seems physically impossible.

 

Anyway, when you choose a gauge, you find that the theory has a bunch of unphysical stuff---stuff that would NORMALLY wreck a theory. But this stuff depends on the gauge that you choose, so we know that the stuff is truly unphysical.

So if I understand you correctly, when the equations are done (and I do remember that example from physics class) the alternative answers are impossibilities like 0 or -1, and since we KNOW that there are more than 0 or -1 dimensions we use the other answer which is invariably 10. Is that the gist of it?

 

I have another question ( I know you are getting bombarded and I appreciate your time and effort in answering all of this). How do we know that all 6 extra dimensions are smaller? Is there any chance that one or more of these dimensions is too large for us too notice and we just assume it is smaller? I guess I'm asking if there is any specific reason why the dimensions must be smaller.

 

As a sub-question, is it possible that 10 dimensions are just an anomaly of our particular point of reference? By that I mean, could the 10 dimensions operate as smaller, larger, and relative so that there is always a range of 10 dimensions with varying sizes in comparrision to the observer, no matter what size you are, or is it a set and definites series of dimensions that are well explained?

[the tree]

What does it mean for a dimension to be larger or smaller? Or "all twisted up" as I remember a physicist saying to me once. From my understanding of a dimension (kind of like a direction except each must be orthogonal to all the others), that doesn't seem to make sense.

 

Secondly, if 10 dimensions is a prediction, then how can it be tested? Has an experiment been thought up/performed to check it?

[YT2095]

dunno if This helps, but in chem we have ortho, meta, and Para and then we have Cis and Trans (that`s rotational).

 

spatial there is 720 degrees (pilot flying).

[BenTheMan]

Great--so many questions!

 

Take a number, and get your nickle out.

 

1) are there different Types of strings, like different Charges and such so Like strings repel etc...

 

YT---the string is the fundamental object. There are not ``types'' of strings as there are ``types'' or particles. Depending on which string theory you're working in, you can have open string, closed strings, or objects called d-branes.

 

2) How do Photons fit into this plan?

 

One should remember that string theory incorporates quantum field theory naturally. At energy scales where photons become important, the physics is exactly the same. This is one of the reasons why it's so hard to test string theory. You can think of the photon as a string vibrating with a specific frequency. (This is why I said Farsight's ideas were similar to string theory in the other thread, except he's calling `photons' fundamental.)

 

3) how would I go about controlling some of these strings?

 

Well, if you could controll strings, you could controll space-time. One would have to understand the theory very well before that could happen.

 

4) what makes you think this is the correct theory?

 

This is a trick question I think strings is correct because of the fact that so many things naturally fall out of it. In other approaches to quantum gravity, you have to tickle things to make them work correctly. I was in a bar once drinking with Joe Polchinski, and I got him to drink a toast to Loop Quantum Gravity. I was with a group of grad students, and we talked a bit about the LQG program. Joe said that he liked string theory because he felt like he was ``discovering'' things, not ``inventing'' them.

 

This is why I like string theory---things like chiral fermions, number of families, gauge symmetries... From studying the low energy data, we have absolutely no reason for any of the phenomena that we observe to be as they are---the standard model just IS. But string theory offers a nice way to understand WHY we have things like three families, gauge symmetries, and chiral fermions. As far as I know, these things are all put into other gravity approaches by hand, along with the number of dimensions, a cosmological constant, the black hole entropy, ...

 

 

NEXT

 

One dimensional? How is that even possible? A two dimensional world sheet sounds strange enough, like a universe with no depth, but one dimensional seems physically impossible.

 

One dimension just means that you can describe any point on the string with one number.

 

So if I understand you correctly, when the equations are done (and I do remember that example from physics class) the alternative answers are impossibilities like 0 or -1, and since we KNOW that there are more than 0 or -1 dimensions we use the other answer which is invariably 10. Is that the gist of it?

 

I think so.

 

How do we know that all 6 extra dimensions are smaller? Is there any chance that one or more of these dimensions is too large for us too notice and we just assume it is smaller?

 

Well, four of our dimensions are infinite, so it's hard to be larger than that But if you're asking if all of the dimensions have to be the same size, then no---this is one of the papers that I (and tons of other people) am currently working on.

 

As a sub-question, is it possible that 10 dimensions are just an anomaly of our particular point of reference? By that I mean, could the 10 dimensions operate as smaller, larger, and relative so that there is always a range of 10 dimensions with varying sizes in comparrision to the observer, no matter what size you are, or is it a set and definites series of dimensions that are well explained?

 

The size of the extra dimensions has to be stabilized---this is a huge problem, called moduli stabilization.

 

NEXT

 

What does it mean for a dimension to be larger or smaller? Or "all twisted up" as I remember a physicist saying to me once. From my understanding of a dimension (kind of like a direction except each must be orthogonal to all the others), that doesn't seem to make sense.

 

Think of the rope example. One dimension is big (the length of the rope), and one dimension is small and twisted up (the distance AROUND the rope).

 

You can think of all kinds of funny spaces that have a similar property. Imagine a run of the mill torus. (Your profile says you know math, so I'll be a bit technical here.) You can make a torus by identifying the opposite sides of a sheet of paper, so you end up with a compact manifold (i.e. not infinite), upon which you can go in two different directions. Now imagine taking one of those dimensions to be big. If you live on the torus, you know that in one direction you can walk and end up back where you were, but the other direction to you looks essentially flat.

 

Secondly, if 10 dimensions is a prediction, then how can it be tested? Has an experiment been thought up/performed to check it?

 

Well, practically it can't. If you built a particle accelerator the size of the solar system or something, you could probably see evidence of the extra dimensions. But, barring that, you can't really test this assertion. You have to look for other generic predictions of the theory, and try to test those. But string theory is tricky, and it seems like there aren't very many generic predictions.

[Wormwood]

25 cents in the hole...

 

One dimension just means that you can describe any point on the string with one number.

Isn't this just a size issue though? If we were shrunk to a miniscule size where strings were as big as cars, wouldn't they have more than one dimension or is the string some sort of non physical entity that somehow manifests as physical matter?

 

 

Well, four of our dimensions are infinite, so it's hard to be larger than that But if you're asking if all of the dimensions have to be the same size, then no---this is one of the papers that I (and tons of other people) am currently working on.

Well, when I said "larger" I just meant in the same way that we can not detect the smaller dimensions because they are so much smaller. A dimension is just another coordinate in defining location right?

 

The size of the extra dimensions has to be stabilized---this is a huge problem, called moduli stabilization

Could you explain this a bit more? I tried to look it up, but the resources were not very clear. Unless I was reading incorrectly (which is highly likely) there are 10^-500 possibilities?

 

I was in a bar once drinking with Joe Polchinski, and I got him to drink a toast to Loop Quantum Gravity

I believe he teaches at the college here in town (Santa Barbara) right? I might be thinking of someone else, but that name sounds familiar.

[BenTheMan]

Isn't this just a size issue though? If we were shrunk to a miniscule size where strings were as big as cars, wouldn't they have more than one dimension or is the string some sort of non physical entity that somehow manifests as physical matter?

 

No. The string is TRULY one dimensional.

 

Well, when I said "larger" I just meant in the same way that we can not detect the smaller dimensions because they are so much smaller. A dimension is just another coordinate in defining location right?

 

I'm not sure I understand this. If you are asking if all the small dimensions have to be the same size, then no. The experimental limit is somthing like 10^-6 meters or something---that is, if the dimensions were BIGGER than 10^-6 m, we would have seen them in an experiment already.

 

Could you explain this a bit more? I tried to look it up, but the resources were not very clear. Unless I was reading incorrectly (which is highly likely) there are 10^-500 possibilities?

 

Well, there is no mechanism which chooses how the extra dimensions should look. Choosing how they look is done by looking at the answer, and tinkering until we get things right. As the reference you saw said, there are 10^500 ways to do this. This is a huge problem for the theory, because untill we get the mechanism down, we are just using string theory to describe low energy physics, and not actually predicting anything.

 

I believe he teaches at the college here in town (Santa Barbara) right? I might be thinking of someone else, but that name sounds familiar.

 

Yes, he is a professor at KITP, in Santa Barbara. He's also written a very difficult textbook on string theory.

[Wormwood]

You sir are extremely knowledgable, and I thank you for taking the time to entertain my questions. If you can't tell I never went past 101 which only has a minute section about quantum mechanics at the end, and then you watch "What the @%&* Do we know" and call it a semester

 

No. The string is TRULY one dimensional.

That is intellectually painful.

 

I'm not sure I understand this. If you are asking if all the small dimensions have to be the same size, then no. The experimental limit is somthing like 10^-6 meters or something---that is, if the dimensions were BIGGER than 10^-6 m, we would have seen them in an experiment already.

Logically this makes perfect sense. Intuitively, I am imagining all sorts of scenarios which probably aren't valid.

 

Well, there is no mechanism which chooses how the extra dimensions should look. Choosing how they look is done by looking at the answer, and tinkering until we get things right. As the reference you saw said, there are 10^500 ways to do this. This is a huge problem for the theory, because untill we get the mechanism down, we are just using string theory to describe low energy physics, and not actually predicting anything.

I think I get it now. Thanks.

[Martin]

When more people work on a problem, progress is faster.

 

a fact? talk to people in software companies.

 

So the field I think is stagnant, and if the US institutions are downsizing, ...

 

That is what I meant----string cutback in the US. In Europe in the past 2 or 3 years there has been a big shift of funding to NON-string QG. The European Science Foundation has set up agencies especially to support researchers conferences schools in the non-string QG category. I've heard grad students expressing a lot of satisfaction with this. You could say that those millions of Euros COULD have gone to string, since these other approaches are more or less going after the same thing----some unification, some pure quantum gravity-with-matter. But I don't think of the money as "taken away" from string. I think of it as new money. And the institutional job appointments I think of as new appointments, not as taken away.

 

In other approaches to quantum gravity, you have to tickle things to make them work correctly.

 

factual issue there. Do you KNOW what the leading approaches to quantum gravity are? A really important one getting a lot of attention now is Reuter QEG. Do you KNOW what, if any, tickling is required in Reuter's approach to get things to work properly?

Maybe shouldn't be talking about rival approaches if insufficiently informed. My impression was that even about 1990's vintage LQG you had misconceptions based on secondhand info, which not to be ashamed of just natural since string culture tends to deny merit of rivals and you've been surrounded by it.

[BenTheMan]

Do you KNOW what the leading approaches to quantum gravity are? A really important one getting a lot of attention now is Reuter QEG. Do you KNOW what, if any, tickling is required in Reuter's approach to get things to work properly?

 

Martin---I have never read or heard of a quantum gravity phenomenology paper that didn't include the word ``string'' in the title. Either the QG people assume that gravity matches smoothly onto a QFT in the IR, or they naturally get the correct structures out of the theory.

 

If the former, then this is not an improvement over the well-known approach. One still adds a gauge symmetry with no motivation other than an ex post facto justification, probably as a cleverly choses stress energy tensor. String theory offers a significant improvement in this regard.

 

The link you posted the other day seems to be an example of the latter, although they had tau neutrinos coupling to the higgs with a yukawa comprable to the top quark, which is not physical, inasmuch as I understood the scentence. Specifically, this means that one should expect the tau neutrino to be of comparable mass to the top quark. This is already ruled out by experiment. If you'd like, I can give you a page number from the particle data book. I have no problem admitting that I didn't understand most of the words in that paper. I scrolled down untill I saw something that looked familiar, then read about their neutrino sector.

 

Unless you can set the record straight by explaining to me what exactly the authors of the aforementioned paper meant, then I will continue to say that people don't get good phenomenology out of other approaches. I will stand humbly corrected if you can show me where I am mistaken. Otherwise I will continue to tell people who ask me that string theory has a much richer and more successful phenomenology than other QG approaches. This cannot be disputed.

[Martin]

You seem to be shifting ground again, Ben.

What you said is

In other approaches to quantum gravity, you have to tickle things to make them work correctly.

 

I mentioned Reuter's work (Quantum Einstein Gravity, QEG). I pointed you to Reuter some weeks ago and gave a link. There is no "tickling" whatsoever in Reuter's approach. He gets things to work with no extra dimensions and a minimum of extra machinery (basically just well-established tools from quantum field theory known to work when applied to other fields besides gravity.)

 

But in your reply you don't talk about Reuter's work at all! You skid off onto something entirely different, Alain Connes (another thread, another context, a different issue!)

 

My outstanding requests to you are:

A) you flatly said Connes phenomenology predictions were wrong. Get confirmation from one of your profs that you aren't the only person in the world who says that, please. Connes is an important guy and he has risked falsification of Non-commutative Geometry on some experimental predictions. It would be strange if the predictions were already ruled out on "astrophysical grounds", and that the only person in the world who realizes this is an Ohio State grad student. I would love it. So please check with a prof. If you can quote someone on the OSU physics faculty who says Alain Connes predictions are already ruled out by astrophysics you would have my undying gratitude and appreciation.

 

B) But that is not a part of this thread. this time you have suggested all non-string QG approaches require "tickling", significantly more tickling than string. this how you explain your preference.

As far as I know it is simply not a fact. non-string QG approaches normally do not involve replicating the Standard Model (although some have started doing this), they just do gravity with some generic matter fields. And they do gravity-with-matter in a comparatively MINIMALIST WAY.

 

No extra dimensions, typically using some classical foundations already in place.

 

Avoiding cooking up extra dimensions and equipment is one of the core aims.

 

So I suggest you MODIFY what you said, so everyone understands that it is just your opinion based on your limited knowledge of other approaches---you think they take more exta junk and twiddling to work as quantum gravity and I would say the opposite.

 

C) You said when you had some time you would sample Reuter's work, which I strongly recommend. It would give you a much better idea of the leading non-string work these days. It would only take 10-15 minutes to get some notion I'd estimate. Easy to understand for anyone whose taken QFT.

I really like Reuter's work. Alain Connes I gave you simply because you asked for particle physics predictions from a non-string approach, but Connes is not a special interest of mine.

 

So I will give you some Reuter links again, still hoping you will do what you said you would. You should like Saueressig. He is a young German string theorist who has switched over parttime to Reuter QG!

http://arxiv.org/abs/0708.1317

Functional Renormalization Group Equations, Asymptotic Safety, and Quantum Einstein Gravity

Martin Reuter, Frank Saueressig

Based on lectures given by M.R. at the 'First Quantum Geometry and Quantum Gravity School', Zakopane, Poland, March 2007, and the 'Summer School on Geometric and Topological Methods for Quantum Field Theory', Villa de Leyva, Colombia, July 2007, and by F.S. at NIKHEF, Amsterdam, The Netherlands, June 2006

(Submitted on 9 Aug 2007)

 

"These lecture notes provide a pedagogical introduction to a specific continuum implementation of the Wilsonian renormalization group, the effective average action. Its general properties and, in particular, its functional renormalization group equation are explained in a simple scalar setting. The approach is then applied to Quantum Einstein Gravity (QEG). The possibility of constructing a fundamental theory of quantum gravity in the framework of Asymptotic Safety is discussed and the supporting evidence is summarized."

 

just getting an arxiv paper and glancing at would take you a quarter of an hour and give you a better notion what I'm talking about.

 

if you have HALF an our to give to educating yourself about the competition's leading figures, then listen to the first 30 minutes of this talk

http://www.matmor.unam.mx/eventos/loops07/talks/PL3/Reuter.mp3

with the slides already printed out so you can refer to them:

http://www.matmor.unam.mx/eventos/loops07/talks/PL3/Reuter.pdf

[Wormwood]

I thought I was done ...

 

No. The string is TRULY one dimensional.

How is this possible? Does it only exist as time or something? Consider what you're saying; picture an object with only length and no height or width...how could it physically exist?

 

I'm not sure I understand this. If you are asking if all the small dimensions have to be the same size, then no. The experimental limit is somthing like 10^-6 meters or something---that is, if the dimensions were BIGGER than 10^-6 m, we would have seen them in an experiment already.

Unless they are phenomenonally larger than us, is what I was originally asking. For example, if we were smaller than atoms, we wouldn't realize that the atoms actually make up something larger that looks like a solid object because our size wouldn't allow us to percieve the macrocosm in that way that we do at this size. However, there is no scenario I can imagine that couldn't be explained with the 4 dimensions we already know about. I was just extending the same reasoning as to why we can not experience the smaller dimensions, to question if perhaps our dimensions were smaller on some even grander scale.

[Martin]

I had something else to ask about too. This is fun, isn't it wormwood?

 

It seems curious to me that I never see a stringy equation for the quantum state of the geometry of the universe. Ben Dundee would you please comment?

 

The best theory of gravity we have working so far is GR and in GR there is an object g which tells the geometry of the whole universe

 

Is there something like that in the string framework? Never seen it.

 

g is the metric like a distance function that tells distances between places or events

 

so in the classical or pre-quantum picture, g is the state of the geometry of the whole universe.

 

and of course g is also the gravitional field, because gravity = geometry, so the state of gravity is the state of geometry.

 

So it's natural to expect that when you go to a QUANTUM model of gravity, a quantum version of Einstein GR in other words, that in place of a classical state of the geometry of the whole universe you would have some object

say g* which is the quantum state of a dynamically evolving geometry.

 

In a sense, a "wave function" over all possible geometries.

 

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

And this is what all the non-string QGs have. Or almost all (one or two don't have it yet but they are getting there.)

I think it would be a really interesting scientific question, if Ben would comment, why doesn't string have a quantum state of the geometry of all spacetime, or if it does what are some arxiv numbers of papers where one could see such a thing. Thanks.

[BenTheMan]

Martin---

 

There is no "tickling" whatsoever in Reuter's approach. He gets things to work with no extra dimensions and a minimum of extra machinery (basically just well-established tools from quantum field theory known to work when applied to other fields besides gravity.)

 

Are we talking about particle physics or not? And of course he gets the proper number of dimensions because he puts them in by hand.

 

) you flatly said Connes phenomenology predictions were wrong. Get confirmation from one of your profs that you aren't the only person in the world who says that, please. Connes is an important guy and he has risked falsification of Non-commutative Geometry on some experimental predictions. It would be strange if the predictions were already ruled out on "astrophysical grounds", and that the only person in the world who realizes this is an Ohio State grad student. I would love it. So please check with a prof. If you can quote someone on the OSU physics faculty who says Alain Connes predictions are already ruled out by astrophysics you would have my undying gratitude and appreciation.

 

I don't know if you were ever a graduate student, but advisors don't appreciate when students come running to them with questions. I asked you if you knew the work---obviously you are familiar with it, or did you just do a google search? Either way, I will reiterate what I am confused about in the specific paper by Connes, et al. They claim that the top quark and tau neutrino have comparable Yukawa couplings. Yukawa coupling means fermion coupling to a scalar. So the paper implies that the coupling of the top to a scalar (i.e. higgs) is of the same order as the coupling of the tau neutrino, which doesn't make sense to me.

 

At some point in the future I am sure that I will figure things out for myslef, as you are proving most unhelpful in all of this. I am probably wrong in all ofthis, and you are probably right---either that or I am misreading the paper.

 

Maybe he has some more scalars in the theory (i.e. SUSY), in which case the neutrinos can generically couple to them. The paper is mathematically dense, and I can't understand it for the most part.

 

non-string QG approaches normally do not involve replicating the Standard Model (although some have started doing this), they just do gravity with some generic matter fields. And they do gravity-with-matter in a comparatively MINIMALIST WAY.

 

This is exactly the point---dimensions and gauge groups are added by hand, just as in traditional approaches. This is tickling.

 

No extra dimensions, typically using some classical foundations already in place

 

Of course not, becuase you've written down 3+1 on your paper and called it a day.

 

This really isn't that constructive, and I'd really rather hoped to avoid such conversations in this thread.

 

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

How is this possible? Does it only exist as time or something?

 

Nono. It only has a length, no depth or height.

 

I was just extending the same reasoning as to why we can not experience the smaller dimensions, to question if perhaps our dimensions were smaller on some even grander scale.

 

Well, there are no stable orbits in higher dimensions, I think, so we wouldn't have our solar system... I think.

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

In a sense, a "wave function" over all possible geometries

 

Martin---these are things I know nothing about. Apologies.

 

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

More on Yukawa couplings.

 

http://www.physics.upenn.edu/neutrino/jhu/node2.html

 

If the neutrino is a Dirac particle, then it can get a mass by coupling to the higgs. The limit on the Yukawa coupling for the neutrino is 10^-10. The top quark Yukawa coupling is 1.

 

So either I am confusing what Connes is talking about (most likely) or he is off by ten orders of magnitude.

 

Note, Martin---the abstract of the paper only claims to get the structure of the standard model correct, which was the situation in string theory for a long time. It took many smart people many years to understand how to get reasonable couplings out of the theory.

 

Finally, if Connes is talking about a Majorana neutrino, he can get a dimension five operator (the smalles dimension which is gauge invarian). If this is the case, then he is supressing the operator by the cutoff, which is presumably the Planck scale. If this is the case, then perhaps a large coupling is allowed, given the fact that the effective operator has such a large suppresion.

[foodchain]

Ok, I am struggling to keep up but here is some questions.

 

1) Are strings literally to be taken as the most primordial substance?

 

2) Can you isolate a string?

 

3) What does string theory say of dark matter/dark energy?

[BenTheMan]

Hi foodchain---

 

Ok, I am struggling to keep up but here is some questions.

 

1) Are strings literally to be taken as the most primordial substance?

 

Yes. I don't know if I can give a more detailed response.

 

2) Can you isolate a string?

 

In what sense? Specifically, electrons are excitations of strings, and we can isolate them.

 

3) What does string theory say of dark matter/dark energy?

 

String theory can explain dark matter pretty easily. The easy answer is that string theory can give the MSSM, which generally contains a dark matter candidate.

 

Dark energy is a bit more troubling. If we lived in a universe with a negative cosmological constant (it is small and positive), then string theory would have no problem. But a small and positive cosmological constant is tricky in general.

 

Generally, people say ``Ok, somewhere in the 10^500 solutions there's MOST LIKELY a few solutions with a small positic cc. So don't worry about it.'' If this sounds unsatisfying it is. Many people are trying to fix this, but little progress is being made.