Answer your string questions for a nickle!

[Martin]

... a stupendous amount of energy to actually observe a string directly, or indirectly (something like on the order of 1x10^32 J ?).

 

Just as a reference point, Lockheed, Planck energy is 2 x 10^9 J, another way to say it is about 10^19 GeV

Here's Wikipedia about Planck energy, Planck length etc.

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

 

An accelerator to achieve Planck energy would, truly, be stupendous. (comparable size to earth's orbit, maybe better way to say it).

 

It will be interesting to hear Ben's description of how string energy, and string scale, relate to Planck

[Reaper]

Just as a reference point, Lockheed, Planck energy is 2 x 10^9 J.

 

 

An accelerator to achieve Planck energy would, truly, be stupendous. (solar system scale).

 

 

Oh, ok. I think I got plank scale and plank energy mixed up or something (Not even in correct units or numbers either whoops!) .

 

I guess the question should be then, how do they know that they can only be directly observed at plank length?

[BenTheMan]

How did string theorists come to predict such a large amount of energy like that, and why?

 

The scale comes from dimensional analysis, just like in every approach to quantum gravity.

 

In physics, we are often lazy about dealing with problems, or we like to be. So to figure out where things apply, we often use dimensionful analysis, which is a beautiful tool.

 

Let's take an easy theory, like quantum electrodynamics. Quantum electrodynamics has only one parameter in it---the electron's mass. Now, even though this is a mass, we can relate it to an energy scale, and a time/distance scale by using simple relationships like E = mc^2 and L = ct. Usually we just set c = 1 (along with hbar) so that we have E=m and L=t. You can also use the compton wavelength, [math]\lambda = 2 \pi \frac{\hbar}{mc}\rightarrow\lambda \sim \frac{1}{m}[/math], to relate distance and mass.

 

So just having one number sets all of the four dimensionful parameters in your theory.

 

Back to QED. In QED, we are given the electron's mass, or we measure it, and put it in the theory. Once we have the elctron's mass, we can now work out all of the dimensionful parameters in the theory. On what length scale would you expect to find QED effects? The compton wavelength of the electron (which is a bit smaller than the bohr radius, which explains why non-releativistic quantum mechanics works so well for hydrogen). What time scale do you expect to find QED effects? 1/m. And so on.

 

When it comes to gravity, the only dimensionful numbers are the Newton's constant, hbar, and c. We can form a length, a time, and energy, and a distance out of these units, see here. These are the scales where we expect quantum effects to become important. And this holds for ANY approach to gravity, be it loops or strings.

 

Some people have had the notion to fool around with the Newton's constant, G_N. If some of the dimensions are larger than other in the compactified space, then the Newton's constant changes at those lengths. This allows you to change the Planck units, with the possibility of actually SEEING quantum gravity at a particle accelerator. This is the large extra dimensions that people like Dvali, Arkani-Hamed, and Dimopoulos work on. The idea is to leave two or more of the small dimensions ``large'', but not too large. If this is the case, they can evade experimental bounds, and we may see strings in the very near future.

 

About cosmic strings I know much less. I do know that one CAN get cosmic string solutions out of string theory, but in such a way that the current astrophysical bounds can be evaded. The old (c. 1980 ish) cosmic strings are leftovers from some processes that occured in the early universe (topological defects, if you want to impress your friends), and have been ruled out by observation. All I know is that the cosmic strings that come from string theory can evade these bounds by two or three orders of magnitude.

 

These strings aren't a generic prediction of string theory, so NOT seeing cosmic strings is not a way to rule out string theory.

[iNow]

About cosmic strings I know much less. I do know that one CAN get cosmic string solutions out of string theory, but in such a way that the current astrophysical bounds can be evaded. The old (c. 1980 ish) cosmic strings are leftovers from some processes that occured in the early universe (topological defects, if you want to impress your friends), and have been ruled out by observation. All I know is that the cosmic strings that come from string theory can evade these bounds by two or three orders of magnitude.

 

These strings aren't a generic prediction of string theory, so NOT seeing cosmic strings is not a way to rule out string theory.

 

So from a high level (explain it to me like I'm stupid ) what are these "cosmic strings" and how do they differ from the 1d strings I'm used to hearing about? Also, what's all this talk of "bounds?" I'm not sure how that plays a role here. Are bounds like a remnant of some other theory which is held as accurate, or...?

 

 

 

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

So iNow, what i would urge you to do is get busy and ASK LOTS OF QUESTIONS of Ben which just have to do with string----and not with unification and quantum gravity in general. that will give him a chance to talk constructively about what he knows.

 

To be perfectly frank, I know so little that I'm often not even sure where to start when it comes to questions. I'm pretty much standing outside the lecture hall listening through the door while I wait for my own class to begin in the neighboring room.

[BenTheMan]

what are these "cosmic strings" and how do they differ from the 1d strings I'm used to hearing about?

 

First of all, I want to make sure you know I am no expert in this field

 

But, anytime you have symmetry breaking in the early universe, you end up with ``topological defects''. I'm not really sure what the mechanism is that generates these defects, but typically they can be two dimensional (domain walls), one dimensional (cosmic strings) or zero dimensional (monopoles). We can look for cosmic strings and domain walls using something like WMAP, and monopoles we look for with direct searches.

 

Also, what's all this talk of "bounds?" I'm not sure how that plays a role here. Are bounds like a remnant of some other theory which is held as accurate, or...?

 

The best that any experiment can do is put a limit on what it sees---this is why there is no direct experimental proof that the photon is massless, for example. A massless photon is CONSISTENT with the experiments. The current round of experimental bounds on cosmic strings rule out the old type of cosmic strings, the ones left over from symmetry breaking.

 

But the types of cosmic strings which come from string theory evade these astronomical bounds.

[fredrik]

Ben, since my perspective is more from the point of view of scientific strategies, i've got another question, if you think it's just philosophy just ignore it.

 

If I'm allowed to simplify the overview here, let's consider that we deal with the following objects

 

A) theories - a theory is a basically a specific prediction (not a classes or landscapes of prediction). So in a sense a theory is an educated guess.

 

B) framework - a framework is something that put constraints on the possible forms of the theories in the general case. So in a sense a framework implies constraints on the set of possible educated guesses, whose purpose is to sort of make sure we don't waste shots on possibilities are that are definitely wrong.

 

c) strategies - with a strategy I mean like a theory on howto make the guess selections from a given framwork. Ie it's some kind of principle that speculates that there are optimum bets.

 

Now in the overall case, if the stream of theories we produce are unsuccessful, it probably means that either our strategy is poor, or that the framework is too restrictive.

 

Since if I understand you right, there is no strategy in string theory. It's only a framework. Then the only opinion I could have is wether is wether it's too restrictive or not. Ie, if the framework bans options that are in fact possible, then the framework is wrong or bad.

 

*If* the framework is good OTOH, the missing key is the strategy. Of course without intelligent strategy, the defauly strategy IS the "random selection". Which is perfectly fine if the options are not too many.

 

But if the framework IS wrong, it means we need a strategy to select a new framework Which seems to suggest to me that the strategy is more fundamental of the tree objects mentioned? And string theory doesn't have a strategy?

 

The reason why i personally find string theory to be speculative is that I don't quite understand how you can do all this stuff without a formal strategy. It seems like high risk gambling here, and we have no risk analysis. That's how I see it.

 

Question. What is your view on this? IS this philosophy or irrelevant, or does it bother you too? If so, howdo you handle it in your position as string theorist/student without bursting out in frustration? doesn't it bug you?

 

If you can answer this, perhaps I can at least try to understand your business.

 

/Fredrik

[BenTheMan]

What is your view on this? IS this philosophy or irrelevant, or does it bother you too? If so, howdo you handle it in your position as string theorist/student without bursting out in frustration? doesn't it bug you?

 

Many would probably want to call string theory a framework (by your definition of the word) untill there is a plausible vacuum selection mechanism. However, there ARE predictions coming from string theory. The most generic ones are just too hard to test. There are also POST-dictions, like chirality and gauge symmetry.

 

Also note that Faraggi PREDICTED the top quark mass (BEFORE it was measured), based on his heterotic string models, in this paper: http://arxiv.org/abs/hep-ph/9405357. And he was right.

 

So a GENERIC string theory makes SPECIFIC predictions: chiral fermions, gauge symmetries, ten dimensions, etc. These are predictions, and if we don't find ten dimensions at the Planck scale, then string theory is wrong.

 

As for my own opinion of it all---it keeps me in business

 

The reason why i personally find string theory to be speculative is that I don't quite understand how you can do all this stuff without a formal strategy. It seems like high risk gambling here, and we have no risk analysis. That's how I see it.

 

You are asking sociology questions like Martin. String theory is interesting to many people, so they work on it. I can assure you that string theorists never talk about ``gambling'' or ``risk analysis''. When other things become interesting to people, they change fields.

 

The standard model was built in exactly the same way, I'll remind you, allbeit with more guidance from experiment. The quantum gravity enterprise split into two factions---those who tried to start with GR and those that tried to start with quantum mechanics.

 

String theory STARTS with QFT---in some sense string theory is quantum field theory applied to strings. If you don't believe me, try to read Green, Schwarz, and Witten without having had any experience with field theory. I tried---it doesn't work. The one base assumption (yes, only one) is that the fundamental object is a string, and (ok, two) that a consistent quantum theory of strings can be built. From these two inputs, one can derive all of low energy physics, including GR and the standard model. You procede in the same way as quantizing a point particle---path integrals, ghost cancellations, etc., etc. Except what you find is that you don't have any UV problems like you do with point particles (i.e. no renormalization), the IR limit is GR, and you have a spin two mode which naturally pops up and gives you gravitons.

 

So you end up with a UV finite theory describing (among other things) a spin two degree of freedom, which is completely consistent quantum mechanically, and which gives gravity in the IR.

[fredrik]

Thanks Ben. You answered my questions

 

It will be interesting to see how things evolve during the next say 20 years.

 

/Fredrik

[vincent]

You [fredrik] are asking sociology questions like Martin.

 

Precisely! Look fredrik, this sort of research cannot be done in the kind of organized way you`re promoting because we don`t know what's ahead and are constantly getting surprised. What we do know is that string theory has passed every one of the enormous number of tests of self-consistency it has ever been subjected to and is the only theory we have that appears to have all the ingredients necessary to solve all of the outstanding problems in this field of physics. As well, unlike virtually every other approach, string theory was not invented, it was discovered. This is why every other approach looks contrived in comparison. Some of these alternative ideas are interesting in certain respects, but none of them really stand comparison to string taken as a whole. This isn`t just a matter of how open an attitude one has. There are very good reasons why string theory dominates high energy theory and quantum gravity.

[fredrik]

I think I'll choose to not comment further on last comments about "sociology" because I now see that it's not leading anywhere constructive and clearly has no impact on string theory relative to your apparent view. But at least I have acquired an understanding of our apparent disagreement and at least in part I think I understand your objections (but that doesn't mean I agree).

 

I think that if I were to attempt to argue further, it would be either in disagreement with, or outside of your string framework.

 

Let's agree that the future will be the effective arbitrator what theories, frameworks or strategies that will become successful.

 

/Fredrik

[vincent]

I think I'll choose to not comment further on last comments about "sociology" because I now see that it's not leading anywhere constructive and clearly has no impact on string theory relative to your apparent view. But at least I have acquired an understanding of our apparent disagreement and at least in part I think I understand your objections (but that doesn't mean I agree).

 

I think that if I were to attempt to argue further, it would be either in disagreement with, or outside of your string framework.

 

Let's agree that the future will be the effective arbitrator what theories, frameworks or strategies that will become successful.

 

/Fredrik

 

It just seems to me that you`re searching for a way to evaluate string theory and it`s prospects without actually understanding string theory.

[fredrik]

It just seems to me that you`re searching for a way to evaluate string theory and it`s prospects without actually understanding string theory.

 

Perhaps that's it seems like from your viewpoint, but to response, what I am trying to do in this thread is to see if those who advocate strings, can give me a better motivation for the approach.

 

My current information about the string approach doesn't render me satisfied, and the primary and most important point is that it seems to lack a formal strategy. This is serious enough to cripple any remaining features _in my view_, but that I think can not be understood within the string framework as far as I see. If I try to explain it, I think you'll dismiss is at irrelevant to stringtheory. Which is exactly my problem.

 

I do see some interesting things in the string approach, otherwise I wouldn't even spend this time. Also I know there are clever people working on it, and there is no doubt to me they see good reasons, making it more mysterious. I don't understand how they can accept to not have a better strategy. Or perhaps the strategy is the selection in the scientific community. This always applies, but I want something ontop of that.

 

I short, I figured there ought to be a hidden strategy behind the string approach, that only you guys understand. My current understaning is that the string approach is not "complete" until the framework is unarguably "unlikely to be unsuccessful" and until there is a strategy.

 

I see better options atm which I try to work on. But I have been intrigued by apparent similarities between different approaches, which in itself isn't surprising since we are after all trying to describe the same world

 

/Fredrik

 

This is why I was fishing for your M-theory, which I admit I know very little about. I was hoping that you would tell me that the answer to my doubts was supposedly answered by the M-theory - which you are still looking for as I understand. Then I would actually have MORE understanding for you, if the M-theory actually contains a strategy and a more fundamental framwork where the string concept is emergent.

 

I still suspect this IS the true answer though. Even though it was not clear cut obvious from this thread. Still, before you get that final "theory/framework/strategy" I see you are on thin ice from my point of view. But I accept beeing called ignorant for thinking so - it is in fact no contradiction. I take responsibility for my mistakes. I've thought this one through..

 

/Fredrik

[BenTheMan]

what I am trying to do in this thread is to see if those who advocate strings, can give me a better motivation for the approach.

 

Well, with all due respect, we don't answer to you

 

String theory explains quite a bit, and gives a complete picture of Nature. This is why it is interesting.

 

Any other approach to gravity is not as complete as string theory. This is not an objective statement, but a subjective one. It depends on what kinds of things you think are important in a quantum theory of gravity. Background independance (for example) is hard to come by in string theory, and if you're looking to quantize geometry, then you're barking up the wrong tree. (GR is an effective theory anyway, so I don't understand why one would want to do this...). I should point out, though, that this is a moot point because Martin Schnabl has solved string field theory in some simple cases. This essentially means (as far as i know) that there now exists a background independant formulation of string theory.

 

So, one has to ask...are the tradeoffs worth it? We have a vacuum selection problem in string theory, this much is evident. But are you willing to sacrifice the huge successes of string theory because of this?

[Martin]

...

Any other approach to gravity is not as complete as string theory. This is not an objective statement, but a subjective one. It depends on what kinds of things you think are important in a quantum theory of gravity. Background independance (for example) is hard to come by in string theory, and if you're looking to quantize geometry, then you're barking up the wrong tree. (GR is an effective theory anyway, so I don't understand why one would want to do this...). I should point out, though, that this is a moot point because Martin Schnabl has solved string field theory in some simple cases. This essentially means (as far as i know) that there now exists a background independant formulation of string theory.

 

So, one has to ask...are the tradeoffs worth it? We have a vacuum selection problem in string theory, this much is evident. But are you willing to sacrifice the huge successes of string theory because of this?

 

Hi Ben, since you bring up other approaches to gravity, and give your subjective viewpoint on them, I will respond to round out the picture with a different subjective viewpoint.

 

Just to clarify about sacrifice the huge successes. You understand that my desire would be to see the US follow the example of Europe UK and Canada where millions of dollars of support is going into the non-string QG research effort. Interested grad students do thesis in nonstring. then they can find postdocs in nonstring, and there are several (not just one) universities and institutes with faculty positions. But obviously the Europeans have not given up on string either!

String is older and correspondingly bigger. But it is not the only line of QG research. That is the situation I would like to see.

So I would like to see less of a MONOPOLY in the US.

so I am not proposing that everybody stop doing string----I'd like to see a moderate cutback and a reallocation of support and jobs in the US.

 

It is a serious concern, which is why I watch the numbers.

 

From my point of view, then, "sacrifice the huge successes" sounds a bit drastic. I don't propose everybody stop, just that the risk be spread and the investment portfolio diversified so that several different approaches are tried.

Also I, like many people, gauge success of science in terms of PREDICTION.

I see string as a broad philosophy or conceptual framework encompassing many different models. I see no fundamental principle analogous to the GR Equivalence Principle. I see no fundamental equation analogous to the GR field equation. I see no exact predictions for future observation. So I don't actually see a THEORY (I'm paraphrasing something Gerard 't Hooft said which I can find if you want.) But instead a philosophy that might sometime produce a theory. And I don't propose people "sacrifice" anything by stopping the effort. Just make room for some other folks at the table

 

In your perspective, Ben, you stress the subjective criterion of COMPLETENESS which I think means that within this broad philosophy including things as different as Schabl's work on String Field and Volker Braun's work on reproducing Standard Model numbers with string methods---you see people have gotten, over the years, many different desired numbers and results.

 

You bring up a comparison with non-string QG approaches and I guess you would argue that string research should have a bigger piece of the pie (or perhaps all?) because it is more COMPLETE.

 

OK, since you bring up a comparison with nonstring, and I have to respond, I would stress the subjective criterion of PROGRESS, and especially progress towards predictions. The nonstring QG approaches mostly date from 1998 on. They have made remarkable progress in that time. And what they show also, that is significant, is that they are closing in on predictions that can be tested, especially as QG models are applied to cosmology---where large amounts of new data are coming in.

 

On the whole I think your thread is going well, to the extent that it just focuses on STRING and doesn't get off into comparisons with other approaches. So as far as answering people string inquiries, more power to you!

 

thanks.

[BenTheMan]

Whatever.

 

Back to strings.

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