What Is String Theory and Can It Be Understood at a Basic Level?

[_Mayday_]

Hey all,

Firstly I would like to say that I am fully aware that there is a part of the site related to string theory and other related topics, but I feel as this is such a layman question, it probably belongs here. Correct me if I am wrong.

Okay, I am sick and tired of people in my Physics class answering every question with something related to string theory. It is doing my head in, and then they get into very heated discussions about it all. I asked the question today, 'So what is a string?' and nobody could answer, due to the fact that are arguing about things they have no idea about.

I would like to gain a very general understanding of string theory, I mean like what it is and what it entails. I don't plan on going into any detail, just skimming the surface. Does anyone know of a site or maybe another free resource that I could use to achieve this? I am not in this to boost my ego, but as it seems to be the topic that seems to keep coming up I might as well know a little about it. I may be totally wrong, and maybe you can't really gain a basic understanding, maybe you have to go into detail...

Thanks to anyone who helps!

_Mayday_

 

[humanino]

Hi,

you can try :

The (self-proclaimed) Official String Theory Web Site

Wikipedia has many links as well
History of string theoryFirst thing, in the 60's string theory was imagined in order to describe hadrons, those are particles made of quarks and gluons. There are good (technical) reasons to try that. At this time, QFT was not fashionable, QCD (the theory we now know think is correct for that) was unknown. So at this point, those strings are effective, not fundamental. They just allow you to describe efficiently complicated objects.

These attempts quickly ran into many difficulties. Besides, QFT came back into fashion, mostly thanks to the discovery of QCD. String theory became an exotic topic, but some people began to imagine that the difficulties with it might be due to the fact that it is much more general, in particular they hope to be able to quantize gravity with it. From this point of view, strings are the most fundamental building blocks of everything, anything, including matter and space-time.

It was not until the beginning of the 80's and what is called the first superstring revolution that strings became really fashionable, appearing as a serious candidate for a theory of all fundamental interactions (including gravity). A second revolution in the mid 90's occurred when we (mainly Witten) realized that all the different known versions of string theories (annyoing for a candidate unique theory of all interactions) might just be different limits of a more fundamental, yet to be properly written, M-theory. "String" are now generalized multi-dimensional objects having different roles depending on their definition/topology...

Since 98 and the Maldacena's conjecture (built also on influences by Polyakov), we are getting a broader perspective : string theory(ies) might or might not be fundamental as such, but we are extremely close to realize their initial goal of re-formulating gauge theories in terms of string (gravitational) theories. Calculations that are difficult to perform in one theory (say gauge theory) can be performed in the other one (say string theory), and conversely.

If I may in the following share my personal opinion on this matter.
String theory have provided an extremely fruitfull interplay between mathematics and physics, and it is not really appropriate to ask whether those "strings" are fundamental anymore, they are so general and powerful that any complete fundamental theory of all interaction would probably be some sort of string theory, and benefit from all the work that has been done in those terms for decades. There is not "one string theory" today, string theories are templates for any possible theory. The speculative M-theory is still around and possibly the theory we will get to in the end, but we reached another conceptual level for which this specific issue is not so relevant anymore, at least not now : with such a generality (a quantum theory of all possible D-dimensional objects interacting in all imaginable manners possible), predictive power can only be obtained either by choosing specific implementations of models dedicated to specific problems (where the string might be considered either as fundamental or as effective) or by finding new conceptual/physical constraints from the beginning.

In any case, good luck, the subject is vast and fascinating. Investigate both the history and the technical implementations so that you can make your own opinion on the matter.

 

[_Mayday_]

Thank you for all the time put into that post!

Yeah people started talking about String Theory and CERN. They were saying things like if The LHC produced a black hole then that proves String Theory. I prefer to keep quiet in discussion I have no knowledge in, but I did know for a fact that, that on it's own probably wasn't correct.

I'll check all of that out now, and hopefully I will gain some understanding. I guess it would be great to understand such a big topic. What are the differences between String Theory and Super String Theory? Is it just a difference in dimensions in layman's speak?

Thanks Again!

_Mayday_

EDIT: Great links, thank you just read the first one, really nice introduction. Okay so according to super symmetry, for every boson there is a fermion, how are these two related and where are they relative to each other? I had always though there were 'things' on each side of the string.

 

[csyrony]

> What are the differences between String Theory and Super String Theory? Is it just a difference in dimensions in layman's speak?

I'm not an expert in this, but my understanding is that superstring theories contains supersymmetry, i.e. it contains supersymmetric particles and full symmetry breaking. Supersymmetry is important from the point of view of generating a TOE (theory of everything).

String theories don't necessarily contain supersymmetry. It is possible for instance to devise a string theory based on elementary particles as open strings with their ends on Dirichlet 6-branes that contains only those particles present in the standard model of particle physics and no more. This particular string theory model appears to be almost as arbitrary as the standard model itself.

My limited understanding is also that those string theories that don't contain supersymmetry contain tachyons. Tachyons have never been observed and are widely believed not to exist. If such string theories are correct then that implies that the branes within those theories are intrinsically unstable.

So my personal opinion is that those string theories that are not superstring theories are not going to be found to be correct, but that's just a personal view.

 

[humanino]

superstring theories contains supersymmetry, i.e. it contains supersymmetric particles and full symmetry breaking. Supersymmetry is important from the point of view of generating a TOE (theory of everything)

Indeed. String theories contains only bosonic excitations. Bosons, such as the photons, the massive vector bosons Z and W, or the Higgs boson, or even the hypothetic graviton, are usually interpreted as the "force carriers" of the interactions. String theories (as opposed to SUPER string theories) can NOT have any fermionic particle-like excitations. Fermions, such as the electron, the quarks, the muon... are usually interpreted as "matter particles".

It is possible for instance to devise a string theory based on elementary particles as open strings with their ends on Dirichlet 6-branes that contains only those particles present in the standard model of particle physics and no more. This particular string theory model appears to be almost as arbitrary as the standard model itself.

If I understand correctly, you claim to know of a string theory having fermions (and in fact the standard model) without supersymmetry. I would be delighted to learn that it is true such a string model exists. I am however quite certain it will turn out you confused. There is no fermion without supersymmetry in string theories. This is a very basic feature (advantage of problem depending on your point of view on supersymmetry) of string theories.

 

[humanino]

Okay so according to super symmetry, for every boson there is a fermion, how are these two related and where are they relative to each other? I had always though there were 'things' on each side of the string.

If you ask the question "where are the supersymmetric partners ?", this is an excellent question because we do not seem to find them

But the supersymmetric partners do not lie on the two ends of the same string if that is your question. We know for a fact that supersymmetry must be a broken symemtry, since we do not normally observe all those additional particles (supersymmetric partners). How and where to break this symmetry is vast question by itself (on top of string theories). And supersymmetry is mandatory for string theories, but quite welcome as well in the Minimal extensions of the standard model to bring a few answers to technical questions. If you have questions about "what is the use of supersymmetry outside string theories ?", please post it in another discussion (and probably in the "Beyond the Standard Model" section).

 

[csyrony]

> If I understand correctly, you claim to know of a string theory having fermions (and in fact the standard model) without supersymmetry. I would be delighted to learn that it is true such a string model exists.

No, I'm wrong, sorry, it doesn't exist. There are several string theories that reproduce only the particles of the standard model without supersymmetric particles, but they are supersymmetric string theories. The one I'm thinking of is a type IIA string theory. The pre-specified orientations and intersections of the stable D6 branes break all the supersymmetry to form an arrangement that contains only the particles of the standard model. What I was thinking of was Figure 15.8 of "A first course in string theory" by Barton Zweibach.