# What are the basic postulates of string theory? [closed]

I read the Popular Science books, The Brief history of time (Hawking), The fabric of the cosmos (Greene), and The Grand Design (Hawking), etc. but did could not manage to understand what string theory is, even at a Popular level. I couldn't understand what exactly it is, even at a Popular Science level. !

Can any one please briefly describe what are its' fundamental postulates?

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## closed as too broad by Dilaton, Michael Brown, Qmechanic♦Aug 5 '13 at 14:06

There are either too many possible answers, or good answers would be too long for this format. Please add details to narrow the answer set or to isolate an issue that can be answered in a few paragraphs.If this question can be reworded to fit the rules in the help center, please edit the question.

I am not an expert. I read that Brian Greene's book and what I understood was that they found a mathematical model that fitted well the data of an experiment. But they didn't understand, at the time, what was such model fundamentally implied. Later, some mathematicians found that such model would imply that the elementary particles of the universe were strings, with different fundamental oscillating frequencies. This is what I understood, but let's wait for an expert... – cinico Aug 5 '13 at 14:37
@cinicio described the historical formulation, but in the more common textbook formulation (which is much better), it's usually like this: (1) Assume that everything is made of branes, $n$-dimensional exxtended objects and write down the DBI action . . . (2) Specialise $n=1$ and write down the NG action. (3) Manipulate the NG action into the easy-to-quantise-without-a-light-cone-gauge Polyakov action. (4) Find the properties of this theory . (5) Realise that this doesn't contain fermions, so add in Supersymmetry on the World-sheet, making the RNS string theory . (6) Re(contd.) – centralcharge Aug 6 '13 at 14:35
(contd.)alise that this still contains a tachyon, so introduce a GSO projection to find the Type IIB and Type IIA string theoryies. (7) Realise that these have spacetime supersymmetry, and formulate the GS string theory . (8) Realise the gauge groups of these aren't nice, and find the Type I string theory . (9) And also the Heterotic string theories . (10) Now, realise the Heterotic string theories have mismatched dimensions, and compactify them, resulting in the Type HO and Type HE string theories . (11) Realise that Type IIB is T-dual to the Type IIA . (12) R(contd.) – centralcharge Aug 6 '13 at 14:38
(contd.)ealise that the Type HE string theory and the Type HO string theories are T-dual to each other . (13) Realise that the Type HO string theory is S-dual to the Type I string theory . (14) Realise that the Type IIB is T-dual to itself . (15) Realise that there are only 2 string theories now, Type IIA , and Type HE, and postulate that their S-duals are actually "M-theory" compactified on different manifolds, a circle, and a line segment . (16) Explore holography . (17) Explore AdS/CFT . (19) Now realise BFSS M(atrix) theory . (19) Realise Matrix Strin(contd.) – centralcharge Aug 6 '13 at 14:42
@Dimension10: Great answer! – Peter Shor Aug 8 '13 at 18:28