
I wanted to discuss this topic for a long time. I am not in Cosmology field, although I read lot of books on it. I started reading about String theory through Brian Greene's 'The Elegant Universe' and I was hooked. I read his next book and plenty other cosmology books which had topics on String Theory.
Recently I read "The Trouble with Physics: The Rise of String Theory, the Fall of a Science and what Comes Next" by Lee Smolin and it made me think a lot about what I head read.
One end String theory is very neat, almost perfect solution, on other hand when you read the details of the critics, it makes you wonder whether lot of it was flight or fiction?
I don't think I have denounced string theory, but I have become a skeptic who wants to get some realistic answers.
Regards,
B
Unfortunately you will find few people able to hold a detailed conversation on this topic. String theory (or superstring theory) is incredibly mathematical, way beyond the abilities of most people (including myself). I understand a very basic outline of the theory (I too have read Greene and Smolin) but, like most physicists I know I can't really talk intelligently about the details of the 'theory'.*
* I put the word theory in inverted commas because, until there is some empirical test for the hypothesis then I think 'theory' is still a bit strong. I think it is more in the realms of mathematical philosophy at the moment.....
I've never heard of the 'String theory' and I'm a bit interested could you please give me a quick explanation of the principals and idea's of it?
P.S. if you say Google it I'm going to be so annoyed, because when ever i Google something it takes me almost an hour to find something substantial nowadays
I suggest you get hold of 'Elegant Universe' by Brian Greene from your local library or Bookshop. It will give you a pretty good introduction to the theory.
Alternatively you can try the following:
http://www.pbs.org/wgbh/nova/elegant/program.html
http://www.superstringtheory.com/
In a nutshell, string theory (ST) postulates that particles are made up of unimaginably small "strings". Some of these are closed loops, some are openended. The type of string determines the type of particle. The strings have tension and vibrate.
ST also introduced the concept of branes. Our universe may be located on 1 of these branes in a higherdimensional bulk. Openended strings must have both ends attached to a brane. In some versions of ST both ends need not be connected to the same brane.
There are 5 versions of ST which seemed to be mutually exclusive until Ed Witten came up with Mtheory. Mtheory showed that all 5 versions are in fact the same theory but viewed differently. There's more to Mtheory than that, but that's all you need to know for now.
It must be stressed that ST, despite more than 20 years of having the cream of physics brains working on it, has not yet come up with a single verifiable prediction. The possibilities of the different types of string, their tension, frequency, and how they interact with each other are almost without limit so making any kind of prediction has thus far proved impossible.
This is a very simplistic overview of ST. The links posted by Bikerman explain things in a bit more depth.
Quote: "In a nutshell, string theory (ST) postulates that particles are made up of unimaginably small "strings". Some of these are closed loops, some are openended. The type of string determines the type of particle. The strings have tension and vibrate. "
How "small" are the strings? Planck lenght? Or smaller? Where do they have the tension from? Based on what do they vibrate? How do they explain the appearance or "existence" of space and time"?
This is why I provided the references. A conversation about String Theory here will almost inevitably be
a) Highly speculative
b) Simplified to the point of reductoadabsurdum (ie way too simplified to give any meaningful picture).
c) Wrong in many important regards
Bikerman wrote:  This is why I provided the references. A conversation about String Theory here will almost inevitably be
a) Highly speculative
b) Simplified to the point of reductoadabsurdum (ie way too simplified to give any meaningful picture).
c) Wrong in many important regards 
Ok. I will have some looks on the references provided by you and tomorrow I will ask something related to their exact components.
Bikerman wrote:  Unfortunately you will find few people able to hold a detailed conversation on this topic. String theory (or superstring theory) is incredibly mathematical, way beyond the abilities of most people (including myself). I understand a very basic outline of the theory (I too have read Greene and Smolin) but, like most physicists I know I can't really talk intelligently about the details of the 'theory'.*
* I put the word theory in inverted commas because, until there is some empirical test for the hypothesis then I think 'theory' is still a bit strong. I think it is more in the realms of mathematical philosophy at the moment..... 
Thanks Bikerman. I agree with your statement that lack of empirical test is making this more of hypothesis. When you read Smolin, you realize how many holes this 'hypothesis' really has.
Let's see of LHC can shed some light on it, one way or another... hoping for it.
Regards,
B
anarhistu wrote: 
How "small" are the strings? Planck lenght? Or smaller? Where do they have the tension from? Based on what do they vibrate? How do they explain the appearance or "existence" of space and time"? 
I know nothing. I'm a psychologist.
Quote:  In theoretical physics, Mtheory is a new limit of string theory in which 11 dimensions of spacetime may be identified. Because the dimensionality exceeds the dimensionality of five superstring theories in 10 dimensions, it was originally believed that the 11dimensional theory is more fundamental and unifies all string theories (and supersedes them). However, in a more modern understanding, it is another, sixth possible description of physics of the full theory that is still called "string theory." Though a full description of the theory is not yet known, the lowentropy dynamics are known to be supergravity interacting with 2 and 5dimensional membranes.
This idea is the unique supersymmetric theory in eleven dimensions, with its lowentropy matter content and interactions fully determined, and can be obtained as the strong coupling limit of type IIA string theory because a new dimension of space emerges as the coupling constant increases. 
Source  http://en.wikipedia.org/wiki/Mtheory
[MOD  quote tags and source added  Bikerman]
This is what I understand on the matter.
In 1968 Gabriele Veneziano, an Itallian physicist, was looking for a set of equations to explain the nuclear strong force. Whilst looking through some old books he happened upon an equation written 200 years previously by a Swiss mathematician Leonhard Euler. This equation, once thought to be a mathematicall curiocity, seemed to describe the nuclear strong force.
Whilst scientists were playing around with the equations one physicist, Leonard Susskind, found depth where he could see particals being described, these particals seemed to be able to stretch and vibrate according to the equation, ie. strings that weren't point particals. Obviously his 'farout' idea was rejected and string theory died.
With the discovery of messenger particals (particals of force!!!) scientists were closing in on the Grand Unified Theory. The standard model was deduced to describe three of the four forces. However this theory didn't include the force 'Gravity'.
Behind the standard model, string theory was left to die. String theory had many problems. Mathematical inconcistancies and unseen predictions. The massless partical, a tachyon, wasn't seen in experiments.
String theorists thought that the equations might be describing gravity. The whole scale needed to be shrunk. The Tachyon now seemed to be describing the Graviton (the messenger partical of gravity). This theory now had the oppertunity to describe the universe better than the standard model. It could describe gravity and also had the depth to describe the other forces!
The tiny strings could describe different particals and their propperties much in the same way as a string on a musical instrument. Different frequencies produce different things.
Quantum mechanics and Special relativity are difficult to meld together. String theory is able to calmly bridge the gap.
However, it is said that no experiments can ever be done on such a small scale to disprove string theory!
Also string theory needs something very perculiar for the equations to work... Extra dimensions of space! These dimensions are said to be very tiny and intertwined
http://en.wikipedia.org/wiki/File:CalabiYau.png
The theory is extremely mathematical to get to prooving or disprooving it but it can be described very nicely to get an idea.
Check this lecture out
http://videos.howstuffworks.com/tedconferences/7608superstringtheoryexplainedvideo.htm
Neilos
I've heard of the basics in science fiction. It's highly speculative and based on crazy advanced mathematical theory on the universe, but I think it's possible.
I like to think about the larger issues it raises. Like the idea of Multiverses. Basically, they provide an explanation for why the laws of physics and the constants are so wellsuited for life on Earth to exist. The "theory" of the multiverse, is that there are multiple or parallel universes, with different parameters for the laws of physics. These universes can evolve and reproduce to produce new universes.
I recently read some work by Ray Kurzweil (the futurist) and he talks about how humans are reaching a point where we will expand our intelligence (via nanobots) into space to convert all matter into intelligent computing machines. He touches on Multiverses and how a universe with more blackholes and life could be better than another in their evolution.
It all reminds me of Hitchhikers Guide to the Galaxy (spoiler if you haven't read it)
The way that the Earth is really a computer designed by highly intelligent beings in another universe.
It's very much scifi, but interesting stuff to think about when you are thinking about the unknown.
http://en.wikipedia.org/wiki/Multiverse
http://en.wikipedia.org/wiki/Raymond_Kurzweil
http://en.wikipedia.org/wiki/The_Singularity_is_Near
