string theory

[ayush garg]

can someone tell me more aboot the strin g theory and the quark theory ...... are they related to each other

[ajb]

It is well documented that string theory originally was proposed as a theory of hadrons, this goes back to Veneziano in the late sixties. Around 1973 quantum chromodynamics emerged as the correct quantum field theory description of the strong force.

 

Now, we know a lot more about string theory. In particular we have the gravity-gauge theory duality. Originally this was a "stringy" idea, but can be understood more generally that this. Loosely we have

 

[math]\textnormal{gravity} = \textnormal{gauge} \times \textnormal{gauge}[/math],

 

meaning it is possible to do calculations for quantum gauge theories using quantum gravity theories. One great example of this using Twistor string theory to get atMHV amplitudes.

 

If we include branes in to all this then supersymmetric gauge theories naturally arise.

 

So, anyway there is now a bit of an industry working towards understanding nonsupersymmetric gauge theories in a "stringy context". There has been some effort in developing string theory based methods for analysing QCD.

 

I don't really know much about this in any detail.

[IM Egdall]

Real basic stuff on quantum mechanics, general relativity, and string theory (hope it helps):

What is now called the "Standard Theory" of quantum mechanics describes the behavior of all known phenomena in our universe, with one exception; gravity. Quantum electroweak theory covers the electromagnetic force and the weak force. Quantum chromodynamics covers the nuclear (strong) force. The odd man out is gravity. However, modern attempts at a unified theory which includes the three quantum forces and gravity have proven most formidable.

 

The predictions of general relativity, like all classical physics theories, are "deterministic'. This means that if we know a particle's current location (where it is) and its velocity (where it is headed and at what speed); we can determine exactly where it will be at any time in the future (assuming no outside influences). And in principle, we can do this to arbitrary accuracy. But the Uncertainty Principle of quantum mechanics tells us that we cannot know a single particle's position and velocity to arbitrary accuracy. Per the Uncertainty Principle, the more accurately we know a particle's position, the less accurately we can know its velocity (momentum). And vice-versa!

 

The "Holy Grail" of theoretical physics research today is the unification of quantum mechanics and general relativity under a single construct; termed "quantum gravity." There are a number of approaches; string or M-theory is the most active.

 

Some of the major predictions of string theory include: 1) the overall universe of universes (the "bulk") is made up of 11 dimensions, 10 spatial and 1 time. 2) Our local universe is a "3-brane". There are other branes of various dimensions. 3) All fundamental particles are 1-branes or strings of a finite one-dimensional size: 10^-33 cm. (In quantum mechanics, fundamental particles like quarks are modeled as points having zero dimensions, so they are infinitely small.) The finite size of the string model puts a limit on quantum jitters (vacuum fluctuations), a key to incorporation of relativity spacetime considerations. 4) For every known quantum mechanics standard model particle, there is a new corresponding supersymmetric particle or "sparticle" which has a spin that is different by ½ unit. 5) The four forces of nature (electromagnetic, strong nuclear, weak nuclear, and gravity) are all the same force. Moments after the big bang, as the universe cooled and expanded this force split off in stages into the four separate forces. 6) All matter and force particles except gravitons are open strings. Endpoints of these open strings stick to branes, like the 3-brane of our universe. Thus they are trapped within certain regions. 7) Gravitons are closed strings. Thus they are free to roam throughout our 3-brane universe, and free to leave it! Gravitons leak into other branes and the bulk. This explains why gravity appears weaker that the other three force. 8) The Cyclic Cosmological Model - We are living within a three-brane that collides with a nearby parallel 3-brane every few trillion years (in higher dimensional space). The "bang" from this collision initiates each new cosmological cycle. (Ref: B. Greene, The Elegant Universe, Part IV)

 

Science fiction or science fact? Nobody knows. Now some 40 years old; string theory is still very much a work in progress. Since its beginnings in the 1970's, literally thousands of physicists have worked on it. There has been great progress, especially in the mathematics; yet no single definitive version of string theory has emerged.

 

Edited October 13, 2010 by I ME