String theory… Brilliant I like it, “it’s very beautiful”.

“Elephant”

“What elephant?”

It’s nice to see that our Jedi mind trick worked on you, as a cosmologist, making you think that *you’re* absorbing *us*. Keep thinking that while we swallow you up. And do wriggle a bit while we chew. Makes it more fun. -cvj

tmccort: Its called string theory of M-theory for want of a better name. What people don’t tell you much is that we know it is not a theory of strings. We basically don’t know what it is a theory of at all, in fact. We know that in some regimes, the physics is “stringy”, and in other regimes it is something else, but we have not yet formulated the theory in a way that does not have to refer to one limit (regime) or another for us to properly understand what is going on. This is the biggest and most important aspect of the puzzle of this approach to fundamental physics. So nobody is bothering to find new names until we have better ideas for what the thing is. Not knowing what it is makes it no less dazzling in its scope and physical richness.

The process of doing science is what you are asking about. The media seem uninterested in talking about that and present everything as the “truth for today”. This is one of the reasons we have this blog, in my opinion - to help people see that science is an ongoing dialogue with Nature. So string theory is an attempt to find answers to several questions that arise when you study Nature. We do not know the answers in advance. We formulate a framework within which to answer those questions, and within that framework theories are constructed that take as input what we already know, and -if we’re smart enough and if we’ve done a good job- the theories give an answer to the questions we care about, *and predict further things*, sometimes things that we had not thought of when we set out to ask those initial questions.

This is true in any field of science. String theory is no different. It will take more time to develop it to the point that we can confront it with real observations from Nature. But the scientific procedure that is going on is just as sound as in any other field.

TM. In this context, perturbative just means that you are formulating and calculating the physics in a regime where the strength of the interaction between the individual strings is rather weak. This strength (or “coupling strength”) controls how likely a string is to split into more strings, or join with other strings. This is the regime which gave the theory its name. In later years, we learned a bit about the regime which is not like this, where the strings interact so strongly that they can do very new things to the physics. This regime is so profoundly different that the individual strings are often completley absent from the physics, and the description is best done in terms of other things, such as the branes of which I spoke. This is a much harder regime in which to work, and is the key to understanding the whole story. This is partly because the strings happen to have the abiltiy to dynamically change the coupling and you can get driven to a regime of strong coupling when you started out in weak coupling.

A manifold has a precise definition which I won’t trouble you with. It suffices for the purposes of reading the article too substitute the word “space”, and have in mind a few candidate shapes. The surface of a ball is a two dimensional sphere, and there are higher dimensional versions of such things, which I used in the article. Also, the two dimensional surface of a table top (ignoring the edges for now, imagine its a big table) is another two dimensional example. One is “compact” and the other is “non-compact”. Another one is the surface of a ring doughnut (donut), which is a two dimensional manifold called a “torus”. It has more interesting “topology” than a sphere as you can see by the fact that you can get a loop of string stuck on it (so that the loop can’t contract), where is there is no way of doing so with a sphere.

These are all manifolds. There is a great deal of interest in studying higher dimensional versions of these spaces, with lots of interesting properties. These are spaces which string theory suggests as possibilities for the dimensions of spacetime that we don’t see directly. One of the things string theory must do is give predictions for experimental signatures that give clues to the nature of this internal manifold. As you can see, you can distinguish between them due to how strings propagate on them -getting tangled or not has experimental consequences.

If you want a defintion of a manifold you can do it as follows. In the local neighbourhood of any point on the space, it should be enough to get around by locally treating itlike the tabletop space (called R^n in n dimensions.) Then you need nice conditions on how to translate between such local maps as you move from neighbourhood to neighbourhood. There are sveral good books on this, but that’s the idea.

Cheers,

-cvj

A perterbative theory is one when the person analyzes it takes the original equations describing the equation to have “free” part and an “interacting” part. The free part usually has the mathematical structure closely analogous to the dynamics of a spring, which is easily solved. The “interacting” part is considered to be multipled by a “small” constant (called a “coupling constant”). The theory can then be approximated as the ‘free’ theory, plus a small, “first-order” correction, proportional to the coupling constant, plus an even smaller ’second order’ correction, proportional to the coupling constant sqared, etc… Feynman’s diagrams are a pictoral way ot expressing this ‘perturbation series’, where the free theory is deformed by interactions. This technique is very well understood, but is only valid for interactions that are small relative to the ‘free’ part. Some examples of quantum field theories well suited to perturbative methods are the Weinberg-Glasgow-Salam model of the weak interaction and Quantum Electrodynamics

Non-perturbative theories are ones that do not use the above process in deriving the quantum mechanics above. Quantum Chromodynamics, the currently accepted theory of the strong interaction, is not well described by perturbative techniques, as the ‘interacting’ part is large with respect to the ‘free’ part at low energies (though perturbative theory is useful at high energies for QCD, as shown by Wilcek). Loop quantum gravity makes the argument that since general relativity is not logically seperable into a ‘free’ and ‘interacting’ part, it must therefore NOT be treated with perturbative techniques. Typically, any non-perturbative approach is very mathematically intractable and difficult, though progress is being made on this front, especially using computer simulations.

A manifold is simply a fancy, technical word which roughly translates to ’space’ or ’space-time’ (depending on the signature–i.e., whether it is 4 dimensional [=space] or 3+1 dimensional [=space-time]). The main feature that a manifold has is that it has enough structure to allow calculus to be done on it (directional derivatives are well defined, etc…)

hope that helped!

All this speculation that is being done on strings, branes and supersymmetry is all well and good but it is still only speculation and in the media often taken to be fact.

For example, take the Ptolemaic model of the solar system. The more they worked on it the more accurate it became at describing the motions of the planets, yet the more they worked on it the further it became from having anything to do with reality. When they found problems I guess it just meant that they had more work to do.

I don’t mean to sound flippant, but what’s the difference between the assumption of Geocentrism and the assumptions of strings, branes or supersymmetry really?

Could all this just be an elaborate human construct?

Sincerely,
Interested non-expert