# Is string theory an approximation to QFT?

1. Jul 7, 2009

### Finbar

The historic roots of string theory are in an explanation of the strong force. Nowadays QCD is the accepted theory of strong force. But having heard several lectures on the large N limit (SU(N)) of gauge theories it seems these theories start to looklike string theories in this limit. I believe this idea (that gauge theories look like string theory in the large N limit) may have inspired AdS/CFT...you take the large limit in the orginal paper i believe. One question in QCD is if 3 is close enoght to infinity to take the large N limit as a good approximation? If it is and we take use Ads/CFT we can then use string theory to do real QCD calculations even if strings are not fundemental. In this way Ads/CFT and string theory could be just useful calclation tool but not a fundemental description of nature. We could then be in a situation where QFT is still more fundemental than String theory or(more likely) they are both approximations to something still more fundemental.

2. Jul 15, 2009

### tom.stoer

Thanks for this summary!

To me it seems that there is some correspondence of strings and QFT in some appropriate limit. Therefore it's not clear which theory is more fundamental. The proof of an exact duality is still missing.

In case of an successfully established duality based on a rigororous proof my conclusion would be that both theories are on the same level. Or a decision which one is more fundamental is more a subject of belief and philosophy instead of physics.

In case of a disproof the experiment must decide which theory is closer to the facts. Currently QFT is in the pole position (it may be uggly due to the many free parameters and the incompleteneness in terms of divergences etc., but it allows us to do calculations).

Having neither a proof nor a disproof it's a hint to search for some deeper structure. A very strong argument for this deeper structure is the spin-2 particle in the string spectrum. This is the main hint to take string theory (M-theory ...) as a serious as a candidate for a ToE.

Question: how would this reasoning change if we find a SUGRA that is both finite a capable to reproduce the standard model in the low energy limit? Would we abandon string theory completely - or is there another reason to believe that strings (or M ...) are more fundamental?

3. Jul 15, 2009

### humanino

Strings and Q gauge FT have a long history, an old account of which being available for instance in A. Polyakov's diary "Gauge field and strings" published in the "Contemporary concepts in physics" series (1987).

The simple answer to "why 3=infinity" is that we take the square of the amplitude and end up with roughly 10% accuracy already at first order.

4. Jul 15, 2009

### Bob_for_short

I believe that the renormalizable QFTs can be reformulated to bypass the divergences and thus give a complete physical and mathematical "satisfaction" to researchers. At least one will be able to sort out correct theories from incorrect.

String theory is still far from any reality and seems to me extremely artificial.

5. Jul 16, 2009

### tom.stoer

Do you have any idea how this reformulation may work?

Why don't we take string theory as a kind of machine to produce a certain set of QFTs?

6. Jul 16, 2009

### Bob_for_short

Yes, see my strict results in the independent research section.
None machine can produce the right physics. The string theory is based on a bad idea - artificial cut-off of a vague nature. It is good as a mathematical exercise but it does not correspond to physics.

7. Jul 17, 2009