Interview: Holger Bech Nielsen 

I was given the pleasure of conducting an conversation with one of the fathers of string theory, Holger Bech Nielsen, who was among those to propose that the Veneziano amplitude, was a theory of strings. He was awarded the Humboldt prize, in 2001. He is considered one of the pioneers behind string theory, and one of it’s original contributors.

https://en.wikipedia.org/wiki/Holger_Bech_Nielsen#Work

Nolan Fitzpatrick: In your own words, how did string theory, emerge from the world of hadron physics in the 60s and early 70s?

Holger Bech Nielsen: It was Veneziano (helped by some predecessors) that proposed an extremely simple - mathematically - model for how pi + pi ---> pi omega scattering could be. Then we - Nambu , Susskind and myself - independently of each other found out that Venezianos scattering amplitude was actually an amplitude for scattering of strings. So the models developing the original Veneziano model were actually string models.

Nolan Fitzpatrick: How is string theory different from particle physics?

Holger Bech Nielsen: Particle physics is the notion for the field of studying the interactions and properties of elementary particles such as the hadrons and the leptons etc. while string theory can be applied as a POTENTIAL MODEL for the particle physics in two ways. But although it can be attempted to be used in two ways - namely for the hadrons alone , or for the constituents in the Standard Model - neither of these two application possibilities are really fully convincingly successfully described by string theory. So string theory is in both cases just an ATTEMPT to make a theory. 

Nolan Fitzpatrick: How is string theory a promising candidate to be a theory of everything, or, being able to unify general relativity and quantum mechanics?

Holger Bech Nielsen: If it should turn out that the string theory was indeed a good theory for the constituents in the Standard Model or some successful theory replacing the Standard Model, then one could say that string theory was indeed the theory of everything. String theory looks to almost unavoidably to come to include gravity, so indeed if it could include the three other forces electromagnetic, weak and strong nuclear forces, then it would indeed be a possibility of a theory of everything. As a side remark I should say that the very important property of string theory of giving finite results, would be spoiled easily if one replaces it by some theory in which the string are composed from some bits only yielding an approximate string theory. 

Nolan Fitzpatrick: How does the graviton, the hypothetical force mediating particle of gravitation, play into string theory?

Holger Bech Nielsen: The graviton is a special state of the closed string with mass zero. So the graviton is just a string in a special state. 

Nolan Fitzpatrick: How do the five different versions of superstring theory differ?

Holger Bech Nielsen: The strings that do not have problem with a tachyon but are very promising are superstrings, meaning that there is postulated to be on the string some extra so called fermionic degree of freedom. One might visualize this extra degree of freedom to mean that there exist a ``particle'' or ``pearl'' that can only run ON the STRING. This particle has the quantum mechanical property of being a fermion, which mainly means that two such pearls cannot be say at the same place if in the same internal state. But now the properties and possibilities for in what states these fermionic pearls can be can be varied in various ways. It some different variations of these properties of the fermionic pearls that make the difference betweens the various types of superstrings. There has turned out to be five ways of adjusting these details of the properties which give truly self consistent string theories. It is these five possible adjustments of the details that make up the five famous types of superstrings. For instance in of the superstrings the fermionic pearls always moves along the string in only one direction. In others they can move both ways. 

Nolan Fitzpatrick: Physicists say that superstring theory is 10 dimensional and that M-theory is 11 dimensional. At the heart of string theory, is the idea that there are 6 or 7 extra dimensions, curled up into the Calabi-yau manifold. Could you explain the process of compactification and extra dimensions?

Holger Bech Nielsen: Yes, you are right this is the typical idea, but in the attempt to make viable string models other possible speculations such as branes have been also studied. It may not help so much, but one can at least seek to imagine that if you were extremely small and went out in the direction of one of the extra 6 ( or 7) dimensions then you could come back say from the opposite side. Instead of a Calabi Yau space one might for pedagogical purposes think of a circle. This would correspond to then only one extra dimension. For further pedagogics we could then also replace the truly existing 3 +1 dimensions by only 1. If so the full 2-dimensional world would be like a tube ( et sugerør) and a very small animal would conceive the surface of the tube as 2 dimensional but a big and and less finely less sensitive animal would be extended all around the tube and conceive of only ONE dimension.