The D=6 string-string duality, crucial for allowing the interchanging of the roles of 4-D spacetime and string-world-sheet loop expansion, entails that there is an equivalence between the K-3 membrane action and the orbifold action. Here are some thoughts and reflections.

In the bosonic sector, the membrane action is:

where:

Recall I derived the total action:

which is highly non-trivial since Clifford algebras are a quantization of exterior algebras. Applying to the Einstein-Minkowski fibre-bundle, we get via Gaussian matrix elimination, an expansion of via Green’s-functions, yielding the on-shell action of M-theory in the Witten gauge:

with the kappa symmetry term. With the metric on , and the corresponding coordinates with an antisymmetric 3-tensor. Hence, the worldvolume is:

The bosonic sector lives on the boundary of the open membrane: two copies of , which naturally couple to the U(1) connections .

Now, double dimensional reduction of the twisted supermembrane on:

of

entails that the bosonic sector is that of the heterotic string:

with gauge group indices I = 1, … , 16.

It gets interesting when we consider:

with dimension:

since the worldsheet action:

is now just a sum of three terms:

and the index I = 1, … , 22 labels 22 gauge fields: 16 coming from the internal dimensions of the heterotic string, and the other 6 gauge fields are the KK modes of the metric and antisymmetric tensor. The action has a massless spectrum given by moduli fields corresponding to deformations of the Narain lattice and thus take values in the group manifold:

Now, something fundamentally deep has occurred: all the gauge fields of the action have appeared within a two-dimensional theory, and *not* a three-dimensional theory

*not*a three-dimensional theory

This is precisely the long wavelength limit behavior of the **open** membrane:

the gauge fields are defined in terms of fields which live on 10-dimensional boundaries of **M**-theory

**M**-theory

In the **closed** membrane case:

the gauge fields are defined in terms of **11**-dimensional fields

**11**-dimensional fields

Hence, the gauge fields of the closed membrane must be defined over M3 and not over its boundary, unlike the closed membrane, whose action on is:

where is with the spacetime being .

Hence, the closed membrane action on reduces to:

with:

and

and since surfaces have no one-cycles, it follows that the three-form potential that appears in of the action:

can be expanded in terms of the cocycles of .

For the 22 2-cocycles of , one can decompose in a similar way for the two-form potential:

with I = 1, …, 22 labeling the two-cycles of . So after insertion into , we can derive:

Applying reparametrization invariance, one can set:

where is a worldvolume coordinate, and now one performs a dimensional reduction of:

Here are the key propositions relevant to the** membrane/string duality** of the low energy theory in D=7.

- the kinetic terms for the gauge fields in D=7 supergravity are:

derived by a split of the 4-4 field strength , of the 11-dimensional supergravity action:

from the following term:

- Membrane/string duality in D=7 requires the existence of a point in the moduli space of where all the 22 gauge fields are enhanced via U(1) gauging: this is key to preserving kappa symmetry. Thus, at the point in the moduli space when the 22 two-cycles vanish the following holds:

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