Editing Supergravity (section)

===The end of the SUGRA era===
The initial excitement over 11-dimensional supergravity soon waned, as various failings were discovered, and attempts to repair the model failed as well. Problems included:{{Citation needed|date=May 2016}}

* The compact manifolds which were known at the time and which contained the standard model were not compatible with supersymmetry, and could not hold [[quark]]s or [[lepton]]s. One suggestion was to replace the compact dimensions with the 7-sphere, with the symmetry group [[SO(8)]], or the squashed 7-sphere, with symmetry group [[SO(5)]] times [[SU(2)]].
* Until recently, the physical [[neutrino]]s seen in experiments were believed to be massless, and appeared to be left-handed, a phenomenon referred to as the [[Chirality (physics)|chirality]] of the Standard Model. It was very difficult to construct a chiral fermion from a compactification — the compactified manifold needed to have singularities, but physics near singularities did not begin to be understood until the advent of [[orbifold]] [[conformal field theory|conformal field theories]] in the late 1980s.
* Supergravity models generically result in an unrealistically large [[cosmological constant]] in four dimensions, and that constant is difficult to remove, and so require [[Fine-tuning (physics)|fine-tuning]]. This is still a problem today.
* Quantization of the theory led to quantum field theory [[gauge anomaly|gauge anomalies]] rendering the theory inconsistent. In the intervening years physicists have learned how to cancel these anomalies.

Some of these difficulties could be avoided by moving to a 10-dimensional theory involving [[superstring]]s. However, by moving to 10 dimensions one loses the sense of uniqueness of the 11-dimensional theory.<ref>{{cite arXiv |eprint=hep-th/9805177|last1=Duff|first1=M. J.|title=A Layman's Guide to M-theory|year=1998}}</ref>

The core breakthrough for the 10-dimensional theory, known as the [[first superstring revolution]], was a demonstration by [[Michael B. Green]], [[John H. Schwarz]] and [[David Gross]] that there are only three supergravity models in 10 dimensions which have gauge symmetries and in which all of the gauge and [[gravitational anomalies]] cancel. These were theories built on the groups [[SO(32)]] and <math>E_8 \times E_8</math>, the [[direct product of groups|direct product]] of two copies of [[E8 (mathematics)|E<sub>8</sub>]]. Today we know that, using [[D-branes]] for example, gauge symmetries can be introduced in other 10-dimensional theories as well.<ref name="Blumen">
{{cite journal
 |date=2005
 |title=Toward Realistic Intersecting D-Brane Models
 |arxiv=hep-th/0502005
 |last1=Blumenhagen| first1=R.
 |last2=Cvetic| first2=M.|author2-link= Mirjam Cvetič
 |last3=Langacker | first3=P.
 |last4=Shiu| first4=G. | doi=10.1146/annurev.nucl.55.090704.151541 | doi-access=free | volume=55 | issue=1 | journal=[[Annual Review of Nuclear and Particle Science]] | pages=71–139
|bibcode=2005ARNPS..55...71B|s2cid=15148429
 }}</ref>