Supergravity

In 1976, three physicists from the State University of New York in Stony Brook will work out the details of supergravity: Daniel Z. Freedman, Sergio Ferrara and Peter van Nieuwenhuizen. While they were working with the old gravity theory of Einstein in 4 dimensions, they actually wrote down the first theory of supergravity!

Supergravity theories combine the principles of general relativity with supersymmetry. Supersymmetry, as we know allows fermions to interchange with boson partner particles, and vice-versa. These are called superpartners or sparticles. The superpartners of the fermions, have an "s-" before their name, while the superpartners of the bosons have an "-ino" at the end of their names. All the while, equations are kept in tact. 

Supergravity became an area of interest in the 1970s and higher dimensional theories became an area of interest again after nearly 50 years since the death of Kaluza-Klein theory.

All particles have superpartners in theories that incorporate the principle of supersymmetry. This made supersymmetry easy to adapt into theories of point particles. These partner particles are called sparticles and their spin will differ by a 1/2 integer. 

It became apparent that theories of supersymmetry, in 1976, were the most promising for constructing a unified theory of gravity, particle physics and fundamental forces. This is because this relationship between the bosons and fermions could calm the frenzy of quantum fluctuations. Sadly, as we will see, this attempted merger of gravity with quantum mechanics will actually be met with failure. However, there is certainly a lesson to be learned from this theoretical attempt at unification: higher dimensions. 

Supergravity - 1970s

Supergravity is restrictive, as it places an upper limit on the number of dimensions that it can be consistently formulated in: 11 (as opposed to general relativity, which can be formulated in any arbitrary number of dimensions, though it is often formulated in 4, to match our physical world). This is the simplest way to include matter in a consistent theory of supergravity: formulate it in 11 dimensions. 

1978: Werner Nahm

Showed that 11, was the maximum number of dimensions that a consistent theory of supersymmetry can be formulated in. This is despite the fact that when supergravity was initially formulated at Stony Brook, it was formulated in 4 dimensions. 

1978: Joel Scherk, Bernard Julia and Eugene Cremmer

Showed that supergravity, is most elegant, with 11 as the maximum number of dimensions. This allowed for local supersymmetry and for no fields with spin higher than 2.

There was hope that by constructing various compactifications (a method that makes extra dimensions curled up and small enough to be undetectable) of supergravity in 11 dimensions, a unified theory of nature’s fundamental forces could be constructed: gravity, electromagnetism, and the strong and weak nuclear forces.

The decline of supergravity:

However, there were problems with 11-dimensional supergravity:

• It could not provide an explanation for a phenomenon in the Standard Model, known as chirality, where the laws of physics distinguish between clockwise and counterclockwise.

• It did not provide a consistent theoretical picture of gravity and quantum mechanics. The theory had problems with uniting the forces of nature. The theory was too small to account for all classes of subatomic particles. 

It became clear that supergravity could not be quantized correctly. Supergravity failed because, whenever physicists tried to calculate numbers in this theory, they would arrive at meaningless infinities. The theory had fewer infinities than the Kaluza-Klein theory, however, it was still not renormalizable. Another problem was that the largest symmetry that supergravity could include was called O(8) and it was still too small to contain all of the symmetry of the Standard Model.