# Why we can't build a QFT of gravity?

#### ndung

Why we can not consider the Einstein's equation as a motion equation of gravitation field in flat spacetime background.If we can,we would be able to construct a quantum field theory of gravity with the Lagrangian deduced from the Einstein's equation and the functional integral formalism.

#### ndung

Why we can build Supergravity theory while we can not build QFT of gravity?

Why we can not build QFT for gravity while we can build SUGRA(being a field theory of SUSY)?

#### topsquark

Forum Staff
We can construct a QFT for gravity and we have. The trouble is that, even for a QFT, the number of infinities in a QFT for gravity is pathological. But some look nice: String Theory, N = 8 supergravity, SUSYs, etc. all have features that are attractive but none of them, as far as I know, have any experimental evidence to support them.

-Dan

#### Farsight

Why we can not build QFT for gravity while we can build SUGRA(being a field theory of SUSY)?
For myself, I think it's because there are problems in the way classical electromagnetism is taught, problems in the interpretation of QED, and problems in the understanding general relativity, particularly when it comes to particle physics.

If you were to ask a particle physicist some simple questions like why do an electron and a positron move towards one another and circle each other and why does an electron fall down, you won't get satisfactory answers. When you do, that's when quantum gravity has got half a chance.

#### MBW

Limitless?

My (very) basic knowledge of QFT gives me the impression that a main source of the infinities is the lack of limits.
There is no limit to how small the smallest scale should be,
There is not limit to how high the highest energy should be.

Surely there must be some kind of maximum energy density that can be supported by the fabric of the universe?

I have no proper theory to back up this idea,
Just the analogy that there should be a limit to how much can be crammed in to any "container" before it is full.

(If such a limit exists, then this immediately also puts a stop on the singularity status of a black hole...)

#### ndung

By the way,so,is there not a limit at which SUSY theories lead to normal symmetry QFT?

#### ndung

Other saying,is the normal QFT (for example Standard Model) contained in corresponding SUSY theories?

#### ndung

I mean is the Lagrangian of the corresponding SUSY theories a generalized of normal symmetry QFT?

#### topsquark

Forum Staff
Sorry to get back so late. A SUSY is a QFT. The difference between an "ordinary" QFT and SUSY is that SUSY adds a new symmetry...a symmetry between bosons and fermions. According to a SUSY every fermion (say an electron) has a "super-bosonic" partner (a "selectron"), and visa versa. In fact, my member name (top-squark) is the super partner to the top quark.

No super partners to the particles of the Standard Model have been discovered.

-Dan

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#### ndung

I hear that every symmetry put a restrictiveness on the number of terms of the Lagrangian.Then SUSY adds new symmetries so is more restrictiveness put on the Lagrangian?Then why we know SUSY Lagrangian contains Standard Model Lagrangian?