Gravitons

[RichIsnang]

Why don't we have a QFT for gravity?

Is it because gravity is attractive and the force carrying particles will have to travel out from the centre of mass against the pull of gravity? Which may slow them.

[MigL]

No, not even close.

A QFT of gravity has too many infinities which cannot be gotten rid of ( renormalised ) away.

[questionposter]

Wait, why "don't" we actually have theories for gravity? If we can't normalize gravity, obviously doesn't follow the same mechanics that can allow you to just add up probable energy levels and etc, it follows something different but still similar to the realm of other forces, so why wouldn't someone look into that? Why would they try to force it to coincide with current theories instead of developing a way to bring it into QTF? Maybe gravity isn't comprised of virtual particles at all, but it can still exist as a field mathematically...

Edited April 19, 2012 by questionposter

[Bill Angel]

People are sure working hard on this issue. I have found the book "once before time: a whole story of the universe" by Martin Bojowald to be very informative on this issue. It discusses "loop quantum gravity", a theory that considers how quantum effects would manifest themselves in the behavior of the "big bang". The book also discusses in passing the tricky question of if gravitons do exist, how could they escape from the gravitational field of a black hole?

Edited May 16, 2012 by Bill Angel

[ajb]

Why don't we have a QFT for gravity?

 

In short it is because the standard methods of perturbation theory, as applied to say quantum electrodynamics, fail for gravity. One cannot define the full quantum theory of gravity via a perturbation series like you can for QED.

 

.

A QFT of gravity has too many infinities which cannot be gotten rid of ( renormalised ) away.

 

This is true of perturbative quantum general relativity; we have a non-renormalisable theory. The same may not be true of quantum general relativity or something very similar outside of perturbation theory. We may have asymptotic safety, which would imply that quantum general relativity is a well defined theory, but it is not a theory of gravitons.

[MigL]

Welcome back AJB, it's been a while.

[Bill Angel]

No, not even close.

A QFT of gravity has too many infinities which cannot be gotten rid of ( renormalised ) away.

 

There is an interesting article discussing the mathematics of gravitons in the recent issue of Scientific American.

 

See :Loops, Trees and the Search for New Physics

According to the authors:

 

The unitarity method, however, has allowed us to actually do calculations that were contemplated in the 1980s but seemed hopelessly beyond reach then. We have found that some of the supposed inconsistencies are in fact absent. Gravity does look like the other forces, albeit in an unexpected way—it behaves like a “double copy” of the strong subnuclear force that binds the constituents of nuclei together. The strong force is transmitted by particles known as gluons; gravity should be transmitted by particles known as gravitons. The new picture is that each graviton behaves like two gluons stitched together. This concept is quite strange, and even experts do not yet have a good mental image of what it means. Nevertheless, the double-copy property provides a fresh perspective on how gravity might be unified with the other known forces.