QFT/GR whats the problem? I can't see one

[taylrl3]

Hi,

Im just wondering why Quantum Field Theory is incompatible with General Relativity. From my current understanding they seem to be compatible. I must be missing a crucial piece of information. Please help :-)

Talyl

 

[taylrl3]

Ok, we have potentials and particles. Why can't you have a discreet amount of a field?

 

[diazona]

One of the most commonly quoted reasons is that general relativity is non-renormalizable. In other words, the procedures used to prevent certain calculations from giving infinite results don't work for GR. I'm not (yet) familiar enough with the topic to explain that offhand, but maybe someone else can fill in the details.

 

[taylrl3]

Yea I have heard about this renormalisation problem too. Ill go and see if I can read up on it and come back with a post.

 

[Finbar]

Hi,

So the problem with general relativity is that its perturbativly non-renormalisable. You can see this just from power counting arguments since Newtons constant is dimensionful. This means that when you add counter terms to the action, to cancel the divergences from loop diagrams, these will general not have the same form as the original action. This doesn't stop one treating gravity as an effective field theory though; as long as you only consider process with typical energy E<<M_pl where M_pl is the Planck mass QFT/GR are perfectly compatible.


http://en.wikipedia.org/wiki/Effective_field_theory

One should also note that the breakdown of perturbative QFT at the Planck scale does not necessarily imply that a non-perturbative formulation of gravity as a QFT is impossible.

http://arxiv.org/abs/0709.3851

 

[Creator]

Hi,

Im just wondering why Quantum Field Theory is incompatible with General Relativity. From my current understanding they seem to be compatible. I must be missing a crucial piece of information. Please help :-)

Talyl

Besides the one mentioned , non-renormalisability of Gen Rel., there appears to be some other difficulties.

One is that in QFT the time component has, at least in part, an absolute (Newtonian) character as opposed to the full dynamic (space)time of Gen Relativity.
See here for example: http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TVN-4S6G8XP-2&_user=10&_coverDate=05%2F15%2F2008&_rdoc=1&_fmt=high&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1430883942&_rerunOrigin=google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=ea2de960d543c63799c7fbefc113ff0a
I'm not so sure this problem has been successfully circumvented , even in Quantum Gravity.
However, that is not exactly my area of expertise.
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