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

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Discussion Overview

The discussion centers on the perceived incompatibility between Quantum Field Theory (QFT) and General Relativity (GR). Participants explore various reasons for this incompatibility, including issues related to renormalization and the treatment of time in both theories. The scope includes theoretical considerations and conceptual challenges.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant questions the incompatibility of QFT and GR, suggesting a lack of understanding of a crucial piece of information.
  • Another participant raises the idea of having a discrete amount of a field, questioning the nature of potentials and particles.
  • A participant mentions that GR is non-renormalizable, indicating that standard procedures for handling infinities in calculations do not apply to GR.
  • Another participant agrees with the renormalization problem and expresses intent to research further.
  • One participant elaborates that GR is perturbatively non-renormalizable, explaining that this is evident from power counting arguments related to Newton's constant. They note that GR can still be treated as an effective field theory under certain energy conditions.
  • A later reply discusses additional difficulties, such as the absolute character of time in QFT compared to the dynamic nature of spacetime in GR, suggesting that this issue remains unresolved in Quantum Gravity.

Areas of Agreement / Disagreement

Participants express differing views on the compatibility of QFT and GR, with some agreeing on the renormalization issue while others raise additional concerns. The discussion remains unresolved with multiple competing perspectives presented.

Contextual Notes

Limitations include the complexity of the renormalization problem and the differing treatments of time in QFT and GR, which are not fully explored or resolved in the discussion.

taylrl3
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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
 
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Ok, we have potentials and particles. Why can't you have a discreet amount of a field?
 
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.
 
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.
 
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
 
taylrl3 said:
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.
...
 
Last edited:

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