When does quantum gravity become important?

[HomogenousCow]

Title^

 

[Vanadium 50]

Tuesday, I think.

Can you be a bit more specific?

 

[chill_factor]

Tuesday, I think.

Can you be a bit more specific?

I am also curious as to what scales of curvature, length, time, acceleration and other possible factors make quantum effects important in gravity. Sorry for not using covariant quantities.

 

[HomogenousCow]

Tuesday, I think.

Can you be a bit more specific?

Amusing.
In what kind of a physical situations do our current models (QFT,standard model, GR) fail to predict with accuracy?

 

[dx]

At the Plank scale.

 

[Vanadium 50]

Thanks - much clearer.

There is no system in which both GR and QM are necessary to explain observations. So at the moment this is a purely academic exercise.

 

[dx]

No currently experimentally accessible situations

 

[HomogenousCow]

Is that the source of the difficulties? Having no physical results to go by.
For example SR, GR and QM were all guided by experiments which could not be accounted for at the time with classical theories.

 

[dx]

The following quotations may be relevant:

The theorist's method involves his using as his foundation general postulates or "principles" from which he can deduce conclusions...

But as long as no principles are found on which to base the deduction, the individual experimental fact is of no use to the theorist; indeed he cannot even do anything with isolated general laws abstracted from experiments. He will remain helpless in the face of separate results of experimental research, until principles which he can make the basis of deductive reasoning have revealed themselves to him

It seems unlikely that the usual incremental increase of knowledge from a combination of theory and experiment will ever get us where we want to go, that is, to the Plank scale. Under this circumstance our best hope is an examination of fundamental principles, paradoxes and contradictions, and the study of thought experiments.

The inconsistency between general relativity and quantum field theory emerged clearly as the limitation of quantum field theory. This problem is a theorists' problem par excellence. Experiment provides little guide except for the bare fact that quantum field theory and general relativity both play a role in the description of natural law.

...

As I have indicated, experiment is not likely to provide detailed guidance about the reconciliation of general relativity with quantum field theory. One might therefore believe that the only hope is to emulate the history of general relativity, inventing by sheer thought a new mathematical framework which will generalize Riemannian geometry and will be capable of encompassing quantum field theory.

 

[Bill_K]

In what kind of a physical situations do our current models (QFT,standard model, GR) fail to predict with accuracy?

Situations which in classical and semiclassical GR can be represented only as singularities.
a) The big bang
b) The endpoint of gravitational collapse
c) The endpoint of black hole evaporation
...