I am reading an article called The fundamental nature of space and time by Gerard 't Hooft. On page 3 he writes the following:

Physically, however, the perturbative approach fails. The difficulty is not the fact that the finite parts of the counter terms can be freely chosen. The difficulty is a combination of two features: (i) perturbation expansion does not converge, and (ii) the expansion parameter becomes large if center-of-mass energies reach beyond the Planck value. The latter situation is very reminiscent of the old weak interaction theory where a quartic interaction was assumed among the fermionic fields. This Fermi theory was also “non-renormalizable”.

In the Fermi theory, this problem was solved: the theory was replaced by a Yang-Mills theory with Brout-Englert-Higgs mechanism. This was not just ‘a way to deal with the infinities’, it was actually an answer to an absolutely crucial question: what happens at small distance scales?. At small distance scales, we do not have quartic interactions among fermionic fields, we have a local gauge theory instead. This is actually also the superior way to phrase the problem of quantum gravity: What happens at, or beyond, the Planck scale?

My Question

Can someone explain in relatively laymens terms what we think happens with quantum gravity at or beyond the Planck scale?

I am not a layman persay but I am not at 't Hooft's level either. Something hitting an audience in between would be great.

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    $\begingroup$ If I knew that I would be writing off to the Nobel committee. 't Hooft is (I assume) pointing out problems with the canonical quantisation of gravity. The implication is that GR is a large scale approximation to some more fundamental theory, which could for example be string theory or loop quantum gravity. But no universally accepted fundamental theory has yet emerged. $\endgroup$ Feb 6, 2015 at 7:26
  • $\begingroup$ You know Wheeler's saying "spacetime tells matter how to move; matter tells spacetime how to curve". Although simple, its factual. I was hoping there were a few simple facts about what's happening that could be summarized. But perhaps not. $\endgroup$ Feb 6, 2015 at 8:02
  • $\begingroup$ I guess I will just have to wait until I know enough to seriously read about string theory and LQG to get an answer then. $\endgroup$ Feb 6, 2015 at 8:04
  • $\begingroup$ Stan, I've just noticed this question in the list of related questions. It has a couple of answers from big guns and would make interested reading. Actually it's sort of a duplicate in that it covers the same area. $\endgroup$ Feb 6, 2015 at 8:28
  • $\begingroup$ related physics.stackexchange.com/q/159922 $\endgroup$
    – Phoenix87
    Feb 6, 2015 at 11:40

1 Answer 1


It is the refusal to accept the possibility that the speed of gravity exceeds the speed of light. Planck constant was derived from the speed of light. It is the smallest amount of energy exchange between electrons and charges.

In the case of gravity, the exchange of energy is not to move electrons to higher orbitals but to move mass or the entire nucleus if not the quarks themselves to a new position in space.

Light speed energy exchange fir gravity wont suffice. Such speed is too slow and would cause an abberation. A faster speed and much smaller discrete energy constant is a logical necessity for gravity to work. We also have to consider that the energy exchange is not a constant velocity just like for light but a constant accelaration. Afterall, gravity is an accelarating contraction of space.

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    $\begingroup$ Considering GR says gravity propagates at the speed of light, and that it's the most common (Only, perhaps?) opinion today, what makes you say that? Can you give a source to that claim? $\endgroup$
    – Omry
    Aug 30, 2015 at 3:57
  • $\begingroup$ What source would be acceptable to you considering that most sources as you would say would only suport textbook gr. ? $\endgroup$ Aug 30, 2015 at 5:38
  • $\begingroup$ Any paper in a peer-reviewed journal would be a start. $\endgroup$
    – Omry
    Aug 30, 2015 at 11:09

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