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I will start by stating that I realise that there already exist questions on the compatibility of GR and QM (e.g. here) but my question is different.

In this essay the following argument is made (pg 156):

  1. Free-falling particles do not resist their fall - they move by inertia
  2. Since they move by inertia, no gravitational force is causing their fall.
  3. Hence no gravitational force is causing the gravitational phenomena - instead objects follow timelike geodesics on a non-Euclidean space-time.
  4. If no gravitational force exists then any attempts to quantize gravity are not possible.

What do people think of this argument? Personally, I can see no overt errors, but the conclusion that attempts to quantize gravity are not possible seems so extreme I am unsure.

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closed as primarily opinion-based by Jon Custer, sammy gerbil, honeste_vivere, ZeroTheHero, John Rennie Jul 27 '17 at 11:18

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ What if space itself was quantized? $\endgroup$ – Yashas Jul 26 '17 at 13:39
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    $\begingroup$ It doesn't have to be a force to be quantizable. The classical description of a free particle doesn't involve any force, but still you can quantize it... $\endgroup$ – Luzanne Jul 26 '17 at 14:08
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In the reference you are quoting there is also a refutation of the existence of gravitational waves, in page 161 and 162. The reference list does not go to the LIGO experimental observation of gravitational waves, it ends in publications up to 2012. So it seems the article is pursuing an off the beaten track study of General Relativity, since even Einstein had predicted gravitational waves.

In my opinion a confusion/contradiction arises whenever one mixes frameworks of physics. What are called forces in Newtonian mechanics are connected with what are called forces in quantum mechanics only by the definition of a force as dp/dt. The three known forces of the standard model are not the only forces in quantum mechanics, every Feynman diagram with an exchange particle has a dp/dt. It is only the couplings at the vertices that can be uniquely identified as strong , weak , electromagnetic.

Physicists expect that once a definitive model of the quantization of gravity is settled ( because there exist effective models of quantization of gravity which work), the gravitational force of Newtonian mechanics will be connected with graviton exchanges.

In physics, frameworks emerge from each other smoothly in the overlap regions of the variables' phase space.

Newtonian mechanics is not thrown out because General Relativity gives more accurate results.

Classical mechanics is not thrown out because quantum mechanics explains the nucleus.

The terminology has to be careful in order not to create confusions as in the list above. The concept of force is not a necessary one for generating quantum phenomena. The concept of interactions is. When space itself is involved one cannot expect a similar path in quantization, that is why there is vigorous research on the subject.

Already there exist string theories which can embed the graviton together with the standard model of particle physics, except there are thousand of possibilities and no definite model yet proposed.

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  • $\begingroup$ Just to your first paragraph, I'm sure you are right that the author is 'off the beaten track'. A more recent discussion re gravitational waves by the same author is at doingphysicsright.wordpress.com/2016/02/14/… $\endgroup$ – user1887919 Jul 26 '17 at 14:13
  • $\begingroup$ Also could they not just be separate? i.e. we have gravity and that is about geometry, and then we have quantum mechanics and field theories? $\endgroup$ – user1887919 Jul 26 '17 at 14:15
  • $\begingroup$ the geometry is the underlying level of all physics theories, and physicists will nor rest until a coherent standard model is found for all four forces/couplings hyperphysics.phy-astr.gsu.edu/hbase/Forces/funfor.html $\endgroup$ – anna v Jul 26 '17 at 14:31
  • $\begingroup$ glanced at the reference. He is making the same mistake of ignoring the mass of the test particle which will be distorting the geodesic of the big mass due to its existence. This distortion is what is the interaction in field theory terms. after all all particles have a mass and generate geodesics around themselves. When they hit a stronger geodesic, they still interact, space changes. $\endgroup$ – anna v Jul 26 '17 at 14:37
  • $\begingroup$ @user1887919 The problem with having the geometry part just classical is that in GR the geometry depends on the matter content. Getting a quantum theory of matter and a classical theory of geometry to talk to each other might be even harder than just quantizing GR... See eg arxiv.org/abs/0802.1978 and references therein. $\endgroup$ – Luzanne Jul 26 '17 at 14:44

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