I remember reading about the groundbreaking experiment by Nesvizhevsky (et al. 2001) some 12 years ago using ultra-cold neutrons which showed the first experimental evidence of quantum gravity. It is my understanding that these experiments has been repeated since then in labs around the world and similar results observed(neutrons bouncing off the reflector in a discrete set of quantized energy states rather than varying continuously)and more recently by Kobakhizde(2010), who was testing Erik Verlinde's "entropy theory of gravity". Am I correct about the reproducibility of Nesvizhevsky's results? If so, these experiments seem to suggest that gravity is indeed an actual force after all and not simply an emergent property as posited by GR. But do any of the results of these experiments conflict with the weak/strong equivalence principles? Someone on Yahoo! answers with the handle OzoneGuy claimed that the observations were explainable simply by diffraction. But this could be tested by positioning the diffraction grating to be parallel to the neutron reflector.So it is simply diffraction? Or is there actual evidence of quantum gravity?

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    $\begingroup$ Technically they weren't quantum gravity experiments, they involved quantum mechanics in a gravitational field, which is different because the gravitational part can be treated classically. $\endgroup$ – Mitchell Porter May 15 '13 at 21:56
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    $\begingroup$ Links to a basic description of the work would be appreciated by readers who don't know this name. $\endgroup$ – dmckee --- ex-moderator kitten May 15 '13 at 22:34
  • $\begingroup$ Here's a link to a paper co-authored by Nesvizehvsky describing their experimental results: arxiv.org/ftp/hep-ph/papers/0502/0502081.pdf $\endgroup$ – Mr X May 17 '13 at 17:37
  • $\begingroup$ Mitchell, how can the gravity be treated classically when given the mass/energy of the neutrons is so tiny that the classical gravitational force would be far too weak to have any observable effect. Clearly gravity has an effect on the quantum scale so these experiments demonstrate that gravity has a quantum mechanical basis carried by some unknown particle. $\endgroup$ – Mr X May 17 '13 at 17:43
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    $\begingroup$ @MrX I've edited the link into your post which is better than putting it in the comments. I've also linked to the arXiv abstract page which is generally preferred to giving a PDF link, as many user will want to read the abstract before deciding to download a PDF and the abstract tracks versions updates by default and provides access to the version history. $\endgroup$ – dmckee --- ex-moderator kitten May 19 '13 at 16:57

I wouldn't describe Nesvizhevsky's work as groundbreaking. It's a refinement on a type of experiment that was first carried out 38 years ago (Colella 1975). For a recent review article, see Abele 2012.

It also isn't a probe of quantum gravity. By the equivalence principle, this type of experiment is equivalent to simply accelerating the apparatus in a region where there is no gravitational field.

Quantum gravity becomes relevant at the Planck scale, and nothing in this experiment got anywhere close to the Planck scale.

Abele, 2012, http://iopscience.iop.org/1367-2630/14/5/055010

Colella, Overhauser, and Werner, Phys. Rev. Lett. 34 (1975) 1472

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  • $\begingroup$ You are correct that these experiments do not offer any direct observational evidence for quantized gravity. However, they demonstrate that gravity has an effect on the quantum level as these neutrons showed quantized energy states based on their interaction with the Earths gravitational field. There is even speculation that these neutrons are interacting with some kind of unseen matter: arxiv.org/abs/1004.2981 $\endgroup$ – Mr X May 20 '13 at 19:23
  • $\begingroup$ Also Mr Crowell, take a look at this paper here(section 4.2, pages 78-79):pi.uni-hd.de/Publications/dipl_krantz.pdf $\endgroup$ – Mr X May 21 '13 at 19:42
  • $\begingroup$ >>By the equivalence principle, this type of experiment is equivalent to simply accelerating the apparatus in a region where there is no gravitational field.<< But the experimental data shows evidence for Yukawa forces as well as the possibility of a new kind of interaction in nature! So what's important here is that UCN experiments demonstrate the breakdown of the equivalence principle on the quantum scale. $\endgroup$ – Mr X May 22 '13 at 8:20
  • $\begingroup$ @Mr X, the experiments had nothing to do with any of that. They were just about energy levels. A nucleus has a charge and creates an electromagnetic potential, and electrons in that potential can occupy different energy levels. Here there were neutrons in a gravitational potential and they also had quantized energy levels. But the potential itself was not treated in a quantized way, as it would be in quantum field theory. There is nothing here about gravitons or new interactions, just the behavior of neutron wavefunctions in gravity. $\endgroup$ – Mitchell Porter Jun 19 '13 at 6:45
  • $\begingroup$ Mr Porter, WADR did you read, or at least have a look at the article I linked to above.As Ben Crowell pointed out the original purpose of these experiments(including the original experiment by Colella in '75)was to test the weak equivalence principle. In fact, ultracold neutron experiments were more recently use to test the Entropic theory of gravity proposed by Erik Verlinde in 2009. The results show that gravity is not an emergent property and is a fundamental interaction like EM, and the 2 nuclear forces. $\endgroup$ – Mr X Jun 24 '13 at 3:38

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