Monday, March 23, 2015

No, you cannot test quantum gravity with X-ray superradiance

I am always looking for new ways to repeat myself, so I cannot possibly leave out this opportunity to point out yet another possibility to not test quantum gravity. Chris Lee from Arstechnica informed the world last week that “Deflecting X-rays due to gravity may provide view on quantum gravity”, which is a summary of the paper

The idea is to shine light on a crystal at frequencies high enough so that it excites nuclear resonances. This excitation is delocalized, and the energy is basically absorbed and reemitted systematically, which leads to a propagation of the light-induced excitation through the crystal. How this propagation proceeds depends on the oscillations of the nuclei, which again depends on the local proper time. If you place the crystal in a gravitational field, the proper time will depend on the strength of the field. As a consequence, the propagation of the excitation through the crystal depends on the gradient of the gravitational field. The authors argue that in principle this influence of gravity on the passage of time in the crystal should be measurable.

They then look at a related but slightly different effect in which the crystal rotates and the time-dilatation resulting from the (non-inertial!) motion gives rise to a similar effect, though much larger in magnitude.

The authors do not claim that this experiment would be more sensitive than already existing ones. I assume that if it was so, they’d have pointed this out. Instead, they write the main advantage is that this new method allows to test both special and general relativistic effects in tabletop experiments.

It’s a neat paper. What does it have to do with quantum gravity? Well, nothing. Indeed the whole paper doesn’t say anything about quantum gravity. Quantum gravity, I remind you, is the quantization of the gravitational interaction, which plays no role for this whatsoever. Chris Lee in his Arstechnica piece explains
“Experiments like these may even be sensitive enough to see the influence of quantum mechanics on space and time.”
Which is just plainly wrong. The influence of quantum mechanics on space-time is far too weak to be measurable in this experiment, or in any other known laboratory experiment. If you figure out how to do this on a tabletop, book your trip to Stockholm right away. Though I recommend you show me the paper before you waste your money.

Here is what Chris Lee had to say about the question what he thinks it’s got to do with quantum gravity:
Deviations from general relativity aren’t the same as quantum gravity. And besides this, for all I can tell the authors haven’t claimed that they can test a new parameter regime that hasn’t been tested before. The reference to quantum gravity is an obvious attempt to sex up the piece and has no scientific credibility whatsoever.

Summary: Just because it’s something with quantum and something with gravity doesn’t mean it’s quantum gravity.


Phillip Helbig said...

"Just because it's something with quantum and something with gravity doesn't mean it's quantum gravity."

I'll add this to my collection of pithy aphorisms.

Anonymous said...

"If you place the crystal in a gravitational field, the proper time will depend on the strength of the field".

Surely it depends upon the depth of potential? The strength of the field relates to the gradient in potential, which relates to the gradient in proper times plotted between the floor and the ceiling. If your light clock on the floor runs at the same rate as your light clock on the ceiling, light doesn't curve and your pencil doesn't fall down.

N said...
This comment has been removed by the author.
Sabine Hossenfelder said...


Please read "strength of the field" as a sloppy way to say "position in the background curved by the presence of a large source" as there is neither a field nor a potential here. It is entirely irrelevant to my explanation though. Best,


Anonymous said...

Sabine: OK noted, I do concur with your article. But there's maybe an issue here, and it could be important. See this and note that as per Einstein's Leyden Address, a gravitational field is inhomogeneous space wherein the spatial energy-density or "depth of potential" relates to gravitational time dilation, the first derivative of potential relates to the force of gravity, and the second derivative of potential relates to the tidal force. For a curved background check out Percy Hammond. IMHO the "strong curvature regime" is electromagnetism.

Uncle Al said...
Phys. Rev. 129(6) 2371 (1963), DOI: 10.1103/PhysRev.129.2371
Ultracentrifuge hub vs. rim is inert toward rate of time re rotation.
Harvard Tower experiment. Neutron stars (pulsars) show no EM emission anomalies despite large gravitational potential gradient (~12 km radius, surface ~2×10^12 m/s^2).

Postulates imprison theory. Newton denies c = c and h = h. Fermionic matter (quarks) in spacetime has a symmetry breaking to which massless boson photons are inert. Hugely amplify it and conventionally test for it, DOI: 10.5281/zenodo.15107.
Faculty and apparatus
The experiment. 6.68×10^22 right shoes vacuum free fall non-identically to 6.68×10^22 left shoes (alpha-quartz enantiomorphic unit cells in 4 versus 4 single crystal test masses).

Physics is imprisoned by a locked door. There is an open window. Escape.

Sabine Hossenfelder said...


Thanks for the references, but I really don't think there is an issue here. The authors use a totally standard treatment of gravitational redshift in curved backgrounds, the Schwarzschild metric in particular. If anything is an issue it's that I found this isn't the place to explain how this comes about and thus was very sketchy in my explanation. Best,


Anonymous said...

Sabine, with respect, you're missing the point. See the article:

"...the stage is an integral part of the show, bending and warping around the actors according to the rules of general relativity. The actors — atoms and molecules — respond to this shifting stage, but they have no influence on how it warps and flows around them. This is puzzling to us... In general relativity, it is possible to bend space and time..."

A gravitational field is spacetime is inhomogeneous space, not curved space. An electromagnetic wave is curved space, and we make an electron and a positron out of it in pair production. Google on electromagnetic geometry. The actors are the bending and warping.

Sabine Hossenfelder said...


I couldn't care less what you think a gravitational field is or what you think Einstein though it is. You're not making any sense to me. I am telling you the authors used a totally standard treatment of gravitational redshift. I will delete further obsolete comments of yours. Thanks,


Anonymous said...

Sabine: again with respect, what I'm saying is not obsolete, it's correct. Follow my previous links, and also see Baez:

"Similarly, in general relativity gravity is not really a force, but just a manifestation of the curvature of spacetime. Note: not the curvature of space, but of spacetime. The distinction is crucial".

A gravitational field is inhomogeneous space, not curved space. The electromagnetic field is curved space.

Zephir said...

/*Deviations from general relativity aren’t the same as quantum gravity*/

Why not? Which theory other than quantum mechanics can influence gravity and relativity?

Uncle Al said...

@Zephir, King of the Spews,