Wednesday, August 05, 2015

No, you cannot test quantum gravity by entangling mirrors

Last week I did an interview for a radio program named “Equal Time for Freethought,” which wasn’t as left-esoteric as it sounds. I got to talk about quantum gravity, black holes, the multiverse, and everything else that is cool in physics. I was also asked why I spend time cleaning up after science news outlets that proclaim the most recent nonsense about quantum gravity, to which I didn’t have a good answer other than that I am convinced only truth will set us free. We stand nothing to win from convincing the public that quantum gravity is any closer to being tested than it really is, which is very, very far away.

The most recent example of misleading hype comes, depressingly, from an otherwise reliable news outlet, the Physical Review A synopsis where Michael Shirber explains that
“A proposed interferometry experiment could test quantum gravity theories by entangling two mirrors weighing as much as apples.”

A slight shortcoming of this synopsis is that it does not explain which theories the experiment is supposed to test. Neither, for that matter does the author of the paper, Roman Schnabel, mention any theory of quantum gravity that his proposed experiment would test. He merely uses the words “quantum” and “gravity” in the introduction and the conclusion. One cannot even blame him for overstating his case, because he makes no case.

Leaving aside the introduction and conclusion, the body of Schnabel’s paper (which doesn’t seem to be on the arxiv, sorry) is a detailed description of an experiment to entangle two massive mirrors using photon pressure, which means that one creates macroscopically heavy objects displaying true quantum effects. The idea is that you first let a laser beam go through a splitter to produce an entangled photon state, and then transfer momentum from the photons on the mirrors by allowing the mirrors to bounce. In this way you entangle the mirrors, or at least some of the electrons in some of their atomic orbits.

Similar experiments have been done before (and have previously been falsely claimed to test quantum gravity), but Schnabel proposes an improved experiment that he estimates to be less noise-sensitive which could thus increase the time for which the entanglement can be maintained and the precision by which it can be measured.

Now as I’ve preached numerous times, just because an experiment has something to do with quantum and something to do with gravity doesn’t mean it’s got something to do with quantum gravity. Quantum gravity is about the quantization of the gravitational field. It’s about quantum properties of space and time themselves, not about quantum properties of stuff in space-time. Schnabel’s paper is remarkable in that it doesn’t even bring in unquantized gravity. It merely discusses mass and quantization – no quantum gravity in sight anywhere.

The closest that the paper gets to having something to do with quantum gravity is mentioning the Schrödinger-Newton equation, which is basically the exact opposite of quantum gravity – it’s based on the idea that gravity remains unquantized. And then there are two sentences about Penrose’s hypothesis of gravitationally induced decoherence. Penrose’s idea is that gravity is the reason why we never observe quantum effects on large objects – such as cats that can’t decide whether to die or to live – and his model predicts that this influence of gravity should prevent us from endowing massive objects with quantum properties. It would be nice to rule out this hypothesis, but there is no discussion in Schnabel’s paper about how the proposed experiment would compare to existing bounds on gravitationally induced decoherence or the Schrödinger Newton equation. (Contacted to author and asked for details, but no reply.)

Don’t get me wrong, I think this is worthwhile experiment and one should definitely push tests of quantum mechanics on macroscopically large and heavy objects, but this is interesting for the purpose of testing the foundations of quantum mechanics, not for the purpose of testing quantum gravity.

And the weight of the mirrors btw is about 100g. If the American Physical Society can’t trust its readers to know how much that is, so that one has to paraphrase it with “weighting as much as apples,” then maybe they shouldn’t be discussing quantum gravity.

All together, the paper is a sad demonstration of the utter disconnect between experiment and theory in quantum gravity research.


Phillip Helbig said...

"Penrose’s idea is that gravity is the reason why we never observe quantum effects on large objects – such as cats that can’t decide whether to die or to live – and his model predicts that this influence of gravity should prevent us from endowing massive objects with quantum properties."

What do you think of this idea of Penrose?

Do you think that this idea is not taken seriously enough because he has had some other ideas in the past couple of decades which were too speculative and in some cases have been shown to be wrong?

Bert Morrien said...

About the cat: I always thought that it is us who cannot decide whether it lives or not. Because of that I think the superposition is about the sum of chances of life and death, which is 1. A real superposition of life and death would not only be a contradiction, but because it cannot be observed, you can't tell that it is true or false. Correct?

Sabine Hossenfelder said...


"A real superposition of life and death would not only be a contradiction, but because it cannot be observed, you can't tell that it is true or false."

I think you'll have to clarify what you mean by 'real' before I can answer that question.

Sabine Hossenfelder said...

Hi Phillip,

I think it is almost certainly true that gravitational degrees of freedom induce some decoherence in quantum systems, but I don't think the effect is as Penrose envisions it, and neither do I think it's actually necessary to invoke gravity in this business. Penrose's model is neither particularly useful, nor plausible, nor elegant, and my suspicion is had it been suggested by anybody but Penrose it would have went straight to the graveyard of forgotten ideas. Best,


Bar said...

I take it that APS and you are both relying on a 100g apple which in weight on earth is a one Newton force, right?

Uncle Al said...

"entangle the mirrors, or at least some of the electrons in some of their atomic orbits" Conductive surface mirror or dielectric mirror? Massive quantum objects: neutron star cores (superfluid neutrons, superconductive protons), the LHC's liquid heium bath and magnets, LIGO's mirrors (maintained near ground state, effective microkelvin temperature, about 200 quanta). Advanced LIGO has 40 kg core optics. So? Gravitation-coupled oscillators are interesting. A bismuth dimer (e.g., doi:10.1088/0953-4075/46/9/095101) is not it.
(Au, 196.96657 amu; Bi = 208.9804 amu, nuclear radius scales as cube root of mass; doi:10.1088/0953-4075/46/9/095101 for bond length; ~300 pm)

A commercial apple masses ~ 250 g, 2.4 newtons.

Sabine Hossenfelder said...

The experiment is designed to be operated on Earth at standard altitude yes. Besides that, the weight, for all I can tell, doesn't matter, since the gravitational interaction doesn't play a role. In any case, it wasn't my choice of words, I just picked it up from the headline, but it's not a relevant point.

MarkusM said...

"Penrose's model is neither particularly useful ..."
At least it has triggered the setup of a highly sophisticated experiment:

Penrose recently gave an update on the status of the experiment (01:02-)

Plato Hagel said...


Wonder what Quantum Cognition community thinks about experiement.


MarkusM said...

Plato Hagel,
I don't know. But I can't really see what the connection between quantum cognition and gravitationally induced state reduction should be. The latter (in theory) is a generic quantum effect, as it involves Planck's constant (because it involves the time energy uncertainty relation), whereas in phenomena related with quantum cognition no h-bar appears. That's why they call their findings "quantum-like", to distinguish them from the ones in quantum mechanics. The only commonality at present seems to be the formalism used (which on the other hand may make one suspicious and curious).


Plato Hagel said...

Hi MarkusM,

I think Penrose is looking for signs of concentric rings in the CMB for a when you look at M=0 as a conformal symmetry he is mapping the Riemann sphere onto it and say quantum theory being a massless way in which to experience the world, has to have matter orientations that begin somewhere, so why not at the early universe where we place a big question mark regarding unification?


Plato Hagel said...

If you look back to first video, at approx 2:40 to 2:48 I see signs of the QC affirming such a knowledgeable approach regarding quantum Computing so who are these people one can be drawn too, that insight such

YouTube video ( ) M=0, as conformal symmetry......maximum symmetry...Delta E, Delta T of 30:00 to 31:13

I really like the stereographic modelling being used as the Riemann sphere to map onto the CMB.

Lincoln Carr said...

Hi Sabine, just stumbled on your post. The point is to test quantum mechanics with classical gravity. And the field doesn't originate with Penrose... Although yes he did some speculative work there a while ago on Phillip wrote. In case you're not aware of it, have a look at cavity optomechanics. E.g. . The right theory for QM with Newtonian gravity is hotly debated, lots of PRLs in that area recently.

Sabine Hossenfelder said...


I know - I am just writing on a new blogpost about Aspelmeyer's work. As should be clear from my blogpost above, I was criticizing the synopsis, not the paper.