Wednesday, January 28, 2015

No, the “long sought-after link between the theories of quantum mechanics and general relativity” has not been found

[Image source: iPerceptions.]

The Physics arXiv blog has praised a paper called “On the weight of entanglement” and claimed that the author, David Edward Bruschi, found a new link between quantum mechanics and general relativity. Unfortunately, the paper is mostly wrong, and that what isn’t wrong isn’t new.

It is well known that quantum particles too must have gravitational fields and that measuring these gravitational fields would in principle tell us something about the quantization of gravity. Whenever you have a state in a superposition of two position states, its gravitational field too should be in a superposition. However, the gravitational field of all particles, elementary or composite, that display quantum properties is way too small to be measured. Even if you take the heaviest things that have yet been brought in superpositions of location you are still about 30 orders of magnitude off. I have done these estimates dozens of times.

The only way you can find larger effects is if you exploit secondary consequences of models that are not just perturbatively quantized gravity. For example the Schrödinger-Newton equation that assumes that the gravitational field remains classical even though particles are quantized can have odd side effects like preventing particle dispersion, or reducing the Heisenberg uncertainty. These effects can be somewhat larger, but they are still much too small to be measurable. The problem is always the same: gravity is weak, really weak. Nobody has ever measured the gravitational field of an atom. We measure gravitational fields of large things: balls, mountains, planets.

In the new paper, the author argues that entanglement “has weight.” By this he seems to mean that the full entangled state couples to gravity. It would be more surprising if that wasn’t so, but the treatment in the paper is problematic for several reasons.

The biggest problem is that the author in the paper uses semi-classical gravity. That means he couples the expectation value of the stress-energy to the space-time background, not the operator, which you would do were you using perturbatively quantized gravity. It is remarkable that he doesn’t discuss this at all. He doesn’t mention any problems with this approach (discussed here), neither does he mention tests of the Schrödinger-Newton equation (discussed here). This makes me think that he must be very new to the field.

Using the semi-classical limit in the case discussed in the paper is problematic because this semi-classical approach does not only break down when you have strong gravity. It also breaks down when you have a large quantum uncertainty in the distribution of the stress-energy. Here “large” means that the uncertainty is larger than the typical width of the distribution. This can be formally shown, but it is intuitively clear: In such cases the gravitational field also must have quantum properties. While these deviations from the semi-classical limit do exist at small energies, they are too weak to be measurable. That the semi-classical limit doesn’t work in these cases has been discussed by Ford and others 30 years ago, see for example these lecture notes from 1997, page 34, and the therein mentioned reference of Ford’s1982 paper.

By using semi-classical gravity and then looking at the non-relativistic case, the new paper basically reinvents the Schrödinger-Newton limit. To make this really clear: the Schrödinger-Newton limit in this case is widely believed to be wrong for good reasons. Using it is a non-standard assumption about perturbatively quantized gravity. The author doesn’t seem to be aware of this.

He then points out that the interference terms of the state makes a contribution to the distribution of stress-energy, which is correct. This has previous been done for superposition states. I am not aware that it has previously also been done for entangled states, but since it isn’t measureable for superpositions, it seems rather pointless to look at states that are even more difficult to create.

He then argues that measuring this term would tell you something about how quantum states couple to gravity. This is also correct. He goes on to find that this is more than 30 orders of magnitude too small to be measurable. I didn’t check the numbers but this sounds plausible. He then states that “one could hope to increase the above result” by certain techniques and that “this could in principle make the effect measurable”. This is either wrong or nonsense, depending on how you look at it. The effect is “in principle” measurable, yes. Quantum gravity is “in principle measurable”, we know this. The problem is that all presently known effect aren’t measurable in practice, including the effect mentioned in the paper, as I am sure the author will figure out at some point. I am very willing to be surprised of course.

As a side remark, for all I can tell the state that he uses isn’t actually an entangled state. It is formally written as an entangled state (in Eq (4)), but the states labeled |01>; and |10> are single particle states, see Eq(5). This doesn’t look like an entangled state but like a superposition of two plane waves with a phase-difference. Maybe this is a typo or I’m misreading this definition. Be that as it may, it doesn’t make much of a difference for the main problem of the paper, which is using the semi-classical limit. (Update: It’s probably a case of details gotten lost in notation, see note added below.)

The author, David Edward Bruschi, seems to be a fairly young postdoc. He probably doesn’t have much experience in the field so the lack of knowledge is forgivable. He lists in the acknowledgements Jacob Bekenstein, who also has formerly tried his hands on quantum gravity phenomenology and failed, though he got published with it. I am surprised to find Bei-Lok Hu in the acknowledgements because he’s a bright guy and should have known better. On the other hand, I have certainly found myself in acknowledgements of papers that I hadn’t even seen, and on some instances had to insist being removed from the acknowledgement list, so that might not mean much.

Don’t get me wrong, the paper isn’t actually bad. This would have been a very interesting paper 30 years ago. But we’re not living in the 1980s. Unfortunately the author doesn’t seem to be familiar with the literature. And the person who has written the post hyping this paper doesn’t seem to know what they were talking about either.

In summary: Nothing new to see here, please move on.

[Note added: It was suggested to me that the state |0> defined in the paper above Eq(5) was probably meant to be a product state already, so actually a |0>|0>. The creation operators in Eq(5) then act on the first or second zero respectively. Then the rest would make sense. I’m not very familiar with the quantum information literature, so I find this a very confusing notation. As I said above though, this isn’t the relevant point I was picking at.]


Phillip Helbig said...

"by certain technics" ---> by certain techniques

Phillip Helbig said...

"The author, David Edward Bruschi, seems to be a fairly young postdoc, who looks nice enough on the photo, so the lack of knowledge is forgivable."

Forgivable because he is a fairly young postdoc, or forgivable because he looks nice enough on the photo? :-)

hm said...

In my opinion, being in the Acknowledgments does not imply agreeing with the paper. You can thank someone for interesting discussions, input, ...

Is this in practice different? And is bad to acknowledged be on papers that you do not agree with?

Kenny Chaffin said...

Bad Science is not forgivable because of youth or looks.

caboolture said...

Just copying this across from the Facebook thread on your post!

"Nice write-up. One thing to be aware of is that the single-particle state 01+10 *is* indeed entangled. This is an old result in quantum info ...

Here's the reference: .

The physics is basically this: I send a single photon through a beamsplitter, generating the state 01+10, where 0 is no-photons and 1 is 1-photon. I go to a point where the modes are spatially-separated, then in each output interact the mode *locally* with an atom-cavity system and generate a two-particle entangled state, e.g. ge+eg where g is ground, e is excited state. Since the interactions are local, the entanglement must have been present in the initial state. QED.

This floated around as a blackboard proof since at least the mid-to-late 90's, that I'm aware of, but it was first formally written up by van Enk in the paper above."

L. Edgar Otto said...

Imagine if you will a beautiful Greek stature youthful with no defects or missing arms and proper fingers and toes, in the resonance within our hearts for symmetry before the ravages of time as the sculptor comes closer to a dynamic living copy we do not see in the depth of hidden anatomy there is classical beauty?

Sabine Hossenfelder said...


Thanks, fixed the typo. I meant he is young and it just seems like he didn't have anybody pointing him towards the relevant literature. This has happened to me too. If you've been around for long enough you have a better idea of what is new and what isn't. Best,


Sabine Hossenfelder said...


As I said, it's not bad science per se, it's lack of knowledge of what has been done before. This is very common for newcomers in any field. You're right in that it doesn't a priori have much to do with age, but it's a matter of experience and that tends to be correlated with age. Of course, he should have read my blog, then he'd have known ;) Best,


johnduffield said...

I think it's improper to be so sneeringly critical about some young guy's paper on your blog.

Sabine Hossenfelder said...


Regarding acknowledgements. You are right of course that being in the acknowledgements does not imply agreeing with the paper. I have been in the acknowledgements of many papers that I didn't agree with. But in these cases at least I raised my objections and they were mentioned. If you're in the acknowledgements of a paper that is missing central information it clearly looks like you either didn't read the paper or failed to point out essential flaws when you heard of the idea.

Either way, one shouldn't overinterpret acknowledgements. I just found it strange that there isn't a single person mentioned in the paper, neither in the references nor in the acknowledgements who actually works on pheno qg.



L. Edgar Otto said...

The problem from loop QM approach is the denial or reachable concept of naked singularities. From the string approach that ultimately there is no privilegrd characterization in nature as if the universe is entirely defined my a mythical landscape where all is accident and random.
These are viable approaches and if both are absolutely true one needs not exclude the other nor as physical description be the same thing.
From Eddingtons view there are no mixed states. From Dirac's view varience of states can evolve coherently. Both theoreticians were concerned with the dimensionfree constant.
We can see an organism, some Venus or Apollo in surface or three space as a developing condensed n-dimensional structure.
In the big picture something like a density matrix is not needed to distinguish the low from the high dimensions.
A linear code itself is intrinsically n-dimensional.
|c> or <c|can be mirrored where kontiguity is independent and conserved and bra or ket is Dirac or Eddington braCketing things interchangeably.
How else can the bulk of nuclear mass come from the kinetics of its stable and hidden particles?

Phillip Helbig said...

"I meant he is young and it just seems like he didn't have anybody pointing him towards the relevant literature."

That's what I suspected, and I'm sure you meant no harm. However, particularly in conjunction with a tweet on the same page (I don't follow twitter, so only occasionally see tweets quoted on web pages), suppose some macho physicist had said that a young, misguided female postdoc looked OK in the photo so should be forgiven of her ignorance. :-|

L. Edgar Otto said...

Sabine, whatever I am doing I more and realize I am lucky not to have early on been trained in our deteriorating schools.
You always seem to have to reply to those who cannot quite read what you are actually saying and they are not embarrassed in the shallowness that in effect misses the science.
I keep saying I will move on to other projects. But I just saw the density matrix idea and its link to renorminalization. Sounds more like excuses to me than science. Best.

Sabine Hossenfelder said...


I see. Sorry about this. I actually added this part of the sentence at the very end, mostly because I thought I have to update the photo on my homepage, and I can see now that it appears in an awkward arrangement...

Yeah, whatever. You know, if somebody's only interest in my writing is based on the fact that I say somebody looks nice on a photo, I can't help them. People who want to complain will always find something to complain.

So please accept my sincere apologies for this unfortunate sentence.



Sabine Hossenfelder said...

Updated that, better now?

joel rice said...

It seemed to me that the Pound-Rebka experiment would be a neat way to get at an aspect of quantum gravity that is measurable - since they measure the redshift of photons, and one would expect that if one puts a double slit there then one would still expect to see the usual interference pattern, the way Feynman explains it in QED -the strange theory... Isn't gravity acting on actual photons rather than possible paths ?

Sabine Hossenfelder said...

Joel: Yes, and yes, one can do this, but this doesn't test quantum gravity, it tests quantum mechanics in a classical background field - unless you manage to measure the gravitational field of the particles sent through the double slit (I would suggest to take something heavier than photons), and that again is too small to be measurable.

Phillip Helbig said...

"Updated that, better now?"

You still say that he is hot! :-)

Sabine Hossenfelder said...

I say he looks nice on a photo. That's more a praise for the photographer ;o)

Uncle Al said...

"30 orders of magnitude too small to be measurable" "in principle" Eötvös experiments are 5×10^(-14) difference/average sensitive, 90 days' summation. Largest divergence: Be - Ti, 0.0023974 net active test mass fraction. Observe 100% active mass as mass-equivalent energy.

Vacuum supersonic expanded 1 kelvin racemic 4-oxa-D_3-trishomocubane molecular beam; chirped pulse FT microwave rotational temperature. Trace chiral vacuum toward matter gives non-degenerate spectra. Quantum gravitation and standard model symmetry breakings are diagnosis not violation, thus baryogenesis.

[(Eötvös /_\mc^2)(molar mass)]/[(molecules/mole)(Boltzmann's constant)] = temperature
(0.540478)(molar mass) = kelvins
(0.540478)(148.202 g/mol) = 80 kelvins divergence, 8000%


Uncle Al said...

arXiv:1501.02671 "Is Spacetime Countable?"
"In this picture, Einstein's Special Relativity, suitably modified to accommodate an expanding Universe, can be reinterpreted as a theory where only the instantaneous shapes of configurations count."

If God is geometer, physics must know if He wears shoes and cares about looking glass milk.

joel rice said...

Bee - Of possible relevance to QG.
Einstein assumes spacetime. I would like to derive it. Those octonion thingies do not assume spacetime, but are merely compatible with it. Keeping in mind that Dirac Algebra is a direct product of quaternions one can consider that since octonions include quaternions then OxO includes HxH. Complexify HxH and you have the usual Dirac Algebra .
One would want the 4-vector interpretation. A relationship of 2 particles always includes a spacetime algebra. The rest of the direct product OxO refers to "internal coordinates" which we can never measure, and would prefer to ignore or 'forget'. Now we have 4-vectors to play with. So if spacetime is this 'relational' it seems to have the wiggle room for the universe to expand. It seems to involve a place to introduce 'mass' - a phenomenological notion, which is not present originally. It might have some bearing on what kinds of theories are acceptable, perhaps.
It isn't circular because the minkowski signatures of complex octonions are not 4-vectors, only compatable with them formally. Its more like a tetrahedron with a special vertex.
Is that possibly a useful idea ?

L. Edgar Otto said...

Unfortunately, your comment is mostly wrong, and what is not wrong is not new.
In an observable uniform incompressible there can be emergence which means science as phenomenology. But what is seen began as crystal ice, an indeffinite span of Dirac's nilpotent algebra. Quintessence is the unlikely persisting quantum like mediator. But of a thousand things we are born with that even the least of us know your irony as negation cloaks rather than confines what we do not know of intrinsic worlds that we might find there but forever hidden. Science can rest running in place or with the flow, but is that progress covering everying with thought as nothing or its power less than nothing?

Sabine Hossenfelder said...


Please read our comment rules. I am not a public ask-the-expert-forum. This has nothing to do with you in particular, I simply don't have time for this. If you want to discuss your own ideas or seek feedback, this is NOT the place to do it. Thanks,


David Brown said...

Consider this test for a publication that presents a possible huge breakthrough on quantum gravity: If there is no mention of Milgrom in the references, the publication is not a huge breakthrough. Why? Milgrom is the Kepler of contemporary cosmology. Kroupa is not deluded. Google "Pavel Kroupa" and carefully study the results.

Meir Amiram said...

"However, the gravitational field of all particles, elementary or composite, that display quantum properties is way too small to be measured.. Nobody has ever measured the gravitational field of an atom", well, unless inertia is the response of the quantum particle to the gravitational field of itself, in which case when we measure the inertial mass of a quantum particle we implicitly measure the (fully) classical gravitational field of that particle, in which case many have measured the gravitational field of quantum particles without even knowing...

David Schroeder said...

It looks like this David hasn’t felled the Goliath of Quantum Gravity – not even scratched its armor bearer. This divisive giant, keeping Relativity and QM apart, taunts and defies all who try to take it on. But, no doubt, someday an intellectual warrior, or team of warriors, will find a weak spot in the giant’s defenses, and finally bring unity to these two great pillars of modern physics.

Stuart said...

Quantum gravity hides in plain sight. It hides in the concept of the event.Here is how one might formulate a union of QT and GR. Let us first understand the premises of each theory. The fundamental concept of QT is the wave packet from which one can derive the uncertainty principle and the superposition of states. In GR the fundamental concept is the event. An event is a displacement vector in Minkowski space. It exists in a certain region of space-time beyond which it vanishes. Thus one can consider an element of Minkowski space as a pulse of space-time in which a smooth connected Lorentzian manifold acts as the background space and also the vacuum state. Here a vacuum state is a state devoid of an event. We then break it down into its Fourier components. If we perform this process we end up with a four vector expressed as a wave packet in which waves superimpose to
form the event and beyond the event they cancel out.
This is the basis of a new approach to quantum gravity which has explained the Dark Sector and has eliminated singularities in GR. Published papers can be found here

MarkusM said...

"Even if you take the heaviest things that have yet been brought in superpositions of location you are still about 30 orders of magnitude off."

Does this include macroscopic wavefunctions ?

L. Edgar Otto said...

I am not sure which 30 orders Sabine refers to. But it is obvious this applies on all observable scales naturally from number theory as does CC 120 and the like. Our ideas of dark stuff is a generalization of a quarter of 36 - 6 in a 3x3 diagonals grid with an ambiguous center of 4 objects not necessarily infinite, zero, or continuous.

L. Edgar Otto said...

Uncle AI,
Although Eridos made logical proofs on the frontier of such graph theory it is not clear that the experiment would be any more observable than gravons or waves until our intuitions for these matters are resolved.

Robert L. Oldershaw said...

BICEP-2 bites dust.

Anyone ready for some new ideas?

Uncle Al said...

@L. Edgar Otto

8% divergence is big for exactly degenerate spectra. Screw μwave, go Eötvös. 20 grams each of α-quartz single crystal enantiomorphic test masses comprise 6.68×10^22 pairs of opposite shoes - half in 3(1), half in 3(2) screw sense; 0.999726 net active mass fraction (ignore electrons). (0.999726)/(0.0023974) = 417-fold sensitivity increase, 2.6 decimal places. Physics 14 significant figures assumes vacuum is achiral toward matter. Observe falsifying 16 sig figs in that apparatus. 1) Dark matter is Milgrom acceleration. 2) Baryogenesis happened. 3) No prior observation is contradicted.

Look. Math alone is not enough.

BICEP-2? Bummer. Photons are not matter, above.

Sabine Hossenfelder said...

MarkusM: Don't know what you mean with 'macroscopic'. It means large molecules, some TeV, or even the odd virus if you want to push it - still off. Another direction trying to push it is not large masses but small distances (think 1/r^2), but that doesn't work either. It also adds considerable complications trying to measure it because then other interactions tend to mess up the gravity signal. Best,


MarkusM said...
This comment has been removed by the author.
MarkusM said...

Admittedly I was somewhat vague.
E.g. superfluidity, superconductivity, BEC (atom lasers), SQUIDs.
Sth. where several/many particles acting as a single wavefunction and a big effective mass are involved. I was wondering if an effective mass would be the same as a mass belonging to a single "fundamental" particle in respect to QG effects.

Sabine Hossenfelder said...

The short answer to your question is no. You need the state to be in a superposition of LOCATION. So you'd have to pipe your BEC through a splitter. People have looked at effects in BEC for other reasons though.

L. Edgar Otto said...

Uncle AI, sounds like the experiment would not be as hard to build as the LHC. But that assumes, as a few thinkers have proposed recently, that the universe of space is discrete and countable. So math is enough to not only cover everything but shows where it breaks down as square roots on any scale pattern that persists. Automorphism I believe is the established term.
You defend the experiment by offering us measures or permutations which certainly can fine tune a general graph to a deeper degree of statistical accuracy. Which also seems a reasonable method that leads to evidence hopefully as convincing experiment.
I am surprised a 4x4 graph of dimers has 36 coverings while if the measure of inconsistency for an 8x8 graph as a measure of complexity jumps to almost 13 million.
While an electron can be ignored in a sense it is the ultimate dust particle. This suggests to me we could be talking about something a little more complex but analogous to gravity, let us say "dark gravity".
Kea once said she was amazed I got so far by just counting but we have to pin down the lesser scale first in neutrino mixing.
I do not reply to you personally so thanks for the reply. Like so many I've encountered on line our intuitions have applications of which there is debates on the frontiers. We seem to have the same language under it all. I read the appendix of the graph theory book so can only say I found the application to physics quite independently up to unsolved problems and find it reassuring to my sanity and that our explorations as science are not closed.
Objectively yours, Lo Dicho - to use a rare Spanish neutral particle meaning: what has been said.

MarkusM said...
This comment has been removed by the author.
MarkusM said...

"So you'd have to pipe your BEC through a splitter."
Shouldn't that be doable. There are a couple of papers that discuss such experiments. (G-search: beam slitter BEC).

Sabine Hossenfelder said...

Yeah, presumably that's doable, but I don't see how that would be better than piping single atoms through a beam splitter. Besides this, even a typical factor of 10^9 doesn't get you anywhere close to measuring the de-localization of the gravitational field. Which, to emphasize it once again, is an effect widely believed to not be present anyway. Best,


Tienzen said...

This issue was discussed at the following blogs. , , , .

L. Edgar Otto said...

I had an admittedly metaphysical thought that if there not more than an 8x8 matrix at least summed up to 10x10 the age of the universe would be 12,988,816 light years.
Intuitively there are some measures not yet seen that existing observational evidence suggests are likely and possible.

Zephir said...

The misunderstanding is in definition of the entanglement subject. The entanglement has indeed its "weight" - it's just much larger than the above study assumes, as not only gravity participates on it. In wider sense all hadrons in atom nuclei are entangled into state of condensate via gluons and Yukawa forces and the energy of these forces contributes to the mass of hadrons significantly.