Tuesday, September 27, 2016

Dear Dr B: What do physicists mean by “quantum gravity”?

[Image Source: giphy.com]
“please could you give me a simple definition of "quantum gravity"?


Dear J,

Physicists refer with “quantum gravity” not so much to a specific theory but to the sought-after solution to various problems in the established theories. The most pressing problem is that the standard model combined with general relativity is internally inconsistent. If we just use both as they are, we arrive at conclusions which do not agree with each other. So just throwing them together doesn’t work. Something else is needed, and that something else is what we call quantum gravity.

Unfortunately, the effects of quantum gravity are very small, so presently we have no observations to guide theory development. In all experiments made so far, it’s sufficient to use unquantized gravity.

Nobody knows how to combine a quantum theory – like the standard model – with a non-quantum theory – like general relativity – without running into difficulties (except for me, but nobody listens). Therefore the main strategy has become to find a way to give quantum properties to gravity. Or, since Einstein taught us gravity is nothing but the curvature of space-time, to give quantum properties to space and time.

Just combining quantum field theory with general relativity doesn’t work because, as confirmed by countless experiments, all the particles we know have quantum properties. This means (among many other things) they are subject to Heisenberg’s uncertainty principle and can be in quantum superpositions. But they also carry energy and hence should create a gravitational field. In general relativity, however, the gravitational field can’t be in a quantum superposition, so it can’t be directly attached to the particles, as it should be.

One can try to find a solution to this conundrum, for example by not directly coupling the energy (and related quantities like mass, pressure, momentum flux and so on) to gravity, but instead only coupling the average value, which behaves more like a classical field. This solves one problem, but creates a new one. The average value of a quantum state must be updated upon measurement. This measurement postulate is a non-local prescription and general relativity can’t deal with it – after all Einstein invented general relativity to get rid of the non-locality of Newtonian gravity. (Neither decoherence nor many worlds remove the problem, you still have to update the probabilities, somehow, somewhere.)

The quantum field theories of the standard model and general relativity clash in other ways. If we try to understand the evaporation of black holes, for example, we run into another inconsistency: Black holes emit Hawking-radiation due to quantum effects of the matter fields. This radiation doesn’t carry information about what formed the black hole. And so, if the black hole entirely evaporates, this results in an irreversible process because from the end-state one can’t infer the initial state. This evaporation however can’t be accommodated in a quantum theory, where all processes can be time-reversed – it’s another contradiction that we hope quantum gravity will resolve.

Then there is the problem with the singularities in general relativity. Singularities, where the space-time curvature becomes infinitely large, are not mathematical inconsistencies. But they are believed to be physical nonsense. Using dimensional analysis, one can estimate that the effects of quantum gravity should become large close by the singularities. And so we think that quantum gravity should replace the singularities with a better-behaved quantum space-time.

The sought-after theory of quantum gravity is expected to solve these three problems: tell us how to couple quantum matter to gravity, explain what happens to information that falls into a black hole, and avoid singularities in general relativity. Any theory which achieves this we’d call quantum gravity, whether or not you actually get it by quantizing gravity.

Physicists are presently pursuing various approaches to a theory of quantum gravity, notably string theory, loop quantum gravity, asymptotically safe gravity, and causal dynamical triangulation, for just to name the most popular ones. But none of these approaches has experimental evidence speaking for it. Indeed, so far none of them has made a testable prediction.

This is why, in the area of quantum gravity phenomenology, we’re bridging the gap between theory and experiment with simplified models, some of which motivated by specific approaches (hence: string phenomenology, loop quantum cosmology, and so on). These phenomenological models don’t aim to directly solve the above mentioned problems, they merely provide a mathematical framework – consistent in its range of applicability – to quantify and hence test the presence of effects that could be signals of quantum gravity, for example space-time fluctuations, violations of the equivalence principle, deviations from general relativity, and so on.

Thanks for an interesting question!


Paul Titze said...

Hi Bee,

"Physicists are presently pursuing various approaches to a theory of quantum gravity, notably string theory, loop quantum gravity, asymptotically safe gravity, and causal dynamical triangulation, for just to name the most popular ones. But none of these approaches has experimental evidence speaking for it. Indeed, so far none of them has made a testable prediction."

Agree. What do you think of Schiller's Strand Model (Vol 6)? http://www.motionmountain.net/

Cheers, Paul.

Sabine Hossenfelder said...


I haven't looked at it, sorry.

naivetheorist said...


why does the column have a video of a kid appearing to torment a cat on a trampoline (i assume that no cat enjoys being bounced around like that although bing a 'dog person', i don't know what a cat might find to be enjoyable or unenjoyable)?


akidbelle said...

Hi Sabine,

thanks for an interesting answer.

I bet Schrodinger's cat is trying to escape the black hole without information loss...
The time travel-creeping loop is pretty nice, and apparently the girl has a lot of fun!


richard.jowsey said...

Dear Dr B, referring to your "except for me" paper, what did you mean by the expression "the quantum phase that we currently live in"? Are you speaking of the exponential phase angle...

Phillip Helbig said...

"Nobody knows how to combine a quantum theory – like the standard model – with a non-quantum theory – like general relativity – without running into difficulties (except for me, but nobody listens)."

Two citations (one a self-citation). :-|

Max Tegmark once told me, regarding his own work, that the fewer citations a paper has, the more important it is. :-)

There is probably some truth to this. By citing a paper which proposes to solve a problem many have famously not solved, the citer is siding with someone who might not come out on top. People are afraid.

Usually, when something wrong is published, someone shoots it down within a couple of months or so. Papers with few citations which have not been shot down should get some sort of special reward. Hhmmm...that's an idea.

Sabine Hossenfelder said...


The image was my random association to space-time fluctuations. The more general reason I have an image in every post is that without a designated image, facebook and G+ will grab a random image which appears on the site, which is most often either some irrelevant icon from the sidebar or some commenter's avatar, neither of which I want to decorate my post with. But look how the poor cat is trying to escape, lol. Ok, I think it's funny. Best,


Sabine Hossenfelder said...


This should make sense if you read the full paper. "Phase" refers to "phase" not as in a complex exponent but as in "phase-transition". The point of the model in the paper is that "quantumness" is a phase of matter/fields (anything in the Lagrangian), and that when gravity is strong, both gravity and matter aren't quantum, hence they can be combined classically and no contradictions ever occur. Best,


Matthew Rapaport said...

Haven't we got a virtual quantization of space-time in the various Planck values?

Sabine Hossenfelder said...


Sorry, I don't know what you mean.

Uncle Al said...

"without running into difficulties (except for me, but nobody listens" New theory is politics. Beautiful symmetries require parameterization versus observation[1]. The universe is emergent not intrinsic[2]. Breaking time reversal symmetry demonstrably creates strong arrow of time chirality[3].

"problem with the singularities" Black holes are (2 + L_P)-dimensional event horizons. No internal volume, no singularity re 0.2 second resolution LIGO event GW150914 with meager -3/(30 + 35) = -4.6% incremental binding energy and absent angular momentum glitches. Equilibrium was hugely too rapid, clean, and gentle.

Observe trace chiral spacetime using chemistry wherein it is not unpostulated.

[1] doi:10.1016/0550-3213(81)90361-8; Erratum, doi:10.1016/0550-3213(82)90011-6
doi:10.1016/j.disc.2013.02.010, arXiv:1109.1963
[2] doi:10.1016/0009-2614(90)87240-R
[3] doi:10.1103/PhysRevA.82.043811
doi:10.1103/PhysRevD.71.057501, arxiv:hep-ph/0501282

Unknown said...

Déborah And Kitten - Playing In Trampoline (24 seconds long)

Kris Krogh said...

Hi Bee,

Great video! Is that your daughter Lara?

Cheers, Kris

Sabine Hossenfelder said...

No, it's not my daughter. I tried to take a video of her trampoline jumping at some point, but every time I pointed the phone at her, she stopped because she thought I was making a photo... Also, we don't have a cat.

David Duffy said...

Do you have thoughts on Diosi-Penrose and related ideas about gravity and collapse in QM?

Herr Weh said...

Dear Dr. B.,

Thank you for the link to your paper, this was very interesting. While I'm not in a position to scientifically judge your idea - I'm neither physicist nor mathematician - it does make intuitive sense to me.

But even while I like myself be guided by intuition, what I eventually need is someone telling me "well, it might make sense to you, BUT ..." (or, "good, BECAUSE ..."). That's how I learn. Also, it's how I try to navigate all the latest hypes and fringe theories - not only in HEP.

I hoped to find some further discussion of your idea but unfortunately, my searches came up empty. Are there other physicists thinking in the same direction, any papers discussing your or a similar approach?

That said, my intuition has a fairly good track record, if that helps at all :-) Regards,

asnant said...

Dear Dr B,

Why is it such that "the gravitational field can’t be in a quantum superposition"?


Sabine Hossenfelder said...


In general relativity the gravitational field can't be in a quantum superposition because the mathematical apparatus doesn't contain any such structures. It's just the wrong theory for that.

JimV said...

"when gravity is strong, both gravity and matter aren't quantum"

This statement bothered me because it seemed to mean that black-body radiation in strong gravity would produce the "ultraviolet catastrophe", but I guess by strong you mean infinite? If not, this idea might be testable by comparing frequency spectra of large stars to quantum predictions.

Thanks for an interesting answer (my immediate thought on ending the article, although I was not the first to post it).

Perekatifield said...

In your article (https://arxiv.org/abs/1208.5874) there seem to be a typo, "unitary" instead of "unitarity":

Black hole evaporation however seems to violate unitary which is
incompatible with quantum mechanics.

Sabine Hossenfelder said...


Indeed, thanks for pointing out!

Rob van Son (Not a physicist, just an amateur) said...

"Black hole evaporation however seems to violate unitary which is incompatible with quantum mechanics."

I understood it was more complex.

Looked at from the outside, nothing ever crosses the horizon. The evaporation is seen as originating from a thin layer of material completely outside of the horizon. Unitarity is not violated.

From the viewpoint of an infalling observer, there is no evaporation, just empty space until she crushes into whatever is at the center. That too does not violate unitarity.

As far as I can understand the current theoretical debate (which is not very far), the problem is about the fact that you can create two particles, one inside and one outside the horizon, that are maximally entangled to a third particle outside the horizon. Black hole evaporation can get the "inside" particle out of the horizon with its entanglement intact. This would violate the rule that no more than two particles can be maximally entangled.

Sabine Hossenfelder said...


Hawking radiation is not created at or nearby the horizon.

Having said that, your explanation is pretty confused. The particles that are created in the Hawking radiation are always entangled across the horizon. You can *not* get the inside particle out (without violating locality and/or causality), that's the problem.

Uncle Al said...

Quantitative geometric chirality divergence of single crystal alpha-quartz given
doi:10.1063/1.532988, doi:10.1063/1.1484559 and QCM software.
Theoretical slope is -2 exactly.

CHI = 1 is perfectly divergent. The "fuzz" is sampling artifact. A sample sphere cannot perfectly symmetrically encapsulate the crystal lattice. Sampling radius increment density decreases toward 10^15 atoms (computation time varies as atoms^2). A 5 gram Eötvös test mass is 5×10^22 atoms, log(radius) = 7.88 radius [Å].

Overly abundant gravitation anomaly theory must be falsified. LOOK

M*P*Lockwood said...

What is the best way to officially submit a "Dear Dr B" question?

Sabine Hossenfelder said...


You can either submit it here in the comments, or send me a note on twitter or on facebook, or an email. Just as advance warning: I take on very few of the questions I get. A good question should fulfill the following criteria: a) of interest for many readers b) a question I can answer c) be clearly phrased and d) not a question that Google can answer in 2 seconds. Best,


Neil Southall said...


In your paper you express Newton's constant in terms of the Planck mass saying this is in 3 dimensions. Is the value dimension dependent?

Sabine Hossenfelder said...

The value doesn't, the dimension does.

Neil Southall said...

Sorry dimension does what?

Sabine Hossenfelder said...

Sorry. The mass-dimension of Newton's constant depends on the number of space-time dimensions.

Neil Southall said...

Thanks B

Je Kl said...

I have a question on black hole evaporation. So the idea is that once a pair of virtual particles appears, one escapes away while the other one (with a negative energy) falls into a black hole and decreases its energy. So, it is possible that in the end a black hole evaporates. While it all makes sense, is it not as likely that a virtual pair has an opposite fate: positive energy particle falls into a black hole, while a negative one escapes and decreases the overall "outside universe" energy? This way, you may see this thermal emission
counterbalanced with a similar "energy sucking" and black hole lives forever...