# Is it possible that there is no theory of quantum gravity? [closed]

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Short question: What published works and what research groups are investigating the possibility of inexistance of gravity at quantum level?

Long question: I am no phisicist, but of what I read and understand of the subject, no one has been able to fit graviton into the standard model, it seems that the equations won't renormalize, thus it would be useless for predictions. Gravity also is undetectable at quantum level, it seems because the force is too small. Also, about every book about the subject traces a strong distinction between "quantum level" and "classical level", almost if they were different worlds that obeys different laws of nature, and how they relate to each x other is an open question. Finally, at least two sources I have seen states that Einstein's General Relativity is the most well veryfied thoery of all physics, because of something related to pulsars.

Thus there are two distinct "worlds": in one gravity is pretty much settled, while in the other, it is neither detectable nor mathematically explainable in the most succesful model. From that, the most logical and simple explanation I can have is that simply there are no such thing as quantum gravity, and that it is a purely classical phoenomenon that arises from whatever may be the relation of the both "worlds".

I like to imagine that space coordinates of both worlds are quite not the same, although very similar locally, thus, if classical world emerges from a projection of the quantum world, like a shadow, gravity are bumps in the surface where that shadow is projected (holographic principle comes in my mind now).

It does't seems to be the mainstream view of the problem, with millions of dollars spent in trying to find the mythical graviton (that is what a physicist of INPE, Brazilian's spacial research institute, told me yesterday). Thus, I whant to know, what are the works, and what people shares with me this view of the problem?

## closed as primarily opinion-based by Abhimanyu Pallavi Sudhir, Brandon Enright, John Rennie, Kyle Kanos, tpg2114♦Feb 26 '14 at 17:00

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

• "Finally, at least two sources I have seen states that Einstein's General Relativity is the most well veryfied thoery of all physics, because of something related to pulsars." I believe that your sources meant to say that GR is the most well verified theory for $\textbf{gravitation}$ that we have now. If we just see in pure figures, I believe that QED is the most well verified one. – Hydro Guy Feb 21 '14 at 13:28
• What if there is no quantum gravity? 50 years of string theory goes down the toilet, that's what. – David H Feb 21 '14 at 14:27
• (1) non-renormailzable theory is not useless, it is a low-energy effective theory. In a broder sense, gravity and SM are already unified below the Planck scale; (2) by the COW experiment, we could predict and vertify the effect of gravity in quantum mechanics at low-energy; (3) what we are missing is a theory capable at arbitary energy scale, if there is no such theory, it is contradict with the logic of QFT (Landau pole, renormalization, etc), it implies we do not have a logical consistency theory anyway – user26143 Feb 21 '14 at 14:38
• it seems that the equations won't renormalize I think that String Theory solves that problem. Lubos are you there? every book about the subject traces a stron distinction between "quantum level" and "classical level" that seems a wrong view, we're being able to observe quantum effects at larger scales than before. – jinawee Feb 21 '14 at 14:51
• en.wikipedia.org/wiki/Induced_gravity – Mitchell Porter Feb 21 '14 at 20:40

The theory was put forward that semiclassical gravity (a classical gravitational field generated by quantum matter) was indeed the correct theory, something of the form

$G_{ab} = \langle T_{ab}\rangle$

With $G$ the Einstein tensor and $\langle T\rangle$ the expectation value of the stress energy tensor of the matter. It's a theory that works fine enough, but you encounter the following weirdness :

Get some matter in some state $\frac{1}{\sqrt{2}} |1\rangle + \frac{1}{\sqrt{2}} |2\rangle$ , where the state 1 is an energy distribution in some region of space, and state 2 is another energy distribution in a second region of space. The gravitational field will be that of half the energy in region one and half in region 2, or something along those lines.

Now perform a measure. The state collapse to $|1\rangle$, and now all the energy is in region 1. You just created a discontinuity in the gravitational field.

This by itself isn't enough to discard the possibility, but it is certainly suspiscious.

I seem to recall that some kind of experiment was performed in the 70's that involved a mass and torsion balance that would change depending on the result of a quantum measurement, and the experiment did not yield any result in favor of semiclassical gravity.

You might want to check those things, too :

http://backreaction.blogspot.fr/2012/01/real-thought-experiment-that-shows.html

http://papers.rqgravity.net/EppleyHannah.pdf

• Plus, you don't get conservation of energy, i.e., $\nabla_{\mu}\langle T^{\mu\nu}\rangle\not=0$. That's bad. – Alex Nelson Feb 21 '14 at 14:15

One may have many science fiction or fantasy projections of how physics might be. In the real world physics and physicists use mathematical theories as tools to model experimental observations.

Two landmarks guide the modelling of data at present .

1) The validation of Quantum Mechanics as the underlying framework that describes with great accuracy the microcosm of elementary particles and predicts with great accuracy future behavior in new experiments.

2) The formulation and validation, to a great extent, of the Standard Model of particle physics.

Number 1) has lead to the realization, in contrast to your hand waving shadows science fiction scenario, that all classical theories are emergent from the quantum mechanical level of fields and particle interactions. This emergence can be rigorously derived using mathematics, not words. Even before quantum mechanics it had been mathematically shown that Thermodynamics, a beautiful classical theory with its differential equations and postulates, emerged from statistical mechanics, the mechanics of large number of particles, again in a well derived mathematical manner.

Number 2) is within quantum mechanic and quantum field theory, but in addition the standard model uses the mathematics of symmetries arising from group transformations and gauge invariance. These symmetries under transformations exist in classical electrodynamics , but find their great usage in quantum electrodynamics and the behavior of elementary particles in forming the standard model of particles.

The success of the standard model and the symmetries it obeys guides the thinking of most physicists about General Relativity ( a classical theory) which also has similar symmetries . The goal is to have one unified model for all interactions at the quantum level, thus the proposal and search for the graviton as the corresponding particle to the photon of the gravitational interaction.

So it is the mathematical consistency of the validated experimentally models that describe the elementary particles that leads to the expectation of a quantized gravity. Call it a beauty postulate.

Another strong reason to expect quantization of General Relativity is that classically it has infinities, like the Big Bang and the black holes, singularities where values predicted to be measured go to infinity. This problem for electrodynamics was solved by quantum mechanics: the 1/r behavior of the potential between two charges lead to infinities/singularities which disappeared with the quantum mechanical formulation of quantized energy states. One may say that the classical "nature abhors a vacuum" has been transformed to "nature abhors infinities". In general terms it is the Heisenberg Uncertainty Principle connected to the probabilistic formulation of quantum mechanics that is the tool to control infinities. Thus one expects that a quantization of General Relativity will dispense with infinities/singularities.

For all these reasons I do not know, am not aware, of serious physicists working with assumptions of the order you imagine. There already exists a mathematical framework that accommodates both the group structures of the standard model and a quantized general relativity , string theories. In plural because as yet no unique successful model exists to be chosen out of a plethora of possibilities, but theorists are working on the problem. There do exist less popular researches on gravity which still quantize it but asking for discrete space time ( as loop quantum gravity) or 't Hooft's deterministic model but I believe anybody working with gravity accepts some form of quantization, mainly for the reasons stated above.

• You give arguments about why gravity should be quantized, while the question shows that he/she thinks that there is no need for a theory that encompasses both general relativity and quantum theory. I don't think that anyone seriously thinks that there shouldn't be such a theory (may be with some exceptions like Dyson), but whether gravity should be quantized is not that obvious in my opinion. For example both landmarks in your answer are formulated and tested with no gravity or negligible gravity. – MBN Feb 21 '14 at 15:46
• @MBN Yes, that is why I call it the "beauty" postulate. By our education we find symmetries beautiful and search for them ever since the eightfold way, I guess. It is this that gives physicists a "need" for quantizing gravity. Non physicists opinions are really science fiction at present. Our expectations will be falsified if it is proven that there exist no gravitons, or some strong contradiction with galacti orbits or CMB or some such. For me actually the elimination of infinities is a more satisfying objective, and quantization does that. – anna v Feb 21 '14 at 15:51
• So, because of beauty, alternatives are only worth investigating when the "beautiful" is falsified? Also, I do believe the theories should be unified, just not necessarily with gravitons... Can't there be a non-particle approach? – lvella Feb 21 '14 at 16:05
• Why do you think people study mathematics and physics after all and go into theoretical physics? They are attracted to order the known observations into mathematical models, and beauty is part of mathematics from ancient times ( the music of the spheres. Like explorers with a compass they investigate the land and the compass at the moment is symmetries and group structures and gauge transformations etc . At the end of the nineteenth century it was differential equations of the Maxwell type that were sought with enthusiasm, and that led to Schroedinger and Klein Gordon equations. – anna v Feb 21 '14 at 16:21
• @annav: What I meant was that there is a difference between trying to find a theory that contains both QM and GR, and trying to quantize GR. The second is just one way to go. And I don't see what beauty has to do with it. Modifying QM can also be very aesthetic. But my objection was that the two points that you make are that we should not touch QM because of the success. But the success (even the formulation) is only where gravity can be ignored. So it is not very convincing. – MBN Feb 21 '14 at 16:49

Following the link on Mitchell Porter comment, I found that there are at least 2 research branches followed by physicists (or maybe one is a generalization of the other) that deals with gravity as an emergent force from the interaction of the particles, and not a fundamental force itself:

From both Wikipedia's articles and the reactions to this question, it seems that this idea faces strong resistance from physics community. Coincidence or not, the reaction seems pretty much like what Boltzmann faced in his time when upholding that entropy was a statistical thing, and that matter was made of molecules.