The two cornerstones of modern physics - general relativity and quantum mechanics - have profound contradictions in describing the essence of spacetime: general relativity regards spacetime as a smooth dynamic stage, while quantum theory implies its inevitable fluctuations at the microscopic scale. What is even more confusing is that in the Standard Cosmological Model (Λ CDM), dark matter (26.8%) and dark energy (68.3%) dominate the evolution of the universe as "unknown components", but there is always a lack of basic theoretical explanations.
Currently, string theory and loop quantum gravity, which serve as focal points, although can be accessed through high-dimensional continuous spacetime and discrete background independent spacetime, both face core challenges such as experimental verification, mathematical tools, and theoretical consistency
Energy scale gap
The quantum gravity effect is significant at the Planck scale (~10 ⁻³ ⁵ meters), while the current strongest particle accelerators (such as LHC) can only reach 10 ⁻¹⁶ times the Planck energy and cannot directly excite gravitational quantum states.
Example: To observe the interaction of gravitons, it is necessary to create an extreme energy density equivalent to the formation of a black hole, far beyond laboratory capabilities.
2. The observability ambiguity of theoretical predictions
Mainstream theories such as string theory and loop quantum gravity lack clear predictions for observable signals at low energy scales. For example:
The additional dimension compactification effect predicted by string theory may only show faint traces when high-dimensional symmetry is broken, but the specific form is still unclear;
There is still no quantitative model for how the macroscopic continuity of the spatiotemporal discreteness proposed by quantum gravity in circles emerges.
Controversy over the Interpretation of Indirect Evidence
Even if anomalous phenomena are discovered through cosmological observations such as early cosmic inflation and the black hole information paradox, it is difficult to rule out interference from other physical mechanisms such as dark energy and axion fields.
So, are there any other paths to explore this field? I will describe this in the form of a question:
1.Does the 'quantum algebra' of spacetime exist?
If spacetime exhibits non commutative geometry at the Planck scale, can its algebraic structure spontaneously decoherence into a classical differential manifold through symmetry breaking? Does this decoherence process encode the topological memory of dark matter?
2.Who is the cause and who is the result?
If the interaction between matter and spacetime is essentially a bidirectional 'self referential feedback' (matter excites spacetime defects, spacetime defects constrain matter motion), does this mean that 'quantum gravity' must be a self consistent closed theory that cannot be decomposed into 'matter field+background spacetime'?
3.Is dark matter an unknown particle? Or is it the 'quantum scar' of space-time?
If dark matter is not a weakly interacting massive particle (WIMP), but a statistical residue of spatiotemporal topological defects (such as the Hawking evaporation product of transient wormholes), does its velocity distribution naturally satisfy the observed 'cold dark matter' characteristics?
4.How to use the "desktop experiment" to touch the Planck scale?
If the quantum fluctuations in spacetime have fractal structures, do their low-frequency modes leave measurable noise at the macroscopic scale (such as LIGO interferometer arms)? Is this noise homologous to the quantum origin of Hawking radiation?
5.Is spacetime a byproduct of computation?
If quantum gravity is ultimately proven to be the 'hardware' of a universal quantum computer, and spacetime is its' software '(i.e. dynamically generated through quantum entanglement gates), does this mean that the essence of physical laws is a manifestation of computational complexity?al quantum computer, and spacetime is its' software '(i.e. dynamically generated through quantum entanglement gates), does this mean that the essence of physical laws is a manifestation of computational complexity?
Experimental probes of quantum gravity involve probing the pre-inflationary epoch of the Universe. As was found by the BiCEP2 experiment, the main background for detecting such signals is the electromagnetic response of interstellar dust, whose composition isn't known precisely enough in order to subtract its contribution to any signal.
This paper: Preprint Probing pre-inflationary anisotropy with directional variati...
seems to present additional ways of probing the pre-inflationary epoch; the qurestion is what are the backgrounds...
In the long and rather comprehensive the thread question “What do you think is the experimentally feasible path of quantum gravity?´ preface most of problems in mainstream physics at attempts “to quantize the GR” are pointed, which really exist, and are fundamentally in vain, since that is fundamentally impossible even theoretically, and, of course – experimentally,
QM and the GR are fundamentally incompatible, it is impossible in QM, where the space XYZ coordinates/variables are also the space operators, which are applicable so only in a flat space, and, besides, fundamentally cannot have any dynamic properties – while in the GR space/time spacetime is dynamic, i.e. contains energy,, momentum, etc.; something similar is with t-coordinate that is [really strange, but this is outside this discussion] energy operator that acts in “flat time” also.
At that the problem above really doesn’t exist - really, as that is rigorously scientifically shown in the Shevchenko-Tokarevsky’s Planck scale informational physical model, two main papers are Preprint The Informational Conception and Basic Physics
, [ in the paper section 2.9 “Mediation of the forces in complex systems” can be passed since this is more comprehensively given in section 6.“Mediation of the fundamental forces in complex systems” in other paper below] , andhttps://www.researchgate.net/publication/383127718_The_Informational_Physical_Model_and_Fundamental_Problems_in_Physics .
- more see the links above, concretely the pointed section 6 [though to understand what is in this section, and why that is correct it is necessary to read the linked papers],
- any fundamental problems at the quantization don’t exist – Gravity is fundamentally nothing else than some fundamental Nature force, which is similar in some traits to the Electric Force; so – as all other Forces Gravity acts in the Matter’s fundamentally absolute, fundamentally flat, fundamentally continuous, and fundamentally “Cartesian”, spacetime that has metrics (at least) [4+4+1]4D (cτ,X,Y,Z, g,w,e,s,ct).
So the “classic” and quantum Gravity theories will be developed, as that happened with Weak, Electric, Nuclear/Strong, Forces, while – as that is shown in the initial models in section 6 above - at least Gravity, Electric, and Nuclear Forces on Planck scale act by the same scheme.
Including the quantum nature of Gravity rather probably can be experimentally observed yet now at experiments with photons, again see the section 6, and corresponding references; reDzennn comment in “Distinguishing classical from quantum gravity through measurable stochastic fluctuations”
https://phys.org/news/2025-03-distinguishing-classical-quantum-gravity-stochastic.html 5 passages is relevant in this case
Cheers
"general relativity and quantum mechanics - have profound contradictions in describing the essence of spacetime: general relativity regards spacetime as a smooth dynamic stage, while quantum theory implies its inevitable fluctuations at the microscopic scale."
Actually that would be like saying: "Google maps does not take into account that the world is a globe. Why are the streets not curved?". The problem is that many seem not to understand that first of all the base is a different scale and then the object of investigation is also different: While GR investigates the geometry of space, QM is rather interested in quantization.
Calling GR and QM 'contradictory' is like being angry that your microscope doesn't show weather patterns.
Recent SS post in https://www.researchgate.net/post/Is-it-true-that-physicists-do-not-want-to-understand-quantum-mechanics/329 , including the “PS” passage , [this thread isn’t indexed] is relevant to this thread question.
Cheers
I think the experimental feasibility path of quantum gravity is bright because quantum mechanics helps us to profound solutions to most of our technical problems in this century.
“…I think the experimental feasibility path of quantum gravity is bright ….”
- if this relates to solutions to most of our technical problems , then it is fundamentally impossible. Gravity is extremely weak fundamental Nature force, and, though, of course, gravitational interactions on QM scale are for sure “quantized”, however that is quite negligible on a few tens orders by magnitude larger other fundamental Forces background impacts.
Nonetheless quantum nature of Gravity with a well non-zero probability can be experimentally, see proposed yet in 2007 experiment inhttps://www.researchgate.net/publication/215526868_The_informational_model_-_possible_tests the section 2.1.2. “Monochromatic photon beam distortion”.
Cheers
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