Connecting quantum physics with gravity

Scientists do not fully understand quantum gravity because the two most successful theories in physics—quantum mechanics and general relativity—are fundamentally incompatible and no experiment has yet observed quantum gravitational effects directly[1][2][3].

Incompatibility of Theories

Quantum mechanics is spectacularly accurate for tiny particles and the other three fundamental forces, but it assumes a fixed, unchanging backdrop of space and time[1][4]. General relativity, on the other hand, describes gravity as the curvature of spacetime itself—a flexible "stage" warped by mass and energy[1][5]. Attempts to combine these frameworks mathematically lead to paradoxes, infinities, and results that do not make physical sense, especially at extremely small scales such as those near the center of black holes or at the origin of the universe[4][1].

Lack of Experimental Evidence

Quantum gravity effects are expected to occur only at the Planck scale—distances around \( 10\^{-35} \) meters and energies far beyond current technology[1][2]. This scale is completely inaccessible to direct measurement with any available experiments, which means scientists have little or no empirical data to guide theory development[2][6]. Recent experimental advancements, like detecting weak gravity on microscopic objects, edge toward probing quantum gravity but remain far from its truly quantum regime[6].

Ongoing Research and Hope

Numerous theoretical approaches exist—such as string theory and loop quantum gravity—but none have been confirmed and it's not obvious what observational evidence would definitively support one over the other[2][4]. Bridging these gaps remains one of the most important and challenging problems in all of modern physics[3][1][2].

In summary, **scientists do not understand quantum gravity** because quantum mechanics and general relativity fundamentally disagree at small scales, no experimental data yet tests quantum gravity directly, and a successful theory merging both remains elusive[1][2][4][6][3].

Sources

[1] Quantum gravity - Wikipedia https://en.wikipedia.org/wiki/Quantum_gravity

[2] Is gravity quantum? Experiments could finally probe one of physics ... https://www.nature.com/articles/d41586-025-02509-7

[3] The quest for quantum gravity: why being wrong is essential to science https://www.sciencefocus.com/space/the-quest-for-quantum-gravity-why-being-wrong-is-essential-to-science

[4] The fundamental problem with gravity and quantum physics https://bigthink.com/starts-with-a-bang/problem-gravity-quantum-physics/

[5] The Trouble with Gravity: Why Can't Quantum Mechanics explain it? https://www.youtube.com/watch?v=NsUm9mNXrX4

[6] Scientists closer to solving mysteries of universe after measuring ... https://www.southampton.ac.uk/news/2024/02/quantum-gravity.page

[7] Why is quantum physics impossible to understand? : r/AskPhysics https://www.reddit.com/r/AskPhysics/comments/1bjxgnq/why_is_quantum_physics_impossible_to_understand/

[8] why do physicists keep trying to add gravity as a force to quantum ... https://www.reddit.com/r/AskPhysics/comments/1d7zrou/why_do_physicists_keep_trying_to_add_gravity_as_a/

[9] Why Quantum Gravity Is Controversial - 4 gravitons https://4gravitons.com/2024/08/16/why-quantum-gravity-is-controversial/