Quantum theory and Einstein’s general relativity are the two main pillars of modern physics. Quantum mechanics makes highly accurate predictions in the realm of the very small, while general relativity does the same on a cosmic scale. Unfortunately, the two theories are incompatible, leading vast numbers of theoretical physicists to attempt to unite them in a theory of quantum gravity, with a notable lack of success.
If you’re familiar with the spacetime model I’ve been describing in this blog, it’s easy to see the reason for this lack of success. The gravity of general relativity is an emergent phenomenon, while quantum mechanics deals with fundamental entities. Here’s what Wickipedia says about emergence.
In philosophy, systems theory, science, and art, emergence is a phenomenon whereby larger entities arise through interactions among smaller or simpler entities such that the larger entities exhibit properties the smaller/simpler entities do not exhibit.
Emergence is central in theories of integrative levels and of complex systems. For instance, the phenomenon of life as studied in biology is an emergent property of chemistry, and psychological phenomena emerge from the neurobiological phenomena of living things.
In a previous post I explained how gravity and the other forces arise from the interactions of spacetime quanta, which I call points. The gravity of general relativity emerges from this quantum activity.
A huge problem with general relativity is that, since it’s a classical theory with a continuous spacetime, it leads to singularities or infinities. These don’t exist in our quantum spacetime. They become problems when an effective theory, which general relativity is, is applied outside its realm of applicability. Wickipedia again:
In science, an effective theory is a scientific theory which proposes to describe a certain set of observations, but explicitly without the claim or implication that the mechanism employed in the theory has a direct counterpart in the actual causes of the observed phenomena to which the theory is fitted. I.e. the theory proposes to model a certain effect, without proposing to adequately model any of the causes which contribute to the effect.
Thus, an effective field theory is a theory which describes phenomena in solid-state physics, notably the BCS theory of superconduction, which treats vibrations of the solid-state lattice as a "field" (i.e. without claiming that there is "really" a field), with its own field quanta, called phonons. Such "effective particles" derived from effective fields are also known as quasiparticles.
No one would attempt to find a theory of quantum fluid dynamics, since fluid dynamics is an emergent phenomenon. Similarly, it seems obvious to me that the search for quantum gravity—actually, quantum general relativity—is a wild goose chase.
As I’ve said many times, the main thing keeping physicists from progressing beyond their current confusion is knowledge of the quantum nature of spacetime.