Integrated Gravity and Quantum Theory
Generalized quantum gravity is a physical theory that combines the principles of general relativity and quantum theory. This theory is expected to provide a satisfactory description of the microstructure of spacetime on the so-called Planck gauge, where all the fundamental constants c (the speed of light in vacuum), ℏ (the reduced Planck constant), and G (the Newtonian constant) of compositional theory together form units of mass, length, and time.
This standard is so far removed from current experimental capabilities that it is almost impossible to conduct empirical tests on quantum gravity proposals following the standard path.
In most (although not all) quantum gravity theories, the gravitational field itself is also quantified. Due to the current theory of gravity, namely general relativity, which describes gravity as the curvature of matter and energy over spacetime, the quantification of gravity seems to imply some kind of quantification of spacetime, quantum spacetime.
Due to the fact that all existing physical theories rely on classical (non quantum) spacetime backgrounds, this not only brings extreme technical difficulties, but also poses profound methodological and ontological challenges for philosophers and physicists. Although quantum gravity has been the subject of physicists' research for nearly a century, philosophers have only just begun to explore its philosophical implications.
The elegant graphic paradox of Dutch artist Escher has been highly regarded by many people, especially philosophers, physicists, and mathematicians.
His work "Rise and Fall" relies on visual illusions to depict situations that are actually impossible. Other works may be contradictory in a broad sense, but it is not impossible.
Relativity describes the connected arrangement of objects, although in this arrangement, gravity operates in an unfamiliar way. Quantum gravity itself may be like this, the placement of an unfamiliar but connected familiar element.
Or perhaps it is more like "rising" and "falling", an impossible structure that may seem reasonable in some details, but cannot be combined into a connected whole when using existing construction materials.
If the latter is true, then the construction of quantum gravity theory may require completely unfamiliar elements. No matter what the final outcome is, the current situation is a change, and there are many competitive ways to compete for awards.
Equally important is to note that awards are not always the same, and string theorists seek a unified theory of all four interactions that can explain algebraic and other previously unexplainable properties such as elementary particles.
Other methods are more gentle, seeking only to align general relativity with quantum theory without necessarily citing other interactions.
The problem of quantum gravity may mean very different things for different researchers, and what constitutes a possible solution for one group may not meet the conditions for another group.
Given that quantum gravity has not yet existed as a useful physical theory, there is reason to question whether philosophers need to be involved at this stage.
The mission of a philosopher will be somewhat different from the mission faced when dealing with a more or less confirmed theoretical system, such as classical Newtonian mechanics, general relativity, or quantum mechanics. In this situation, people usually make assumptions about the physical rationality of theories or theoretical structures, and introduce the ontology and possible epistemological results of the theory, attempting to understand what the theory tells us about the essence of space, time, matter, causal relationships, and so on.
The theory of quantum gravity is plagued by a series of technical and conceptual questions, doubts, and problems, making it largely unsuitable for this interpretive approach. As far as string theory is concerned, there is no real "theory" to speak of, only some clues pointing to the hope that many people will one day become an applicable and consistent physical theory.
Philosophers interested in broader and more open exploration methods will find many things worth considering, and it is entirely possible that future physics philosophers will face very different flavor problems due to the special properties of quantum gravity.
Quantum gravity provides a unique opportunity for philosophers of physics to make active contributions, rather than just analyzing what physicists have already established in philosophy.
This sentiment has been responded to by some physicists, especially Carlo Rovelli (known as the central designer of the method of ring quantum gravity), who complained that he expects philosophers not to limit themselves to "discussing and polishing current scattered physical theories, but to take risks and try to look forward".
This raises an important point that although we consider general relativity and quantum theory to be "good" theories from a philosophical perspective, in a very practical sense, they are not the entirety of the story and can be broken in extreme norms.
The encounter between gravity and quantum theory, and the difficulty of reconciling quantum theory and gravity into a certain way of quantum gravity, stem from the surface incompatibility between general relativity (Einstein's theory of gravitational relativity) and quantum field theory (which describes the structure of the other three forces (electromagnetic force and the interaction between strong and weak nuclei). Why does it present an incompatible situation.
General relativity is described by Einstein's equations, which appropriately constrain the curvature of spacetime (Einstein tensor on the left-hand side) due to the presence of mass and other forms of energy, such as electromagnetic radiation (stress energy momentum tensor on the right-hand side).
In the process of doing so, they managed to incorporate traditional Newtonian gravitational phenomena, such as the attraction of two or more massive objects to each other, and also predicted some new phenomena, such as the twisting and redshift of light rays by these objects (which have been observed) and the existence of gravitational radiation.
Until recently, with direct exploration of gravitational waves, this was only indirectly observed through the reduction of binary pulsar periods.
In general relativity, mass and energy are treated in a simple classical way, where "classical" refers to physical quantities such as the strength and direction of various fields, as well as the orientation and velocity of particles, that have confirmed numerical values. These quantities are represented by tensor fields, which are sets of (real) numbers associated with each spacetime point.
The stress, energy, and momentum Tab (x, t) of the electromagnetic field at a certain point (x, t) are functions of the three weights Ei, Ej, Ek, Bi, Bj, Bk of the electric and magnetic fields E and B at that point. These quantities, in turn, determine one aspect of the "curvature" of spacetime through Einstein's equations, which is a set of numbers Gab (x, t), which is also a function of the metric system of spacetime.
The metric gab (x, t) is a set of numbers associated with each point, which gives the interval between adjacent points. According to the world model of general relativity, there is a metric spacetime manifold whose curvature is limited by the stress energy momentum distribution of matter.
All physical quantities, the value of the electric field X weight at a certain point, the spacetime scalar curvature at a certain point, and so on - have confirmed values given by real numbers (relative to complex or imaginary numbers). Therefore, in the above sense, general relativity is a classical theory.
The problem is that our fundamental theories about matter and energy, which describe the interactions between various particles through electromagnetic forces and strong and weak nuclear forces, are all quantum theories. In quantum theory, these physical quantities generally do not have confirmed numerical values.
In quantum mechanics, the orientation of electrons can be specified with arbitrary high precision, but the value lies in the loss of specificity in describing their momentum, resulting in a loss of velocity. In the quantum theory of electromagnetic fields known as quantum electrodynamics (QED), there are also uncertainties related to the electric and magnetic fields associated with electrons.
Physical quantities are described by quantum states, which give probability distributions of many different values. An increase in the specificity of a characteristic (such as orientation, electric field) (narrowing dispersion) can lead to a decrease in the specificity of its typical conjugate characteristics (such as momentum, magnetic field). This is an expression of Heisenberg's principle of indeterminacy.
Under the background of quantum gravity, the wavering theories are called "space-time foam". Similarly, if people focus their attention on a few points in space, it will not have a clear trajectory.
On the surface, the incompatibility between general relativity and quantum theory may seem insignificant. Why not directly follow the QED model to quantify the gravitational field, similar to the method of quantifying the electromagnetic field. This is more or less the path it has taken, but it has encountered extraordinary difficulties.
Some physicists believe that these "just" technical difficulties are related to the irretrievability of gravitational interactions and the subsequent failure of perturbation methods that have been proven useful in general quantum field theory. However, these skill issues are closely related to a series of challenging conceptual difficulties that both physicists and philosophers are interested in.
The conceptual difficulty largely stems from the nature of gravity's interaction with each other, particularly the equivalence between gravity and inertial mass, which allows people to represent gravity as a characteristic of spacetime itself, rather than a field propagating in a (forced) spacetime context. When people attempt to quantify gravity, certain characteristics of spacetime are subject to quantum perturbations.
In the classical quantification of gravity, several quantities (roughly the intrinsic and extrinsic curvatures of three-dimensional space) were separated and quantified as orientation and momentum variables. Given the uncertainty principle and the probabilistic nature of quantum theory, people have a picture that touches upon the shaking of space, just like the shaking of electric and magnetic fields in QED.
However, general quantum theory presupposes a clearly defined classical context on which to define these oscillations. Therefore, people not only encounter difficulties in providing mathematical features for quantifying programs (how to consider these oscillations in useful spatiotemporal structures), but also in providing conceptual and physical explanations for the theory that occurs, if successful.
A wavering norm seems to imply a wavering causal structure and spatiotemporal ordering of events. In this situation, how can we define isochronous conversion connections in quantum theory.
The conceptual requirement for this problem is to have a conceptual solution. Advocate for the synthesis of ontology. This approach requires an analysis of the ontological picture of the two component theories of quantum gravity in order to correctly evaluate their consistency.
Ontology refers to the primary and autonomous structures from which all other features and connections in a theory are constructed.
A proper and brief examination of the ontological constraints imposed by general relativity and quantum field theory reveals a serious and serious connection. General relativity abandons the fixed motion structure of spacetime, making positioning interconnected. However, in quantum field theory, the fixed flat background is part of its ontological foundation, from which the gauge features of the theory are derived.
As we can see, quantum field theory touches upon quantum oscillations near points, while general relativity touches upon point neighborhoods that utilize lubrication. Anyway, in order to combine the foundations of these two ontologies, it is necessary to demolish one of them and build a new one.
The best way to address this serious connection is to focus attention on the necessary principles of each theory. The gravitational properties of ubiquitous coupling are essential, but he pointed out that this does not require continuity, so it is possible to preserve the former and discard the latter without causing structural inconsistency, thus allowing for the drastic shaking of quantum theory.
Quantum field theory requires a fixed context to locate the quantum field and establish causal structures. But he pointed out that a relational statement about positioning can perform such a function, allowing the fields to be positioned relative to each other.
In this way, people can imagine a diffraction covariant quantum field theory that does not involve a reference to the field located at each point on the spatiotemporal manifold. The resulting composite entity (a violently shaken, coupled quantum gravitational field) will be what the quantum theory of gravity should describe.
Although this approach may sound reasonable on the surface, it is not easy to put it into practice during the constructive stage of theoretical construction.
The causal body approach aims to provide a structure for quantum gravity theory, with the idea of developing a general methodology that respects the key features of general relativity and quantum theory. General relativity is considered a dynamic (non probabilistic) causal structure, while quantum theory is considered a probabilistic (non dynamic) dynamics.
Causal body is an entity that encodes everything that can be accounted for in theory. Part of the problem here is that Hardy's approach assumes that ontological criteria hold on the Planck criterion. However, it is entirely possible that these two input theories will collapse at higher energies.
Not only that, the skill difficulty in establishing the quantum field theory of diffraction invariance that he proposed (physically practical) has proven to be an insurmountable challenge so far. A key aspect missing from the structure is the concept of what is an observable variable.
They need to be interconnected, but this still leaves the problem very open. The idea of obtaining development through appropriate criteria of quantum gravity constitutes the foundation of a special issue.