To date, one of the most profound problems in theoretical physics is the reconciliation of general relativity and quantum mechanics. Can a new model of quantum gravity, which has both excited and sparked debate among physicists, rewrite the theories of physics? Regarding the “Hořava model” recently born at the Lawrence Berkeley National Laboratory in the United States, scientists at Texas A&M University conducted further research and reached a balanced conclusion. They published their results and points of contention in the August 24 issue of the journal Physical Review Letters.
A New Dawn for Quantum Gravity
Quantum gravity is primarily an attempt to merge quantum mechanics with general relativity to describe the quantization of the gravitational field, forming a corner of the Theory of Everything. However, how to combine them, how to maintain the correctness of both at microscopic length scales, and what kind of verifiable predictions any candidate theory of quantum gravity can provide remain unresolved questions in current physics. Unfortunately, the energy and scales explored by quantum gravity are beyond what could be observed under previous laboratory conditions. Although it is possible to test these theories through astronomical observations, such cases remain rare exceptions, and clues for the development of quantum gravity theory have consistently failed to materialize.
Not long ago, a new model of quantum gravity proposed by string theorist Petr Hořava of the Lawrence Berkeley National Laboratory caused a significant stir in the physics community. This original-thinking Czech physicist, who first introduced the idea of brane cosmology into superstring theory, has sparked both immense excitement and continuous controversy among physicists because this new theory could have a massive impact on Einstein’s general relativity and potentially resolve classic contradictions in theoretical physics. Another crucial point is that this model can be tested using experimental methods.
The team at Texas A&M University subsequently conducted in-depth research on this and noted in their report that Hořava’s theory would have a broad impact on the solutions of general relativity. On the other hand, their report simultaneously led to a new contradiction: the “Hořava model” actually reproduces general relativity at unobservable scales, scales that are even larger than the universe itself. Physicists believe that the research findings in the report are of extremely important significance for testing the model, and a significant amount of work is currently needed to digest the trade-offs involved.
Incompatibility of the Pillar Theories
This long-standing and obvious contradiction in theoretical physics proves that even successful theories within the same field can have fundamental structural conflicts.
General relativity, established by Einstein in 1916, successfully describes gravity. By accounting for the influence of mass on the curvature of spacetime (related to Einstein’s equation E=mc^2), it provided complete explanations for three previously inexplicable phenomena: the perihelion precession of Mercury, the gravitational redshift of light, and the bending of light. To this day, in astronomical observations, the degree of agreement between experimental data and the predictions of general relativity is far higher than that of any competing theories. Therefore, for a long time, few have doubted the correctness of the gravitational discourse provided by general relativity.
If general relativity is one of the two pillars of modern physics, then quantum theory—the fundamental theory through which we understand elementary particles and condensed matter physics—is the other. Starting from the Dirac equation of relativistic quantum mechanics established by Dirac, quantum mechanics expanded into various forms of quantum field theory. It successfully explains the quantum behavior of the other three fundamental forces acting at microscopic scales, including electromagnetism and the strong and weak nuclear forces. Only the quantum nature of gravity remains undescribed by quantum mechanics.
In short, general relativity is applicable to large-scale structures (stars, planets, galaxies), while quantum mechanics describes the other three fundamental forces acting at the microscopic scale. Their contradiction lies in the lack of compatibility between the quantized description of matter and the geometric description of spacetime, involving theoretical development and subtle logical tugs-of-war.
To date, humanity has not yet obtained a theory of quantum gravity that can be called complete and self-consistent. This theory needs to provide an adequate description of the interior of black holes and the conditions of the very early universe, where gravity and the associated spacetime geometry must be described in the language of quantization. Many potential candidate theories have been in development for a long time, but each has problems to solve, and all face the issue of being unable to verify theoretical predictions through experiments. As for the highly popular “Hořava theory” of 2009, it only gains the qualification to move from an “attempt” to being “official” if its concepts can be successfully applied within the framework of general relativity.
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