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Invisible particles in the universe: How do gravitons interact with gravity?
Gravity has always been a fascinating subject in physics. With the in-depth study of quantum gravity theory, the existence of the hypothetical particle graviton has attracted more and more attention. Gravitons are believed to be the fundamental particles that mediate gravitational interactions, but there is still no complete quantum field theory to support this hypothesis.
According to theory, the graviton should be a massless spin-2 boson because gravity has an extremely long range and appears to travel at the speed of light.
Theoretical basis of gravitons
The concept of graviton began with the exploration of the quantization of gravitational interactions. Like other natural forces such as electromagnetism, strong force and weak force, the graviton is predicted to be a fundamental particle. In theory, the existence of such a particle would enable a quantum description of gravity, while existing models of physics, including the Standard Model, cannot adequately explain the quantum nature of gravity.
A successful graviton theory should be able to reduce to general relativity in the classical limit, while general relativity can be reduced to Newton's laws of gravity in the weak field limit. This provides new insights into the fundamental structure of the universe.
Historical BackgroundThe concept of graviton first appeared in 1916, when Einstein first explored quantized gravitational radiation. The term "graviton" was first used by Soviet physicists in 1934, and reintroduced in a lecture by Paul Dirac in 1959. Classical physicist Pierre-Simon Laplace had predicted long before Newton that gravity was mediated by particles.
Similar to his predictions about photons, Laplace's understanding of the graviton was not connected to quantum mechanics or relativity, as these theories were not yet available during his lifetime.
Gravitons and the renormalization problem
The Feynman diagram approach works well for describing graviton interactions, but ultraviolet divergence occurs when you get into situations with at least two loops. These infinite calculations cannot be eliminated because quantized general relativity cannot be regularized in perturbation theory, which leads physicists to produce unpredictable results when calculating the probability of gravitons being emitted or absorbed.
This flaw suggests that to describe behavior close to the Planck scale, we need a more unified theory than quantized general relativity.
Comparison with other forces
Gravitons play a key role in general relativity, defining the spacetime in which events occur. In some descriptions, energy changes the "shape" of spacetime, and gravity is a consequence of that shape. This view sometimes makes it difficult to understand gravity as an interaction between particles.
Unlike the Standard Model, general relativity is considered to be background independent, that is, it does not depend on a specific space-time background. This makes it an open question whether background independence should still be maintained when building a theory of quantum gravity.
Relationship between energy and wavelength
Although gravitons are thought to be massless, they still carry energy. The energy of the graviton is not yet clear. If the graviton has mass, the relationship between its wavelength and mass-energy will be calculated. In this context, the Compton wavelength of the graviton is at least 1.6×1016 meters, equivalent to about 1.6 light-years.
This wavelength-mass-energy relationship can be calculated using the Planck-Einstein relationship.
Experimental observations
While it is theoretically possible to detect gravitons, individual gravitons cannot be unambiguously detected using any realistic detector due to their extremely low interaction cross section with matter. Even if a detector were designed to be as large as Jupiter, it is expected that only one graviton would be detected every decade in the best case scenario.
Another possibility for detecting gravitons is to use quantum sensing. Still, the gravitational waves observed by LIGO and the Faurecia collaboration are not designed specifically to detect gravitons, but the observations could provide clues to some of their properties.
Difficulties and unresolved issues
Most theories that include gravitons have serious problems. Attempts to extend the Standard Model or other quantum field theories by adding gravitons often run into theoretical difficulties approaching or exceeding the Planck energy range. This is due to the infinite problem caused by quantum effects, which makes it impossible to regularize gravity.
Some physicists have even suggested that replacing particles with strings could be a solution.
How will future research on gravitons affect our understanding of gravity, and the fate of the universe, remains an open question.
