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Unifying gravity and quantum theory remains a significant goal in modern physics. Despite the success in unifying all other fundamental interactions (electromagnetism, strong force and weak force) with quantum mechanics and many attempts at explaining a "quantum gravity," scientists are still coming up short. Still, some believe we are getting closer to determining whether these two theories can be combined or whether they are truly incompatible.

A major contender for proving or disproving whether gravity is quantum is Richard Feynman's proposed experiment to test if gravity can entangle two massive objects. In theory, such entanglement would indicate quantum behavior. While it was not actually feasible to do this experiment in 1957, when Feynman came up with the idea, new scientific advances are bringing it closer to reality.

However, a new study, in Nature, claims that it's a little more complicated than this. The researchers involved in the study determined, through their calculations, that entanglement is not necessarily evidence of quantum gravity—and that classical gravity can generate this entanglement in some cases too.

"Although entanglement can be used to provide evidence for the quantum nature of gravity, contrary to that considered previously, this is not unambiguous and is, instead, fundamentally a phenomenological issue: it depends on the parameters and form of the experiment," the study authors explain.

The team says that the key is using quantum field theory. The current view is that classical gravity can involve only local operations and exchanges of classical information (LOCC), meaning that it should not produce entanglement, because that would be "unphysical" and require information to travel faster than the speed of light. But, when the team combined quantum field theory for matter with classical gravity, this is not what they found.

"Here we show that local classical theories of gravity can, in fact, generate quantum communication and, thus, entanglement. The arguments and theorems for classical gravity operating only as LOCC implicitly treat matter in standard quantum mechanics. However, to the best of our knowledge, matter obeys quantum field theory (QFT), and when this is taken into account, we show that a classical gravity interaction naturally gives rise to quantum communication," they write.

The team explains that this quantum communication arises from virtual matter propagators, instead of the assumed virtual graviton propagators. They claim that prior theorems took too restrictive a view on what the gravitational interaction consists of. They say that, while quantum gravity involves only virtual graviton propagators, quantum field theory also involves virtual matter propagators.

According to their calculations, both can lead to entanglement. And so, entanglement in experiments, like Feynman's, is not unambiguous evidence for quantum gravity.

Luckily, Feynman's experiment is still useful. Although both classical gravity and quantum gravity appear to produce entanglement, they do so at different strengths. The strength depends on parameters, like mass and the duration of the experiment, and whether the effect was quantum or classical may still be discernible. Still, the findings in this study might have complicated the lifework of at least a few physicists.

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