A question that not even Albert Einstein was able to resolve in his lifetime gained a new chapter at the end of 2025. Study published in the journal Naturein October, reignites an old debate in modern physics: after all, does gravity need to be quantized to produce phenomena such as quantum entanglement?
The traditional answer – that, yes, this was the only possibility – has just gained a convincing counterpoint.
The authors, professors at the Department of Physics at the University of London, demonstrated that under certain conditions – when matter is described by quantum field theory –, Even gravity treated in a completely classical way can generate quantum entanglement between superimposed masses.
The conclusion does not overturn the idea of quantum gravity, but it changes the interpretation of one of the most promising experimental strategies for detecting it.
Superposition experiments can generate entanglement without quantum gravity, scientists say
In recent years, research groups in several countries have dedicated themselves to a particular type of experiment: placing two small masses in quantum superposition and observing whether the gravitational interaction between them produces entanglement. Many physicists argue that this entanglement would be an “unequivocal sign” of the quantization of gravity.
Now Joseph Azizi and Richard Howl, from the University of London, tell Nature that this argument was incomplete. The statement, according to them, ignored an essential ingredient: quantum field theorywhich describes matter more realistically than traditional quantum mechanics.
The new work reveals that, when matter is treated with this more complete structure, interaction channels mediated by virtual particles of matter emerge.
These channels, independently of gravitons, can transmit quantum information between masses and generate entanglement. This means that classical gravity can indeed be involved in a process that results in a quantum effect, even without being quantum itself.
The authors also show that this type of entanglement grows with masses, distances and times in a different way than what would be expected if the mediator were a quantized gravitational field.
The implication is immediate: the simple detection of entanglement no longer serves as definitive proof of quantum gravity. Future experiments will need to measure how this entanglement varies with different parameters, not just whether it exists.
It will be necessary to demonstrate that the observed characteristics do not fit into the classical+QFT mechanism described by the authors. This makes experiments more challenging, requiring larger superpositions, longer coherence time and extremely precise measurements.
And what does this mean for quantum physics?
Imagine the following hypothetical scenario: two dancers are on a dark stage. Each of them can be in two places at the same time, as if they were duplicated in the scene.
None of them can talk or combine steps. The only thing that connects them is very soft, almost imperceptible music. In other words: gravity.
Even so, their choreography is perfectly synchronized, as if one responded to the other without direct communication. This deep synchrony is the metaphor of quantum entanglementwhich many physicists believed could only arise if gravity was itself a quantum phenomenon.
But the new study suggests that view was incomplete. According to the authors, the matter itself – which and quantum in nature – can, under certain conditions, generate this “invisible synchrony” between the dancers, even when the music that guides them continues to be completely classical.
