Fundamental physics faces a stubborn paradox: the current impossibility of reconciling the rules of the quantum world with those of gravity, despite a century of efforts. This theoretical divergence persists without scientists achieving a unified vision.
Since Einstein's work, gravity has been interpreted as a deformation of spacetime caused by mass. In parallel, quantum mechanics describes interactions at the subatomic scale via quanta, such as the photon for the electromagnetic force. The hypothesis of a graviton, a particle mediating gravity, remains speculative due to extreme detection difficulties.
In the 1950s, Richard Feynman imagined a thought experiment where an object, like an apple, is placed in quantum superposition, existing in multiple states simultaneously. He believed that if this object interacted gravitationally with another, it would demonstrate the quantum nature of gravity. This proposal long guided research on unifying theories.
Joseph Aziz and Richard Howl, from the University of London, have recently overturned this conception. Their calculations indicate that quantum entanglement between objects can occur even if gravity is classical, without recourse to gravitons. They mention virtual matter processes that, by interacting with the gravitational field, enable this entanglement. Their approach broadens perspectives on fundamental interactions.
In this model, virtual particles, although ephemeral, play a key role in facilitating entanglement via classical gravity. Permitted by quantum principles, these temporary entities create correlations between objects, partially simulating the effects of quantum gravity. However, the intensity of these correlations is lower, which could help distinguish the two scenarios in future experiments.
The practical implementation of Feynman's experiment represents a considerable technical challenge, as superposition states are highly sensitive to decoherence. Research groups in the UK, Austria, and elsewhere are attempting to overcome these obstacles, but progress is slow. Feasibility relies on isolating quantum systems from external disturbances, a demanding task with current technologies.
The results of Aziz and Howl, published in
Nature, offer innovative avenues for exploring the links between gravity and quantum. Although they do not rule out the possibility of quantum gravity, they highlight alternative entanglement mechanisms, enriching the scientific debate on harmonizing physical laws.
Quantum superposition
Quantum superposition is a fundamental principle where a particle or system exists in multiple states simultaneously until a measurement is made. This state is described by a wave function, which represents the probabilities of different possible configurations. For example, an electron can have a spin that is both 'up' and 'down', and it is only at the moment of observation that one of the possibilities is realized. This phenomenon defies classical intuition, where objects have well-defined properties at all times.
The idea of superposition dates back to the early days of quantum mechanics, with experiments like Schrödinger's cat, which illustrates the paradoxes associated with this state. In practice, superpositions are observed in isolated systems, such as cold atoms or superconducting circuits. They enable applications like quantum computing, where qubits exploit this property to perform parallel calculations. However, maintaining superposition requires a highly controlled environment to avoid decoherence.
Decoherence occurs when the quantum system interacts with its environment, causing the wave function to collapse and the loss of superposition. This process explains why macroscopic objects, like an apple, do not appear to be in superposition states in everyday life. Physicists work to minimize these interactions in laboratories, using cooling and isolation techniques to preserve quantum states longer.
Understanding superposition is essential for advancing fields like quantum cryptography and ultra-precise sensors. It paves the way for new technologies while raising philosophical questions about the nature of reality. Research continues to explore the limits of this phenomenon and its role in the universe at different scales.
The role of virtual particles
Virtual particles are conceptual entities in quantum physics that briefly appear during interactions, without having permanent existence. They are permitted by the Heisenberg uncertainty principle, which allows energy fluctuations over very short durations. Within the framework of quantum field theory, these particles mediate fundamental forces, like electromagnetism, where virtual photons facilitate the interaction between electric charges.
Unlike real particles, virtual ones cannot be directly detected, as they do not obey the laws of energy and mass conservation over long periods. Their presence explains phenomena like the Casimir effect, where they create an attractive force between two close metal plates. They also explain certain properties of the quantum vacuum, which is not empty but filled with permanent fluctuations.
Regarding gravity, if it were quantum, virtual gravitons would be assumed to mediate the interaction. However, recent work suggests that even with classical gravity, similar effects to entanglement can be found. These virtual particles interact with the quantum fields of objects, creating correlations without requiring gravitons. This expands the possibilities of interaction in a non-quantum framework.
The study of virtual particles helps to understand deep aspects of physics, like Hawking radiation or dark matter. Although conceptual, they have practical implications in the development of new theories and technologies. Research continues to better understand their nature and influence on the observable universe.