**Science News**

###### Curated by RSF Research Staff

# Scientist Propose Feasible Test of Quantum Gravity

Unified physics is the unitary description of phenomena from quantum mechanics to general relativity that is fully consistent across both regimes. The realization of a singular model that can describe fundamental behaviors and properties at the smallest scales to the largest scales was found in the study by physicist Nassim Haramein Quantum Gravity and the Holographic Mass. In the study, Haramein finds a unifying equation that describes the origin of mass as a function of quantized spacetime geometry at the Planck scale---giving the exact mass for astronomical-sized black holes and quantum-domain objects like the proton---how this quantum spacetime geometry produces the coupling force to bind hadrons via quantum gravity, and how relativistic Lorentzian invariance makes the quantum gravitational interaction appear exponentially weak within a few Planck lengths from the charge-radius of the proton.

Despite the remarkable success of the new model in unifying physics, many physicists remain on-the-fence, as no matter how elegant an equation and model may be, its predictions must be tested experimentally before it can be truly considered as valid. Note that the proton-charge radius predicted in the QGHM study was subsequently verified experimentally after its publication. Now, two teams working independently have designed an experimental protocol to test the "quantumness" of gravity---proving that gravity and quantum mechanics can be reconciled.

The first team is a pairing of Chiara Marletto of the University of Oxford and Vlatko Vedral of National University of Singapore. The second is an international collaboration. In the papers, both published in *Physical Review Letters*, the research teams describe how inducing quantum entanglement via gravitational-field interaction of two particles can demonstrate that indeed gravity has a quantized nature at the most fundamental scales---as demonstrated by Haramein in QGHM where geometric and holographic computation of tiny electromagnetic oscillators known as Planck spherical units generate the precise mass and gravitational interaction of objects.

**More information:** C. Marletto et al. Gravitationally Induced Entanglement between Two Massive Particles is Sufficient Evidence of Quantum Effects in Gravity, *Physical Review Letters* (2017). DOI: 10.1103/PhysRevLett.119.240402 , https://arxiv.org/abs/1707.06036

**ABSTRACT**

All existing quantum-gravity proposals are extremely hard to test in practice. Quantum effects in the gravitational field are exceptionally small, unlike those in the electromagnetic field. The fundamental reason is that the gravitational coupling constant is about 43 orders of magnitude smaller than the fine structure constant, which governs light-matter interactions. For example, detecting gravitons—the hypothetical quanta of the gravitational field predicted by certain quantum-gravity proposals—is deemed to be practically impossible. Here we adopt a radically different, quantum-information-theoretic approach to testing quantum gravity. We propose witnessing quantumlike features in the gravitational field, by probing it with two masses each in a superposition of two locations. First, we prove that any system (e.g., a field) mediating entanglement between two quantum systems must be quantum. This argument is general and does not rely on any specific dynamics. Then, we propose an experiment to detect the entanglement generated between two masses via gravitational interaction. By our argument, the degree of entanglement between the masses is a witness of the field quantization. This experiment does not require any quantum control over gravity. It is also closer to realization than detecting gravitons or detecting quantum gravitational vacuum fluctuations.

Sougato Bose et al. Spin Entanglement Witness for Quantum Gravity, *Physical Review Letters* (2017). DOI: 10.1103/PhysRevLett.119.240401 , https://arxiv.org/abs/1707.06050

**ABSTRACT**

Understanding gravity in the framework of quantum mechanics is one of the great challenges in modern physics. However, the lack of empirical evidence has lead to a debate on whether gravity is a quantum entity. Despite varied proposed probes for quantum gravity, it is fair to say that there are no feasible ideas yet to test its quantum coherent behavior directly in a laboratory experiment. Here, we introduce an idea for such a test based on the principle that two objects cannot be entangled without a quantum mediator. We show that despite the weakness of gravity, the phase evolution induced by the gravitational interaction of two micron size test masses in adjacent matter-wave interferometers can detectably entangle them even when they are placed far apart enough to keep Casimir-Polder forces at bay. We provide a prescription for witnessing this entanglement, which certifies gravity as a quantum coherent mediator, through simple spin correlation measurements.

Read more at:A possible experiment to prove that gravity and quantum mechanics can be reconciled

RSF-related post:

**Entanglement of quantum clocks by gravity:**

Time forms the basis of our every experience, yet it remains a challenging factor to define in a consistent scientific model. Most advanced models of unified physics considers time as an emergent property, not an intrinsic attribute of the physical world. This partly emerges from the treatment of time in general relativity, where time becomes inextricably merged with space and becomes relative to ones frame of reference.

In general relativity, it is commonly imagined that any given reference frame uses a latticework of clocks to record events, where each location in space has a corresponding idealized clock. The clocks can then be used to locate events in spacetime. In this clock latticework picture, clocks are considered as external objects that do not interact with the rest of the universe. This raises some considerations, as within the framework of the connected universe we understand that "everything affects everything else", so how accurate is it to consider our clocks as behaving independently from the surrounding system?

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