stochastic fluctuations affect gravity

New Study Explores How Stochastic Fluctuations Can Differentiate Classical and Quantum Gravity

A study recently published in Physical Review Letters suggests an experimental pathway to resolving the fundamental question of whether gravity adheres to classical or quantum mechanics.

Introduction: The Gravity Dilemma

For decades, physicists have grappled with the enigmatic nature of gravity. Unlike the electromagnetic, strong and weak nuclear forces, gravity remains resistant to  unification within the quantum framework.

Alternative Approach: Moving Beyond Graviton Detection

Instead of attempting to formulate a complete theory of gravity or detect individual gravitons—the hypothetical carriers of gravitational force—the researchers adopt an alternative approach.

Serhii Kryhin's Perspective

"In recent years, multiple proposals have emerged aiming to experimentally determine the nature of gravity. However, their execution remains highly challenging. Our goal was to devise a more practical experiment capable of at least falsifying the notion that gravity is classical," stated Serhii Kryhin, a third-year graduate student at Harvard University and co-author of the study.

Reframing the asking Question: Observable Differences Between Classical and Quantum Gravity

Rather than asking whether gravity must be quantized, the researchers reframed the question to seek measurable distinctions: "What observable differences would indicate the necessity of quantizing gravity?"

Quantum vs. Classical Fluctuations

"The concept is straightforward yet has remained overlooked until now. If gravity is inherently quantum, it should facilitate the entaglement of distnat matter due to its long-range nature. Conversely, if gravity is purely classical, such entaglement would be impossible," explained Vivishek Sudhir, Associate Professor at MIT and co-author of the study.

Stochastic Fluctuations in Classical Gravity

The fundamental observation is that if gravity is classical, it must generate unavoidable stochastic fluctuations. These fluctuations arise as a necessary consequence of resolving an inherent inconsistency—without them, classical gravity's determinism would contradict quantum mechanical principles.

Quantum vs. Classical Gravitational Fluctuations

The ingenuity of this approach stems from recognizing that these fluctuations would induce a measurable phase shift in the cross-correlation spectrum, distinguishing classical gravity from its quantum counterpart.

Weak Quantum Fluctuations

"Quantum fluctuations inherently emerge as variations in the dynamic degrees of freedom within general relativity. The key distinction between quantum and classical gravitational fluctuations lies in their magnitude—quantum effects, being relativistic in nature, are exceptionally weak and therefore extremely difficult to detect," explained Kryhin.

Larger Classical Fluctuations

"On the other hand, for classical fluctuations to be theoretically viable and consistent with our current understanding, they would have to be considerably larger," remarked Prof. Sudhir.

Theoretical Model: Interplay Between Quantum and Classical Domains

The researchers present a theoretical model describing the interplay between quantum and classical domains within the Newtonian limit of gravity, where classical gravity and quantum matter coexist.

Quantum-Classical Master Equation

The researchers formulated a quantum-classical master equation governing the joint evolution of quantum matter and classical gravity. Additionally, they derived a Hamiltonian for Newtonian gravity's interaction with quantum masses using two distinct approaches: Dirac's constrained systems theory and the Newtonian limit of gravity.

Modified Newton's Law and Stochastic Effects

Subsequently, they derived a modified quantum Newton's law incorporating stochastic gravitational effects and identified the unique correlation patterns between two gravitationally interacting quantum oscillators.

Markovian Master Equation

Through this mathematical framework, they derived a closed Lindblad equation—a Markovian master equation—governing quantum matter interacting with classical gravity. This equation introduces a parameter,  ε, where ε ≠ 0 Signifies classical gravity and ε = 0 denotes quantum gravity.

Identifying Measurable Quantities

Through their analysis, the researchers uncovered critical insights, establishing that a coherent theoretical model of classical gravity coupled with quantum matter is achievable, challenging earlier claims to the contrary.

Distinct Fluctuations in Classical and Quantum Gravity

Their computations indicate that classical gravity generates fluctuations fundamentally different from those of quantum gravity, with a distinct, experimentally verifiable signature.

Phase Shift in Quantum Harmonic Oscillators

When two quantum harmonic oscillators engage gravitationally, a hallmark phase shift of  π (180 degrees) emerges in their cross-correlation spectrum at a defined detuning from resonance, signaling classical gravity.

Proposed Experiment: The Quantum Cavendish Experiment

To validate these theoretical predictions, the researchers propose a quantum analogue of the historic Cavendish experiment, employing two highly coherent quantum mechanical oscillators coupled via gravity.

Measuring the Phase Shift

The characteristic phase shift could be identified by accurately quantifying the cross-correlation of their movements.

This approach stands out due to its experimental viability. Unlike previous proposals requiring macroscopic quantum superpositions, it leverages correlations between quantum oscillators, achievable with present or near-future technology.

Theoretical Implications: Self-Consistent Interaction Between Gravity and Quantum Matter

Professor Sudhir explained that semiclassical gravity models often disregard the influence of quantum fluctuations of matter on classical gravitational dynamics. In contrast, their framework enables a self-consistent interaction between classical gravity and quantum matter.

Empirical Evidence and Its Potential Impact

Empirical evidence supporting the classical nature of gravity would have far-reaching consequences for our understanding of fundamental physics.

Challenging the Quantum Gravity Paradigm

"The notion that gravity must be quantum is widely accepted, yet its precise implications remain elusive," remarked Kryhin.

Potential Reassessment of Fundamental Physics

"Extensive efforts have been dedicated to formulating a quantized theory of general relativity, leading to the development of string theory as a significant outcome. However, if experiments confirm that gravity is classical, a fundamental reassessment of our ontological understanding of the universe will be necessary."

Challenges Ahead: Formalism Development and Technological Feasibility

While this research provides a fresh taken on a decades-old problem, the team acknowledges that significant hurdles remain, including formalism development, model refinement, and the technological feasibility of the proposed experiment.

Technological Challenges in Sensitivity and Measurement

"Experimentally, achieving the necessary sensitivity for a conclusive test required the precise integration of two gravitating masses, advanced noise isolation, and highly refined measurement techniques," Kryhin concluded.

Source

Explore the Future of Gravity Research!

A groundbreaking study challenges conventional physics by exploring how stochastic fluctuations could distinguish classical and quantum gravity. Discover how researchers are redefining our understanding of gravity's fundamental nature!

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