Photon Correlations Ignore Graviton Statistics, CERN Physicists Find

CERN physicists have demonstrated that photon correlations within a closed optical resonator are unexpectedly insensitive to the statistical properties of gravitons. Building on the Hanbury-Brown Twiss effect, a technique developed through experiments in the 1950s, the research team analyzed how these photon correlations respond to the hypothetical particles mediating gravity. The work reveals that the super-Poissonian statistics expected from these gravitons cannot be inferred from photon measurements, even in principle. Unlike gravitational waves from massive objects, the source of these gravitons isn’t massive objects, but rather zero-point fluctuations of the gravitational field amplified by the evolution of spacetime curvature.

A technique utilizing the Hanbury-Brown-Twiss effect is now being repurposed in the search for elusive gravitons, the hypothetical particles that mediate gravity, as CERN physicists explore the quantum nature of gravitational interactions. This application of the technique, originally demonstrated through experiments in the 1950s, focuses on scrutinizing the interactions between gravitons and photons. The research investigates the insensitivity of photon correlations to the degrees of second-order coherence of the gravitons.

Current understanding of gravitational waves centers on massive astrophysical events like black hole mergers as primary sources; however, a growing body of theoretical work suggests a more subtle origin for background gravitational radiation. This connection between optical measurements and fundamental gravitational particles is driving new investigations into these elusive particles. The research reveals that the Hanbury-Brown Twiss correlations of the photons are insensitive to the degrees of second-order coherence of the gravitons, implying that inferring graviton statistical properties from photon measurements may be fundamentally limited. This perspective, built upon the success of the adiabatic paradigm of structure formation, posits that large-scale curvature inhomogeneities are accompanied by a diffuse background of gravitons. The resulting entangled states lead to specific correlations, and the team’s work seeks to connect the quantum coherence of photons to that of gravitons.

CERN physicists are applying a technique used in astronomical observations, the Hanbury-Brown Twiss effect, to scrutinize the interactions between cosmic gravitons and photons within a closed optical resonator featuring perfectly-reflecting walls. This approach connects established optical methods with the search for these fundamental particles. Researchers are investigating the insensitivity of photon correlations to the degrees of second-order coherence of the gravitons, demonstrating that the statistical properties of the gravitons cannot be inferred from the intensity correlations of the cavity modes, even in principle.

This insensitivity isn’t a roadblock, but rather a key insight into the nature of these primordial gravitons. The team’s investigation focuses on frequencies ranging from aHz to THz, where the spectral energy density of these gravitons is customarily measured. These limits have been slowly but steadily improving in the last three decades; while the different releases of the WMAP collaboration did set upper limits, recent determinations suggest even smaller values. Although we shall be using natural units, Dyson originally suggested it would eventually be possible to detect single gravitons at higher frequencies.

Their investigation centers on scrutinizing the interactions between cosmic gravitons and the fundamental mode of a quantized electromagnetic field confined inside a closed optical resonator boasting perfectly-reflecting walls, a setup designed to reveal correlations. The study suggests that the statistical properties of the gravitons cannot be inferred from the intensity correlations of the cavity modes, even in principle.

CERN physicists are now applying an analytical tool to characterize these elusive particles: the Hanbury-Brown Twiss (HBT) effect, a technique used in optical astronomy since experiments conducted in the 1950s. This approach focuses on the statistical properties of gravitons, attempting to discern their spectral energy density, a measure of energy per logarithmic frequency interval. The team’s analysis, centered on photons interacting within a closed optical resonator with perfectly-reflecting walls, has yielded a counterintuitive result. These calculations reveal that the spectral energy density of the diffuse backgrounds is customarily measured in terms of the averaged multiplicity of gravitons. The research indicates that detecting single gravitons may eventually be possible at higher frequencies, as Dyson suggested, and explores the connection between photon and graviton quantum coherence, questioning whether the photons’ HBT correlations are sensitive to the degrees of second-order coherence of the gravitons, and ultimately, what the statistics of photons within a Fabry-Pérot cavity reveal about the underlying quantum states.

Pulsar Timing Arrays & Interferometer Limits

Current gravitational wave detection relies heavily on instruments like Pulsar Timing Arrays and large interferometers, each probing different frequency ranges of spacetime ripples. Pulsar Timing Arrays, for example, monitor minute shifts in the arrival times of radio pulses from millisecond pulsars, effectively acting as galactic-scale gravitational wave detectors; recent data continues to refine limits on low-frequency gravitational radiation. Simultaneously, ground-based interferometers like LIGO and Virgo are sensitive to higher-frequency waves generated by mergers of compact objects. However, new research suggests fundamental limits to these detection methods when applied to the search for individual gravitons, the hypothetical quantum particles of gravity. Dyson originally proposed it would eventually be possible to detect single gravitons at higher frequencies. The team’s calculations reveal the spectral energy density is customarily measured as a function of the averaged multiplicity of gravitons.

Utilizing natural units, the calculations demonstrate the effect diminishes as the averaged multiplicity of gravitons increases.

This finding suggests a fundamental limit to how photon measurements can reveal characteristics of gravitons; the statistical properties of the gravitons cannot be inferred, even in principle, from the intensity correlations of the cavity modes, implying a need for entirely new diagnostic tools to probe the quantum realm of gravity.