A dancer on stage casts a shadow from the spotlight. From that shadow you can guess the rhythm. But entangled (linked by an invisible thread) gravitons — the tiniest packets of gravitational waves (ripples in spacetime) — perform a shadowless dance. These particles were born at the moment of inflation (the superfast expansion of the universe, predicted by Alan Guth) a split second after the Big Bang. Their state is squeezed: as if a dancer perfectly kept their balance, but their steps were unpredictable. Physicists wanted to see this hidden rhythm through light.
To do this, scientists proposed using quantum optics (the science of light at the particle level). Photons are placed inside an ideal cavity of two mirrors (a resonator), and the gravitational background — a faint ripple that stretches space (the expansion of the universe), discovered by Edwin Hubble — is passed outside. If gravitons affect the photon number, then measuring coincidences (correlations) of light particles could reveal their statistics. This method is akin to how in the 1950s Serge Haroche (a Nobel laureate in quantum physics) developed precise ways to track light in cavities.
However, calculations showed: the photon number in the resonator does not change. Symmetry forbids gravitons from adding or subtracting light. So the photon correlations stay the same as if there were no gravitons. Their dance fundamentally casts no shadows. It's not a matter of weakness (though the coupling is tiny — about 10⁻¹³), but of the very structure of the laws. Information about the superposition of graviton states does not transfer to light.
[callout]Even with a perfect resonator, you can't catch a gravitational signal this way. This route to a quantum theory of gravity (the unification of quanta and gravity) is closed, but scientists are seeking detours — for example, via invisible particles like dark photons, or through ultraprecise instruments (interferometers) capable of detecting single gravitons. Meanwhile, the dance goes on, and scientists are trying to spot it in the polarization of the cosmic microwave background — ancient light that has journeyed from the Big Bang.
The correlation method they tried to use was originally invented to measure stellar diameters — and only later was its quantum meaning understood.
In Carl Sagan's novel 'Contact,' they search for artificial signals by analyzing pulse statistics. In reality, we're trying to catch a natural quantum signature, but it slips away, like shadows cast by invisible dancers.