⚛️ general relativity

Comments on graviton detection

This paper argues that detecting single gravitons or quantum noise from squeezed states does not prove the quantization of the gravitational field, as classical gravitational waves can produce identical detector outputs.

Original authors: Daniel Carney

Published 2026-02-04

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Daniel Carney

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Question: Is Gravity Made of Tiny Particles?

Imagine the universe is filled with invisible waves, like ripples on a pond. We know that light is made of tiny particles called photons. Because gravity is a force like light, most physicists guess that gravity is also made of tiny particles called gravitons.

But here is the problem: We have never actually seen a single graviton. We have seen gravitational waves (huge ripples from colliding black holes), but we haven't proven that these waves are made of individual "grains" of gravity.

The author of this paper asks a very specific question: If we build a machine that "clicks" when it catches a single graviton, or if we see weird "quantum noise" in our detectors, does that prove gravity is quantized (made of particles)?

The surprising answer is: No.

The "Clicking" Detector Analogy

Imagine you have a very sensitive microphone (a detector) in a room.

Scenario A (Quantum): You throw a single, tiny pebble (a graviton) at the microphone. It makes a "click."
Scenario B (Classical): You blow a very gentle, steady stream of air (a classical wave) at the microphone.

The author argues that if the air stream is just right, it can make the microphone "click" in exactly the same way the pebble does. Even if you build a detector that only clicks when it absorbs energy, a classical wave can trick it into clicking just as often as a particle would.

The Takeaway: Just because a detector clicks doesn't mean a particle hit it. A classical wave can mimic the behavior of a particle perfectly in this scenario.

The "Sub-Poisson" Secret (The Real Proof)

So, how do we prove something is a particle and not a wave? In the world of light (photons), scientists found a trick.

Imagine you are counting how many times a light bulb flickers in one minute.

Classical Light: If you have a steady stream of light, the flickers happen randomly, like raindrops hitting a roof. The math says the "noise" (how much the count varies) is always equal to or higher than the average number of clicks.
Quantum Light: If you use a special "squeezed" state of light, you can make the flickers happen with such perfect timing that the noise drops below the average. It's like a drummer hitting a snare drum with such perfect rhythm that the gaps between hits are smaller than physics should allow for random rain.

This "sub-Poisson" behavior (noise lower than the classical limit) is the smoking gun. It proves the field is quantum.

Why This Doesn't Work for Gravity Yet

The author explains that while this trick works for light, it is impossible to use it for gravity right now. Here is why:

Gravity is incredibly weak: Imagine trying to hear a whisper (a single graviton) in a hurricane. Our detectors (like LIGO) are huge, but they are still tiny compared to the strength of gravity.
The "Efficiency" Problem: To see the "perfect rhythm" (sub-Poisson statistics) in gravity, your detector needs to catch almost every graviton that hits it.
- For light, our detectors catch about 90% of the photons.
- For gravity, our detectors catch roughly one in a trillion trillion trillion gravitons.
The Result: Because we catch so few, the "noise" from our own detector (the static in the microphone) completely drowns out the perfect rhythm of the gravitons. We only see the "super-vacuum" noise (the loud, messy part), which a classical wave could easily explain. The "sub-vacuum" part (the quiet, perfect rhythm that proves it's quantum) is too small to measure.

The LIGO Example

The paper does a detailed math check on LIGO (the giant laser detector). It asks: "If we had a super-squeezed gravitational wave, could LIGO see the quantum signature?"

The answer is a hard no. Even in the most extreme, impossible scenario where the gravitational wave is perfectly squeezed, the signal that proves it's quantum is about $10^{-43}$ times smaller than the noise LIGO already has. It's like trying to see a single grain of sand on a beach from a satellite in space.

The Conclusion

The paper concludes with a simple, technical statement:

We can build a detector that clicks for a single graviton.
We can build a detector that sees quantum noise.
But, a classical gravitational wave can produce the exact same data in both cases.

Therefore, observing these signals does not prove that gravity is quantized. To prove gravity is made of particles, we need a different kind of experiment (perhaps tabletop experiments or looking at the early universe) that can distinguish between a classical wave and a quantum particle in a way that isn't drowned out by the weakness of gravity.

In short: Just because we can hear the "click" doesn't mean we've found the particle. The wave could be faking it. And right now, our ears aren't good enough to tell the difference.

Technical Summary: Comments on graviton detection

Problem Statement
While the quantization of the gravitational field is theoretically plausible by analogy with other fundamental fields, direct experimental evidence for the existence of the graviton remains lacking. A common intuition suggests that detecting a single graviton (a "click" in a detector) or observing quantum noise in gravitational wave detectors (such as LIGO) would constitute proof of field quantization. This paper addresses the question of whether such observations can actually distinguish between a model where the gravitational field is a quantum operator ( $\hat{h}_{\mu\nu}$ ) and a model where it is a classical stochastic field ( $h_{\mu\nu}$ ) coupled to a quantum detector. The author argues that contrary to some recent literature, neither single-graviton detection nor the observation of quantum noise in squeezed states provides definitive evidence for the quantization of gravity.

Methodology
The paper employs a comparative analysis of two theoretical models:

Quantum Model: The detector is a quantum system coupled to a quantized gravitational field (or electromagnetic field in the analog case).
Classical Model: The detector is a quantum system coupled to a classical, non-dynamical external field drawn from a probability distribution $P_{cl}$ .

The methodology proceeds in three stages:

Electromagnetic Analogy: The author first revisits the photoelectric effect and photon counting. Using the Glauber-Sudarshan $P$ -representation, it is demonstrated that the rate of discrete "clicks" in a photodiode is identical in both the quantum and classical models, provided the classical field amplitude is treated as a random variable. The paper establishes that "clicks" alone cannot distinguish the two; only specific correlation statistics (variances) can.
Non-Classicality Tests: The paper analyzes conditions under which quantum models produce statistics impossible for classical models. Specifically, it examines sub-Poisson statistics (variance in click counts $\Delta N^2 < \langle N \rangle$ ) and sub-vacuum noise levels in linear detectors. These phenomena arise from "non-classical" states (where the $P$ -function is negative) and cannot be reproduced by any classical probability distribution of fields.
Gravitational Application: These arguments are extended to gravitational wave detectors. The author calculates the expected output for:
- Graviton Counting Detectors: Devices that absorb a single graviton (e.g., via the Gertsenshtein effect).
- Linear Strain Detectors: Devices like LIGO that measure the strain amplitude $h$ directly.
- LIGO Specifics: A detailed calculation using the input-output formalism for LIGO is performed, modeling the interaction between a squeezed gravitational signal and the detector's mechanical mirror, accounting for damping and noise power spectral densities (PSD).

Key Contributions and Results

Equivalence of Single-Event Detection: The paper proves that a detector "clicking" upon the absorption of a single graviton yields the same statistical output in a classical stochastic field model as in a quantum field model. Just as the photoelectric effect does not prove the existence of photons (without further correlation tests), a single graviton detection does not prove the existence of gravitons.
The Efficiency Barrier for Counting Detectors: While sub-Poisson statistics in photon counting can prove field quantization, the author demonstrates that for gravitons, this is practically impossible. The deviation from Poisson statistics scales with the square of the detector efficiency ( $\eta^2$ ). Given that gravitational coupling is extremely weak ( $\eta \sim 10^{-33}$ in current proposals), the signal is swamped by detector noise, and any observed statistics will appear Poissonian regardless of the field's quantum nature.
Classical Explanation of "Quantum Noise": The paper addresses the claim that squeezed gravitational states could produce observable "quantum noise" in LIGO. The analysis shows that a squeezed state produces two effects:
- Sub-vacuum noise: A reduction in noise below the vacuum level in one quadrature.
- Super-vacuum noise: An increase in noise in the conjugate quadrature.
  The super-vacuum noise (the "large" noise component) can be fully explained by a classical random signal. Only the sub-vacuum component requires a quantum explanation.
LIGO Calculation: A specific calculation for LIGO reveals that while a sufficiently squeezed graviton state could theoretically reduce the detector output noise below the classical limit, the magnitude of this effect is vanishingly small. The ratio of quantum to classical output noise is estimated to be $1 - 10^{-43}$ . This reduction is far below the intrinsic quantum noise of the detector itself, rendering the signature unobservable.

Significance and Claims
The paper claims to provide a "simple technical statement": a classical gravitational wave can produce the same output data in a detector as a quantized gravitational wave, even in scenarios involving single-particle absorption or squeezed states.

The significance of this work is to correct a misconception in the literature regarding what constitutes evidence for the quantization of gravity. The author emphasizes that:

Observing a single graviton or "quantum noise" is not sufficient evidence for quantization.
Distinguishing the two requires observing specific non-classical correlations (like sub-Poisson statistics or sub-vacuum noise) that are impossible for classical fields.
Due to the extreme weakness of the gravitational coupling, the efficiency of current and proposed detectors is too low to observe these distinguishing features. The "quantum-only" signatures are effectively washed out by the detector's own noise.

Consequently, the paper concludes that realistic measurements of gravitational radiation cannot currently distinguish between classical and quantized fields. The author suggests that future efforts to prove gravity's quantization must look elsewhere, such as tabletop experiments probing virtual graviton exchange or measurements of the Cosmic Microwave Background (CMB), rather than relying on direct detection of gravitational waves or single gravitons.