A sealed box, a cat, a vial of poison, and a quantum trigger that may or may not fire. As long as Schrödinger’s thought experiment has circulated, it has pointed to a gap in our picture of nature. Atoms and subatomic particles can occupy two states at once. Tables, people, and cats cannot. Somewhere between a single particle and a living animal, the both-at-once description gives way to ordinary, one-thing-at-a-time reality. Physicists call that fading of quantum overlap decoherence. In their paper “Experimental exclusion of a generalized Károlyházy gravity-induced decoherence model,” Nicola Bortolotti, Kristian Piscicchia, Alessio Porcelli, Matthias Laubenstein, Simone Manti, Antonino Marcianò, Federico Nola, and Catalina Curceanu show that a generalized form of one of the oldest gravity-based explanations for decoherence no longer fits the data.[1]
Let’s start with the cat, because the image is useful. In quantum theory, a tiny system can be described as a wave that holds several possibilities together concurrently: an atom can be here and there, or a photon can take two paths. Schrödinger’s point was that if those rules applied without limit, a carefully arranged chain of events could put a whole cat into a state that was both living and dead. Everyday life never displays that concurrence. Decoherence is the process that removes quantum concurrence and leaves a single outcome.
There is more than one explanation for decoherence. The most common one is contact with the surrounding environment. Air molecules, stray light, heat, and vibrations interact with an object in a quantum superposition. Those interactions differ depending on the object's possible states. After enough interactions, the superposition is gone, and the object behaves like an ordinary classical object in one state. This is consistent with laboratory results.
That doesn’t mean that the converse is true. Whether superposition would still disappear if a quantum system were cut off from air, light, heat, and vibration remains open. Some theories of decoherence say that a system in several states at once will, on its own, end up in one state, even if nothing in the surroundings disturbs it. They also say this happens faster for more massive objects. Other theories point to gravity. Mass curves spacetime, so if quantum rules cannot define that curvature precisely, a large superposition may be unstable and collapse to one state. These ideas can be discussed together, but they are different theories with different predicted outcomes. A measurement can show which predictions fail.
The gravity-based account at issue begins with Frigyes Károlyházy, a Hungarian physicist writing in the 1960s. He asked how precisely one can measure a length if both quantum uncertainty and gravity are taken seriously. If the measuring object has too little mass, quantum uncertainty makes its position unclear, so the measured length is unclear. If it has too much mass, it curves spacetime and changes the region being measured. Balancing those demands, Károlyházy found that the smallest possible error in a length or time measurement grows as the measured length or time increases. He treated that limit as a property of spacetime, not as an imprecise measurement. In Károlyházy’s model, spacetime geometry fluctuates at random. Those fluctuations are built into the metric, the mathematical object that defines distances and times.
A small quantum system is affected only weakly. A large object is affected much more strongly. Its parts occupy different places, so they pick up different random shifts from the fluctuating geometry. The phase relationship that holds a superposition together is lost, and the object decoheres. That is why, in Károlyházy’s theory, a Schrödinger-cat superposition of a macroscopic body decoheres.
Károlyházy’s 1966 paper influenced later gravity-related decoherence models. It also shares a premise with many attempts to combine quantum theory and gravity: that a minimum meaningful length, set by Planck-scale constants, limits how precisely position can be defined.
In the original model, the metric fluctuations were required to obey a wave equation, as gravitational waves do. The generalized model tested in the research drops that requirement. Károlyházy’s uncertainty relation still fixes the time dependence of the fluctuations. The model describes how much spacetime fluctuations at one point resemble those at another. A single distance, RK, sets that resemblance. If RK is small, even nearby points fluctuate differently. If RK is large, the fluctuations stay similar across a laboratory. RK is not fixed by the original theory; the experiment constrains it.
In this model, random changes in spacetime give charged particles a random push. A charged particle that is pushed around emits radiation. That radiation would be very faint, spread over a range of energies rather than concentrated at one, and stronger if RK is smaller. In a real detector, it would come from atoms in the material, especially from protons. Ordinary quantum theory does not predict this extra radiation. If the model is right, a large mass of metal should emit a small extra flux of high-energy photons. The experiment looked for that radiation.
The researchers searched for that extra radiation at Italy’s National Institute for Nuclear Physics (INFN) at the Gran Sasso National Laboratory. The laboratory sits under enough rock to reduce the flux of cosmic-ray muons by about a million. (Cosmic-ray muons impact a detector and produce extra counts: direct hits, secondary particles, and a smear of background radiation. That background can hide, or mimic, the faint radiation the generalized Károlyházy model predicts.) They used a high-purity germanium detector surrounded by copper and lead. The detector and shielding were also the sample: if spacetime fluctuations shook charged particles in those materials, the Germanium would absorb some of the emitted radiation. They analyzed about 62 days of data collected by the VIP Collaboration, a group that uses underground detectors at Gran Sasso to test basic rules of quantum mechanics. They used the 1.3 to 1.6 MeV energy range, where a computer model of the known radioactive background accounts for almost all recorded counts.
The researchers calculated the radiation pattern predicted by the generalized Károlyházy model, corrected it for the detector's efficiency in recording photons from each part of the apparatus, and used a Bayesian analysis to determine how much extra radiation the data could still contain. In the model, that extra amount fell as 1/(RK)2. No extra radiation was found. At 95 percent confidence, RK must be greater than 4.64 meters. That limit is more than ten times stronger than the previous experimental bound of about 11 centimeters.
The 4.64-meter limit matters because the model already had an upper limit.
For this theory to explain why ordinary objects aren't in two places at once, RK cannot be too large. If it is, spacetime fluctuations are too smooth over everyday distances and do not force a large object into one state fast enough. A published argument of that kind, using a 10-micrometer graphene disk becoming definite within 0.01 seconds, requires RK to be less than 1.98 meters.
The Gran Sasso data require RK to be greater than 4.64 meters, contradicting the 1.98-meter upper limit. The model cannot avoid producing extra radiation while still accounting for ordinary objects occupying one place at a time, so we rule it out. A related collapse model that predicts the same radiation is ruled out for the same reason.
This result does not settle whether gravity plays any role in the shift from quantum behavior to ordinary classical behavior, but it does rule out the generalized Károlyházy account, in which spacetime fluctuations both suppress large superpositions and produce extra high-energy radiation. That account is inconsistent because it needs RK to be below 1.98 meters and above 4.64 meters at the same time.
The exclusion also tightens the next round of theories. Other collapse models and other quantum-gravity ideas start from the same premise: that length cannot be measured with unlimited precision. If a later model predicts extra radiation from ordinary materials, that radiation must be weaker than this search could see, or it must appear in a different energy pattern, or the link it draws between localization and emission must be different.
The paper does not discuss quantum computers, but it also implies that this model would set a limit on how large a coherent quantum device could be, because the same spacetime fluctuations that make a macroscopic object definite would also destroy the superpositions a processor needs. That limit is gone with the model. The result does not solve the engineering problem of keeping a machine stable. It means this gravitational mechanism is not an extra law of nature standing in the way.
The apparatus is the same kind used in underground rare-event searches: a high-purity Germanium detector, heavy shielding, and a detailed model of leftover radioactivity. Better control of that background helps those searches and, at the same time, makes tests of collapse models and gravity-induced radiation more sensitive.
The result applies to this generalized Károlyházy model, not to every gravity or collapse theory. Other forms of spacetime correlation remain possible, and other collapse models are separate proposals, some of them already constrained by radiation and interferometry experiments. Gravity could still enter in a different way. The researchers say the next step is the same method with better detectors and lower background. They also note that the 1.98-meter theory bound is conservative: a smaller object, or a shorter time for it to become definite, would lower that upper limit and make a related model harder to reconcile with the data.
Schrödinger’s cat is a demand to identify where the both-at-once description stops. The researchers have shown that the generalized Károlyházy account cannot do that job: it is too strong to match the germanium spectrum and too weak to keep everyday objects definite. The search for whatever does cause that transition continues, with this version of the model excluded.
This article used AI to craft the contents and is shared at no charge for educational and informational purposes only.
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[1] https://six3ro.substack.com/p/curiosity-might-kill-the-schrodingers
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