Quantum noise is an intriguing concept in physics, representing a fundamental uncertainty inherent in the fabric of reality. Even in the most silent conditions—where phones are muted and thoughts are calmed—this noise persists, manifesting as random fluctuations that defy complete comprehension. This phenomenon remains one of the most puzzling challenges in contemporary science.

Despite its prevalence, physicists struggle to provide a satisfactory explanation for quantum noise. The governing equation of quantum theory, the Schrödinger equation, is deterministic and does not account for such randomness. To unravel the origins of this noise, additional principles must be considered.

For proponents of the Niels Bohr perspective, the act of observation is critical. The Schrödinger equation outlines various potential outcomes for a particle, but it is only when a measurement is made that a specific result materializes, seemingly chosen at random. This inherent unpredictability leads to different outcomes for identical particles, resulting in uncontrollable variances in fundamental processes. Bohr viewed quantum noise as an intrinsic part of reality—an ongoing process of creation sparked by observation itself.

Skeptics, such as Einstein, found this interpretation both poetic and illogical. Questions arise about the nature of the observer and the act of observation. For decades, scientists and philosophers have sought a more concrete explanation, exploring two main avenues. One possibility is that quantum noise, much like everyday noise, has an underlying meaning that is currently beyond our understanding. Despite its apparent randomness, it may stem from deterministic processes obscured from our view, potentially arising from a multitude of parallel universes. In this view, noise indicates our specific reality, with the random behaviors of particles signifying the uniqueness of our universe.

The alternative perspective posits that quantum noise is indeed meaningless, and quantum mechanics is fundamentally indeterministic. In 1986, physicists GianCarlo Ghirardi, Alberto Rimini, and Tullio Weber proposed that noise occurs independently, without any triggering event, potentially manifesting once every 100 million years for individual particles.

Within this ongoing debate, the Ghirardi-Rimini-Weber (GRW) theory stands out. While not the only indeterministic interpretation of quantum mechanics, it uniquely highlights indeterminism as observable noise rather than relegating it to a subquantum realm. It is also one of the few interpretations that can be empirically tested, presenting a data-driven inquiry into the foundational question of whether the universe is fundamentally deterministic.

The GRW theory suggests that noise randomly affects particles, causing them to localize in one of their possible positions. This phenomenon must occur infrequently; otherwise, particle behavior would continuously deviate from the Schrödinger equation. However, when such noise does occur, its effects can be significantly amplified through quantum entanglement—the mysterious connections between particles. A disturbance to one particle resonates with all those it is entangled with, explaining why we observe quantum effects on a particle level but not in everyday objects.

The influence of noise extends beyond spatial positioning, also affecting other properties indirectly. In the well-known thought experiment involving Schrödinger's cat, the cat's state (alive or dead) is determined by the arrangement of particles in its body, which the GRW mechanism would resolve.

Moreover, the GRW theory clarifies the concept of observation. To observe is to correlate a particle with a large apparatus, thus exposing it to the noise that interacts with that apparatus. While Bohr suggested that our observations create reality, the GRW theory posits that our observations merely enable the omnipresent noise to influence objects that would otherwise remain unaffected.

The GRW framework, akin to quantum mechanics, is a mathematical construct and does not specify the underlying nature of reality. The original vision was a particle-based ontology, yet the theory is compatible with various models, including fields of smeared-out matter. Its defining characteristic is the spontaneous nature of noise, which does not necessitate any external triggers, opening up the possibility that the universe could be fundamentally composed of noise.

This minimalist perspective, championed by physicist John Bell in 1987 and further explored by Roderich Tumulka, presents the universe as a collection of disconnected moments, much like random flashes in a dark auditorium. Our observations are part of this randomness, and while it appears continuous, the absence of a particle's existence between observations raises questions about our conventional understanding of particles.

While this view may not be universally accepted, it proposes a potential resolution to the conflict between quantum entanglement and relativity. Quantum mechanics suggests that entangled particles instantaneously influence one another, which contradicts the relativity principle that prohibits instantaneous effects. However, in the GRW framework, the independent nature of noise means that the timing of events is irrelevant, allowing for coherence between these two theories.

The GRW theory may even shed light on other challenges in physics, such as the second law of thermodynamics, which posits that entropy tends to increase in closed systems. Traditional explanations suggest that disorder is simply more probable than order. However, David Albert argues that the distribution of states in the phase space could account for the observed increase in entropy, with quantum noise potentially providing the necessary disturbances that prevent reversals to ordered states.

For the theory to hold, the noise must be spontaneous and irreducible, and it should significantly disrupt particles to prevent any reversals. Among various interpretations of quantum noise, GRW uniquely meets these criteria.

Despite its promise, no experimental evidence has yet confirmed the predictions of the GRW theory. It should exhibit various unusual effects, such as random deviations in electron trajectories, but studies have shown that such phenomena occur far less frequently than initially estimated. Similarly, gravitational wave detectors like LIGO observe exceedingly low noise levels, further challenging the theory.

If GRW does operate, it may require modifications to the original model, such as considering how a particle's mass influences its susceptibility to noise. The theory remains viable, yet the absence of definitive signals raises doubts about its foundational indeterminism.

If determinism ultimately prevails, it would not eliminate noise but rather redefine its origins. With the laws of physics, we could trace noise back to specific particle trajectories, suggesting that noise is woven into the very fabric of reality, shaping the complexity of the universe.

In the laboratory, physicists strive to isolate and analyze the fundamental aspects of nature, yet this quest often leads to a deeper inquiry about the nature of noise itself. The distinction between signal and noise blurs, prompting philosophical reflections on existence. “To God all is signal,” mused professor Bart Kosko, yet perhaps it is more accurate to say that, to a higher power, all is noise; only humanity imposes meaning onto the chaos.

George Musser is a writer specializing in physics and cosmology, author of *Spooky Action at a Distance and The Complete Idiot’s Guide to String Theory. He has served as a contributing editor for Nautilus and spent 14 years as a senior editor for Scientific American, winning several awards for science writing.*