Reality Without Observers? Why Quantum Theory Still Needs a Witness

Lead: Vlatko Vedral's Portals to a New Reality challenges the long-standing Copenhagen interpretation by arguing that quantum physics does not require observers — that measurement is merely 'stuff interacting with stuff.' This article revisits that claim, clarifies what Copenhagen actually says, and explains why attempts to banish observers tend to reintroduce them.

What Copenhagen Actually Says

The Copenhagen view, most closely associated with Niels Bohr, does not insist that human minds magically create reality. Rather, it emphasizes that quantum theory provides no unique, observer-independent way to carve the world into separately existing objects. When a measuring device and a system interact they become entangled; because Planck's constant is finite, there is no unambiguous, objective decomposition of that entangled whole into a distinct 'particle' and a distinct 'apparatus.'

Put simply: interactions come first, and any division into objects is a choice imposed on those interactions. That choice determines what counts as a definite measurement outcome. Bohr warned that quantum theory excludes, in principle, the sharp separation between the behavior of objects and the means of observation that classical physics assumes.

Many-Worlds, Decoherence, and the Observer Problem

Proponents of Many-Worlds, including Vedral in spirit, argue that measurements are ordinary quantum processes: a system interacts with an apparatus, the universal wavefunction evolves unitarily, and all outcomes occur in different branches. Critics reply that this picture can't produce the single, definite outcomes we actually observe without invoking a partition between system and apparatus.

Decoherence is often offered as the mechanism that makes branches look classical, since phase relations leak into the environment and off-diagonal terms in the reduced density matrix become negligible. But decoherence presumes a split between system and environment. That split — what to treat as the environment and what to track — functions like an observer's choice: if we decide to monitor the environment, the decoherence effect disappears. Thus decoherence alone does not eliminate the need for a criterion that singles out outcomes.

Where Observers Reappear

Attempts to remove observers entirely tend to smuggle them back in via the partitions, definitions, or practical choices required to interpret the theory. Hugh Everett's original relative-state formulation spoke about observers branching into different states of having observed outcomes, which highlights that a notion of 'observer' — broadly construed as a chosen partition and record-making process — is implicit even in Many-Worlds accounts.

Moreover, a sole universal wavefunction evolving in Hilbert space is an abstract mathematical object. Without a rule for associating parts of that object with the things and agents we see and use in experiments, you lose empirical contact: no particles, cups, cats, or physicists appear in the theory unless you provide a way to identify them.

Bohr, Vedral, and What This Means Practically

Vedral's research program seeks to push quantum effects to larger, more macroscopic systems and to treat observers themselves quantum mechanically. That is a legitimate and exciting scientific agenda — but it does not by itself remove the need to specify how to partition an interaction into system and apparatus if we want definite experimental outcomes.

Bohr never denied that macroscopic apparatus and observers can be described quantum mechanically. His point was epistemological and methodological: given quantum theory, the classical categories we use to describe experiments are not delivered by the formalism, so we must impose them contextually if we are to obtain meaningful results.

QBism: Agents and Decision-Making

One alternative that takes the role of observers seriously is QBism (Quantum Bayesianism). QBism recasts quantum states as tools for agents' expectations, rooting probabilities in personalist Bayesian decision theory and emphasizing agents' active role in generating outcomes. This approach keeps the observer central but reframes that centrality in pragmatic, decision-theoretic terms rather than mystical language.

Conclusion

No interpretation has settled every question. Many-Worlds offers a bold, unitary picture but struggles to explain why observers see single outcomes without invoking partitions. Copenhagen emphasizes the contextual, choice-like nature of defining systems and apparatuses. QBism places agents and choices at the heart of the theory. Vedral's push to extend quantum mechanics to ever-larger systems is important, but it must grapple with the persistent need for partitions or criteria that perform the role we associate with 'observers.'

"The challenge is not to eliminate observers but to make explicit the pragmatic acts by which we carve experience out of interaction."

Originally published at Nautilus. Image credits: Tasnuva Elahi; Natalya Kosarevich, cybermagician, and zombiu26 / Shutterstock.