It from Bit: Is Reality Made of Information?

In 1989 the physicist John Archibald Wheeler compressed a lifetime of thinking about quantum mechanics into three words: it from bit. The idea is that every physical "it" — a particle, a field, perhaps spacetime itself — ultimately derives its existence from the answers to yes-or-no questions posed by measurement. Information, on this view, is not something we extract from an already-existing world. It is what the world is made of.

Why a physicist would say something so strange

Wheeler arrived here through the quantum measurement problem. Quantum theory does not tell us what a system is before it is measured; it tells us the probabilities of the answers we will get when we ask. His famous delayed-choice thought experiment (later carried out in the laboratory) sharpened the puzzle: whether a photon's history looks "wave-like" or "particle-like" can depend on a measurement choice made after the photon has already passed through the apparatus. Wheeler's reading was that no elementary phenomenon is a phenomenon until it is a registered phenomenon — the past is not fully written until the question is asked.

From slogan to research program

"It from bit" can sound like mysticism, but several precise lines of work give it real content:

• Information is physical. Rolf Landauer showed that erasing a bit of information has an unavoidable thermodynamic cost. Information is not an abstraction floating above physics; it is always carried by, and constrained by, physical stuff.
• Reconstructions of quantum theory. Researchers have derived the full structure of quantum mechanics from simple information-theoretic postulates — for example, that a single elementary system carries a limited amount of information, or that any two states can be continuously and reversibly connected. If the theory follows from rules about information, those rules may be the deeper layer.
• Quantum information science. Entanglement, teleportation and quantum computing treat the quantum state primarily as a resource for processing information — and they work.

What it does not (yet) say

Information is always information about something, and for someone. That is precisely the tension: if bits are fundamental, what are they bits of, and who asks the questions? Wheeler's own answer was a "participatory universe" in which observers are part of how reality comes to be — an answer many physicists find more suggestive than satisfying.

That unresolved tension is exactly where my interest lies. If quantum theory is fundamentally a theory of information, then the role of the observer is not an embarrassment to be explained away — it is a clue about how a definite, shared physical world emerges at all. The next notes follow that clue: through the measurement problem, the observer-centred interpretations it inspired, and the physics of how a classical world crystallizes out of quantum information.

Further reading: J. A. Wheeler, "Information, Physics, Quantum: The Search for Links" (1989); R. Landauer, "Information is Physical," Physics Today (1991).