The conceptual breakthrough initiated by Heisenberg (who was mentored by Niels Bohr), and firmed up with contributions from Max Born, Wolfgang Pauli, Paul Dirac, Erwin Schrdinger and others, makes it clear that the world of the very small that of photons, electrons, atoms and molecules obeys rules that go against the grain of our everyday physical reality.

Take an electron that is emitted at Point A and is detected at Point B. One would assume that the electron follows a trajectory, the way a baseball does from a pitchers hand to a catchers mitt. To explain experimental observations, Heisenberg rejected the notion of a trajectory for the electron. The resulting quantum theory deals in probabilities. It lets you calculate the probability of finding the electron at Point B. It says nothing of the path the electron takes. In its most austere form, quantum theory even denies any reality to the electron until it is detected (leading some to posit that a conscious observer somehow creates reality).

Since the 1950s, scientists have tried to make quantum theory conform to the dictates of classical physics, including arguing for a hidden reality in which the electron does have a trajectory, or suggesting that the electron takes every possible path, but these paths are manifest in different worlds. Rovelli dismisses these attempts. The cost of these approaches is to postulate a world full of invisible things.

Instead, in Helgoland Rovelli explains his relational interpretation, in which an electron, say, has properties only when it interacts with something else. When its not interacting, the electron is devoid of physical properties: no position, no velocity, no trajectory. Even more radical is Rovellis claim that the electrons properties are real only for the object its interacting with and not for other objects. The world fractures into a play of points of view that do not admit of a univocal, global vision, Rovelli writes. Or, as he puts it, Facts are relative. Its a dramatic denunciation of physics as a discipline that provides an objective, third-person description of reality.

This perspective blurs the distinction between mental and physical phenomena. Both are products of interactions between parts of the physical world, Rovelli says. In arguing that the mind is itself the outcome of a complex web of interactions, Rovelli takes on dualists who distinguish between the mental and the physical and nave materialists who say that everything begins with particles of matter with well-defined properties.

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In Quantum Physics, Everything Is Relative - The New York Times