'You really believe the Moon is not there ...?'

 

The Moon and Measurement: Einstein's Question Revisited

“Do you really believe the Moon is not there when you are not looking at it?”

Einstein’s famous quip was no mere rhetorical flourish. It was a technical objection to the implications of quantum mechanics, directed at the Copenhagen view that unmeasured observables possess no definite values. He was objecting not just to philosophical idealism, but to the notion that physical entities as massive and permanent as the Moon could, in any serious sense, lack a determinate position until observed. For Einstein, such an idea was a reductio ad absurdum of quantum orthodoxy.

The technical heart of his concern lies in the quantum treatment of position and momentum. Quantum theory does not assign definite values to these quantities simultaneously. The best one can obtain is a wavefunction or density matrix encoding a probabilistic distribution, constrained by the uncertainty principle. So what, then, is the Moon's quantum state when no one is measuring it?

To sharpen the issue, let us consider a thought experiment: imagine a Moon entirely isolated from its environment — no light, no gravity gradients, no cosmic radiation, no air molecules. A true quantum island. Suppose we measure its position very precisely at time t = 0, localising its wavefunction into a very narrow peak in the position basis. We have collapsed it into something close to a position eigenstate.

From this point forward, if the Moon is truly isolated, it evolves according to the unitary Schrödinger equation. But a position eigenstate is not a stationary state of the free Hamiltonian — it contains a wide spread of momenta. The result is that the wavefunction begins to spread over time. The Moon’s centre-of-mass position becomes increasingly uncertain as its wavefunction expands. This is not unique to the Moon — it is observed in experiments with electrons, atoms, and even large molecules like buckyballs in quantum interference setups. It is the standard behaviour of a delocalised quantum object.

If we now wait long enough (in practice way longer than the age of the universe for an object the Moon's size) — again, ignoring all interactions — and perform a second position measurement, quantum mechanics says we could in principle find the Moon almost anywhere compatible with its initial momentum spread. Perhaps on the far side of the Earth from where it was first observed. This is not classical orbital motion: this is pure quantum uncertainty in the absence of localisation, an indication that like bound electrons, in this scenario the moon does not really orbit classically. In effect, the Moon's wave function jumps on observation (to a new positional eigenstate).

Repeated measurements could reveal positions all around its orbital path, disconnected from any classical trajectory. It is absurd, and yet entirely within the predictive structure of quantum theory — if the Moon is isolated and we would wait long enough.

But of course, it never is. The Moon is bathed in photons from the Sun, bombarded by particles from cosmic rays, and continuously interacting with the Earth’s gravitational field. These environmental interactions entangle the Moon’s quantum state with the rest of the universe. This is decoherence.

Decoherence is the process by which the off-diagonal elements of the Moon’s reduced density matrix — representing quantum superpositions between macroscopically distinct positions — decay rapidly. The key result from decoherence theory is that such superpositions do not persist for large systems. The Moon’s enormous mass and surface area make it highly susceptible to environmental measurement. Even photons from the cosmic microwave background — with energy on the order of microelectronvolts — suffice to localise its position in femtoseconds.

If you model the Moon as a sphere of radius 1,700 km exposed to the 2.73 K CMB, you can estimate that over 10³⁰ photons strike it every second. Even if only a minuscule fraction scatter coherently, the decoherence timescale for a 1 cm position superposition is vanishingly small: 10^–20 seconds or less. And that is the most conservative estimate, not including solar photons, infrared thermal emission, and gravitational interaction with the Earth. The Moon is, in quantum terms, being continuously measured by the universe.

This constant decoherence dynamically selects a preferred basis — the so-called pointer states — which are robust under environmental monitoring. These states are highly localised in both position and momentum: quasi-classical states. The result is that the Moon appears, and indeed behaves, as though it always has a definite position and trajectory. Decoherence does not require human observers, nor does it invoke collapse. It merely shows that the rest of the universe acts as a measuring apparatus.

Einstein’s rhetorical question still stands, but it has a modern answer. Yes, the Moon is “there” when we are not looking — not because quantum mechanics gives it a determinate position by fiat, but because the environment ensures its continual localisation. The Moon does not jump, because the cosmos is watching.