Zurek and his colleague Jess Riedel have been able to calculate how fast and extensive this proliferation of quantum copies is for a few simple situations, such as a dust speck in a vacuum flooded by sunlight. They find that, after being illuminated for just one microsecond, a grain of dust a micrometre across will have its location imprinted about 100 million times in the scattered photons.
It’s because of this multiple imprinting that such objects seem to have objective, classical-like properties at all. Ten observers, say, can separately measure the position of a dust grain and all agree that it’s in the same location. Each observation consumes a different replica of the grain in the reflected photons. In this view, we can assign an objective position to the speck not because it truly ‘has’ such a position (whatever that means), but because its position state can imprint many indistinguishable replicas in the environment. What we take as obvious common sense turns out to have secure yet far-from-obvious underpinning in quantum theory.
There’s a seemingly bizarre corollary to this picture. When we measure a property of a system by probing its replica in the environment, we destroy that replica. Might we then potentially use up all the copies by repeated measurement, so that the state can’t any longer be observed? Yes we can: too much measurement will ultimately make the state seem to vanish.
What Quantum Darwinism tell us is that, fundamentally, the issue is not really about whether probing physically disturbs what is probed (although that can happen). It is the gathering of information that alters the picture. Through decoherence, the Universe retains selected highlights of the quantum world, and those highlights have exactly the features that we have learnt to expect from the classical world. We come along and sweep up that information – and in the process we destroy it, one copy at a time.
