A recent tweet by Sean Carroll has me thinking.
Quantum decoherence is said to occur when a particular quantum system becomes entangled with its environment, that is to say, as information about the quantum system spreads throughout the environment, that system undergoes at least an apparent wave function collapse. It stops behaving like a wave and more like a particle.
But it’s possible to have two quantum particles in their wave stage interact and become entangled, apparently without decohering. Of course, since they are entangled, a later measurement of one of the particles in the relevant way will give us information, not just on the particle being measured, but also information about the other particle. Once measured, from our point of view, both particles will have decohered, collapsed from a wave to a particle.
Which raises the question, from particle A’s perspective, when A becomes entangled with particle B, wouldn’t B have decohered (at least for whatever properties are entangled)? That is, wouldn’t B’s wave function have collapsed for A? And wouldn’t the same be true from particle B’s perspective in relation to particle A?
All of which is to ask, is entanglement what we see from an interaction we don’t partake in, but decoherence what we see from one that we do? Put another way, can we say that entanglement is decoherence from the outside, while decoherence is entanglement from the inside?
If not, why not? If so, what does this mean for interpretations of quantum mechanics outside of the many worlds and relational ones?
h/t James of Seattle (who replied to Carroll with a question I wouldn’t mind seeing an answer to)