Just how do we get from the quantum world of empty space and invisible energy to the concrete, orderly world that we perceive with our senses? We might now have an answer, and it would make Charles Darwin proud.

From the standpoint of quantum mechanics, the world as we experience it is just an unexplained illusion. Every single atom in the universe is more than 99% empty space, so how do we manage to see and feel the world as something solid and continuous? That might seem to be a pretty basic question, but it has stumped scientists and engineers for more than a century, and only now are we close to a possible answer. Hold on tight - we're diving into some serious quantum weirdness.

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At its most basic, the answer relies on a process called "decoherence", in which the quantum states of subatomic particles transition from the quantum to the classical world. Amazingly, the process is very similar to Darwin's notion of natural selection. Decoherence holds that the vast, vast majority of quantum states are inherently unstable, and they collapse and disperse when they interaction with the environment. A selection process operates in which some of the quantum states manage to arrive at a final, stable state that can survive transmission through the environment without collapsing. These stable forms are known as pointer states.

Once the pointer state is achieved, the low energy quantum state can then create a higher energy duplicate of itself that can be perceived at the macroscopic scales with which we're familiar. What that basically means is the only parts of the quantum world that we ever experience are the subatomic particles that have arrived at their pointer states. To borrow the evolutionary notion of survival of the fittest, the rest of the quantum world is unfit to copy itself to the higher energy states that we perceive.

As with just about anything else in quantum mechanics, this theory is far from proven, but there is some experimental evidence to support these ideas. Admittedly, these experiments are almost incomprehensible to the layperson (I've read through the explanation several times and still don't really understand it, but go here if you want to try to puzzle it out). Still, this represents a huge step toward understanding how we get from the classical world that our senses experience to the quantum world that physics insists must exist.

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[Physics Review Letters]