Here's a (perhap unfairly harsh) rebuttal (warning: stray into their other blogposts at your own peril). To summarize, Motl's critique is that the experiment doesn't really show anything that...
Exemplary
Here's a (perhap unfairly harsh) rebuttal (warning: stray into their other blogposts at your own peril). To summarize, Motl's critique is that the experiment doesn't really show anything that can't be explained by ordinary quantum mechanics (I'm not familiar enough with the experiment to verify Motl's claim).
However, that's not really the point I'd like to make. I want to highlight the following apocryphal story attributed to Wittgenstein (roughly transcribed from this talk).
A friend sees Ludwig Wittgenstein on a street corner, lost in thought, and says "What's bothering you, Ludwig?"
"I was just wondering why people said it was natural to believe the sun went around the center of the earth rather than the other way around."
"Well, that's because it looks like the sun goes around the earth."
Wittgenstein thinks for a moment, then responds: "Tell me, what would it have looked like if it'd been the other way around?"
There is a natural tendency to assume that the classical realm is real, and the quantum realm only applies at small distances. But that's silly: in terms of sheer predictive power, quantum mechanics is actually a much better theory than classical mechanics. And according to quantum mechanics, when quantum objects interact with other quantum objects, they don't become less entangled -- they become more entangled!
So how do we explain the seemingly classical world we live in? Well, one proposition is Many Worlds. Those carefully prepared quantum states quickly decohere with the environment, entangling the lab with the experiment (this is true regardless of interpretation). In Many Worlds, if one could somehow "see" outside the environment (ie, outside the universe), there would be multiple "worlds", all in superposition; in other interpretations of QM (but not every other), the wave function somehow "collapses" instead (quantum darwinism is in the latter category).
(Perhaps surprisingly, Many Worlds actually makes the fewest assumptions: there is the wave function, and that's it; the existence of multiple worlds is just a consequence. Collapse-based interpretations need to tack on extra machinery to explain the collapse.)
To be clear, I don't mean to denigrate the authors of the paper for their work. Frankly, I support any amount of work on the foundations of quantum mechanics (in my mind, the ontology of quantum mechanics is one the Big Questions). But ultimately, when we want to understand what's truly fundamental, it's either turtles all the way down -- particle and subparticles and subsubparticles -- or it's information. Embracing the classical world won't help us here, but embracing the quantum world might.
The experiments are, unfortunately, not likely to offer many insight or opportunities for surprise; the result can be predicted with very high confidence long in advance. QD is roughly about demonstrating the internal (theoretical) compatibility of unitary quantum dynamics with the appearance of classicality (and, in particular, wavefunction collapse). Since classicality/collapse are prerequisites for performing laboratory measurements in the first place, such measurements can’t really tell you anything new.
The best argument for performing these experiments probably is that they are good exercise; directly illustrating QD can require some very difficult measurements that will push the boundaries of existing laboratory techniques. A counterargument is that, conditional on not attempting to make measurements with uncertain outcomes (say, because searching for new fundamental physics is just so hard/expensive), the proper exercise for pushing the boundaries of experimental ability is best devised by the experimentalists themselves, and that illustrating intrinsically theoretical work is mostly theatre. I see merit in both arguments.
So these experiments don’t teach us anything new about QD, nor do they eliminate any possible ways that QD might be wrong. QD is most like an internal self-consistency check on quantum theory, vaguely like how a proof of Yang–Mills existence and mass gap would give some mathematical justification for why the currently unrigorous QFT techniques are so accurate and useful for predicting the results of experiments.
And what does that mean? If the dust grain is imprinting the position information on 100 million photons, what is the mechanism for this and why don't those photons quickly decohere from the...
It’s because of this redundancy that objective, classical-like properties exist at all. Ten observers can each measure the position of a dust grain and find that it’s in the same location, because each can access a distinct replica of the information. In this view, we can assign an objective “position” to the speck not because it “has” such a position (whatever that means) but because its position state can imprint many identical replicas in the environment, so that different observers can reach a consensus.
And what does that mean? If the dust grain is imprinting the position information on 100 million photons, what is the mechanism for this and why don't those photons quickly decohere from the information? They're just photons, after all.
Reading further, this is what the experiments mentioned in the article are attempting to pin down.
Here's a (perhap unfairly harsh) rebuttal (warning: stray into their other blogposts at your own peril). To summarize, Motl's critique is that the experiment doesn't really show anything that can't be explained by ordinary quantum mechanics (I'm not familiar enough with the experiment to verify Motl's claim).
However, that's not really the point I'd like to make. I want to highlight the following apocryphal story attributed to Wittgenstein (roughly transcribed from this talk).
There is a natural tendency to assume that the classical realm is real, and the quantum realm only applies at small distances. But that's silly: in terms of sheer predictive power, quantum mechanics is actually a much better theory than classical mechanics. And according to quantum mechanics, when quantum objects interact with other quantum objects, they don't become less entangled -- they become more entangled!
So how do we explain the seemingly classical world we live in? Well, one proposition is Many Worlds. Those carefully prepared quantum states quickly decohere with the environment, entangling the lab with the experiment (this is true regardless of interpretation). In Many Worlds, if one could somehow "see" outside the environment (ie, outside the universe), there would be multiple "worlds", all in superposition; in other interpretations of QM (but not every other), the wave function somehow "collapses" instead (quantum darwinism is in the latter category).
(Perhaps surprisingly, Many Worlds actually makes the fewest assumptions: there is the wave function, and that's it; the existence of multiple worlds is just a consequence. Collapse-based interpretations need to tack on extra machinery to explain the collapse.)
To be clear, I don't mean to denigrate the authors of the paper for their work. Frankly, I support any amount of work on the foundations of quantum mechanics (in my mind, the ontology of quantum mechanics is one the Big Questions). But ultimately, when we want to understand what's truly fundamental, it's either turtles all the way down -- particle and subparticles and subsubparticles -- or it's information. Embracing the classical world won't help us here, but embracing the quantum world might.
Perhaps a fairer view: FAQ about experimental Quantum Darwinism from Jess Riedel, one of the proponents of the idea cited in the article:
And what does that mean? If the dust grain is imprinting the position information on 100 million photons, what is the mechanism for this and why don't those photons quickly decohere from the information? They're just photons, after all.
Reading further, this is what the experiments mentioned in the article are attempting to pin down.