## Where did the weirdness go?

Posted by Arjendu on May 5, 2008

Nature just featured an excellent article on “one of the great conundrums of modern physics: the quantum–classical transition” by Philip Ball, at the semi-popular level, talking about the mystery of where and how weird quantum mechanical effects go — that is, why we know they exist, but don’t see them in daily life.

Most of my work is concerned with some aspects of this, and when I try to explain this to my research students, I talk about it as follows: If atoms are quantal, and we are made of atoms, why don’t we behave quantum mechanically? And if it’s a matter of size or complexity (nonlinearity of the system concerned) or temperature and influence of the environment on the system (as is believed) then how and when does the change from quantum mechanics to classical mechanics happen as a function of these properties? Is the change smooth or abrupt — that is, do we go from very quantal to somewhat quantal (and what does that look like?) to classical, or does it go from quantum-classical immediately? Is the transition monotonic — that is, do we only go from quantum to less quantum as we change parameters in one direction, or do you have regions of more quantum-ness and less quantum-ness? How does the quantum dynamics reflect behavior in the classical dynamics? Etc. (Some more discussion of these issues is on my research web-page.)

These are entirely fascinating questions as fundamental physics, but quantum effects are not only cool, they are impressively powerful, and very useful sometimes, so the practical question is: where can we find them?

As a theorist, I wonder about right measure of quantum-ness: How quantum is a given state? How do you measure the difference between a classical distribution and a quantum distribution? I’d like to be able to discuss all this in some sort of abstract way so I can understand the topology, the geography, really, of the quantum-classical boundary. And of course, if I do find an effect, how do I translate this into something an experimentalist might measure?

As the school year heads into the home stretch, I’m getting excited again about getting some uninterrupted (well, relatively uninterrupted, let’s be honest) time to make some progress on these questions again. It’s been a long and complicated year, and I’m looking forward to the comfort and joy (and pain, yes, and pain) of grappling with some of these intriguing questions with more focus soon.

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