Applied Physics: Don’t Try This At Home Dept.

Via Synthstuff, this:

Seeing this reminds me of a passage from Jeremy Bernstein’s now sadly out-of-print lovely little book Quantum Profiles. In the first of the three scientist encounters, Bernstein talks to the now-legendary Irish physicist, John Stewart Bell.  Bell is most famous for Bell’s Theorem, which provides one of the clearest and most powerful theoretical bases for demonstrating the physical reality of the counter-intuitive claims of quantum mechanics.*

In the conversations Bernstein reported, the two men did speak about that work.  But Bell also thought about the gap between abstract knowledge and the rich messiness of the real world.  He talked about bicycles.

I’m reporting from memory here, and distant recollection at that, but the essence of what Bell told Bernstein was that bicycles are completely comprehensible machines.  They are wholly Newtonian — classically mechanical devices, whose behavior can be expressed in purely deterministic mathematics, with none of the uncertainty that so bedeviled Einstein’s view of quantum mechanics.

And yet, actually modelling the motion of a bicycle in all its detail, Bell said, was so difficult that it was practically impossible to predict the actual motion of a real bicycle coaxed into action by a live human being.  There was just too much — every moving part, the flux of forces produced by a moving body pumping legs up and down, swaying and twisting and generally getting the bike to do much less amazing stuff than you can see in the video above.  The best one could hope for, Bell told Bernstein, was a mathematical description of a riderless bicycle.

Bernstein’s book is a dozen iterations or more of Moore’s law old at this point, so the quantitative limitations Bell was pointing out in his call for a bit of physicist’s modesty impinge a lot less than they did then.

But what Bell meant is still true, or so at least it seems to me, treading a bit out onto a much deeper philosophical argument than I want to engage right now.  Mechanics wholly constrains what our bicycle virtuoso shows us in the video above.  It does not, however, capture all the properties of the experience that the bicycle artist or his audience enjoy.

*In slightly more detail:  In the famous Einstein Podolsky Rosen (EPR) paper, the three authors complained that “no reasonable definition of physical reality could be expected to permit” that measurements on one part of a quantum system could determine (predict) the results of a measurement on another, distant part of the same system.  Bell’s Theorem provides the theoretical apparatus to demonstrate that reasonable or not, quantum mechanics does violate at least one of the assumptions that Einstein and his colleagues posited as “reasonable.”  Experiments to test whether Bell’s argument actually corresponds to the world we live in have consistently shown that quantum mechanics is consistent and does describe events in the world correctly, and that Einstein’s loathing of quantum mechanics, though heartfelt, turned on fundamental beliefs about the aesthetically necessary construction of reality that turn out not to be true.

Image:  Man riding the bicycle in the centre of Kraków, Poland; author: Nova; date: 30.04.2005; permission: GFDL

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