Wednesday, January 13, 2010

Tipping Time of a Quantum Pencil

I ran across this article in Eur. J. of Phys. and it reminded me of several other articles that I've read on this very topic. This is, of course, a rather familiar problem to many physics students. It involves the a pencil balanced vertically on its tip. So classically, it is in an unstable equilibrium. The problem is to use quantum mechanics, or the Heisenberg Uncertainty principle in particular, to find the tipping time for the pencil. The application of the HUP invokes the fact that the exact position of the top of the pencil can have a natural fluctuation that will tip it off the vertical axis.

The latest paper that I'm aware of on this topic deals with a very detailed calculation of calculating the tipping time of a quantum rod[1]. In this calculation, the author showed that the classical problem can be recovered when the Planck constant goes to zero, and draws the conclusion that:

.. the tipping of the quantum rod can be understood as having been triggered by the uncertainty in angular momentum engendered by localization of the initial state...

The article is a bit difficult to follow, and I didn't get any direct value of the tipping time.

The more interesting papers that I've found earlier on the same topic are much more illuminating than this one. A paper by Don Easton presents a caution for people who tries to apply QM as the basis of the tipping time[2]. His calculation of the tipping time, using QM, gives a humongous number: 0.6 million years. He examined why some posted solutions actually gave a balancing time of the order of 3 seconds, and why those treatment may be faulty.

Another paper that cautioned the use of the HUP in calculating the tipping time is a paper by Shegelski et al.[3] Here, they caution that one can't just use the HUP alone, and they also compared this to the faulty application of the WKB approximation to this problem.

Fascinating! Certainly something that I read in bed before going to sleep! :)


[1] O. Parrikar, Eur. J. Phys. v.31, p.317 (2010). You can also get a free copy of the paper within the first 30 days of online publication at this link.
[2] D. Easton, Eur. J. Phys. v.28, p.1097 (2007).
[3] M.r.A. Shegelski et al., Am. J. Phys. v.73, p.686 (2005).


Kenny said...

Thanks for this highly informative post! I will be citing this blog as a read for my students.

Anonymous said...

trying to present this problem.
I am totally at sea about which solution i should follow