I was writing my thesis, coupling phonons to quantum
tunneling, just when Leggett and Caldeira came out with their
``macroscopic quantum tunneling'' ideas. They coupled harmonic
oscillators (phonons) to quantum tunneling, but made the coupling to
low--frequency phonons much larger than I could get from my microscopic
models. By doing so, their model developed lots of bizarre behavior:
later work showed that as the coupling was increased, there was a phase
transition at which the tunneling stopped (the environment ``collapsed
the wavefunction'', keeping the particle in one of the two potential
wells). Around this time, Mark Stiles was writing his thesis on helium
scattering off surfaces, and I was pestering him about his phonon
coupling: his helium atoms coupled just like Leggett's heat bath, which
seemed obviously unphysical and likely to lead to weird answers...
Finally a couple of years ago, in a conversation with Risto Nieminen
in Helsinki, I finally understood: atoms hitting from above could
transfer a net momentum (force monopole), where my atoms tunneling inside
the material could not (force dipole). This implied
that an atom tunneling to and fro from a scanning tunneling microscope
(STM) tip would be a microscopic system where phonons would give Tony's
weird effects. Ard Louis transformed this idea into a proposal for an
experiment, which although ambitious (a low--temperature AFM/STM
connected via strip lines to a picosecond laser) should be able to see
the phase transition in the quantum coherence predicted by theory.
Schrödinger's cat now does not need to couple to a metal to have its
wavefunction collapsed: it can just brush up against the surface of the box.
James P. Sethna, email@example.com
Statistical Mechanics: Entropy, Order Parameters, and Complexity, now available at Oxford University Press (USA, Europe).