I am aware and well read on the decoherence approach to understanding how conglomerations of micro quantum systems will tend to lose quantum coherence via interaction with the environment. The cross terms in the density matrix for the system will tend to zero (due to the partial trace operation), leaving us with a (diagonal) mixed state, representing a classical form of uncertainty rather than a quantum form of uncertainty (we don't know which state the system is in, but it is definitely in a state). My reading on decoherence has largely been Zurek's papers. (for instance http://arxiv.org/abs/quant-ph/0306072). At a conference recently, however, somebody mentioned to me the "thermodynamic argument" for why macroscopic quantum states can't exist. I have a vague sense of what he meant, based on thermodynamics and probablility, but I am not clear on the exact argument or its basis. Is it different from decoherence theory, or essentially the same? What is the thermodynamical reasoning for why MQS states are not viable?