quantum solution to the arrow of time dilemma?

physorg |  Entropy can decrease, according to a new proposal - but the process would destroy any evidence of its existence, and erase any memory an observer might have of it. It sounds like the plot to a weird sci-fi movie, but the idea has recently been suggested by theoretical physicist Lorenzo Maccone, currently a visiting scientist at MIT, in an attempt to solve a longstanding paradox in physics.

 

The laws of physics, which describe everything from electricity to moving objects to energy conservation, are time-invariant. That is, the laws still hold if time is reversed. However, this time reversal symmetry is in direct contrast with everyday phenomena, where it’s obvious that time moves forward and not backward. For example, when milk is spilt, it can’t flow back up into the glass, and when pots are broken, their pieces can’t shatter back together. This irreversibility is formalized through the second law of thermodynamics, which says that entropy always increases or stays the same, but never decreases.

This contrast has created a reversibility paradox, also called Loschmidt’s paradox, which scientists have been trying to understand since Johann Loschmidt began considering the problem in 1876. Scientists have proposed many solutions to the conundrum, from trying to embed irreversibility in physical laws to postulating low-entropy initial states.

Maccone’s idea, published in a recent issue of Physical Review Letters, is a completely new approach to the paradox, based on the assumption that quantum mechanics is valid at all scales. He theoretically shows that entropy can both increase and decrease, but that it must always increase for phenomena that leave a trail of information behind. Entropy can decrease for certain phenomena (when correlated with an observer), but these phenomena won’t leave any information of their having happened. For these situations, it’s like the phenomena never happened at all, since they leave no evidence. As Maccone explains, the second law of thermodynamics is then reduced to a mere tautology: physics cannot study processes where entropy has decreased, due to a complete absence of information. The solution allows for time-reversible phenomena to exist (in agreement with the laws of physics), but not be observable (in agreement with the second law of thermodynamics).

In his study, Maccone presents two thought experiments to illustrate this idea, followed by an analytical derivation. He describes two situations where entropy decreases and all records of it are permanently erased. In both scenarios, the entropy in the systems first increases and then decreases, but the decrease is accompanied by an erasure of any memory of its occurrence. The key to entropy decrease in the first place is a correlation between the observer and the phenomenon in question. As Maccone explains, when an interaction occurs between an observer and an observed phenomenon that decreases the entropy of the correlated observer-observed system, the interaction must also reduce their quantum mutual information. When this information is destroyed, the observer’s memory is destroyed along with it.