Summary of research
The everyday world is one of macroscopic variables. We talk about what the inner temperature of a perfect steak should be, the minimum pressure of a diving tank at which we should return to the surface, and how to optimally furnish a new apart- ment according to the available floor area. These macroscopic variables describe the gross state of the world; a coarse-grained interpretation of the universe. We never discuss what the exact configuration (position, velocity, interactions, etc.) of molecules that make up perfectly cooked steak should be. This exact configu- ration is called the microstate of the perfect steak. If we had a specific microstate that corresponds to a perfect steak, and then swapped around a few molecules we would get another, different, microstate. However, these two microstates would most certainly taste the same. The fact that we would not be able to distinguish these microstates by taste, is the reason why we care more about macroscopic states in an everyday description of the world. Macroscopic variables are those we can reliably measure and use to distinguish different systems from each other.
The macroscopic laws of thermodynamics were largely developed during the 18th and 19th centuries. These laws describe how macroscopic variables like temperature, pressure, and volume behave with respect to each other. As the foun- dation of thermodynamics, lie the four laws of thermodynamics, which describe how heat, energy, and entropy behave under various circumstances. Many famous statements, which even non-physicists are familiar with, come from these laws: "energy can never be created nor destroyed, only change form", "it’s impossible to cool a system to absolute zero", and "perpetual motion machines can not be created".
However, these laws were first postulated at a time where we did not know that the world was built up of elementary particles like atoms and electrons. They were formulated using macroscopic variables, which are just coarse-graining of the underlying microscopic variables. With the rise of statistical and quantum mechanics and a massive improvement in technological capabilities, we began to be able to detect and measure microstates directly. The natural question that arose was; How do we explain the empirically observed laws of thermodynamics, from the underlying microscopic behavior? In some cases, this was not too difficult. For example, the first law of thermodynamics, the conservation of energy, is deeply connected to time translation symmetry via Noether’s theorem. In other cases, it was not so straightforward. The second law implies an arrow of time in physics, but how can the reversible microscopic dynamics of particles lead to irreversible macroscopic phenomena? One particular paradox which this thesis focuses on is Maxwell’s demon. The resolution of this paradox revealed a deep connection be- tween information and the laws of physics. This has had a large effect on physics, to such a degree that some researchers consider information to be the most fun- damental constituent of the universe, rather than quarks or strings. The bulk of my research is based on the relationship between information and thermo- dynamics, and how to optimize these information processing systems.