Internal Convection

It sounds amazing but even in ultracold gases, there is heat. And just as ordinary thermodynamics predicts, even ultracold heat flows from hot to cold. Nonetheless, it is the mechanism by which it does so which is truly astonishing. 

When bosonic gases are cooled to very low temperatures, most of the atoms gather in the lowest energy state and build the so called Bose-Einstein condensate. Whereas all particles not part of the condensate behave like we expect from any gas - they flow towards colder regions, it is the condensate behavior which surprises: As a flux of absolute cold it is attracted by heat and flows towards regions of higher temperature. 

A similar effect is known from superfluids and named internal convection. In our recent publication we theoretically investigate internal convection in ultracold atomic vapors and suggest that it might provide the opportunity to experimentally study heat transport on the mesoscopic scale. 

Canonical Transformations and the Theory of Everything

Everything is described by the laws of physics - and if this is not so, we have to change the laws of physics. That is the fundamental approach of physical research and as the final goal, physicists want to describe not less than everything. In practice this always comes down to finding the right set of equations to be applied in the right mathematical formalism. Concerning different set of equations, the universe seems to be pretty creative: Mechanics is governed by ordinary differential equations, Electromagnetism by partial differential equations and Quantum Mechanics even uses operator-equations in infinitely dimensional Hilbert-spaces. Things look differently when it comes to the right formalism: In fact, there is actually not much more than Hamiltonian (Quantum-)Mechanics. You may ask - so what? - if physicists search for a theory of everything - and for everything there is a Hamiltonian, then they are on a good way. Well kind of. Indeed, most if not all of current physical research is looking for the right Hamiltonian, and of course clever methods to solve it. There is a Hamiltonian for the Standard-Model, and people are working hard to extend it in order to treat Supersymmetry, Quantum Gravity, String Theory etc. etc. What puzzles me is now the following: In principal you can always transform your set of coordinates, e.g. it should not matter if you define the origin of your coordinate system to be here on my desk or at any other point in the universe. In general, besides choosing the point of origin, there are many other, more complicated and sometimes even time-dependent changes of coordinates, and within Hamiltonian Mechanics they are all equivalent and called canonical transformations. Among all possible canonical transformations there is always a really special one, namely the transformation to the solution of the Hamiltonian equations of motion. In this very special set of coordinates, the Hamiltonian is always equal to zero! Surprised? You sure should be. I just told you that many physicists are looking for the right Hamiltonian description of the universe and here is the solution: It is zero! - So all the work for nothing? - Well, certainly not, there still remains the question of the Canonical transformation - and about that, I honestly do not know anything. But my real point is: Somehow we all know that there is one preferred set of coordinates, namely space and time. We are free to move in space and we are forced to move forward in time. This very subjective experience contradicts Hamiltonian Mechanics  where any choice of coordinates is equivalent. Consequently, I would guess that in order to explain the true foundations of the world surrounding us we have to go beyond Hamiltonian theory. In what sense - I really do not have any idea - but I would be very happy to hear from anyone who has some!

Superfluidity

When some fluids (for example liquid helium) are cooled to very low temperatures, strange effects occur: Suddenly, the fluid can move without friction, can pass through capillaries so tiny that no other fluid is able to run through, or it can even leak out of a half-filled bucket. Because of these superb properties, such fluids are called superfluids.

Superfluidity was first discovered by Kapitza and Allen and Misener in 1938. For explaining the quantum nature of this phenomenon, Lev Landau was awarded the Nobel Price in 1962. 

When browsing the web I found the following series of videos (produced in the 1960s). They very nicely show the truly amazing properties of superfluid Helium. So watch and be ...flabbergasted... (always wanted to use this word :-)

Second Law of Thermodynamics - violated?!

source: www.podpocalypse.com

The second law of thermodynamics is one of the fundamental axioms of physics - and it prevents very strange things from happening: The second law of thermodynamics assures that a broken glass will never reassemble and jump back onto the table it fell from - or like in the picture next to this post: A burst balloon will never blow itself up again.
So what is special about it? - It breaks time-reversal symmetry. This is surprising, because any underlying theory, like (quantum-)mechanics does not care about the direction of time. Every process can in principal run backwards and no one would notice. Finally, because every thermodynamic system underlies the laws of (quantum-)mechanic, there is no obvious reason why there should be such a thing as a definite arrow of time.
Nonetheless, time obviously has a direction - we all get older and nobody would seriously doubt that. Consequently the second law of thermodynamics is a solid fact - or is it?
There is actually a (quite prominent) paper which states that they observed a violation of the second law of thermodynamics. They achieved this by going back to very small system sizes. Here, single scattering events between different particles are directly observable and on the timescale of such collisions, strange things may happen - like eventually the violation of thermodynamics second law. It is like gambling: On the time scale of a few games, you might actually win, but over a whole evening of gambling, you will always lose!
All in all, we see that thermodynamics is a theory, which predicts the long time behavior of large systems. On short time scales - which are not at all relevant to our every day life - thermodynamics may sometimes fail.
An excellent review on this topic - which was partly an inspiration to this post has been published in nature.

Getting Started 2.0

Besides explaining physical research to a broad audience, this blog serves a second purpose: Highlighting scientific contributions I personally find fascinating and helpful.
With more and more people writing scientific papers, it has become complicated, almost impossible, to keep a complete overview on the ever-growing jungle of new scientific insights. Therefore, in this blog I want to share my personal (indeed very subjective) selection and view of current scientific publications. I explicitly want to encourage you, as the reader of this blog, to leave comments and express your viewpoint on the topics at hand.

Getting Started 1.0

"Theoretical Physics" - "Aww...", "Quantum Thermodynamics" - "even worse". These are just normal reaction of non-scientists (and even some scientists) to my everyday work as a theoretical physicist.
Thereby, I am convinced that the work on the foundations of physics is essential to our everyday understanding of the world surrounding us. Unfortunately, very many people are frightened by the complicated theoretical concepts involved. Nonetheless, in the end very often stands a simple result - which can explained in a couple of simple sentences.
And exactly that is the mission of this blog: Explaining physical research and by that, lifting the fog and reveal new insights and view points.