Temperature - what is it really

In daily use, the concept of "temperature" is identified as the measurable "degree of heat" of a substance. Notably, here already we encounter different temperature scales: From Fahrenheit, Réaumur, Celsius C°, etc., up to the scale expressed in Kelvin, namely the so termed" absolute" temperature. Because of the second Law in thermodynamics, there exists in fact a lowest possible temperature (but seemingly no highest one). In thermodynamics, the concept of a temperature is introduced as a native state variable associated with the change in the amount of heat as a function of a related change in thermodynamic entropy, the latter being the suitable measure of the "disorder" present in a system at thermal (!) equilibrium. In the framework of classical statistical physics, we have the "equipartition theorem", according to which each energy-carrying degree of freedom possesses on the average the same thermal energy. Then, we comfortably can define the temperature in terms of the average kinetic energy of the particles in the system. Matters become already more complicated on the atomic scale, encountering the regime of quantum dynamics:
There, this classical "equipartition theorem" loses its validity. The issue of "heat" becomes now somewhat more complex. For example, does a nanosystem always possess a definite temperature? If so, does the latter fluctuate? In particular, in some quantum systems there may (formally only) appear even "negative temperatures", below absolute zero. Matters become even more obscure when one wishes to describe the influences of the theory of relativity. Knowingly, time and space are then entangled. Even within expert circles, it is then often unclear what the means are for measuring the temperature of a fast moving system. Is a moving body now hotter or colder, or does it maintain the same temperature? This intriguing question has preoccupied Planck and Einstein, Eddington, Landsberg, Van Kampen, and many others: It did spark an ever-continuing controversy over 100 years. Recently, much of this problem could be solved satisfactorily, however with the investigations done in my work group [1, 2, and 3].
"Thermodynamics and Statistical physics of Small Systems":
http://www.physik.uni-augsburg.de/theo1/hanggi/Symposium/symp.html
J. Dunkel and P. Hanggi, Relativistic Brownian motion, Phys. Rep. 471:
1-73 (2009).
J. Dunkel, P. Hanggi, and S. Weber, Nonlocal observables and lightcone-averaging in relativistic thermodynamics, Nature Physics 5,
741-747 (2009).

Category:

Education

Tags:

• physics
• thermodynamics
• science
• temperature
• Maxwell's deamon
• second Law