If you've visited this blog before, you've probably gathered that my two favorite areas of physics are particle/high energy physics and applications of physics to sports. Another area of physics, which I'd like to discuss today, is condensed matter physics, which, according to this Wikipedia summary:
...deals with the macroscopic physical properties of matter. In particular, it is concerned with the "condensed" phases that appear whenever the number of constituents in a system is extremely large and the interactions between the constituents are strong. The most familiar examples of condensed phases are solids and liquids, which arise from the bonding and electromagnetic force between atoms.
As also discussed in the Wikipedia document, the term solid state physics has often been used, but the term "condensed matter" is more general, taking in both solids and fluids. One additional fact from the same source is that, "Condensed matter physics is by far the largest field of contemporary physics... By one estimate, one third of all American physicists identify themselves as condensed matter physicists."
The late, legendary particle theorist Richard Feynman, who also worked on some condensed matter topics, noted the following roughly two decades ago in his book QED:
The branches of physics that deal with questions such as why iron (with 26 protons) is magnetic, while copper (with 29) is not, or why one gas is transparent and another one is not, are called "solid-state physics," or "liquid-state physics," or "honest physics"... The most interesting problems today -- and certainly the most practical problems -- are obviously in solid-state physics (p. 114).
Yet another perspective comes from the UCLA physics department's overview of its condensed matter physics program. I found this information to be surprisingly readable for a non-physicist such as myself, at least in the initial sections before the faculty members' research interests are discussed. According to this document:
...microscopic equations which describe an individual water molecule are of limited use when it comes to describing the macroscopic properties of matter, as are more refined descriptions at the atomic or subatomic scales... An entirely new set of concepts... must be introduced, such as temperature, entropy, and phase. The concept of temperature, for instance, makes no sense for an individual water molecule, but one can hardly understand a macroscopic body of water without it. These properties only emerge at macroscopic scales, which are the most interesting and important scales for humankind.
Further:
In considering such a perspective, one is immediately faced with such questions as why is a solid solid? If you open the door to a sauna and try to push against the steam which streams out, the water molecules near your hand will move out of the way, and your hand (and, eventually the rest of you) will be bathed in steam. This is precisely what you would naively expect of a collection of molecules, each of which acts fairly independently of the others in moving around your hand. However, if you try the same thing with a block of ice, the whole block will move as one. How can this be? The answer is that the water molecules interact strongly with each other in order to form one highly correlated whole in which the oxygen and hydrogen atoms sit as the sites of a rather rigid lattice. The same is true of all crystaline solids. Such highly correlated states are said to be condensed; it is their study which is the subject of condensed matter physics.
(The bold emphases in the above paragraph were added by me.)
What I glean from these descriptions is that condensed matter physics deals with coordinated structures of aggregating molecules. I'll conclude with two interesting examples of condensed matter physics, one of which is likely to be known to the general public, and the other of which won the 2001 Nobel Prize, having been inspired by one of Einstein's lesser-known ideas (at least "relative" to relativity and E = mc-squared).
Somewhere along the line, most people have probably heard the term superconductivity. Non-technical readers will probably want to skip down to the "Applications" section of the linked document, where technologies such as MRI are discussed.
The other example is the Bose-Einstein condensate, for which three researchers won the Nobel Prize. The Physics 2000 project at the University of Colorado, Boulder, has a nice multiple-page tutorial on Bose-Einstein condensates, including interactive, animated demonstrations (two of the three Nobel winners on this topic also happen to be based in Boulder). As you click through the Bose-Einstein tutorial, be sure to scroll down to the bottom of each page, to be sure to see the interactive exhibits (they also may take a little time to load, so be patient).