Since the Large Hadron Collider has been closed for a while, and experiments concerning the search for the Higg's and other hot topics such as the "faster than light neutrinos" (there is still no firm evidence as to the validity of the results, hence the quotes) are not due to start again until March, in the meantime I would like to take advantage of the break to talk briefly about one of physics most interesting concepts: entropy!
Entropy is one of few scientific concepts which have gradually acquired a certain fame in the general public : for instance, in "Whatever works", the Woody Allen film, a charming blonde is suddenly heard proposing a definition of entropy : "entropy is what makes it impossible to put the toothpaste back in the tube " (quoting loosely). Although arguably this isn't much of a scientific definition, it does point out to two things : that in an Allen film, even the silliest girl can discuss complex physical concepts with her (somewhat bemused) boyfriend, and that entropy is in fact linked with a deep truth about the evolution of physical systems, which is almost entirely ignored by most physical equations.
In a nut shell : entropy is practically the only concept in physics which suggests an essential difference between past and future, and requires that time flow only in one direction.
Now first, a preliminary observation is necessary to understand what I mean by saying that almost all of physics ignore the difference between past and future: most theories in physics are "time symmetrical", meaning that the laws they describe would work just as well if time flowed backwards... In other words, nothing in their equations specifies a direction of time for their predictions to be valid: they all lack a "time arrow".
And this is where the Second Law of Thermodynamics kicks in, yelling at the top of its voive: "The entropy of an isolated system tends to a maximum." Meaning that, for an isolated system (and we may assume our Universe is isolated), you can say that the "time arrow" will point from a state with less entropy to one with maximal entropy, from past to future.
So what has this got to do with toothpaste ? Well, translating the statement "it is much harder to put toothpaste back in a tube than to squeeze it out" in physical terms gives something along the lines of "making a system evolve from a state of high entropy (toothpaste spilled out) to one of low entropy (toothpaste back in tube) requires work, while the contrary happens spontaneously (of course, in reality, you do have to squeeze the tube a bit, but it is comparatively easier than to put it back).
So now you know why entropy is so important to physicists, but you still don't know what entropy is... Traditionally, entropy is associated with a measure of "disorder" : a common if slightly inaccurate statement of the Second Law in popular science is that a child's room spontaneously evolves towards a state which maximises its untidiness, where untidiness, or "disorder" is taken to be synonymous with entropy, and to go back to toothpaste, the spontaneous direction of change will be from toothpaste in the tube to toothpaste spilled out of the tube ("order" to "disorder").
However, now that this definition of entropy as "disorder" has been introduced, it is also important to bear in mind that this is just destined to give a "feel" for entropy, and entropy is actually a physical variable, measurable and justified theoretically by statistical thermodynamics, and not a mystical concept (which "disorder" is to some extent), so that although the Second Law gives a "time arrow", this doesn't mean that entropy explains the flow of time, but merely that it is one of the only laws of physics which requires time to flow in one direction in order to function, which for a physicist, believe it or not, is already reason enough to rejoice!