## Monday, February 14, 2005

### The Arrow of Time

Rudolf Julius Emanuel Clausius

There is but one kind of entropy change. Entropy change is due to energy dispersal to, from, or within a system (as a function of temperature.), measured by microstate change: S = kB ln [microstates final / microstates initial ].

I should back up here and mentioned that Peter Woit seems to be coming out in the open and explaining somethings that have been not so clear before?

Perter Woit:
Penrose also carefully lays out areas in which his point of view differs from the general consensus of most theoretical physicists. An example is his emphasis on the importance for cosmology of understanding why the universe had such low entropy at the Big Bang

What is strange today that with this thought on the subject of entropy.

Lubos Motl:
This is what allows the early gas to clump (and seemingly create a more order state) without violating the 2nd law of thermodynamics: the gravitational entropy overcomensates the decrease of the entropy.

OK, so why was the beginning of the Universe a low-entropy state? The best explanation we have is inflation. It simply explodes the size of the Universe. During inflation, the total entropy of the Universe grows, but much more slowly than how it would grow otherwise, without inflation
.

if one did not understand the early universe consideration here, and the idealization of supersymmetry, could we have found a association to low orders of entropy since this early time would have been very topologically considered and part of a continuum?

Entropy and the second law of thermodynamics

Entropy is no mystery or complicated idea. Entropy is merely the way to measure the energy that spreads out in a process (as a function of temperature). What's complicated about that? Entropy change, S, measures how much energy is spread out in a system, or how spread out is the energy of a system (both always involving T). The first example in our text was melting ice to water at 273 K where S = q/T. So, in that equation, it's easy to see that q (the enthalpy of fusion) is how much "heat" energy was spread out in the ice to change it to water. What's the big deal?

Update: Reading Peter Woit's blog today he linked Sean Carroll's "Arrow of Time article," so I thought it most apropriate to link it from here as well, since I am on the topic.

Sean Carroll:
Jennie and I do the following thought experiment -- if it weren't for inflation, what would be a "natural" state for the universe to be in? Different people have addressed this question, with different answers; Roger Penrose, for example, has suggested that it would be a lumpy universe full of black holes. Our answer is almost exactly the opposite -- the only natural state is empty space. This is basically because gravity makes everything unstable, and the entropy of any given configuration can always be increased by just expanding the universe by a huge factor. Sure, black holes will form, but they will ultimately evaporate away. If you let the universe evolve forever, it will ultimately get emptier and emptier (generically).