## March 24, 2011

### From Eternity to Here by Sean Carroll

Why is the future different than the past? The fundamental rules of physics seem as symmetric in time as they are for different directions of space, yet you can't unscramble an egg and you can't remember the future. The answer seems to lie in the concept of entropy, which tends to increase over time in a closed system unlike energy and momentum which stay constant. Yet, entropy is one of these concepts that lead to quite a bit of confusion, sort of like (and sometimes for the same reasons as) probability. Sean Carrol's wonderful book tackles the mysteries of entropy and arrow of time. Here are some entropy puzzles and some further reading:

Entropy puzzles:
• If you find yourself in a universe with low entropy it is as likely to have come from a higher entropy state as it is to evolve into a higher entropy state. So how do you know you are not a Boltzmann brain?
• Take a box with a partition in it, with gas A on one side, gas B on the other side, and both gases are at the same temperature and pressure. Remove the partition. If gas A and B are different gases, there is an entropy that arises due to the mixing. If the gases are the same, no additional entropy is calculated. What if you thought they were the same gas and years later it was discovered that they happened to be two different isotopes? (See Gibbs paradox and E.T. Jaynes' paper). More generally this microstate / macrostate business seems completely user defined and arbitrary, so how can it have real physical effects?
• In a reversible system there must be just as many paths that decrease the entropy as that increase the entropy. Why don't we observe as many of the first type as the second?
• A gas squeezed in the corner of a room will tend to spread thereby increase its disorder and entropy. If we add an attractive force like gravity matter seems to clump together rather than spread out. How does clumping together increase entropy?
• A rotting plant turns into dust and gas which increases disorder and entropy. A seed turns a bunch of gas and dust into a full grown tree which seems to decrease entropy. This can only happen because the seed is not a closed system and is using the energy from the sun and ends up increasing the overall entropy of the universe at the end. When, how, and why does this type of thing happen?