The Arrow of Time
Sean Carroll has a really interesting post about a paper he’s written on cosmology and the arrow of time. The “arrow of time” refers to the idea that even though most microscopic physics (e.g., single particles and quantum mechanics) looks the same whether time runs forwards or backwards, macroscopic physics (e.g, smashing a plate or cooling a cup of coffee) has a definite “direction” for time to flow. So, while I could run a movie of a single electron forwards or backwards, and you couldn’t tell the difference, you’d probably realise something was wrong if fragments of broken china suddenly formed into a whole plate! The arrow of time exists because of entropy (basically the “disorder” of a system) and the second law of thermodynamics which says that entropy always increases. So, in our example, a broken plate is more disordered than a whole one, so for it to spontaneously fix itself would decrease entropy, which is not allowed.
His paper is about explaining why our universe started off with low entropy, how inflation of the universe fits into the picture and how (on a really big scale) the whole universe may look the same whether run forwards or backwards. His summary (in his post) is really quite good (with lots of links for the uninitiated) so I won’t say any more - go and have a read! If you’re feeling adventurous, you can even get a sneak preview of his paper before the rest of the physics world.