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If there is one thing that is nearly always on my mind, it is time. Trying to cram 30 hours into 24 hours each day tends to do that. But beyond constantly thinking of our own personal time issues, the actual subject of ‘time’ is one of the deepest and most difficult concepts to define and understand. One person trying to do that right now is physicist Sean Carroll.

A gifted communicator of difficult cosmological concepts, Carroll summarised where he’s coming from in a recent article in Wired:

I’m trying to understand how time works. And that’s a huge question that has lots of different aspects to it. A lot of them go back to Einstein and spacetime and how we measure time using clocks. But the particular aspect of time that I’m interested in is the arrow of time: the fact that the past is different from the future. We remember the past but we don’t remember the future. There are irreversible processes. There are things that happen, like you turn an egg into an omelet, but you can’t turn an omelet into an egg.

And we sort of understand that halfway. The arrow of time is based on ideas that go back to Ludwig Boltzmann, an Austrian physicist in the 1870s. He figured out this thing called entropy. Entropy is just a measure of how disorderly things are. And it tends to grow. That’s the second law of thermodynamics: Entropy goes up with time, things become more disorderly.

But there’s a missing piece…which is, why was the entropy ever low to begin with? Why were the papers neatly stacked in the universe? Basically, our observable universe begins around 13.7 billion years ago in a state of exquisite order, exquisitely low entropy. It’s like the universe is a wind-up toy that has been sort of puttering along for the last 13.7 billion years and will eventually wind down to nothing. But why was it ever wound up in the first place? Why was it in such a weird low-entropy unusual state? That is what I’m trying to tackle.

Carroll goes into much more depth on the topic in his recently released book, From Eternity to Here: The Quest for the Ultimate Theory of Time (Amazon US and UK). You’ll also have your mind hurt nicely by these TED videos of Carroll discussing the arrow of time at the University of Sydney in December 2009: Part 1, and Part 2 (I originally embedded them here, but it seems they autoplay so I removed them). Not exactly light viewing, but fascinating stuff.