Arrow of time and origin of universe

Sean at Cosmic Variance has put together a very nice FAQ about the arrow of time, in which he answers the following questions:

1. What is the arrow of time?
2. But entropy decreases all the time; we can freeze water to make ice cubes, after all.
3. So what’s the big deal?
4. And how do we reconcile them?
5. Wasn’t this all figured out over a century ago?
6. Is the origin of the Second Law really cosmological? We never talked about the early universe back when I took thermodynamics.
7. Does inflation explain the low entropy of the early universe?
8. Does that mean that inflation is wrong?
9. My theory of (brane gasses/loop quantum cosmology/ekpyrosis/Euclidean quantum gravity) provides a very natural and attractive initial condition for the universe. The arrow of time just pops out as a bonus.
10. What is the entropy of the universe?
11. If you don’t understand entropy that well, how can you even talk about the arrow of time?
12. Are black holes the highest-entropy states that exist?
13. So what does a high-entropy state look like?
14. Could the universe just be a statistical fluctuation?
15. Don’t the weak interactions violate time-reversal invariance?
16. Doesn’t the collapse of the wavefunction in quantum mechanics violate time-reversal invariance?
17. This sounds like a hard problem. Is there any way the arrow of time can be explained dynamically?
18. Why do we remember the past, but not the future?
19. Why do we conceptualize the world in terms of cause and effect?
20. Why is the universe hospitable to information-gathering-and-processing complex systems such as ourselves, capable of evolution and self-awareness and the ability to fall in love?
21. Why do you work on this crazy stuff with no practical application?

Here is the answer to question number 4:

The observed macroscopic irreversibility is not a consequence of the fundamental laws of physics, it’s a consequence of the particular configuration in which the universe finds itself. In particular, the unusual low-entropy conditions in the very early universe, near the Big Bang. Understanding the arrow of time is a matter of understanding the origin of the universe.

And, of course, the answers to questions 18, 19, and 20 are The arrow of time! Very interesting piece. Take a look!

Tags: arrow of time, cosmology, entropy, origin of universe