12 February 2006
Here’s my candidate for the deepest scientific mystery of all time. It’s a question that any one of us can ask, but which the greatest minds of physics have not been able to answer.
What is the provenance of time’s arrow? or, Why is the relationship of this moment to the future qualitatively different from its relation to the past?
In our experience, we can remember the past, but we do not know what the future will hold. And it is not just our subjective human experience that is asymmetric: For example, we observe that things mix, but they never un-mix. We see a fire burn, turning a log into carbon dioxide and water vapor, while warming the living room; but we never see carbon dioxide and water vapor come together to make a log, as the room gets a little cooler.
There is the Second Law of Thermodynamics, which applies to all such examples and provides a quantitative measure of something about the natural world that is always increasing, never decreasing (entropy).
But it is fair to ask where the Second Law comes from. It is not a fundamental law, but a derived law. In fact, it is derived from interactions that are all time-symmetric. Every collision, every orbit, every exchange between two particles is described by physical laws that treat past and future identically. If you watch a movie of Humpty Dumpty falling from his perch and breaking into a thousand pieces, you have no trouble telling whether the movie is running forward or backward. But if you watched a movie of any small set of Humpty Dumpty’s atoms during this event, you would see them bouncing around in a way that could happen in a backward direction just as easily as forward.
All of the ‘microscopic laws’ of physics treat time symmetrically. There is no distinction between future and past. (This is as true in quantum as in classical physics.) The macroscopic behaviors are supposed to be mere collections of many microscopic behaviors, and yet they make obvious distinctions between future and past.
Does all of our universe have the same arrow of time? What would we experience if we came across a region where time is flowing the other way?
If the Big Bang expansion of the universe were to reach a point where gravity pulled it back, and we began contracting toward a Big Crunch, would we still perceive time going the same way, and dread our future annihilation; or would we then be experiencing the future the way we now experience the past, and we’d call it the Big Bang? (This was a claim by Stephen Hawking, which he later renounced.)
These are open scientific questions, which you have as good a chance of understanding as anyone else in the world. Here are some thoughts that others have had on the subject: