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Monday
Oct252010

The Secret Of Misery

Check out the funky video attached to this post. A water droplet appears to "undrop" itself from a twig. It's just a video run in reverse. Why is it that this process feels instinctively unnatural? Is it at all possible to setup the water mass in the lake in such a precise way that a drop will suddenly emerge and latch onto the twig as shown? Here's a surprise for you: the equations describing this water droplet cannot distinguish between forward and backward in time! there are absolutely no reasons from the perspective of the microscopic physics for this process not to happen in either directions: dropping is equivalent to "undroping"… What then creates this perceived asymmetry? An asymmetry suggesting that the water droplet dropped into the lake, instead of the reverse; an asymmetry that distinguishes past from future… Why does time appear to come with an arrow pointing into a future? Why do we age uncontrollably and helplessly, and perceive a distinction between past and present, and an uncertain future?

The field of Thermodynamics - often called Statistical Mechanics in our modern era - is one of the oldest disciplines of Physics. It is also the most robust - the one that changes least over time (no pun intended) through the discovery of new laws of Nature. Thermodynamics deals with statements about the macroscopic bulk behavior of physical systems that involve a very large number of constituents. The air in a room contains an unimaginably large number of molecules moving around erratically across a large expanse. We don't care about where a particular molecule is at a particular time. We care about averaged quantities that can be used to quantify the whole mass of the air. So, we talk about, for example, the temperature of the air instead: roughly speaking this is the average energy per molecule - basically a statistical assessment of the state of the air. 

There's another very important macroscopic quantity that is particularly useful for quantifying physical systems with a large number of constituents: entropy. Entropy is a measure of the level of disorder in a system; the more the disorder, the higher the entropy. If you take the air in a room and pack it in a small box, you've decreased its entropy: the molecules have less room to be disordered in and hence are relatively more ordered. The water droplet on the twig falls down into the lake and dissipates its molecules within the large mass of the lake's water: the droplet mixes with the lake's waters and increases its entropy by reaching out to a more disordered state. As we age and eventually drop dead, the molecules in our body - which are in a rather highly ordered state (specially in my case) - will disintegrate as we decompose and become fertilizer.. our entropy increases in this rather grim scenario as well.

One the laws of Thermodynamics proposes that the entropy of the universe cannot decrease; it must increase or, at best, stay the same. Hence, Nature is determined to increase disorder all around us. You can view this as due to the following: a more disordered state involves more underlying possibilities; so, it is statistically more likely to find yourself in such a state. The water droplet prefers to drop instead of "undroping" because that would increase its entropy - the number of possibilities for its molecules that can then roam freely the waters of the lake. We could in principle - with enough charisma - reverse the process by having the water from the lake jump up onto the twig. But that will require us to expand large amounts of energy as we carefully prepare the water in the lake with the proper initial conditions; and expanding energy will require burning calories;  and burning calories amounts to a net increase in the entropy of the universe since it involves dissembling the molecules of the food we consume… No matter what you do, the disorder in the universe will not decrease. 

Pretty interesting, isn't it? Complex systems - particularly the universe - will tend to increase their level of disorder, driven by statistical considerations. And that is precisely what creates the asymmetry between the past and the future! Did we just understand why time comes with an arrow pointing into a future? Not really. The universe apparently started in a highly ordered and hence unlikely state, creating this uncontrollable drive toward resettling itself into disorder; hence we perceive an arrow of time. We now understand, given the initial state of the universe billions of years ago, why time moves forward in one direction. That is certainly quite amazing. But why did then the universe start in such a highly unlikely state? If the most disordered state is the most likely one, why didn't things start off in such a maximally disordered state in the first place! There then would have been no need for this misery due to a constantly advancing time that buries our past and memories into dust… things would have been disordered dust from the outset… We owe our very existence to that initial highly unstable, unlikely, fine tuned, and ordered state of the universe 14 billions year ago; but we also can trace to it all the pain we suffer with the passing of time.

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