Sometime back I promised to give you one more reason why my children remind me about the universe. But, thanks to them, I could not get around to writing about it till today. That is the problem with children, you do not get time for much else. Don’t get me wrong. I am not saying parenthood is without its moments. The other day, I was carrying Aman around in a store, while my wife was looking admiringly at all the tablets that were on display. I was talking to him and in a bout of fatherly affection, I hugged him tight, and all of a sudden I felt a certain warmth envelop my heart. The problem with these moments are they are very fleeting. This time it ended with my wife telling me that the warmth was not from the inside, but because, Aman threw up some of the formula he just drank, onto my shirt. I digress.
Do you remember the term Entropy? I remember it from my Chemistry text book being defined as “Entropy is the measure of randomness in a system” accompanied by a diagram having two boxes, one in which small circles are arranged closely together on one side of the box, and the other box having these circles evenly spread out. The first one is supposed to have low entropy, because it is more ordered, while the second one has higher entropy, as there is less order in it. As with rest of Chemistry, I had no clue what it meant. I vaguely remember wondering, why this is even considered an important concept, but was practical enough to memorise it since it was an easy definition, and could earn me a couple of marks if it came up in my examination.
A few years back when I actually learnt what entropy meant I was blown away. In many places, entropy is defined in terms of orderliness. The higher the orderliness, the lower the entropy . A common example is that of a room neatly arranged, with all the things in their places. If you do not take any effort towards the room for some time, it will soon end up in an unordered state, what with your leaving a bowl here, a book there, a shirt on the couch, a drinking cup elsewhere and so on. Another example given is on the lines of dropping a little bit of ink into a glass of water. Initially the ink is focussed at the spot where it was dropped, but slowly it spreads out turning the entire water blue. In both these cases, the room, or the glass full of water, initially there is low entropy (high orderliness), then the entropy slowly increases (orderliness decreases). (I know that this definition of entropy is confusing, so I will try to use orderliness more frequently in the rest of this post, occasionally translating it into entropy.)
The examples are given to explain one of the fundamental laws of physics: second law of thermodynamics, which is that the entropy of a closed system will never decrease. By closed, they mean a system that does not interact with any other object outside the system. And the examples of the room and a glass of water, is meant to tell you that a room never gets organised on its own (unless you put in some effort), and though you will see ink spreading out in water uniformly, you will never see all those ink molecules come together as one blob in any part of the glass of water. But these examples are merely analogies. Not really an example of entropy. For, if you really did nothing to the room, say you locked your house and went on a vacation, the room would be as it is when you return. The example of ink in water, can be turned around with the example of oil in water. If you drop some oil into water, stir it well, the oil will spread. But once you stop stirring, the oil will accumulate on the surface of the water.
Further, this definition of orderliness stuck somewhat odd to me. After all, who decides what orderly is. If I think a spilt cup of milk on my couch is an integral feature of an orderly room, then surely orderliness can increase. So, this was never really convincing to me. Thankfully, there is a better definition.
Imagine this. You heat some water in a bowl, and you see steam coming out of it. The steam is hot. And so is the water. Have you thought of heating some milk with this heat from the steam, and the heat of the bowl of water? After all, we were told that energy can be neither created nor destroyed. So we should be able to use that energy again. If you try, you will notice that you can achieve some heating of the milk, but not really much.
The question is, can energy be recycled? The problem is that there is useful energy and the not-so-useful energy. For energy to be useful, it has to be focused in a very small area. When you heat a bowl of water, you do not lose energy. Only that it becomes more spread out. As steam and as heat in the water. The more spread out it becomes, the less useful it becomes as an energy source.
Thus an easy way to think of Entropy is to think of the amount of useful energy in a system. The way this energy and orderliness can be coupled is this: If energy is arranged in an orderly fashion, it is useful energy. If it is spread out in a random manner, then nothing much can be done with it. So if you want to increase orderliness in a place A, say, to get all those atoms in a particular arrangement, then you need to spend some useful energy elsewhere drawn from a place B. And always, the quantum of orderliness gained in place A, is less than the orderliness you lost in place B. Which means, in the overall perspective orderliness always goes down. Going back to our confusing definition this means entropy will always go up.
What you see above are two orderly arrangements of a gazillion number of atoms (of course, so are you and me). These atoms could have been arranged in a gazillion to the power of gazillion number of other ways. But very few combinations will actually form a human being that can grow, eat and function purposefully. Thus this very high orderliness in my house should mean orderliness must have reduced somewhere else. Where did my children steal that orderliness from?
It comes from the sun. In fact all the orderliness on earth comes from the sun. It is the energy of the sun that gives us all our life. So it is the useful energy of the sun that we are converting into useless energy. The energy from the sun has high orderliness, low entropy. The energy that we release (as heat) has low orderliness, high entropy. And the closed system I am talking about is the entire universe. Our universe as a whole has a finite amount of useful energy. At some point in future, all that useful energy will be lost. So, even if we could be immortal, there is a point after which you cant eat to get energy, because there are no plants. No plants because there is no sunlight. No sunlight because, there are no stars. No stars because energy is not focused at any point any more anywhere in the universe. It is all spread out in such a manner, that the entire universe has one consistent, very cold, temperature. At that point, nothing can be done. No turning back. Life cannot exist any more.
I know it sounds all bleak, but that future is very far away. We have bigger worries like the sun dying out in the next 5 billion years. So, the universe ending up with maximum entropy need not keep you awake at night. But entropy is important because, it is one idea that seems to explain why time always flows from past to future and not the other way (more on it some other time). Another interesting aspect is that, if entropy always increases towards the future, then that means, our universe must have begun in the past with very low entropy. That is why we are able to have so much useful energy. It is as if the universe is like a clock that has been wound up, and is slowly unwinding. At some point it will unwind completely and all that is left is a clock that does nothing. So how did the big bang start off the universe with low entropy?
I really do not have an answer for that (though scientists know it). But what fascinates me is that it is this low entropy start that big bang gave to the universe making life possible in this universe. You can read a short story by Isaac Asimov on this topic here.