Why does time move forward

Enthropy

Many scientists cite enthropy as the reason why time only moves in the direction we are used to.

Enthropy is a measure if disorder in a system - the degree of chaos, if you like. The less ordered the system, the higher its enthropy. Enthropy level grows with time, but it can never dwindle. The law of enthropy goes like this: the enthropy of an isolated system can never decrease.

Science divides all processes into reversible and non-reversible. A process is considered reversible if it doesn't change the level of enthropy. Here is an example: a whole glass has a low level of enthropy - it is a complete and integral object. Once we break it, its enthropy grows. We cannot make this process go backwards unless we manage to turn back time, reversing the vector of enthropy change.

One more example: if I decide to mix milk and coffee, I will end up with a homogenous drink - coffee with milk. The molecules of milk spread evenly among the coffee molecules. It is not that self-evident, actually, at least not on the microscopic level.

If I were a milk molecule, i could find myself anywhere inside the glass. The exact spot would be determined by factors influencing me at the moment of my getting into the glass.

For example, I could end up in the left part of the glass. And so could you (if you were a molecule). The probability of you and me both ending up in the left half equals 1/2. That's a really high probability, actually. That's how it would look like.

Only that doesn't happen. Once we pour milk into the coffee, we always end up with an equal distribution of milk in coffee (or coffee in milk).

This paradox results from the difference in scale between the microscopic and the macroscopic worlds. On the microscopic level, a molecule does have a pseudo-choice where to go. But there are simply too many molecules. For each new molecule, the probability of it being located in the left part of the glass together with all the others is reduced by half.

Here is an example: the number of water molecules in a standard 200ml glass is about 6 septillions - it is a number with 24 zeros, and it is written like this -

6 000 000 000 000 000 000 000 000. The probability of all those molecules ending up in the same left side of the glass equals roughly one divided by 2 to the power of 6 septillions.Because of such a tremendous difference in scale the microscopic and macroscopic worlds should be studied separately.

The second law of thermodynamics

...or, rather, its physical interpretation. In an isolated system enthropy never decreases - we have learned that earlier from the law of enthropy. The fact that all processes can only go in one direction, in turn, sets the forward direction on the axis of time.

The second law of thermodynamics is particularly popular among anti-evolutionists: those folks insist that the theory of evolution is completely wrong, since if enthropy cannot decrease and the time is always directed towards an increase of chaos, then how did we come about? how could life even appear?

They are partly right: humans are incredibly intricate organisms, and life is a complex structure. But let us not confuse complexity and disorder. I, a human being, am complex; the Universe is unimaginably complex. In order to give a detailed description of a human, you need lots of variables, an immense amount of information. But a glass of coffee or a glass of milk are simple systems, and each of them is not hard to describe.

There's coffee here, coffee there, and also here - there's coffee everywhere.

When we start pouring milk into the glass with coffee, the system becomes more complex; now we need much more advanced structures to describe the state of this liquid. But it's only an intermediate state. Liquids start mixing, becoming ever simpler, until they turn into uniform coffee with milk. Now it's a simple structure again, since it can be described by just one phrase: coffee with milk. At the same time, enthropy kept growing throughout the process, since the relative location of molecules of coffee and milk became ever more chaotic.

Complexity and disorder are completely different things. The emergence of more complex structures can easily coincide with growing enthropy. At some point the Universe left its original simple state and started to get more complex and expand. That event is what forces time to move in one direction only - in the direction of an inevitable end. Humans, animals, plants - all living things on the planet and all the beautiful things that exist in the Universe - are no more than swirls of milk among coffee molecules, complex structures that will inevitably become simpler and more uniform. At some point, when enthropy in the Universe reaches its maxium, all macroscopic processes will cease. At that moment the Universe will stop expanding; in fact, our Universe itself will end.

So what next?

Nobody knows. In order to give a specific answer, we would need to know what happened at that moment of origin. Whatever transpired in that fateful instance made our Universe the way it is and made it move where it is moving.

In this article we described things unrelated to time - and we did that on purpose. We did not talk about the space-time continuum, or about how time is linked to the relativity theory, or about the influence exerted by gravity on time. The purpose of this article is to make you look at time from a different angle and to provide you with food for thought.