eli5 What is entropy and how does it helps to understand time in a scientific way?

Question

I read that entropy is directly related with thermodynamics but is the concept of entropy different if I want to understand the perception of time?

We know that the transformation of matter never regress. If you throw a cup of glass in the floor it will break into pieces but will never become a entire cup again on its own even if you gather the pieces together. If entropy did not existed and if we could see the broken cup rearrange into a full cup again could we perceive time going backwards?

Do entropy make things easier to kill and destroy than create them? A house take months to be built but an explosion can destroy it in seconds for example.

Another example things get dirty but cannot get clean and shiny by their own. Does that mean that things getting deteriorated and dirt with time is a sign of entropy as well?

So time ( or the perception of time) exists because of entropy which makes us perceive it always going forward but never the other way around?

In: Physics

Answer 1

Strictly speaking, you *would* eventually see the glass spontaneously reassemble itself. The laws of thermodynamics are probabilistic, not mechanical, and when you frame them the right way they basically just say “if you pick a random state, you’ll mostly find really common states”.

But in practice, the probability of even slight violations of thermodynamics are so low as to be effectively zero for all practical purposes. For example, there is no physical law preventing you from flipping 100 heads in a row with a coin. But if you and every single other person on Earth did it once a second for your whole lives, it’s still overwhelmingly likely none of you would ever get all 100 heads (the chance of even one success is one in a few hundred billion). And this system only has 100 degrees of freedom – real world thermodynamic systems have trillions of trillions.

Part of why it appears to be fully one-way right now is that our Universe is very far from its “most likely” (i.e., high-entropy) state. Imagine that, say, you *started* with 70 coins on heads and 30 on tails, and flipped coins at random over time – you’d see a very steady and quick decline towards 50 heads and 50 tails, and would in all probability never return to 70 heads in any sane period of time.