Entropy is a core concept in Physics. Entropy is connected to the state of randomness in a system. The Second Law of Thermodynamics, the boss of all laws states that the Entropy of the Universe always increases i.e the Universe always tends to get more random.
If given enough time, a monkey who is typing gibberish on computer would write all of the Shakespearean novels in exact chronological order by mere chance.
If a room is filled with gas, each individual particle of the gas will interact with each other and the walls. There will be a lot of collisions. During these collisions, the particle will be distributed in all parts of the room. But, if monitored long enough, there can be a point in time when the particles arrange themselves all at a small point, and the whole room is almost empty. It is probable but highly unlikely. This is used as a possible explanation for big bang. That all the energy in the universe concentrated into a small point just because time is indefinite and given just enough time, anything could happen. It is not exactly relevant with modern studies but it is inspirational. And, we haven't got to the good part of this theory yet.
This theory is the explanation of Second Law of Thermodynamics. That maybe, it is just a mere coincidence that our universe tends to increase its entropy i.e the sense of randomness. It happens because the highly unlikely but still probable case of the whole universe just coincidentally contracting to a point is now reverting. But, the Second Law of Thermodynamics explains each and every process in our real life. Energy is lost in every system in the form of heat. When we move, friction acts and heat is generated. When a refrigerator works, the inside gets cold but the outside release more heat than the one released from inside. Things tend to act in the favor of increasing randomness. But till when.
The moment the universe starts its re-collapsing, maybe another coin flip of a chance, the world will no longer follow the Second Law of Thermodynamics. It will become the exact opposite. When we walk, heat will not be produced, it will be absorbed. The fridge will generate more heat and the energy in the system would absorb it. In simple terms, whatever we did the way we do it, we wouldn't be able to do it that way.
The explanation to how this anti-world would behave is through Reversed Time. Time is moving forward, linear, but the perception of that time would be the opposite of how we experience time moving right now. A dog wouldn't be able to move forward the way it moved. It would move as if moving backwards through time. All living would be different. If put rightly, it will also result in the anti-aging where you get younger as you get older.
This concept is also used in the movie Tenet by Christopher Nolan. It is on the top of my mind-blown theories list.
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