Arrow of Time
Classical Mechanics
Level
2
Video clips, when played in reverse, appear unnatural. Broken things fix themselves, toothpaste goes back into the tube, and puddles freeze to form snowmen.
Why are such phenomena not observed in forwarding time?
They violate the law of conservation of energy
They violate Newton’s laws of motion
Systems become more disordered with time
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The measure of disorder is called entropy . A bedroom can be considered as a system. If no energy or matter can comes in or goes out of the room, then it is an isolated system. Suppose the room is very organized initially. The clothes are arranged neatly in the wardrobe, books are stacked well on the shelves, and the bed is made well. It is ordered and has low entropy. As time passes, the room becomes disordered. The wardrobe, bookshelves, bed all become messy. The entropy of the room increases.
Disordered systems (messy room) have more microstates compared to ordered systems (organized room). That is, there are more ways in which a room can be messy compared to the number of ways a room can be neat and tidy. An (isolated) organized room becomes messy, but an isolated messy room does not become organized.
Similar to the room, all isolated systems tend to become more disordered with time. Since there are more microstates for a puddle than a snowman, a snowman is likely to turn into a puddle, but not the other way around.
If energy is supplied from outside, then it is possible to decrease the entropy of the system. We can transform puddles into snowmen, but we need to spend time and energy to make the process happen. It does not happen spontaneously, and we do not see puddles becoming snowmen on their own.