Why does the universe have an arrow of time?
Quick Answer
Time seems to move in one direction because entropy tends to increase in isolated systems. The universe started in a very low-entropy state, so many processes are irreversible.
The Full Story
Most fundamental physical laws work almost the same forward and backward in time, but everyday life is not symmetric because of thermodynamics. A dropped glass can shatter, but the shards do not spontaneously reassemble, because that would require a dramatic decrease in entropy. The early universe was unusually ordered, which set the direction for increasing entropy ever since. This gives us the practical sensation of time flowing from past to future. The concept is closely tied to the second law of thermodynamics, formulated in the 19th century.
Key Facts
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"Why can we remember the past but not the future?"
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