Unlike our daily experience, the world of elementary particle physics is mostly symmetrical in time. Run the clock backward on your day and it won't work; run the clock backward on a process in particle physics and things are just fine. However, to preserve certain fundamental aspects of space-time the Standard Model predicts that certain reversible events nevertheless have different probabilities, depending on which way they go.
Finding a direction of time in exotic particle transformations: Entangled B mesons show that time has an arrow on the smallest scales by Matthew Francis, Ars Technica
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