Meditações - o Tempo e a física quântica

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Write down the fundamental equations of quantum mechanics and run them backwards, they work just as well. Schrödinger's equation, the laws of quantum electrodynamics, the Standard Model interactions: every single one is time-symmetric. A film of two particles colliding and bouncing apart looks physically valid whether played forwards or in reverse. Nothing in the microscopic laws of physics privileges one direction of time over the other.

And yet you have never watched a broken egg reassemble itself. Heat has never spontaneously flowed from a cold object to a hot one. Memories only form of the past, never the future. The macroscopic world has an unmistakable arrow of time, and its origin, at the quantum level, is one of the most contested problems in theoretical physics.

The standard answer invokes entropy and the second law of thermodynamics: systems evolve toward more probable states, and disordered states vastly outnumber ordered ones. But this only pushes the question back, why was the universe in an extraordinarily low-entropy state at the Big Bang in the first place? That initial condition is not explained by any current theory; it is simply assumed.

More recently, physicists have probed a subtle exception: CP violation, the discovery that certain particle decays behave slightly differently depending on whether you swap particles for antiparticles and mirror the spatial coordinates. Combined with charge symmetry, this implies a faint but real asymmetry in how the universe treats time at the quantum level. Whether this microscopic bias is the seed from which the macroscopic arrow of time grows, or merely a curiosity unrelated to it, remains an open and fiercely debated question. We experience time's direction as the most obvious fact of existence. We have no idea where it comes from.