Physicists sometimes deny the reality of irreversible time and consider it an illusion, a psychological phenomenon. In a letter of condolence written in 1955, Einstein said this: ...the distinction between past, present and future is only an illusion, although persistent. A curious way to comfort those who have lost a beloved one. His reasons for saying this were the following:
· In Newton equations of universal gravitation, if the sign of the variable representing time is changed, the equations don’t change. If we look at the film of a gravitational process, the theory predicts that we won’t be able to detect if the projection was made in the right sense or in reverse.
· The same is true of Maxwell equations, which describe the behavior of electromagnetic waves.
· The same is true of Einstein equations of General Relativity, which replace Newton equations to describe gravity.
· The same happens with the Schrödinger equation, the basis of quantum mechanics.
But there is a problem: the equations mentioned do not make all of physics. The second principle of thermodynamics implies the existence of an arrow of time. In 1928, in a book titled The nature of the physical world, the inventor of this term (Arthur Eddington), said the following: if your theory [opposes] the second law of thermodynamics... [it will] collapse in deepest humiliation.
Every physical theory is a simplified abstraction where some parts of reality have been eliminated. If the irreversibility of time is one of those simplifications, it is not surprising that the final result is always reversible. In real events, however, there is no abstraction or simplification. All the physical theories, including the second law of thermodynamics, must be applied together. If this is done, the alleged temporal symmetry goes away.
of the first applications of Newton’s theory describes the fall
of an apple. If a film being projected shows several pieces of an
apple on the ground, which suddenly set in motion and gather in a single fruit,
which then rises upwards until it gets attached to a tree, would we doubt that it
has been projected in reverse? The fact that we don’t is a consequence of the
second law of thermodynamics.
|Newton and his apple|
· This also applies to the movements of celestial bodies. Imagine a recording of Mercury moving in its orbit, with the sun visible. By studying the solar sunspots we could deduce whether the film is projected correctly or in reverse. Sunspots are a consequence of thermodynamic phenomena.
· Radioactive decay is another example of a theoretically reversible process that in practice is irreversible. In fact, the proportion of uranium-238 and lead-206 in a rock provides a reliable method to calculate its age. The chain of disintegrations from uranium to lead is far more likely than the reverse chain, although physical theories affirm that both things could happen in theory.
· Whatever Schrödinger equation says, the Copenhagen interpretation of quantum mechanics requires an irreversible time. If a photon hits an electron with some energy, the electron is left in two overlapping spin states. If the spin is measured, the quantum superposition collapses into a positive or a negative value. This process involves a direction of time: first comes the impact of the photon, then the electron in two superimposed states, finally a measurement and a quantum collapse. The reverse process cannot happen.In these examples, when all of physics is taken into account without excluding thermodynamics, the supposed reversibility of time disappears. Apparently physicists put their theories above reality, doing the opposite of what the scientific method demands. Not even great men like Einstein were exempt.
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