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| Albert Einstein |
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.
·
 |
| Newton and his apple |
One
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.
·
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.
El mismo artículo en español
Thematic thread on Time: Next
Manuel Alfonseca
The
origin of life is an event that most likely happened only once in Earth's
history. It is impossible to repeat it, so as to study how it happened,
therefore it is not a scientific fact. It is a historical fact.