Aware of this problem, physicists in favor of
the reversibility of time have answered in different ways: it has been said
that the second principle of thermodynamics is a fictitious, subjective law
that does not conform to reality; a mental illusion; an approximation; a
consequence of the initial conditions of the universe. It has been hypothesized
that, if the universe were cyclic, the arrow of time would be reversed during
the contraction stage. (This theory has been abandoned). To escape the problem,
Stephen Hawking proposed a universe without initial conditions in his book A Brief History of Time. It is curious, this desire
to defend at any price the reversibility of time, when it was precisely Hawking
who proposed the existence of an arrow of time in black holes, which rather
than being permanent, would disintegrate.
In 1928, a year after inventing the term the arrow of time, Arthur Eddington challenged
the physicists who defend the reversibility of time with the following
devastating words: If your theory is found to be against the second
law of Thermodynamics... there is nothing for it but to collapse in deepest humiliation
(The Nature of the Physcal World,
1928).
This dilemma has a name, which was invented in
1876 by Johann Joseph Loschmidt: the paradox of
irreversibility, which can be described as follows:
·
According to the laws of mechanics that we know, there does not seem to be an arrow of
time in the microscopic world. If we accept the reductionist
hypothesis, there shouldn’t be one in
the macroscopic world.
·
But according to experience and thermodynamic
experimentation, there is an arrow of
time in the macroscopic world.
·
Consequently, mechanics
and thermodynamics must be incomplete, for they reach incompatible conclusions.
As Alfred North
Whitehead said, a clash of doctrines is not a disaster, it is an
opportunity (Science and the Modern World, 1967).
Are the laws of
mechanics and the microscopic world really reversible, as asserted by those who
deny the existence of the arrow of time? It is not so clear. Let us see:
1.
Newton's mechanics not only explains the movements
of celestial bodies, but also much closer phenomena, such as the fall of an
apple, whose irreversibility is fully evident. Imagine you are
shown a film where several pieces of apple are on the ground, which suddenly
start moving and come together at the same point, turning into a piece of fruit
that flies upwards, until it stops hanging from a branch of a tree. Would we
have any difficulty in detecting that the direction of time has been reversed when
projecting this film? No, for this example makes use, not just of only Newton's
mechanics (which explains the motion of the apple when it is falling), but also
of the second principle of
thermodynamics, which tells us that the change from a disordered state
(the pieces of the apple) to an ordered state (the apple hanging from the
branch of the tree) cannot take place spontaneously.
Even in
celestial bodies it is possible to detect if the film showing their movements has been
inverted. Imagine a recording of Mercury's orbit where the sun can
be seen. By looking at the movement of the sunspots (which are a consequence of
thermodynamic phenomena) it is possible to find out if the image is correct or
has been inverted. Once again the interaction of mechanics and thermodynamics makes
time irreversible.
2.
There are reversible
chemical reactions, such as the dissolution of calcium carbonate in
carbonated water, which when taking place in the opposite direction is
responsible for the existence of stalagmites and stalactites. But there are also
irreversible reactions, such as the precipitation of barium
sulfate when mixing two solutions of sodium sulfate and barium chloride. Also
in this case, detecting the correct direction of projection of a film is very
simple.
There are
also irreversible
nuclear reactions, such as the series of disintegrations of
uranium-238 ending in lead-206. The inverse chain is so unlikely, that the
analysis of the proportion of these two substances in the same rock provides a
means of ascertaining its age.
3.
Quantum mechanics also contains indications that
time is irreversible, such as the measurement problem: if we measure the
state of a particle in a superposition of quantum states, it collapses in one
of them with a certain probability. The reverse phenomenon, however, never happens.
Another
indication is the CPT symmetry. So far we know no violation of this
symmetry, which consists of simultaneously changing the sign of the charge of a
particle, its parity and the direction of time. However, a violation of CP
symmetry has been detected in the disintegration of a kaon-0: in one out of
every billion disintegrations, this particle becomes a positive pion, an
electron and an antineutrino, while its normal disintegration turns it into a
negative pion, a positron and a neutrino. If the CPT symmetry is true, as it is
suspected, the violation of the CP
symmetry implies that time is irreversible.
4.
As noted by the biologist Stephen Jay Gould (Wonderful
life, 1989), the history of the evolution of living beings is asymmetric:
as the number of species grows, the types of organization decrease. There would
be, therefore, an arrow of evolutionary time (Time's
arrow, time's cycle, 1988).
 |
| Roger Penrose |
The same post in Spanish
Thematic thread on Time: Preceding Next
Manuel Alfonseca
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