CHAPTER III
IRREVERSIBILITY
So far, we have seen that the physical laws essential to
the determination of the course of the universe from its present
momentary condition are all reversible. From this it might be
concluded that all physical laws must in consequence be
reversible, and that, therefore, there can be no essential
difference between the real universe and the reverse universe.
And this much is true, that, provided we examine the motions of
the particles of matter, everything that happens in the reverse
universe can be described in terms of the physical properties of
matter as we know them.
But at the same time, if we take the most ordinary events
of the real universe and attempt to find out what is the
corresponding event in the reverse universe, something
strange will at once impress us about the reverse universe.
Take this, for example: a ball rolls down a staircase, bounces a
little at the bottom, and finally stops. In the reverse universe the
initial condition is the ball at the bottom, on a floor near, the foot
of a staircase. The heat energy in the floor collects at one point
underneath the ball, so as to push the ball suddenly upward.
Each time that the ball falls back to the floor this process is
repeated, until finally the floor throws the ball on to the first stair.
The stairs, each in turn, throw the ball in a similar manner up
the staircase, till finally the ball stops at the top. The molecular
vibrations in the ball, floor, and staircase, had previously been
so arranged that concentration of energy would happen at a
particular spot and time, while the ball so moved that it just
happened to be at those spots exactly in time.
So it will be with the occurrences corresponding in the
reverse universe to almost any common occurrence in the
physical world of our experience. Everything seems to be
perfectly explicable In terms of physical laws, but at
the some time the combinations of motions seem to have
something utterly strange about them. Hence there is some
point of difference between the real universe and the reverse
universe, and hence there must be some property of the real
universe that is irreversible. This irreversible property is found
in what is called the second law of thermodynamics. This,
taken in its most general aspect, amounts to this; that the
energy of the universe is constantly running down to one
common level. In other words, where energy of the same variety
is present in different degrees of concentration, those differences
will be equalised, and energy of a still higher-level or to a greater
amount must become dissipated in order to re-create these
difference[s] of concentration. Of the various varieties of energy,
all kinds tend to turn into heat, which is the least concentrated
form of energy; and, even though some of that heat may be
re-converted into some other form of energy, still, at each step,
some energy is irretrievably lost in the form of heat.
This physical law, as well as all those which are derived
from it, is irreversible. Furthermore, only such physical laws as
are derived from the second law of thermodynamics are
irreversible; so that this law constitutes the sole difference
between the real and the reverse universe. Where, in the real
universe, energy runs down to a common level, it follows that,
in the reverse universe, energy tends to build itself up into
different levels.
We may say, then, that the characteristic irreversible
part of the universe consists in this, that energy tends to evolve
(or devolve) from molar motion of extremely large masses,
which is the most concentrated form of energy, to a condition
in which all energy is in .the form of heat, which is the least
concentrated form, and at a uniform concentration, that is to
say, at a constant temperature throughout:. A final condition
would result in which a dead level of energy would be reached,
and after that nothing further could ever happen In the universe.
The fact, for instance, that perfectly elastic collisions of
large masses of matter do not occur, but that such collisions
are inelastic, is a direct consequence of the second law of
thermodynamics. The characteristic of an inelastic collision is
that some of the molar kinetic energy of the colliding bodies is
lost by the impact. This lost kinetic energy is changed into heat,
which is always produced by an inelastic collision. This is in
strict accord with the second law of thermodynamics. In the
reverse universe, on the contrary, an impact would be an
occasion for heat to be converted into molar motion, thus
increasing the total amount of kinetic energy. Such a collision
we may call super-elastic, and not within our experience.
Again, the resistance offered by one body to another,
whether in the form of friction or otherwise, is but an example
of the second law of thermodynamics, being another case of
change of molar energy into heat. In the reverse universe, the
very opposite process would take place. Accordingly we find
as might be expected, that the laws of friction, etc., are
irreversible.
Many chemical reactions are irreversible, though some
are reversible. As a general rule, the irreversible chemical
reactions are cases of conversion of chemical energy into heat,
in accordance with the second law of thermodynamics. So with
all irreversible processes.
In the case of a machine, the ratio of the energy obtained
to the energy put in (usually expressed as a percentage) is
called the mechanical efficiency of that machine. The remaining
energy, that the machine has lost, becomes heat. The second
law of thermodynamics, expressed in terms of mechanical
efficiency, means that all physical phenomena have a
mechanical efficiency of less than 100%. The reverse universe,
on the contrary, is distinguished from the universe of our experience
in that the mechanical efficiency of its phenomena is over 100%
Again, to express it In another way. Suppose two bodies,
one at a temperature of 0° Fahrenheit, the other at a temperature
of 200°. The only available heat-energy in those bodies would be
the amount represented by 200 degrees in the hotter body. At the
same time, the colder body being -460 degrees above absolute
zero, there is unavailable energy, which, according to the second
law of .thermodynamics, cannot be reached, amounting to 460
degrees in each of the two bodies. If both bodies have the same
mass and specific heat, the energy which, under the second law
of thermodynamics, is available for conversion into other forms of
energy, could thus be represented by 200, while the total
heat-energy In the two bodies would be represented by
460+660-1120. The ratio of available to total energy in this case
would be 200:1120, or 5:28. In other words, only 18% of the total
heat-energy is available for conversion. The second law of
thermodynamics states, not merely that not all the available
energy can actually be used for any purpose except heat, but also
that all energy in an available form (a form other than heat, or else
heat-energy in the form of a difference of temperature) tends to
turn into unavailable energy, that the amount of available energy
in the universe is constantly decreasing.
In the reverse universe we have a different situation, since
the second law of thermodynamics is irreversible. Even the
heat-energy below the temperature of the coldest bodies in the
environment is not merely available, but constantly drawn on. The
same immense fund of energy which in the real physical universe
is constantly stored up and unavailable, now ceases to be
unavailable, but becomes a reserve fund of energy with which
difference of concentration of energy is constantly being built up.
Under the second law of thermodynamics a reserve fund of energy
is constantly stored up in the form of heat and never afterwards
touched; under the reverse of that second law, on the contrary, we
start with this reserve fund of energy and constantly draw on it to
build up energy-differences.