CHAPTER II
REVERSIBLE LAWS

Now we shall attempt to find out what are the physical
laws which subsist in this imaginary "reverse universe." To
start with the simple laws of mechanics, we have it given in
the real universe that a body retains its velocity unless there
is some external force to change that velocity. Now, as there
can be no change of velocity in the reverse universe without
a corresponding change of the reverse velocity in the real
universe, and since all forces in both universes are
respectively equal, it follows that this same law of motion
applies also in the reverse universe. In other words, the
law of inertia is unchanged by the reversal in time, and is
therefore what we bay call a reversible physical law.
The second law of motion is that change of
momentum is proportional to force impressed. Now,
following the reasoning which we have already followed
in the case of accelerations, the rate of change of
momentum remains unchanged in our reverse universe.
Furthermore, we have already seen that, in the reverse
universe, the force impressed on a body remains
unchanged. Hence it follows that the second law of motion
subsists in the reverse universe, and is therefore reversible.
The third law of motion is that to every action (force)
there is as also an equal and opposite reaction. This law is
also obviously reversible, since in the reverse universe
neither the magnitude nor the direction of forces is altered.
Energy being the same In the reverse as in the real
universe, it similarly follows that the law of the conservation
of energy holds in the reverse universe, and is therefore
reversible in time. The same holds true of of the law of the
conservation of matter.
One of the principal methods by which motion can be
changed is by impact. An impact may be elastic or inelastic.
In the case of the very smallest particles of matter, that kind
of collision only is possible which loses no energy, but In which
the kinetic energy remains the same as before, that is to say,
an elastic collision; for, in the case of ultimate particles, none
of the original energy can be changed into internal motion of
the particles. Hence only elastic collision is possible in the
case of ultimate particles; and it is not difficult to show that,
in the case of elastic collision, the reversed final velocities
of the same masses will cause, as an effect of the impact,
the reversed initial velocities. Now, since all matter Is made
up of these particles, whatever they may be, and therefore
all collisions of bodies of matter are made up of countless
elastic collisions of ultimate particles, it follows that, in the
reverse universe, where impact occurs, all particles of
matter follow the same course as they would in the real
universe under the same initial conditions. Hence the laws
of impact, when brought down to ultimate particles, are
perfectly reversible and also remain unaltered in the
reverse universe.
The various laws of attraction and repulsion that are
found to subsist in the objective universe, such as
gravitation, electrical and magnetic attraction and
repulsion, etc., dealing as they do with the directions of
forces, must also remain unchanged In the reverse
universe. Similarly with many other general physical laws.
Even the laws of reflection and refraction of light will
remain unaltered in the reverse universe, and are therefore
perfectly reversible.
As a result, we may say in general that, looking to the
ultimate particles of matter, enough physical laws subsist in
the reverse universe to determine, from the positions and
velocities of all particles of matter at a given instant, the entire
past and future of the universe. The result is that, given those
physical laws which we assume to remain always true, if we
should imagine that, in the real universe, at one given moment,
all particles of matter should, while retaining their respective
positions, reverse their velocities, it would follow that this
would be enough, of itself, to make all particles of matter trace
back their previous positions in the reverse order and thus, as
it were, create a reverse universe.