Arrows of time

Where is all this taking us? I began by talking about the direction
in which time flows. Remember this is not a true direction in the
sense of North or South, or even a direction in time; it is a direction of
time and can only point in one of two (opposite) directions. There
are two ways of choosing such an arrow: we can either consider
two events and ask which one happened first or, by considering
a quantity that is changing, we can choose an arrow of time to be
pointing in the direction of increase or decrease of that quantity.
It is often claimed that the reason we ‘see’ time flowing in the
direction that we do is because our brains, like any other physical
system, must obey the second law of thermodynamics. Thus the
psychological arrow of time must always point in the direction of
increasing entropy. This is extremely dubious. To suggest that
the entropy in our brains is increasing is wrong. Like any other
biological system, our brains utilize energy to maintain their low
entropy state. To a good approximation, the entropy in our brains
remains constant for most of our lifetime.
The second law of thermodynamics gives us an arrow of
time which seems to be more general and less subjective than
the psychological arrow of time that you and I seem to have
built into our consciousness. We therefore define what is called
a thermodynamic arrow of time, which always points in the direction
of increasing entropy. Since we always see entropy around us
increasing, then by design the thermodynamic arrow will point in
the same direction as the psychological arrow. What if one day entropy began to decrease everywhere in
the Universe? We would say that the thermodynamic arrow has
flipped over. Whatthen happens to the psychological arrow? Does
it now point in the opposite direction? Do we now see sugar
undissolving, packs of cards unshuffling and cigarette smoke
collect up from all around a room and focus in on, and disappear
into, the tip of a lit cigarette?
The answer, some believe, is no. It is here that they
appeal to the notion that our thought processes, which define
the psychological arrow, are chemical processes in the brain, and
like any other physical system, should be subject to the second
law. If for whatever reason, entropy begins to decrease everywhere,
then that includes our brains (and thought processes) and the
psychological arrow would flip over too. I amnot so sure because,
as I mentioned earlier, I believe that our brains strive against
the tide of increasing entropy outside. It is far from clear to me
what would happen inside our brains if entropy began to decrease
everywhere else.
There are two further arrows of time I should mention which
reflect different types of irreversible process in physics. The first
is the quantum measurement arrow. As long as a quantum
system, such as an atom, is left alone and we do not attempt to
measure its properties, it remains fully reversible in the sense that
processes that go on inside it could happen forwards or backwards
in time. Howeveronceweattempt to probe the system (usingsome
experimental apparatus such as a detector to measure the position
of an atom say) a definite direction in time is chosen. Certain
properties are permanently altered by the act of measurement.
Recent research into the meaning of quantum mechanics
suggests that the quantum measurement arrow is very similar
in origin to the thermodynamic arrow. Another way of defining
increasing entropy is through loss of information. By saving a
file on computer you are creating order and decreasing entropy
locally. The reverse happens when you erase a file. You are
losing information and entropy increases. It is now emerging
that the quantum measurement arrow comes about because of a
similar loss of information on the subatomic level. In technical jargon it is said that quantum coherence leaks out into the
environment surrounding the quantum system when it is probed,
thus increasing its entropy. This loss of quantum information
is a bit like the way a hot object leaks heat out into its cooler
environment.
Finally, I should mention a fourth arrow of time in the light
of recent experimental findings. It is called the matter/antimatter
arrow. In a rather subtle experiment carried out at the CERN
particle accelerator in 1998 it was discovered that it is slightly
more likely for antimatter to convert into matter than the other
way round. The experiment, known as CP-LEAR (which stands
for charge parity experiment in the low energy antiproton ring) is
not cut and dried. Rival research groups around the world have
yet to be convinced. But if correct, it suggests that if you were
to start off with an equal amount of matter and antimatter, in the
form of subatomic particles called kaons, then at a later time there
should be fewer antimatter kaons than normal matter kaons. This
provides us with an arrow of time at the level of these particles,
pointing in the direction of diminishing antimatter.