Environmental Engineering Reference
In-Depth Information
things go the other way with the passing of time. This idea is central to the study
of thermodynamics where the disorder in a system is a measurable quantity called
entropy. The total entropy of the Universe appears always to increase with time. It
is possible to decrease the entropy (increase the order) of a part of the Universe,
but only at the expense of increasing the entropy of the rest of the Universe by a
larger amount. This net disordering of the Universe is in accord with our perception
that time has a direction. We cannot use natural processes to “wind the clock back”
and put the Universe into the state it was in yesterday - yesterday is truly gone
forever. That is not to say that the laws of physics forbid the possibility that a
cup smashed on the floor will spontaneously re-assemble itself out of the pieces
and leap onto the table from which it fell. They do not; it is simply that the
likelihood of order forming spontaneously out of disorder like this is incredibly
small. In fact, the laws of physics are, to a very good approximation, said to be
“time-reversal invariant”. The exception occurs in the field of particle physics where
“CP-violation” experiments indicate that time-reversal symmetry is not respected
in all fundamental interactions. This is evidence for a genuine direction to time
that is independent of entropy. Entropy increase is a purely statistical effect, which
occurs even when fundamental interactions obey time-reversal symmetry.
Thermodynamics gives us a direction to time and periodic events allow us to
measure time intervals. A clock is a device that is constructed to count the number
of times some recurring event occurs. A priori there is no guarantee that two clocks
will measure the same time, but it is an experimental fact that two clocks that are
engineered to be the same and which are placed next to each other, will measure,
at least approximately, the same time intervals. This approximate equivalence of
clocks leads us to conjecture the existence of absolute time, which is the same
everywhere. A real clock is thus an imperfect means of measuring absolute time
and a good clock is one that measures absolute time accurately. One problem with
this idea is that absolute time is an abstraction, a theoretical idea that comes from
an extrapolation of the experimental observation of the similar nature of different
clocks. We can only measure absolute time with real clocks and without some
notion of which clocks are better than others we have no handle on absolute time.
One way to identify a reliable clock is to build lots of copies of it and treat all the
copies exactly the same, i.e. put them in the same place, keep them at the same
temperature and atmospheric conditions etc. If it is a reliable clock the copies
will deviate little from each other over long time intervals. However, a reliable
clock is not necessarily a good clock; similarly constructed clocks may run down
in similar ways so that, for example, the time intervals between ticks might get
longer the longer a clock runs, but in such a way that the similar clocks still read the
same time. We can get around this by comparing equally reliable clocks based on
different mechanisms. If enough equally-reliable clocks, based on enough different
physical processes, all record the same time then we can start to feel confident
that there is such a thing as absolute time. It is worth pointing out that in the 17th
century reliable clocks were hard to come by and Newton certainly did not come to
the idea of absolute time as a result of the observation of the constancy of clocks.
Newton had an innate faith in the idea of absolute time and constructed his system
of mechanics on that basis.
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