Wolfram Alpha:

```CPT Symmetries
----------------

Symmetries play an extremely important role in physics.  According to
Noether's theorem a symmetry in a physical experiment suggests that
there is a conserved quantity.  Thus, conservation laws and symmetries
are very closely linked.  There are 3 important symmetries that have
been identified.  These are the symmetries of CHARGE CONJUGATION (C),
PARITY (P), and TIME REVERSAL time (T).

Charge Conjugation Symmetry (C):

Reversing the electric charge and all the internal quantum numbers.

Parity (P):

Parity involves a transformation that changes the algebraic sign of
the coordinate system.  A parity transformation replaces such a
system with a type of mirror image.  Parity is an important idea
in quantum mechanic al systems because the wavefunctions which
represent particles can behave in different ways upon transformation
of the coordinate system which describes them.  In general, if a
system is identical to the original system after a parity transformation,
the system is said to have even parity.  If the final formulation is the
negative of the original, its parity is odd.

Conservation of parity implies that it is impossible to distinguish
right from left and clockwise from counterclockwise. The laws of
physics are therefore indifferent to a mirror reflection.

Time reversal (T):

In simple classical terms, time reversal just means replacing t by -t,
inverting the direction of the flow of time. Reversing time also
reverses the time derivatives of spatial quantities, so it reverses
momentum and angular momentum.Newton's second law is
quadratic in time and is invariant under time reversal. It's invariance
under time reversal holds for either gravitational or electromagnetic
forces.

There are examples in the nature where each of these symmetries
can be violated individually.  It was thought for a time that a
'stronger' symmetry could be created by combining C and P.  This
became known as CP SYMMETRY.  However, although the electromagnetic
and strong force have been shown to obey CP symmetry, the weak force
does not.  One example cited is the β decay of Cobalt-60.  As a
result, physicists looked for a yet stronger symmetry.  This came
in the form of CPT SYMMETRY.  Based on strong theoretical arguments
that are very complex, it is believed that when C, P and T are combined
a profound symmetry is produced that is consistent with all known
experimental observations. ```