Many physics processes have a property known as
parity invariance.
This means that the probability of a particle
process occurring is exactly the same as the probability of the
same process occurring with the position vectors and directions of
travel of all particles reversed.
What does it mean to reverse a position vector?
Choose any point as your position vector origin and draw a line
from the origin to the position of an object. That is the position
vector of the object. A parity transformation about that origin
would relocate the object at a point found by flipping that
position vector so it goes the same distance from the origin but
in exactly the opposite direction. It turns up to down, left to
right, and front to back!
This seems odd because we are used to thinking of
up as physically very different from down, but if we reverse
everything then the position of the earth changes, too. Consider,
for example, a collision of two spheres in space. There would be
no way you could tell by looking at a movie whether you were
watching an actual collision or a parity reversed simulation of
the collision, each would look equally plausible.
Parity invariance is true for
