Symmetry in common usage is a kind of vague
term like most terms and common usage.
We use it flexibly.
The idea of symmetry that has turned out to
be extremely fruitful in mathematics and physics
and in the fundamental description of nature
is a precise distillation of some aspects
of the common usage.
So it’s not unnatural to call it symmetry
but it’s something very precise that we
can describe.
When I say what it is it’ll sound kind of
mystical but it’s actually I’ll spell
lit out and you’ll see what I mean.
So symmetry in the sense that’s turned out
to be fruitful in mathematics and physics
and fundamental investigations is change without
change.
Now you might be puzzled.
What does that have to do with symmetry?
Well consider a circle.
A circle is a very symmetric object.
You can rotate it around its center by any
angle and although every point on the circle
may move, the circle as a whole doesn’t
change.
And that’s what makes it symmetric in the
intuitive sense.
You can change it.
You can make changes on it which might have
changed it but although they transformed each
part of it, don’t transform the thing as
a whole.
So that’s what makes a circle a symmetric
object.
An equilateral triangle for instance you can’t
rotate through any angle and get the same
thing.
It’ll change.
If you rotate it one-third of the way around
though by 120 degrees it goes over into itself.
If you rotate around the center by 120 degrees
it’s the same equilateral triangle.
Whereas if you take some lopsided triangle
it’ll never go back to itself until you
come all the way back to a trivial transformation.
That doesn’t change anything.
So change – so a triangle has less symmetry
than a circle according to this concept but
some symmetry.
And so you start to see how this concept of
change without change matches the intuitive
notion of symmetry.
The great advantage of that definition is
that you can apply it in very broad context
not only to describing the symmetry of objects
but to describing the symmetry of physical
laws or the symmetry of equations.
So, for instance, the theory of relativity
is a statement of symmetry that if you change
the way the world looks by moving past it
at a constant velocity you change the appearance
of everything that’s happening.
But the underlying laws are still valid.
That’s the assumption of the theory of relativity
that drives it and makes it powerful.
The idea that the laws of physics don’t
change as a function of time is also a symmetry
because it means you can change when you start
your clocks and although the time stamps you’ll
give to each event look different the underlying
equations will be the same.
That’s the way of staying that the laws
of physics don’t change.
And similarly with that the fact that the
same physical laws apply at different places
is a symmetry because you can change your
position without changing the way the laws
work.
So symmetry is a very powerful constraint
on our description of the world that nature
seems to respect in many ways.
Now the kind of symmetry that leads to quantum
chromodynamics or general relativity or quantum
electrodynamics is mathematically considerably
more complex but it’s the same idea.
So there are transformations of the equations
that change the different terms in them.
So they might change an electric field into
a magnetic field or a magnetic field into
a combination of electric and magnetic that
change the way the equations look but don’t
change their consequences.
So the equations look quite different – some
parts have moved over to the left and some
parts have moved over to the right and some
things have been multiplied in funny ways.
But their consequences, their content is exactly
the same.
That’s the kind of equations that are like
the circles among equations are ones that
have this symmetry property and those are
the kinds of equations that turn out to be
the ones that appear most prominently in our
fundamental description of nature.
It’s an extraordinary thing but that’s
not only true but that’s how we got to the
equations in the first place.
