[MUSIC PLAYING]
It was the spring of 1948,
and the world's top physicists
converged for a conference
in the Pocono Mountains.
Included among them was the
brilliant, bongo playing
physicist, Richard Feynman.
One of the biggest
problems in physics
was describing
quantum interactions
in a way that's consistent with
Einstein's special relativity.
The possibilities and potential
problems seemed endless.
Feynman presented a
simple diagrammatic scheme
for keeping track of and
calculating the particle
interactions.
That's it!
These powerful tools
in physics became
known as Feynman diagrams.
After World War II,
physicists wanted
to further develop
the theory that
explained electromagnetism--
why like particles repel each
other, and opposite charges
attract.
This theory called quantum
electrodynamics or QED,
attempted to calculate
the probabilities
of all possible outcomes
of particle interactions.
But there were two seemingly
insurmountable problems.
First, writing
down the equations
meant keeping track of all
possible particle interactions,
including virtual ones--
a grueling, hopeless exercise
for even the most organized
and patient physicist.
Second, when quantifying
the scattering
amplitudes-- the probability
for how particles
come together,
disperse, or transform
into other particles--
the calculations
break down, producing
infinite values.
Feynman's 1948 diagram
was a simple visual way
to account for and ultimately
quantify all the interactions.
Think of this Feynman diagram
as a story of a particle
interaction in space and time.
The straight lines are particles
of matter, like electrons.
And the wavy lines
are particles that
convey forces, like photons.
In this simple interaction,
two electrons scatter.
Let's rotate this diagram.
Now the arrows are
pointing the opposite way,
and represent positrons,
or anti-electrons,
moving and exchanging a photon.
Each section of the
diagram corresponds
to part of the QED equation.
The stories of these scatterings
begin at the in and out states
of the interaction.
Across those points,
energy, momentum, and charge
should be conserved.
Things get a little
weird in the middle
since these intermediate
virtual particles can disappear
and even go back in time.
This means that any number
of bizarre interactions
could happen in this space.
There are infinitely
many possibilities
for how a particle
interaction can occur,
but Feynman diagrams allows you
to chop off the possibilities.
And calculate the result at
whatever level of precision
you want.
[MUSIC PLAYING]
When Feynman first
introduced his diagrams,
his peers remained
perplexed by how
to use them in their own work.
Fellow physicist Freeman
Dyson translated the diagrams
into mathematics
that researchers
could understand and work with.
He also showed how
infinities could
be converted into finite
values through renormalization.
Soon the diagrams
were everywhere,
transforming modern
theoretical physics.
But after a time,
their limitations
became increasingly evident.
When it comes to collisions
of subatomic particles--
like quarks and gluons--
thousands of diagrams are needed
to calculate the relatively
simple scattering amplitude.
Some physicists are now
working on a geometric approach
for scattering amplitudes,
called the amplituhedron.
It all started at that 1948
conference in the Poconos.
Feynman diagrams helped
unlock the underlying
physics of our universe
on the smallest scale.
