Hello!
Antimatter: what is it really, and how can
we use quantum mechanics to predict its existence?
First up, here’s a lesson on everything
you need to know about quantum electrodynamics
in 60 seconds.
Richard Feynman proposed the Feynman diagram
in 1948 as one method of visualising the otherwise
totally bizarre interactions of particles,
forces and fields at the quantum scale.
Suppose we have two electrons travelling towards
each other with some velocity v.
This motion can be thought of as “as we
go forwards through time, the two electrons
move closer in space”.
So, if we now draw a set of axes, with space
going horizontally and time going vertically,
we can represent this electron motion by two
lines pointing diagonally upwards.
Taking horizontal cross sections of these
two lines going upwards on the time axis shows
the electrons moving closer together in space.
We know that since the two electrons are both
negatively charged, and that like-charges
repel, that the electrons will eventually
be pushed away from each other by an electromagnetic
force.
So, we can represent this repulsive motion
by pointing the lines away from each other.
But wait, what happened in the middle!?
Quick side note: here’s a lesson on everything
you need to know about quantum field theory
in 20 seconds.
Forces, such as the electromagnetic force,
are mediated at the subatomic level by particles
called “gauge bosons” or “force-carrying
particles”.
In the case of the electromagnetic interaction,
this boson is called the photon.
Since the photon only exists for a tiny instant
of time, and (for purely theoretical reasons)
cannot be captured while in transit between
the two electrons, the photon is known as
a “virtual” particle.
Side note over, and now we can complete our
Feynman diagram:
The electrons move apart from each other because
a virtual photon, mediating the electromagnetic
force, propagates between them.
In effect, the photon is what “tells”
the electrons to be repelled from one another.
So our diagram is complete.
But the fact that two electrons repel each
other is pretty well-established in physics,
so this diagram isn’t particularly exciting.
But quantum mechanics is far, far more weird
than this.
For two reasons:
The first being some super-complicated quantum
voodoo that I couldn’t possibly explain
because I don’t completely understand,
The second being the fact that, in special
relativity, travelling close to the speed
of light causes space and time to get mixed
up, such that what looks like space to you
might appear as time to me (more on this in
a later video),
And so the space and time axes on this Feynman
diagram are completely arbitrary, and in fact
the diagram can be rotated any number of degrees
and still make complete physical sense.
This is crazy.
So what if we put the diagram on its side
like this?
Hang on, that electron is moving backwards
through time.
Side note number two:
All subatomic particles have a quantum-mechanical
property called spin, and they act very similar
to spinning objects in the “real” world.
When we have a normal electron travelling
forwards through time it will accelerate away
from negative charge, accelerate towards positive
charge and spin in (let’s say) a clockwise
direction.
This pattern of acceleration is what leads
us to conclude that the electron is itself
negatively charged (like-charges repel, opposite
charges attract).
Suppose now we play the video in reverse with
the electron travelling backwards through
time.
Now, it will accelerate towards negative charge,
accelerate away from positive charge and spin
in an anticlockwise direction.
In other words, this time-travelling electron
will appear to us to be positively charged,
with the same mass as the electron, but with
its spin (and most other quantum properties)
reversed.
This mirror image, so Feynman proposed, is
what we know of as “antimatter”: normal
matter, travelling backwards through time.
End of side note two.
So, we now know that this particle is a positively-charged
anti-electron, or “positron”.
From this diagram, we can see that the electron
and the positron don’t repel each other
like the electron and the electron did, but
rather smack right into one another (presumably
because of their opposite charge).
When they do, they disappear from the diagram
entirely.
In their place on the timeline has now appeared
a single photon.
This phenomenon you may know as “matter-antimatter
annihilation”: matter and antimatter come
into contact and mutually annihilate to produce
pure energy (in this case, in the form of
a gamma photon).
(Angels and Demons fans please contain your
excitement)
We can see from this quantum-mechanical symmetry
law that the photon produced in the annihilation
is just the same as the photon exchanged between
the two electrons in the first diagram, but
viewed with the space and time axes reversed
(weird).
This photon propagates uninterrupted for a
while, before it too disappears and is replaced
once again with another electron and positron.
This phenomenon you may know as “pair production”:
pure energy (in this case in the form of a
photon) spontaneously produces a pair of matter-antimatter
particles.
All of this is allowed because of Einstein’s
energy-mass equivalence relation: annihilation
converts the mass of a particle and an antiparticle
into energy, pair production converts energy
into the mass of a particle and an antiparticle.
Rotating the diagram a full 180 degrees from
its original position demonstrates another
more obvious feature of positrons: they mutually
repel each other, just like the electrons
did.
So, just by taking one of the simplest laws
of physics there is (the fact that like-charges,
such as in electrons, repel each other) and
exploiting the space-time symmetry laws that
are inherent in quantum electrodynamics, we
have been able to not only predict the existence
of antimatter, but even derive most of its
major properties and physical interactions.
In other words, by just rotating a really
simple diagrammatic representation of an equation,
we have been able to predict at least three
new laws of physics.
This is the power of quantum mechanics, and
the genius of Richard Feynman.
