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.