If you've studied any physics you know that
like charges repel and opposite charges attract. But why? It's as though this thing - electric charge
- is as fundamental a property of an object as its mass. It just sort of ... exists. Well it turns out if you dig deep enough,
the fundamental-ness of charge unravels, and in many things, including mass itself,
unravel with it. Although many mysteries remain in physics,
at least our understanding of electricity and magnetism seems pretty complete. The math that describes it - Maxwell's equationsÂ
or quantum electrodynamics - seem to wrap it up nicely. Except that all of electromagnetism is powered
by a single property: electric charge. And neither Maxwell’s equations nor QED
say a thing about what electric charge really is. It seems to just be a property that particles
can have or not have - as fundamental as mass. Like we’ve got to the exasperated end of
a child’s train of but why? But why?”, where the only answer is: "just
because." The idea of “fundamental” feels like the
physics version of “just because”. But actually, in the case of electric charge
we have at least one or two more “but why’s” with which we can annoy the universe. Today we're going to ask them,Â
and the answers will take us  through the birth of Particle Physics, and, in fact, through the birth of the universe
itself. As with much of modern physics, this story
begins with Werner Heisenberg, whose epiphanies birthed quantum mechanics. At some point, Heisenberg turned his remarkableÂ
intellect to the newly discovered neutron. He was suspicious of its similarity to the
proton - they’re practically twins in the atomic nucleus, occurring with similar numbers
and almost the same mass, with the only major difference being our mysterious friend - electric
charge. The neutron seemed like a chargeless,Â
or neutral proton, hence the name. Heisenberg wondered if the two particles were
in fact just different states of a single particle which he called the nucleon. At this point we already knew of particles
that had internal states. For example electrons have this thing called
spin - a quantum analogy to angular momentum. Spin can take on discrete values; in the electronsÂ
it can be +1/2 or -1/2, loosely corresponding to the spin axis being aligned or anti-aligned
with your measurement device, which we’ll call “up” and “down” states. And spin is conserved - flip an electron’s
spin and the difference has to be transferred by a photon. So if the protons and neutrons are just two
states of the same particle, Heisenberg reasoned that they may be differentiated by a property analogous to spin, governedÂ
by analogous mathematics. Thus, in 1932 Heisenberg proposed aÂ
new fundamental property of matter:Â Â Isospin, a contraction of isotopic or isobaric spin, depending on
who you ask. In this theory the proton would be the “up”
state with isospin 1/2, and the neutron would be the down state with isospin -1/2. By introducing this new conserved quantity, Heisenberg started to makeÂ
sense of the relationship between protons and neutrons. For example, with this choice of their relative
isospins, it made sense why nuclei prefered to have roughly equal numbers of protons and
neutrons, and at the same time allowed precise predictions of the outcome ofÂ
collisions between these particles. But for isospin to really do its job, it needed
to explain the most obvious difference between protons and neutrons - which is to say electric
charge. Charge would have to depend on isospin, which
could mean that charge is not a fundamental property after all. Fast forward a few decades. Our particle colliders advanced, leading to
the discovery of weird new particles. So many of them, in fact, that physicists
struggled to make sense of this so-called particle zoo. But there were some clues. For example, some of these particles had very
similar masses but very different electric charges, which I hope reminds you of the proton
and neutron. So maybe each of these groups were really
a single particle in different states - with different isospins. The case for isospin was solidifying. But what exactly was the connection between
isospin and electric charge? Well that mystery was solved independently
by Kazuhiko Nishijima and Murray Gell-Mann. Peering into the depths of the particle zoo,
they noticed another pattern. There seemed to be a family of particles that
were only created in pairs. Similar to how the electron and positron are
only created in pairs in order to conserve electric charge. But these new particles weren’t doing this
to conserve charge, nor isospin, nor any other known property. This suggested a brand new conservedÂ
quantity, which was altogether strange. In the same way that isospin followed the
same mathematics as regular quantum spin, this new property seemed to obey the math
for our old friend electric charge. They called it Hypercharge. Nishijima and Gell-Mann discovered an even
deeper pattern. Electric charge, isospin and hypercharge were
intimately connected across all particles. In fact, it seemed that electric charge was
just isospin plus half of hypercharge. To be pedantic, that’s the z-component of
isospin, but we’ll put that complication aside for now. The conservation of fundamental properties
defines which interactions are possible and which are impossible. Charge alone couldn’t explain the patterns
of interactions and particle types observed in the particle zoo. However hypercharge and isospin seemed to
do a much better job - suggesting that these may in fact be more fundamental than charge. But there remained a mystery. Not every combination of isospinÂ
and hypercharge were possible. It was Murray Gell-Mann who first noticed
this. Plotting particles according to their isospin
and hypercharge revealed peculiar geometric patterns. For example some groups of eight particles
formed hexagons, and one group of ten particles formed a triangle. Except that the triangle was missing the bottom
corner. No big deal - Gell-Mann just hypothesized
an undiscovered particle - the omega baryon - with the right isospin and hypercharge to
fill that hole. Sort of like how Mendeleev had used holes
in the Periodic Table to predict the existence of unknown elements. And when the omega wasÂ
discovered by experimentalists, Gell-Mann got his Nobel prize. Isospin and hypercharge seemed to be “deeper”
than electric charge. But the geometric relationship between these
two new properties hinted that there may exist even deeper physics yet; even moreÂ
fundamental rules which explained why they should be constrained in these ways. Again, it was Gell-Mann who figured this out. He recognized that these patterns were actually
representations of a mathematical symmetry known as SU(3). Unfortunately we can’t get into the gory
details of symmetry groups in this episode, but in short, Gell-Mann realized that he couldÂ
make sense of the geometric symmetry if nucleons themselves were not elementary particles,
but rather made up of smaller components, which he dubbed quarks. He showed that isospin and hypercharge were
just emergent properties that reflected the different types of quarks that make up one
of these particles. Starting with experiments at the Stanford
Linear Accelerator Center in 1968, the reality of quarks quickly became conclusive. So after all this hard thinking it turns out
that isospin and hypercharge were as much mathematical abstractions as was electric charge. There must be something deeper - something
that lives in the hearts of these quarks and other elementary particles - that governs
these differences between particle groups, and that also governs electric charge. The quark model for nucleons led to a descriptionÂ
of the strong nuclear force via this SU(3) stuff to give us quantum chromodynamics. But that’s a story for another time. It may seem like the strong force led us astray
- but actually it points to the answer. For one thing, the early approaches of HeisenbergÂ
and Gell-Mann and others are exactly what we need - it's just that they were applied
to the wrong force of nature. And it's by unraveling one of the forces of
nature that we can explain electric charge - but it's not the strong force, it's not
even electromagnetism. The secrets of electric charge are actually
hiding in the last, most obscure of the quantum forces - the weak force. It’s also the weirdest force of all, and we need to consider two ofÂ
its weirdest properties. First, the weak force can transform particles intoÂ
other particles - something no other force can do. Second, it only works on left-handed particles. That second really does sound weird, and it
is - but it's the thing that's going to connect all of this back to quantum spin, which is
sort of where we started. One consequence of quantum spin is this thing
called chirality, which is sort of the projection of spin in the direction that a particle is
moving. It's more complicated, obviously, but that'll
do for now. Particles can have right-handed chirality
if their spin is clockwise relative to their momentum vector and left-handed chirality
if it's counter-clockwise. Only particles with left-handed chirality
feel the weak force. For example, the electron has both a right-
and left-handed component. Only the left-handed component can emit one
of the weak-force carrier particles - the W boson - and in doing so transform into a
neutrino. Remember that Heisenberg imagined that the
proton and neutron were differentiated by this new conserved quantity, isospin. We can play the same trick with the electron
and neutrino. So it turns out that the new conserved quantity
behaves eerily similarly to isospin. We're going to call it weak isospin. Only left-handed particles have it, and so
it has an intimate connection to the quantum spin. Weak isospin is effectively the charge of
the weak force, carried by these W bosons. To fully explain weak interactions we need
a second charge - this one carried by the Z boson. It acts more like electric charge, so we'll
be imaginative and call it weak hypercharge. And here's the weird thing: weak isospin and
weak hypercharge are mixed in exactly the same way as Gell-Mann’s versions of these
quantities. Which is to say, electric charge equals weak
isospin plus half weak hypercharge. And we know these weak versions of isospin
and hypercharge must be fundamental because they are properties of elementary particles
that can't be broken into smaller pieces. Particles like the electron, the neutrino,
and even the quark. That's right, quarks feel the weak force and
obey the same rule for their electric charge. It turns out that our old strong-force versions
of isospin and hypercharge in the composite particles of the particle zoo emerge from
their different quark content, but are driven by these more "real" weak-force quantities. Let me summarize where we've got to: the chargeÂ
that drives electromagnetism is governed by the charges that drive the weak force. So does that mean that electric charge is
not really fundamental? We need to question what fundamental really
means. What we’ve learned is that electromagnetism
and the weak force are deeply connected. Or, should I say, were connected. These two forces were once united in what
we call the electroweak force, whose charges were the same weak isospin and hypercharge
that we just discovered. Something happened to that force in the very
early universe to force these charges to only take on a specific combination of values - the
combination that we now observe as electric charge. That event - the breaking of electroweak symmetryÂ
- created the weak and electromagnetic forces as we know them today. So we now know that electric charge is a sort
of shadow of the ancient fields from the birth of the universe. Very soon we’ll follow this thread deeper
to fully understand why these fields separated, and how, in this process, the Higgs field
was also created. Which, by the way, grants mass to elementary
particles - yet another supposedly “fundamental” property”. But is it any more fundamental that theÂ
dubiously fundamental electric charge? And if mass isn't fundamental, then what is? We’ll find out soon as we continue to unravel
the tangled symmetries of spacetime. Just a couple of things before we go: FIrst we’d
like to tell you about Subcultured, a brand new documentary series from PBS that exploresÂ
the lesser known communities and folx that have had a major impact on the mainstream
and answers questions like: What can somebody who is completely blind teach you about video
games? How did anime become a 25 billion dollar industry? And why are these guys putting underwear on
goats? You can check it out over on PBS Voices, link
in our description, and let them know (politely) that Space Time sent you. And the second think is just our thanks. Thanks  for watching. Clicking that linkÂ
when you get the notification really helps the show grow. As does subscribing - just tap that little
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supportive, you could always join us on Patreon. We have a really great community - so come
hang out with us on the discord and on our monthly Zoom chat. And if you're already a Patreon supporter
- well thank you! We're hypercharged to have you with us. But today I want to give an extra specialÂ
thank you to David Taiclet, who's supporting us at the big bang level. David, the swiss are famous for their chocolatiersÂ
and for their giant particle colliders. Well, we arranged a collaboration and have
put together a nice box of assorted exotic hadron flavoured candy asÂ
a token of our gratitude. My favorites are the fudge-filled hexaquarks,
which are one of the most delicious candidates for dark matter. And the rho meson truffles .. but eat them
before they decay in 10^-24 seconds. But maybe just skip the chocolate-dippedÂ
Kaons - they taste a little strange. Thank you David, as you can tell, you are
supporting some very important work.
Was this episode harder to understand than usual, or is it just me?
help I can’t feel my toes