music Big fleas have little fleas upon their backs
to bite 'em. Little fleas have lesser fleas, and so ad infinitum. The Victorian era mathematician Augustus De
Morgan wrote these words as an allegory of the self-similarity -- the idea that things
of vastly different size are often superficially similar...basically it means that patterns
repeat themselves -- for example solar systems and atoms. The idea of self-similarity applies to many
fields of human knowledge, but it has a special place in our modern understanding of the smallest
bits of matter. The ancient Greeks philosophers were among
the first to think seriously about the question of the building blocks of the universe, with
Empedocles offering the four elements of air, fire, water and earth, and Democritus suggesting
tiny, indivisible, objects called atomos, from which our word atom originates. In the last couple of centuries, we have made
great strides in our understanding of just what is the smallest objects in the universe:
first molecules, then atoms and protons, neutrons and electrons and now finally quarks and leptons.
Quarks are found inside protons and neutrons and the most familiar lepton is the electron. So where do we stand? Are quarks and leptons
the final word? Or are they just a way station in our journey to find the smallest constituents
of the universe? Well what do we know? The picture here off
to my left embodies our current best understanding. There are six quarks, with kind of silly names
called up and down, charm and strange, top and bottom. The up and down quarks are found
in ordinary matter, but the others are unstable, existing for only short fractions of a second
before decaying and found only inside particle accelerator collisions. In addition, we have three charged leptons,
called electrons, muons and taus, as well as three neutral leptons called neutrinos,
called the electron neutrino, muon neutrino and tau neutrino. So let's take a look at this graphic. We
have up and down quarks and electrons that make up matter. So why are there four other
kinds of quarks and two other charged leptons? When the muon was found, Nobel Prize winning
physicist I.I. Rabi is reported to have said "Who ordered that?" It turns out that there are commonalities
in the quarks and leptons. The charm quark has the same charge as the up quark, but it's
heavier. Similarly, the top quark has the same charge, but is heavier still. In the
same way, the strange and bottom quark have the same charge as the down quark, with the
ones on the right having a higher mass. This pattern is repeated with the charged leptons
as well. It's a little trickier to talk about the neutrinos, but they are paired with
their respective lepton, so we can group them in a comparable pattern. You'll notice that there appear to be three
distinct carbon copies of the quarks and leptons and we number them 1, 2 & 3. The quarks and
leptons of generation 1 are found in ordinary matter and the others are only found in accelerators.
Nobody has any real idea why there are other 2 other generations, but in my opinion this
is the first real evidence suggesting that quarks and leptons might not be the final
story in the saga of the search for the ultimate building blocks. There is a historical precedent for this pattern.
In the 1860s, Dmitri Mendeleev invented the periodic table of chemical elements. You may
remember from chemistry class. Each column consists of elements with similar chemical
properties. For instance, Helium, Neon, Argon, Krypton & Xenon are noble gasses that interact
very little. In contrast, the column of hydrogen, lithium, sodium, potassium, and so on, are
all very reactive substances. In short, the columns are similar. In contrast, as we move down the columns,
the atomic mass of the elements grows heavier. Although this wasn't clear when the periodic
table was proposed, these commonalities were the first indication of atomic structure.
We now can explain these patterns as atoms of a particular column having similar arrangements
of electrons and the increase in mass comes from having more protons and neutrons in the
atomic nucleus. We can contrast the quark and lepton periodic
table with the chemical one. In the chemical table, columns are chemically-similar and
moving down the rows means more mass, while in the quark and lepton version, the rows
are similar particles, with the mass increasing as you go from row 1 to 2 to 3. This pattern is suggestive of the idea that
perhaps something new is to be found inside quarks and leptons. But you should be careful...I
used the word "suggestive" on purpose. There is no agreement by scientists that these
particles must contain smaller particles within them. Indeed, the Standard Model, which is
our best theory of the subatomic world, treats the quarks and leptons as point-like particles,
with zero size and nothing inside them. So what has our experimental work told us?
Well, when we've looked for constituents of the quarks and leptons, we've never found
any evidence supporting the idea. We know that if quarks and leptons have a size, that
size is smaller than one ten thousandth the size of a proton, smaller than 10-19 meters.
That's about as small compared to an atom as an atom is compared to you. There have been theories advanced for particles
smaller than quarks and leptons. The name for these tiny hypothetical particles is preons.
However, the preon idea is not a popular one. Don't believe it. But it's a cool idea. Another idea for which there is no experimental
evidence is the theory of superstrings. According to this idea, there are objects called strings
that are the smallest constituents of matter. These tiny strings are like little sticks
of spaghetti or little hula hoops that vibrate. The familiar particles are just different
vibrations of the strings, with an up quark perhaps being a B-flat and an electron being
an F-sharp. Again, don't believe in superstrings, but it's also a really cool idea. So, what is the answer? Well, frankly, I don't
know. The best response to that question came from Einstein, when he said "If I knew what
I was doing, then it wouldn't be called research." Still, the search for the ultimate
building blocks has a history of over two millennia and scientists will continue to
look for them. The patterns of the periodic table of quarks and leptons are highly suggestive
and we will continue to study this timeless question.
