Honestly, I got interested in quarks because of Lore
Fitzgerald Sjöberg's love song about everyone's
favorite Icelandic techno diva:
She's small and she's odd, like a lepton or quark
Oh Björk, oh Björk, oh Björk
Which doesn't exactly rhyme if you pronounce everything right,
but who am I to complain? If you enjoy this kind of thing,
you should check out Lore's Book
Of Ratings.
Along with electrons and their subatomic cousins, quarks
are pretty much the tiniest 'things' that we know of. All
the atoms making up the regular 'stuff' in the universe have
only three ingredients: protons and neutrons, with electrons
flitting distantly about them. In the sixties it was discovered
that protons and neutrons have even smaller components, the
six flavors of quark, held together by the delightfully named
gluons. Quarks have also been found to group together into
a variety of other, more exotic particles only encountered
in very high-temperature environments like the center of the
sun. Physicist
Murray Gell-Mann found the word 'quark' in James Joyce's
Finnegan's Wake. (Is this what particle physicists like to
read in their free time? Yikes.) Somewhere in the book, seabirds
give "three quarks for Muster Mark", as in three
cheers combined with an onomatopoetic imitation of a seabird
call. Richard Feynman wanted to name them 'partons', as in
Dolly, but Murray Gell-Mann won. Did I mention that this was
the late sixties?
So how small are we talking about here? First of all, we
need an imaginative grip on how small atoms are. Bill
Bryson has my favorite comparison: the width of an atom
is to the thickness of a sheet of paper as the thickness of
a sheet of paper is to the height of the Empire State Building.
Another good comparison I've seen: If an apple were the size
of the Earth, its constituent atoms would be the size of grapes.
According to the Internet, a human hair is about one million
carbon atoms wide. An HIV virus contains about a hundred million
atoms total. An E coli bacterium has about a hundred billion
atoms. As befits their greater complexity and size, a typical
human cell contains about a hundred trillion atoms.
But so here's the thing. An atom may be unimaginably tiny,
but it's stupendously huge compared to its nucleus. Imagine
a medium-sized atom, say oxygen, scaled up so that its radius
is the distance from the Earth to the moon, about ten billion
inches. On this scale, the nucleus would be about ten thousand
inches wide, the length of a golf course. A proton would be
about a thousand inches wide, the length of a football field.
On this scale, a quark would be one inch wide.
It utterly freaked me out to learn that atoms are almost
entirely empty space. They hang together tightly because of
the magnetic attraction between electrons and protons, but
don't let the illusion fool you. Anything not subject to electromagnetism
passes right through 'solid' objects as easily as you can
pass through a baseball stadium without touching home plate.
The sun pours trillions of neutrinos through your body every
second without your noticing. The same goes for the dark matter
thought to comprise most of the universe's mass. We know about
dark matter because it interacts with the galaxies via gravity,
but gravity is extremely weak. For all I know, this Starbucks
is as full of dark matter as it is of neutrinos. One reason
modern physics is such a tough pill to is swallow is that
it asks you to accept that more and more of the world's significant
happenings are turning out to be invisible, intangible and
resistant to our intuition. The good news is, our intuition
is flexible and easily reprogrammed to suit new information.
It helps when scientists invest their work with a sense of
fun, like Murray Gell-Mann did in coining the word quark.
Super Mario Brothers is counterintuitive and weird, but that
doesn't stop it from being fun. Why not particle physics?
Quarks come in six types, or flavors, and for each one there's
an antiquark. If you cram enough energy into a small enough
space, it can convert itself into matter, which is the point
of Einstein's E=mc2. One way energy can convert
itself into mass is in the form of quark-antiquark pairs.
Alternately, if a quark meets its associated antiquark, they
mutually annihilate, releasing energy in the form of a high-energy
photon. Here are the six quark flavors, in ascending order
of mass:
up quark |
anti-up quark |
down quark |
anti-down quark |
strange quark |
anti-strange quark |
charm quark |
anti-charm quark |
bottom quark |
anti-bottom quark |
| top quark |
anti-top quark |
Particle physics is a veritable banquet for jargon lovers
like me. Physicists describe and predict quarks' behavior
mathematically using what they refer to as the flavor quantum
numbers, whose names include strangeness, charm, bottomness,
topness, isospin and weak isospin. If I ever release an album
of instrumental posthuman electronica like Selected Ambient
Works Volume 2, I want to use those terms for the track titles.
Sometimes I love particle physics just for the sheer unintended
poetry of statements like these, from around the Internet:
The quark model groups together particles by isospin and
strangeness.
The flavorless mesons are linear combinations of several
quark-antiquark pairs, including up-antiup.
The kaons form two doublets of isospin. One doublet of
strangeness +1 contains the K+ and the K0. The antiparticles
form the other doublet.
The first breakthrough was obtained at Caltech, where a
cloud chamber was taken up Mount Wilson, for greater cosmic
ray exposure.
Two different neutral K mesons, carrying different strangeness,
can turn from one into another through the weak interactions,
since these interactions do not conserve strangeness.
Since neutral kaons carry strangeness, they cannot be their
own antiparticles. There must be then two different neutral
kaons, differing by two units of strangeness.
Isolated quarks are never found naturally - not
since a gazillionth of a second after the Big Bang, anyway.
In the world we inhabit, quarks always appear bound together
in groups of two or three. If you want to study quarks, you
need to rip some protons or neutrons apart and watch what
happens. At particle accelerators like the eagerly-anticipated
Large
Hadron Collider, gigantic magnets smash atoms and their
constituent parts into each other as hard as possible, making
all kinds of weird new particles spray out. As
the Particle Adventure site puts it:
It is as if you stage a head-on collision between two strawberries
and get several new strawberries, lots of tiny acorns, a
banana, a few pears, an apple, a walnut, and a plum.
Fun fact about another humungous accelerator, the ominously-named
Relativistic Heavy Ion Collider: it has a component called,
I kid you not, the 'South Muon Arm Eyebrow.' Isn't that a
song by Anti-Pop
Consortium? And isn't the Relativistic Heavy Ion Collider
what the Death Star uses to vaporize planets?
Gravity and electromagnetism both have infinite range, but
their strenth falls off dramatically with distance. The strong
force holding quarks together works exactly the opposite way.
Its range is extremely short, but within that range the strong
force gets stronger as the quarks get pulled even a miniscule
fraction of a millimeter further apart. We won't be seeing
a free quark roaming around, so far as we know, because by
the time its separation from other quarks is wide enough to
be observable, the strong force's potential energy becomes
high enough to yank new quarks and antiquarks into existence
via E=mc2. These new quarks immediately bind to
their closest neighbors. One analogy here is to a piece of
string - if you pull the two ends far enough apart, eventually
the string breaks, making two 'new' pieces of string. Or think
of pulling on a soap bubble until it pulls apart into two
'new' bubbles. Einstein and others have been saying that energy
and matter are different aspects of the same thing, but the
news hasn't remotely begun to sink in yet, certainly not on
me.
Think of a proton or neutron as an elastic bag with three
hard tiny quarks inside. Inside their bag, the quarks jostle
freely about. If you try to pull a quark out, the bag stretches
and resists. I always imagined an atomic nucleus as just sitting
there while the electrons whiz around it, but the nucleus
actually turns out to be a lively place. The strength of the
force holding quarks together is so intense as to suggest
that they're constantly whipping around inside their bag at
nearly the speed of light. Most of a proton's or neutron's
mass comes from the ferocious energy in the gluon field between
the quarks, not from the quarks' own miniscule masses.
So what's happening in the middle of the sun that makes all
that light and all those neutrinos? The same thing that happens
inside particle accelerators: atoms get smashed into each
other very hard and very fast, spraying out lots of new particles.
These particles smash into other particles, spraying out more
particles, and so on. The process is a lot like the explosion
of a hydrogen bomb, but on a much bigger scale, and with enough
hydrogen fuel to be able to keep exploding for another good
five billion years or so. Plenty of time to find another sustainable
energy source that big.
Stars like our sun start out as big balls of mostly hydrogen
gas that collapse in on themselves due to gravity. In the
center of a star, gravity crushes together the hydrogen atoms
so closely together that their nuclei fuse into helium. Sunlight
comes from the fusion of four hydrogen nuclei into a helium
nucleus, two positrons (antimatter partners of electrons),
two neutrinos, and a bunch of photons. These photons are
the ones that heat us, light us, and power all of the world's
plants (and thus, indirectly, everything that eats the plants,
on up to us.) The oil and coal we dig up is mostly rotted
plant matter from millions of years ago, and its chemical
energy is basically sunlight in a conveniently portable storage
medium.
So if burning fossiziled plankton and ferns is stored fusion
power, why can't we just cut out the middleman? Why don't
our cars run on the fusion of hydrogen into helium? There's
plenty of hydrogen in the water, and helium is as harmless
a waste product as you could ask for. The problem is that
it takes a high temperature and/or ridiculously extreme pressure
to make atomic nuclei overcome their mutual electromagnetic
repulsion and come into the short range of the strong force.
Driving around with a tank full of gasoline is dangerous enough;
imagine driving around with a reactor whose interior is at
ten million degrees. Sadly, we shouldn't be holding our breath
for the fusion-powered cars.
When the universe was only a few tens of microseconds old,
the temperature everywhere in space was so high that all matter
is thought to have been in the form of melted quarks and gluons.
The point of those humungous particle accelerators is to recreate
the tumultuous primeval conditions prevailing in all of space
in the earliest gazillionths of a second of time. The people
at the Relativistic Heavy Ion Collider think that they've
produced quark-gluon plasma. Some burned-out compact stars
out there in space are so compressed by their own weight that
there might be quark matter in their interior. Hypothetical
compact stars composed mostly or entirely of strange quarks
are known as 'quark stars' or 'strange stars'. Cue
Jerry.
When a star runs out of hydrogen to fuse into helium, it
collapses spectacularly in on itself, briefly triggering a
new and much more violent fusion reaction, a titanic explosion.
When this happens to the Sun in five billion years, the expanding
ball of hot gas will vaporize the planets immediately, leaving
behind a big wad of gas glowing dully in the infrared. When
bigger and hotter stars die, the explosion can leave behind
a bigger, hotter and denser remnant known as a compact star.
The difference between a compact star and the sun is like
the difference between ordinary solids and gases. You could
land on the surface of a compact star and walk around, at
least before you got squashed flat. The
racistly-named but nonetheless beautiful Eskimo Nebula is
illuminated by the white dwarf star at its center. Inside
the white dwarf is usually carbon and oxygen nuclei in a sea
of unattached electrons, burning white-hot, thus the name.
A lone star like the Sun is actually unusual. Most stars
come in systems of two or more. Astronomers have identified
several binary systems with one normal star and one white
dwarf. As they mutually orbit one another, the white dwarf
sucks hot gas off the other star's surface. As the white dwarf
gets hotter and denser, electrons
react with protons in its interior to form more neutrons.
Without electrical repulsion between electrons and protons
to hold them apart, the nuclei collapse even further in on
themselves. At this point, a couple of different things might
happen. If the star's core is made of relatively lightweight
fusion remnants like oxygen and carbon, the subatomic particles
undergo an escalating wave of more violent collisions, resulting
in runaway fusion that blasts the star apart and flings its
innards deep into space. It's hot enough in a supernova explosion
to fuse big wads of protons and neutrons together into things
like gold and uranium. You can imagine, then, why gold and
uranium are so hard to come by in quantity.
If a collapsing compact star's center is mostly magnesium
or heavier elements, then the collapse inwards continues and
accelerates through the explosion. As electrons slam into
the nuclei, they keep turning more protons into neutrons.
Because they're not magnetic, neutrons are easier to cram
together, and as the equilibrium shifts towards heavier, more
neutron-rich nuclei, these nuclei become larger and less well-bound.
At a critical density called the 'neutron drip line', the
nuclei fall apart into loose protons and neutrons - imagine
all the bags rupturing. Eventually we reach a point where
the entire star is as dense as a normal nucleus.
If a room-temperature nucleus is like a pebble in a stadium,
think now of a stadium densely packed with pebbles, each of
which weighs as much as a stadium. We now have what the astrophysicists
call a neutron star. They were predicted mathematically not
long after the neutron's discovery in the 1930s, and were
subsequently detected with telescopes. The
Crab Nebula is a supernova remnant containing the Crab Pulsar,
a spinning (and thus electromagnetically pulsing) neutron
star. Matter falling onto the surface of a neutron star
would strike the surface at a ninety thousand miles a second,
and would be instantly crushed under its own weight into a
puddle less than an atom's width thick. A teaspoonful of neutron
star would weigh a hundred million metric tons.
