In one of his books, Douglas Adams describes someone's state
of mind and a concrete floor as both being "excitingly
cracked." The world we live in is excitingly cracked
too, and for me the most exciting crack of all is the weak
nuclear force. The contradiction inherent in idea of a weak
force is only the beginning of the rich zen absurdity that
awaits within.
The weak force is the least famous of the four fundamental
forces believed to underlie everything that's happening in
the observable world. Gravity gets more press because it happens
on the macroscale, and it's directly accessible to our senses.
The next most familiar force is electromagnetism. Even if
few of us get how it works, we get a lot of firsthand experience
with it at human scale. Electromagnetism holds protons and
electrons together to form atoms, it makes those atoms stick
together in molecules, and it makes the molecules break apart
and recombine in particular ways. Chemistry, light, heat,
electricity, radio waves and x-rays are all electromagnetism
at work. Up until quite recently, gravity and electromagnetism
were thought to account for every phenomenon in the universe.
Once people figured out how to probe the internal structure
of atoms, we found out that there are two more fundamental
forces, named the strong and weak, because they're, no surprise,
strong and weak. Physicists also call them the strong and
weak interactions, because their effect is more complicated
than simple pushing and pulling. The strong force is closer
to the way we imagine electromagnetism working. The same way
electromagnetism attracts protons and electrons together to
form atoms, the strong force holds quarks together to form
protons (and neutrons, and many other things with scifi names.)
And in much the same way that electromagnetism makes atoms
stick together to form molecules, the strong force between
quarks holds protons and neutrons together to form atomic
nuclei. Protons repel each other magnetically, but the strong
force's grip is a thousand times stronger, so it takes a hell
of a lot of energy to pry an atomic nucleus apart. So that's
the strong force.
When they come to the weak force, most physics texts suddenly
become vague and evasive. There's usually a single feeble
sentence about how the weak force mediates certain forms of
radioactive decay. This is true, but it doesn't begin to approach
how delightfully weird the weak force actually is. A better
one-sentence description would be: "The weak force transforms
one kind of particle into another, in ways that utterly freak
us out when we think about them too hard."
So for instance, there's an unstable form of nitrogen called
nitrogen-16. As the components of the nitrogen-16 nucleus
jostle and shift, it's possible for the quarks to come within
the weak force's extremely short range. This is where the
fun begins. A neutron contains two down quarks and an up quark.
In a process known as beta decay, the weak force transforms
one of the down quarks into an up quark. This flavor change,
as it's known, transforms the neutron into a proton, an electron
and an antineutrino. With its new proton, the atom now becomes
oxygen-16, and the electron and antineutrino go zipping off
on their merry way. I don't know if you can transmute lead
into gold this way, but weak interactions underlie the fusion
reactions powering the sun, and they generate a good part
of the heat emanating from the earth's core. There are also
weak interactions happening inside nuclear reactors and in
the upper atmosphere as it gets struck by high-energy particles
from space. There's not very much weak force action in our
daily lives, though that may change as new technological possibilities
reveal themselves. Some
clever person claims to have invented a laptop battery that
runs for thirty years without a charge, exploiting beta decay
of borides in a semiconductor. That would be pretty cool,
except for the 'carrying radioactive isotopes of boride in
my bag' part.
They do use the weak force in hospital PET scanners to produce
a live image of processes in the body. Before you lie down
in the scanner, they inject you with a radiotracer, a compound
containing small amounts of a short-lived radioactive isotope
of oxygen, nitrogen, carbon or whatever. The radiotracers
are incorporated into compounds normally used by the body,
like glucose, water or ammonia. As your body metabolizes the
compounds, the weak force transforms down quarks into up quarks
in the tracer atoms' nuclei, emitting anti-electrons,
aka positrons, in the process. (Thus the name, Positron
Emission Tomography.) The positrons don't get far before they
collide with nearby electrons, mutually annihilating into
pairs of photons that go zipping off in almost exactly opposite
directions. These photons collide with the ring of magnetic
sensors in the PET scanner. Lots of other photons are constantly
whacking into the sensor ring at any given moment, so the
scanner devotes much computer power to ignoring them, looking
only for pairs of photons arriving at the same time on opposite
sides of the ring. The computer then deduces where in the
body the photons originated, and voilá, you get a 3D
animated picture of your organs doing their thing.
Physicists are currently devoting much effort to understanding
how the weak force may be involved in giving the various subatomic
particles their various masses. The idea is that there's an
invisible energy field pervading all of space called the Higgs
field. This energy field gives space a weak force charge,
the way wet air builds up an electric charge during a thunderstorm.
Particles susceptible to the weak force drag against the Higgs
field, the way you drag when you walk in waist-deep water.
In Lisa Randall's analogy, the Higgs field is like a fine
mist of paint. Quarks and electrons interact weakly, so they
'pick up paint', and as they drag against the Higgs ocean,
we see them as possessing inertia, and thus mass. Photons
don't interact weakly, so they pass through the Higgs ocean
unimpeded, making them massless and enabling them to zip along
forever (until they hit something.) Weak force carrier particles,
the W and Z bosons, drag heavily against the Higgs ocean,
so they have a lot of mass as (as force-carrying bosons go)
and they 'peter out' quickly over a severely short range.
That's why the weak force only works on such tiny scales.
The weird thing is that the standard model of particle physics
predicts, and experiments confirm, that at extremely high
temperatures, electromagnetism and the weak interaction turn
out to be two different manifestations of the same force.
Think of the way water and ice cubes appear to be made of
different 'stuff' until you heat them up and see that they
both become steam. The idea is that when it gets really, really
hot, the Higgs ocean vaporizes. With no paint to drag against,
W and Z bosons become massless, making them indistinguishable
from photons. A picture emerges of an 'electroweak force'
that existed in the furiously hot conditions immediately following
the big bang. As the universe
cooled, the Higgs ocean condensed like steam into water, breaking
the so-called electroweak symmetry into what we experience
as electromagnetism and the weak force. The unification of
the different forces into a single mathematical description
has been a much-desired goal of physicists since Einstein.
The experimental verification of electroweak unification has
stirred much hope that at even higher temperatures, the other
forces might unify as well.
Albert Einstein believed strongly
that the forces were all unified somehow, and he spent many
decades of his life working on it, though ultimately without
success. String theory is motivated by a similar search for
a grand unified theory of the forces. Unfortunately, experimentally
testing for electrostrong unification is way beyond our technological
reach at the moment. Testing for the unification of electrostrong
with gravity isn't even possible on Earth. If we wanted to
build a big enough accelerator, it would have to be in space.
I'm imagining something like Larry
Niven's Ringworld, or Halo.
Making one flavor of particle transform into another is peculiar
enough, but the weak force gets better. Subatomic particles
have a property called spin. I can't get any of my scientist
friends to make clear to me what's doing the spinning, but
mathematically it's a well-understood phenomenon, one that
physicists and engineers use routinely. So far as I can tell,
particles behave as if they're spinning rapidly at all times,
at very particular speeds. Particle spin can be either left-handed
or right-handed - imagine the spin axis as your thumb, and
your curving fingers pointing in the spin direction. For some
as-yet-unknown reason, the weak force acts only on left-handed
particles, not on right-handed particles. This finding suggests
a previously-unsuspected deep asymmetry to the universe's
overall handedness. Why should a fundamental force 'care'
whether a particle's spin is left-handed or right-handed?
No one knows, but I found some intriguing clues in a
fascinating discussion of chirality by David
M Harrison. What follows draws heavily on his article.
Think of a square. If you rotate it ninety degrees, it looks
exactly the same. Even more symmetrical is a circle - it looks
the same no matter how you rotate it. More symmetrical still
is a sphere, which looks the same even if you rotate it off
the page. In the world we experience, there are some very
basic symmetries that appear to apply to every physical happening:
Space translation symmetry. If I do something
over here, it works the same as if I do it over there.
Time translation symmetry. If I do something today,
it works the same as if I do it tomorrow.
Rotational symmetry. If I do something facing
north, it works the same as if I do it facing east.
Scale symmetry. If I do something, it works the
same as if everything is ten times bigger or smaller.
Mirror symmetry. If I do something, it works the
same as if I do it mirror-imaged.
In 1956, Chien-Shiung Wu demonstrated that the weak interaction
is not mirror-symmetric. This was a big shock for physicists,
and for this amateur physics fan as well. Before we can probe
this matter, it helps to understand what mirror symmetry actually
is. David
M Harrison answers a question that's bothered me my whole
life: why do mirrors reverse left and right, but not up and
down?
We imagine that we write a name on a fairly thin piece
of paper. We hold the sign up to a mirror. We see that left
and right are apparently reversed, but not up and down.
Is this because we are looking with two eyes that are aligned
horizontally, our left eye and our right one? No, because
if we look with only one eye, it still looks the same.
We can explore this a little further by constructing a
round disc. We cut a hole in the middle that we can look
through, and we draw four F's on the disc. We hold the sign
up to a mirror and look through the hole. All the F's are
now a mirror image.
What if we hold the sign with the name on it upside down?
The mirror still reverses left and right, not up and down.
Now we position the sign right side up and hold it up to
a light and look through it from the back. We see the text
in a mirror image! So if we hold the sign up to the mirror,
what we see in the mirror looks like what we see looking
through the sign from the back.
Now we take the sign, still looking at it from behind,
and pull the left and right edges towards us. We hold the
bent sign up to the mirror and peek over it at the mirror.
The text in the mirror image is still mirror reversed. But,
in the mirror image the left and right edges of the sign
are curving away from us!
Now we begin to understand that it is front and back that
are being reversed by the mirror. The original has the left
and right edges of the sign curving towards us, while in
the mirror the left and right edges curve away from us.
A further subtlety is that in some sense the surface that
we see in the mirror is the surface that we see when we
look directly at an object, but we are looking at it from
inside!
In other words, when you see something in a mirror, you're
looking at its eversion, a mathematical term meaning 'turned
inside-out'. Another way to think of everting something is:
turning it a hundred and eighty
degrees in a hypothetical fourth dimension of space. Maybe
you can also evert something by turning it around on the time
axis. For instance, the math guys say that if you time-reverse
a photon, you get the same photon, but with opposite spin.
If you time-reverse a black hole,
you get a white hole, ie an inside-out black hole. This
seems to me like it has to mean something.
David M Harrison agrees. He observes that living creatures
are mostly, but not totally, mirror-symmetric. Our heart leans
to the left, our liver is on the right. The left and right
hemispheres of the cortex process different information. Nobody's
face is perfectly mirror-symmetrical. We usually experience
greater symmetry as attractiveness. Leading men like George
Clooney and Brad Pitt have more facial symmetry than character
actors like Paul Giamatti and Jerry Orbach. An easy Photoshop
experiment: Take a photograph of the face of someone you know
and replace the right side of the face with the mirror-imaged
left side (or vice versa.) It changes the person's appearance
dramatically. Mac laptop owners should try the mirror-image
effect in Photo Booth. Creepy!
David M Harrison goes on to point out that life's slight
violation of mirror symmetry happens down all the way down
at the molecular level as well. So for instance, think of
DNA. The 'A' in GATTACA is the amino acid alanine. Like many
larger molecules, alanine comes in two mirror-imaged forms,
left-handed and right-handed. The DNA of all living things
on Earth contains only left-handed alanine, not right-handed.
Most of the organic molecules only exist in one particular
handedness in our bodies. Is there a relationship between
the violation of mirror symmetry by weak interactions and
by life? David M Harrison says: possibly.
Back in the fifties, it was discovered that if you put together
a 'soup' of carbon, water, a little nitrogen, and trace amounts
of other common elements, and you zapped the mixture with
electricity, then amino acids and other simple building blocks
of life would form spontaneously. There's nothing mysterious
at work here. It has to do with the number of electrons in
the outermost orbitals of the Earth's most common atoms and
molecules. Electrons and protons are like the south and north
ends of little magnets. Imagine shaking a box of bar magnets
- eventually, all the north ends would stick to the south
ends, and a structure of bar magnets would emerge. So it goes
with atoms. The speculation is that the 'soup' could have
existed in the oceans of the early Earth, and that the zap,
the shaking of the box, was a lightning strike. The conditions
to assemble amino acids etc into proteins and DNA are more
complex and are still up for conjecture, but we're well on
the way to finding out how life could have arisen by natural
happenstance. The point for our discussion here is that when
you do the soup-zapping experiment, you get equal quantities
of the left-handed and right-handed forms of alanine etc.
So what happened to all the right-handed alanine?
The answer is probably: It was destroyed by cosmic rays.
Outer space is profoundly hostile towards life. It's full
of energetic particles zipping to and fro at close to the
speed of light. One reason we haven't sent anyone to Mars
yet is that cosmic rays tend to break down complex molecules
like DNA. It would take a whole heck of a lot of lead shielding
to keep the astronauts from getting cancer during the long
ride there and back. The Earth's atmosphere shields us from
the worst of the cosmic rays, but it produces some high-energy
particles of its own as radiation bounces off it. Since a
lot of these particles are generated by weak interactions,
they tend to be left-handed. For some reason unknown to me,
left-handed particles are more likely to destroy right-handed
alanine, so we wound up with the 'correct' left-handed alanine
in our genes.
David M Harrison concludes that since the violation of mirror
symmetry in weak interactions can break the symmetry in the
molecules found in living creatures, it could also conceivably
be the basis of our larger-scale mirror asymmetry as well.
I wonder if there's some connection here to the other slight
asymmetries of the universe. A central mystery of big bang
theory is that the early universe contained nearly the same
amount of matter and antimatter, but not exactly the same
amount. The balance is weirdly close: for every billion antiquarks,
the big bang produced about a billion and one quarks. Is
there an anti-universe out there where the balance tipped
the other way, where the weak force only acts on right-handed
particles, and where time is running backwards? Maybe
the weak force will tell us.
© ethan hein 2007 | back
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