FROM 101 Things You Don't Know about Science and No One Else Does Either
BY JAMES TREFIL just three samples
How
Much Is a Kilogram?
When you buy
hamburger in a supermarket, you aren't likely to worry that the weight written
on your package is
incorrect. This
is because there is a system stretching from your neighborhood store to
scientific laboratories
around the world
devoted to making sure that scales are correctly calibrated. Maintaining
accurate standards of
measurement has
always been a traditional responsibility of governments, and today it is a
major scientific
undertaking. But
old-fashioned or modern, the basic idea is the samethe government sets the
standard for weight or
length or
whatever, to which everyone within that government's jurisdiction must adhere.
The oldest such
standard we know of is the Babylonian mina, a unit of weight equal to
about a pound and a half.
The standards
were kept in the form of carved ducks (five mina) and swans (ten mina), and
were presumably used
in balances to
weigh merchandise. In the Magna Carta, King John agreed that ''there shall be
standard measures for
wine, corn, and
ale throughout the kingdom." The marshal of the great medieval fairs at
Champagne kept an iron
rod and required
that all bolts of cloth sold at the fair be as wide as the rod. For most of
recorded history each
country has kept
various different standards for different purposes. In America, for example, we
measure land in
acres, grain
production in bushels, and height in feet and inches. According to the Handbook
of Chemistry and
Physics,
there are no fewer than eighteen different kinds of units called the barrel,
for measuring everything from
liquor to petroleum. There is even a
barrel used exclusively to measure cranberries!
It was, I
suppose, to get away from these sorts of confusions that the nations of the
industrialized world signed the
Treaty of the
Meter in 1875. According to this treaty, ''the" kilogram and
"the" meter were to be kept at the
International
Bureau of Weights and Measures near Paris, and secondary standards were to be
maintained in other
national
capitals. In the United States, they were kept at the Bureau of Standards (now
the National Institutes of
Standards and
Technology, or NIST) in Washington, D.C. The meter was the distance between two
marks on a
length of
platinum-iridium alloy, the kilogram the mass of a specific cylinder of the
same stuff.
But since the
setting of these simple, intuitive standards, advances of technology have made
them obsolete. It's all
very well for
"the" meter to reside in a vault in Paris, but it would be much more
convenient if everybody could
have access to a
uniform standard. Thus the trend has been away from the kind of centralized
standard-keeping
codified in the
Treaty of the Meter and toward standards based on the one truly universal thing
we know aboutthe
properties of
atoms. The development of the atomic clock is one example of such a move, the
new standards for
the meter
another. In 1960 the platinum-iridium bar was discarded and the meter redefined
as 1,650,763.73
wavelengths of a
particular color of light emitted by a krypton atom. Since every krypton atom
in the world is the
same, this
redefinition meant that every laboratory in the world could maintain its own
standard meter. In 1983,
following
further development of the atomic clock, the meter was redefined as the
distance light travels through the
vacuum in
1/299,792,458 second. Again, this standard can be maintained in any laboratory.
But the kilogram
hasn't changed. It's still that same cylinder sitting inside three protective
bell jars on a quartz slab
inside a vault
in Paris. Even in such an environment, however, atoms of other substances stick
to the cylinder's surface. Until 1994 it was cleaned periodically by an old
technician using a
chamois cloth.
(I remember listening to an absolutely fascinating argument at a NIST lunch
over whether or not
removing atoms
by washing was worse than letting gases accumulate on the surface.) When the
United States
wants to check
whether its version of the kilogram still matches the standard in Paris, the
American kilogram has
to be carried
overseas for tests. The last time this was done, in 1984, two scientists went
with it one to carry it, the
other to catch
it if it fell.
This is no way
to run a high-tech society, and there is an enormous push to develop an atomic
mass standard and
put ''the"
kilogram into a museum. One technology that may allow us to do this is the new
technique of isolating
single atoms in
a complex "trap" made of electrical and magnetic forces so that they
can be studied for months at a
time. These
single atoms stay in the traps so long that they acquire names (the first, a
barium atom trapped in
Munich in the
1980s, was called Astrid). It is not too difficult to determine the mass of
individual atoms to high
accuracy; the
problem is counting the number of atoms in a sample big enough to serve as a
mass standard.
The cylinder
that now constitutes "the" kilogram contains approximately
10,000,000,000,000,000,000,000,000
atoms, so even
if we knew how much each one weighed to incredible accuracy, we'd have a real
problem knowing
how many to add.
At the moment, at least five different techniques are being developed to give
the kilogram an
atomic
definition, and I don't imagine it will be long before one of them succeeds.
When this happens, the
kilogram will join the meter in its
museum.
2] Why Does a Protein Have the Shape It Has?
Life is based on
the chemical reactions between molecules, and these reactions depend on
molecular geometry.
This statement
represents one of the most profound truths we know about the nature of life.
Take the
combining of two large molecules as an example. Bonds can be created only
between individual atoms.
Think of the
atoms that could form bonds as small patches of Velcro on a large, convoluted
sphere. For
combination to
occur, the molecules have to come together in such a way that the Velcro
patches are juxtaposed.
The chance of
this happening in a random encounter is pretty small, so chemical reactions in
cells depend on a
third kind of
molecule called an enzyme. The enzyme attaches separately to each of the
molecules taking part in a
reaction,
assuring that each one's Velcro is in the right position and allowing the
reaction to proceed. The enzyme
is not itself
affected in the process. Think of it as a broker who brings together a buyer
and a seller but doesn't buy
or sell anything
himself.
The enzymes in
our cells are proteins, which are long molecules made from a set of smaller
molecules called
amino acids. The
amino acids are assembled like beads on a string, and the resulting protein
then curls up into a
complex shape.
Because there are so many possible combinations of ''beads," the final
assembly can have many
possible shapes,
which makes proteins ideal for the role of enzymes.
Cells work like
this: the DNA in the nucleus contains codes specifying the order of the amino
acid beads that go
into a protein.
The stretch of DNA that contains the blueprint for one protein is called a
gene. Each gene codes for one
protein, and
each protein acts as an enzyme for one chemical reaction. In a particular cell,
as many as a few
thousand genes
may be operating at any given time.
This knowledge,
combined with our ability to manufacture genes and implant them in bacteria,
opens an exciting
possibility. If
we know the chemical reaction we wish to drive, we can figure out the shape of
the enzyme needed
to drive it. If
we know how a specific sequence of amino acids folded up into a protein's final
shape, we can design
a gene to make
that sequence, put it into some bacteria, and brew it up.
But there's a
problem with this notion that has plagued biochemists for the last forty years.
Even if we know the
sequence of
amino acids in a proteinthe order of the beads on the stringwe simply do not
know how to predict the
protein's final
shape. A solution to what is known as the ''protein folding problem"
remains tantalizingly beyond
the grasp of
modern science.
The reason for
this gap in our understanding is simple: there can be hundreds of thousands of
atoms in a single
protein, and
even our best computers aren't good enough to keep track of everything that
goes on when the protein
folds up.
At the moment,
two lines of research are being pursued. The first involves experiments whose
goal is to designate
intermediate
states in the folding process. For example, a long chain might first twist into
a corkscrew, then have
some segments
fold over double, and then fold up into its final shape. By knowing these
intermediate states we can
break the
folding process down into a series of simpler steps. One difficulty with this
approach is that proteins
apparently can
follow many different folding sequences to get to a given final state.
Other scientists
are trying to use clever computing techniques to predict the final shape a
string of amino acids will taketechniques that do not require following each
atom
through the
folding process. For example, computer programs can estimate the final energy
state of different
folding
patterns. Since systems in nature move to states of lowest energy, the
suggestion is that when you find the
lowest energy
state, you have found the final pattern of the molecule. The problem: there may
be many low-energy
states, and it
becomes difficult to know which one the molecule will wind up in.
Another computer
approach involves the techniques of artificial intelligence. Data on known
folding patterns of
amino acid
strings are fed into a computer, which then guesses a folding pattern for a new
molecule based on
analogies to
known proteins. The problem: you can never be sure the guess is right.
Whichever
technique finally brings us to a solution of the protein folding problem, one
thing is clear. When the
problem is solved, we will have
eliminated a major roadblock on the road to manufacturing any molecule we want
3] How Does the Brain ''See"?
Take a moment to
look around you. What do you see? Perhaps you see a room with colored walls,
pictures, doors,
and windows.
Whatever you see, though, one thing is cleara collection of cells in your eye
and brain has converted
incoming light
from the environment into a coherent picture. Over the past few years,
scientists have started to
construct an
amazingly detailed picture of how that process works.
It begins when
light enters the eye and is focused on the retina at the back of the eyeball.
There, in the cells called
rods and cones
(because of their shapes), the energy of the light is converted into a nerve
signal. At this point, the
external
environment ceases to play a role in the process, and the mechanisms of the
brain and the nervous system
take over. The
central question: how are those initial nerve impulses in the retina converted
into an image?
Since the early
twentieth century, scientists have known that the basic units of the nervous
system are cells called
neurons. Each of
the many types of neurons has a central cell body, a collection of spikes
(called dendrites) that
receive signals
from one set of neurons, and a long fiber called an axon through which signals
go out to another
set. The neuron
is an all-or-nothing, one-way elementwhen (by a process we don't understand) it
gets the right mix
of signals from
its dendrites, it "fires" and sends a signal out along its axon. The
problem for brain scientists is to
understand how a
set of cells with these properties can produce images of the outside world.
The first bit of
processing of the data carried by the light occurs in two layers of cells in
the retina (oddly enough, these cells are located in front of the rods
and cones,
blocking
incoming light). These cells are connected in such a way that a strong impulse
will go to the brain from
one set of cells
if it sees a bright dot with a dark surround and a weak signal from that set
otherwise. Another set
of cells will
send a strong signal if it sees a dark dot with a white surround and a weak
signal otherwise. The signal
that goes to the
brain, then, consists of impulses running along axons that, in effect, break
the visual field down
into a series of
bright and dark dots.
These signals arrive
at the primary visual cortex at the back of the brain, and from this point on
the system is
designed to
build the collection of dots back into a coherent picture. The signals feed
into a set of cells in one
particular layer
of the visual cortex, located in the back of the brain. Each of the cells in
that set will fire if it gets
strong signals
from a certain subset of cells in the retina. For example, one of the cortex
cells may fire if it gets
signals
corresponding to a set of dark dots tilted at an angle of 45 degrees. In
effect, this cell ''sees" a tilted dark
edge in the
visual field. Other cells will fire for bright edges, for edges at different
angles, and so on. The output
from these
cells, in turn, is passed on to other cells (perhaps in other parts of the
brain) for further integration.
So the process
of seeing is far from simple. Neurons fire and pass information upward through
a chain of cells,
while at the
same time feedback signals pass back down the chain and affect lower cells.
Working out the details of
this complex
chain is one important area of research. Another is following the chain upward,
to ever more
specialized
neurons. There are, for example, neurons in the temporal lobes (located on the
sides of the brain) that
fire only when
they get signals corresponding to certain well-defined patternsone set of cells
may fire strongly in
response to a
dark circle with a horizontal line through it, another to a star shape, another
to the outline of a square, and so on. Presumably neurons higher up the
chain combine
the output of these cells to construct even more integrated versions of the
visual field.
Thus our image
of the external world is built up in the brain by successive integrations of
visual elements. Where
does the process
end? At one point, some neurophysiologists talked about the existence of the
''grandmother
cell"the
one cell in your brain that would fire when all the stimuli were right for an
image of your grandmother.
While this
simple notion has fallen out of favor, it seems likely that within the next
decade scientists will be able to
trace the
physical connections from the cones and rods all the way to the cells in the
brain that put together the
final picture.
Other scientists are already trying to locate the cells in the frontal lobe
that fire when short-term
memory is
activated. This work starts the next step in understanding the visual
processfinding out how we
recognize an
object whose image has been constructed.
But even when
all these neural pathways have been traced out and verified, the most
intriguing question of all will
remain: what is
the "I" that is seeing these images? To answer that question,
we'll have to know about more than
neural circuitswe'll have to understand
consciousness itself.
