Here They Are, Science's 10 Most Beautiful Experiments [pdf] [10 experiments ppt] [brain ppt 1, 2]

September 24, 2002
By GEORGE JOHNSON

Whether they are blasting apart subatomic particles in
accelerators, sequencing the genome or analyzing the wobble
of a distant star, the experiments that grab the world's
attention often cost millions of dollars to execute and
produce torrents of data to be processed over months by
supercomputers. Some research groups have grown to the size
of small companies.

But ultimately science comes down to the individual mind
grappling with something mysterious. When Robert P. Crease,
a member of the philosophy department at the State
University of New York at Stony Brook and the historian at
Brookhaven National Laboratory, recently asked physicists
to nominate the most beautiful experiment of all time, the
10 winners were largely solo performances, involving at
most a few assistants. Most of the experiments - which are
listed in this month's Physics World - took place on
tabletops and none required more computational power than
that of a slide rule or calculator.

What they have in common is that they epitomize the elusive
quality scientists call beauty. This is beauty in the
classical sense: the logical simplicity of the apparatus,
like the logical simplicity of the analysis, seems as
inevitable and pure as the lines of a Greek monument.
Confusion and ambiguity are momentarily swept aside, and
something new about nature becomes clear.

The list in Physics World was ranked according to
popularity, first place going to an experiment that vividly
demonstrated the quantum nature of the physical world. But
science is a cumulative enterprise - that is part of its
beauty. Rearranged chronologically and annotated below, the
winners provide a bird's-eye view of more than 2,000 years
of discovery.

Eratosthenes' measurement of the Earth's circumference

At
noon on the summer solstice in the Egyptian town now called
Aswan, the sun hovers straight overhead: objects cast no
shadow and sunlight falls directly down a deep well. When
he read this fact, Eratosthenes, the librarian at
Alexandria in the third century B.C., realized he had the
information he needed to estimate the circumference of the
planet. On the same day and time, he measured shadows in
Alexandria, finding that the solar rays there had a bit of
a slant, deviating from the vertical by about seven
degrees.

The rest was just geometry. Assuming the earth is
spherical, its circumference spans 360 degrees. So if the
two cities are seven degrees apart, that would constitute
seven-360ths of the full circle - about one-fiftieth.
Estimating from travel time that the towns were 5,000
"stadia" apart, Eratosthenes concluded that the earth must
be 50 times that size - 250,000 stadia in girth. Scholars
differ over the length of a Greek stadium, so it is
impossible to know just how accurate he was. But by some
reckonings, he was off by only about 5 percent. (Ranking:
7)

Galileo's experiment on falling objects

In the late 1500's, everyone knew that heavy objects fall
faster than lighter ones. After all, Aristotle had said so.
That an ancient Greek scholar still held such sway was a
sign of how far science had declined during the dark ages.

Galileo Galilei, who held a chair in mathematics at the
University of Pisa, was impudent enough to question the
common knowledge. The story has become part of the folklore
of science: he is reputed to have dropped two different
weights from the town's Leaning Tower showing that they
landed at the same time. His challenges to Aristotle may
have cost Galileo his job, but he had demonstrated the
importance of taking nature, not human authority, as the
final arbiter in matters of science. (Ranking: 2)

Galileo's experiments with rolling balls down inclined
planes

Galileo continued to refine his ideas about objects in
motion. He took a board 12 cubits long and half a cubit
wide (about 20 feet by 10 inches) and cut a groove, as
straight and smooth as possible, down the center. He
inclined the plane and rolled brass balls down it, timing
their descent with a water clock - a large vessel that
emptied through a thin tube into a glass. After each run he
would weigh the water that had flowed out - his measurement
of elapsed time - and compare it with the distance the ball
had traveled.

Aristotle would have predicted that the velocity of a
rolling ball was constant: double its time in transit and
you would double the distance it traversed. Galileo was
able to show that the distance is actually proportional to
the square of the time: Double it and the ball would go
four times as far. The reason is that it is being
constantly accelerated by gravity. (Ranking: 8)

Newton's decomposition of sunlight with a prism

Isaac
Newton was born the year Galileo died. He graduated from
Trinity College, Cambridge, in 1665, then holed up at home
for a couple of years waiting out the plague. He had no
trouble keeping himself occupied.

The common wisdom held that white light is the purest form
(Aristotle again) and that colored light must therefore
have been altered somehow. To test this hypothesis, Newton
shined a beam of sunlight through a glass prism and showed
that it decomposed into a spectrum cast on the wall. People
already knew about rainbows, of course, but they were
considered to be little more than pretty aberrations.
Actually, Newton concluded, it was these colors - red,
orange, yellow, green, blue, indigo, violet and the
gradations in between - that were fundamental. What seemed
simple on the surface, a beam of white light, was, if one
looked deeper, beautifully complex. (Ranking: 4)

Cavendish's torsion-bar experiment
 

Another of Newton's
contributions was his theory of gravity, which holds that
the strength of attraction between two objects increases
with the square of their masses and decreases with the
square of the distance between them. But how strong is
gravity in the first place?

In the late 1700's an English scientist, Henry Cavendish,
decided to find out. He took a six-foot wooden rod and
attached small metal spheres to each end, like a dumbbell,
then suspended it from a wire. Two 350-pound lead spheres
placed nearby exerted just enough gravitational force to
tug at the smaller balls, causing the dumbbell to move and
the wire to twist. By mounting finely etched pieces of
ivory on the end of each arm and in the sides of the case,
he could measure the subtle displacement. To guard against
the influence of air currents, the apparatus (called a
torsion balance) was enclosed in a room and observed with
telescopes mounted on each side.

The result was a remarkably accurate estimate of a
parameter called the gravitational constant, and from that
Cavendish was able to calculate the density and mass of the
earth. Erastothenes had measured how far around the planet
was. Cavendish had weighed it: 6.0 x 1024 kilograms, or
about 13 trillion trillion pounds. (Ranking: 6)

Young's light-interference experiment
 

Newton wasn't
always right. Through various arguments, he had moved the
scientific mainstream toward the conviction that light
consists exclusively of particles rather than waves. In
1803, Thomas Young, an English physician and physicist, put
the idea to a test. He cut a hole in a window shutter,
covered it with a thick piece of paper punctured with a
tiny pinhole and used a mirror to divert the thin beam that
came shining through. Then he took "a slip of a card, about
one-thirtieth of an inch in breadth" and held it edgewise
in the path of the beam, dividing it in two. The result was
a shadow of alternating light and dark bands - a phenomenon
that could be explained if the two beams were interacting
like waves.

Bright bands appeared where two crests overlapped,
reinforcing each other; dark bands marked where a crest
lined up with a trough, neutralizing each other.

The demonstration was often repeated over the years using a
card with two holes to divide the beam. These so-called
double-slit experiments became the standard for determining
wavelike motion - a fact that was to become especially
important a century later when quantum theory began.
(Ranking: 5)

Foucault's pendulum

Last year when scientists mounted a pendulum above the
South Pole and watched it swing, they were replicating a
celebrated demonstration performed in Paris in 1851. Using
a steel wire 220 feet long, the French scientist
Jean-Bernard-L?on Foucault suspended a 62-pound iron ball
from the dome of the Panth?on and set it in motion, rocking
back and forth. To mark its progress he attached a stylus
to the ball and placed a ring of damp sand on the floor
below.

The audience watched in awe as the pendulum inexplicably
appeared to rotate, leaving a slightly different trace with
each swing. Actually it was the floor of the Panth?on that
was slowly moving, and Foucault had shown, more
convincingly than ever, that the earth revolves on its
axis. At the latitude of Paris, the pendulum's path would
complete a full clockwise rotation every 30 hours; on the
Southern Hemisphere it would rotate counterclockwise, and
on the Equator it wouldn't revolve at all. At the South
Pole, as the modern-day scientists confirmed, the period of
rotation is 24 hours. (Ranking: 10)

Millikan's oil-drop experiment

Since ancient times,
scientists had studied electricity - an intangible essence
that came from the sky as lightning or could be produced
simply by running a brush through your hair. In 1897 (in an
experiment that could easily have made this list) the
British physicist J. J. Thomson had established that
electricity consisted of negatively charged particles -
electrons. It was left to the American scientist Robert
Millikan, in 1909, to measure their charge.

Using a perfume atomizer, he sprayed tiny drops of oil into
a transparent chamber. At the top and bottom were metal
plates hooked to a battery, making one positive and the
other negative. Since each droplet picked up a slight
charge of static electricity as it traveled through the
air, the speed of its descent could be controlled by
altering the voltage on the plates. (When this electrical
force matched the force of gravity, a droplet - "like a
brilliant star on a black background" - would hover in
midair.)

Millikan observed one drop after another, varying the
voltage and noting the effect. After many repetitions he
concluded that charge could only assume certain fixed
values. The smallest of these portions was none other than
the charge of a single electron. (Ranking: 3)

Rutherford's discovery of the nucleus
 

When Ernest
Rutherford was experimenting with radioactivity at the
University of Manchester in 1911, atoms were generally
believed to consist of large mushy blobs of positive
electrical charge with electrons embedded inside - the
"plum pudding" model. But when he and his assistants fired
tiny positively charged projectiles, called alpha
particles, at a thin foil of gold, they were surprised that
a tiny percentage of them came bouncing back. It was as
though bullets had ricocheted off Jell-O.

Rutherford calculated that actually atoms were not so mushy
after all. Most of the mass must be concentrated in a tiny
core, now called the nucleus, with the electrons hovering
around it. With amendments from quantum theory, this image
of the atom persists today. (Ranking: 9)

Young's double-slit experiment applied to the interference
of single electrons

Neither Newton nor Young was quite right about the nature
of light. Though it is not simply made of particles,
neither can it be described purely as a wave. In the first
five years of the 20th century, Max Planck and then Albert
Einstein showed, respectively, that light is emitted and
absorbed in packets - called photons. But other experiments
continued to verify that light is also wavelike.

It took quantum theory, developed over the next few
decades, to reconcile how both ideas could be true: photons
and other subatomic particles - electrons, protons, and so
forth - exhibit two complementary qualities; they are, as
one physicist put it, "wavicles."

To explain the idea, to others and themselves, physicists
often used a thought experiment, in which Young's
double-slit demonstration is repeated with a beam of
electrons instead of light. Obeying the laws of quantum
mechanics, the stream of particles would split in two, and
the smaller streams would interfere with each other,
leaving the same kind of light- and dark-striped pattern as
was cast by light. Particles would act like waves.

According to an accompanying article in Physics World, by
the magazine's editor, Peter Rodgers, it wasn't until 1961
that someone (Claus J?nsson of T?bingen) carried out the
experiment in the real world.

By that time no one was really surprised by the outcome,
and the report, like most, was absorbed anonymously into
science. (Ranking: 1)