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-Léon Foucault suspended a 62-pound iron
ball from the dome of the Panthéon 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 Panthéon 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 Jönsson of Tübingen) 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)
