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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. Based on the
paper of George Johnson in The
New York Times we list below 10 winners of this polling and
accompany the short explanations of the physical experiments with
computer animations.
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1. Double-slit electron diffraction
The French physicist Louis de Broglie proposed in 1924 that
electrons and other discrete bits of matter, which until then had been
conceived only as material particles, also have wave properties such
as wavelength and frequency. Later (1927) the wave nature of electrons
was experimentally established by C.J. Davisson and L.H. Germer in New
York and by G.P. Thomson in Aberdeen, Scot.
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.
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2. 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.
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3. Millikan's oil-drop experiment
Oil-drop experiment was the first direct and compelling
measurement of the electric charge of a single electron. It was
performed originally in 1909 by the American physicist Robert A.
Millikan. 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 (red in animation) and the other
negative (blue in animation). Since each droplet picked up a slight
charge of static electricity as it traveled through the air, the speed
of its motion could be controlled by altering the voltage on the
plates. When the space between the metal plates is ionized by
radiation (e.g., X rays), electrons from the air attach themselves to
oil droplets, causing them to acquire a negative charge. 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.
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4. 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.
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5. 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.
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6. Cavendish's torsion-bar experiment
The
experiment was performed in 1797–98 by the English scientist Henry
Cavendish. He followed a method prescribed and used apparatus built by
his countryman, the geologist John Michell, who had died in 1793. The
apparatus employed was a torsion balance, essentially a stretched wire
supporting spherical weights. Attraction between pairs of weights
caused the wire to twist slightly, which thus allowed the first
calculation of the value of the gravitational constant G. The
experiment was popularly known as weighing the Earth because
determination of G permitted calculation of the Earth's mass.
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7. Eratosthenes' measurement of the Earth's circumference
At Syene (now Aswan), some 800 km (500 miles) southeast of
Alexandria in Egypt, the Sun's rays fall vertically at noon at the
summer solstice. Eratosthenes, who was born in c. 276 BC, noted that
at Alexandria, at the same date and time, sunlight fell at an angle of
about 7° from the vertical. He correctly assumed the Sun's distance
to be very great; its rays therefore are practically parallel when
they reach the Earth. Given estimates of the distance between the two
cities, he was able to calculate the circumference of the Earth. The
exact length of the units (stadia) he used is doubtful, and the
accuracy of his result is therefore uncertain; it may have varied by
0.5 to 17 percent from the value accepted by modern astronomers.
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8. 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.
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9. 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.
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10. 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.
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