Originally published in GOlden Thread magazine in a 2002 issue, reprinted here for NR viewers.

“Philosophy is written in that vast book which stands forever open before our eyes, I mean the universe; but it can not be read until we have learnt the language and become familiar with the characters in which it is written.”
- Galileo Galilei

New realities emerge when great minds question the unknown. Science, therefore, is the record of comprehending the mysterious. Finding beauty in the pursuit of these mysteries unifies our dual nature of harmony and logic. This is what Robert P. Crease, professor of philosophy at the State University of New York at Stony Brook and historian at the Brookhaven National Laboratory, was looking for when he put out an interesting question to readers of Physics World.  He asked, “What are the most beautiful experiments in physics, and what is the connection between their beauty and their scientific value?”    
The respondents, mostly physicists and scientists, said that a beautiful experiment reveals something about nature with clarity and simplicity that transforms our thinking and behavior.  What was most often cited as beauty involved something called “deep play.” There is a sense that we are actively engaging in something beyond ourselves. It is the place where the Observer and the observed merge into one.  
The list in Physics World was ranked according to popularity. It appears that the simpler the demonstration, the more profound its conclusion. Annotated below are the top ten experiments, covering more than 2,000 years of scientific discovery.
 
  [Picture of pendulum]
10) Foucault’s Pendulum demonstrating that the Earth revolves on its axis: 1851
In Paris, a French scientist named Jean Bernard Léon Foucault suspended a 62-pound iron ball with steel wire, 220 feet long, from the dome of the Panthéon and set it in motion, rocking it back and forth. To chart the movement Foucault attached a marker extending from the ball so that it barely touched a circle of damp sand underneath. Normally what one would expect is for the pendulum to trace the same places over and over again. Yet to the astonishment of the crowd the pendulum appeared to shift positions leaving a slightly different trace with each swing. What was actually happening was that the floor of the Panthéon was rotating.
It takes 30 hours at the latitude of Paris for the pendulum to complete a full clockwise rotation. When Foucault’s pendulum was used in the southern hemisphere it rotated counterclockwise. On the equator it doesn't revolve at all.   Scientists have recently confirmed that the period of rotation at the South Pole is 24 hours to return to its originally traced line.  
     
[Picture of atom]
9.  Rutherford's demonstration of the atom’s nucleus with rotating electrons: 1911
Until this experiment, atoms were believed to consist of large mushy blobs of positive electrical charge with electrons embedded inside. It was called the “plum pudding” model.  Experimenting with radioactivity at the University of Manchester, Ernest Rutherford saw that when tiny positively charged projectiles, called alpha particles, were fired at a thin foil of gold, a small percentage of them came bouncing back. It was as though bullets had ricocheted off Jell-O.  
He subsequently calculated that there was actually only a positively charged mass at the core, now called the nucleus, with negatively charged electrons circling the outside. Quantum theory has updated this a bit, but Rutherford’s general idea is still the best working model around.

[Picture-Two balls rolling down an inclined plane]
8) Galileo's measuring the accelerating velocity of gravity: 1600s
Although not quite as popular as his measuring the equality of falling objects — see experiment 2 — Galileo Galilei proved that there is a progressive acceleration to falling objects.
Up until this time, most people would predict that the speed of a falling object was constant. If you doubled the falling time, the distance traveled would double. However, Galileo showed that when doubling the time of a falling object it travels four times further. He calculated that the distance is proportional to the square of the time.   
Galileo demonstrated this by taking a board about 20 feet by 10 inches and cutting 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 weighed the water that had flowed out, which equaled the measurement of elapsed time. He then compared this with the distance the ball had traveled.

[Picture of angle of shadow on a sphere]
7) Eratosthenes' measurement of the Earth's circumference: Third century B.C.
Eratosthenes, the director of the great library in Alexandria, read that once a year at noon on the day of the summer solstice a special event occurs. In the southern Egyptian village of Syene (now Aswan) a deep vertical well is entirely lit to the bottom by the sun. In other words, the sun is directly overhead casting no shadow. Noticing something different in Alexandria, on the same day at the same time he measured the angle of the sun to deviate from the vertical by 7.2 degrees.
That was all the information he needed to measure the circumference of the Earth. Like any keen observer of the sky, he figured the planet must be spherical in nature, with a circumference of 360 degrees. It just so happens that 7.2 is exactly one-fiftieth of 360.  He estimated from travel time that the cities were 5,000 “stadia” apart and therefore he concluded that the Earth must be 50 times that size, or 250,000 stadia.
Actually Alexandria and Aswan are about 500 miles apart.  Therefore one stadium is equal to one-tenth of a mile; so Eratosthenes’ converted measurement is 25,000 miles.  Amazingly enough the actual circumference of the Earth is 24,860 miles at the poles, and 24,902 miles at the equator where it is slightly flattened and bulged.

6) Cavendish's experiment showing the force of different gravitational fields using a torsion bar: 1798
      Part of Isaac Newton's theory of gravity is that the attraction between two objects increases with the square of their masses and decreases with the square of the distance between them. Scientists have confirmed this inverse square law of gravitation many times at macroscopic distances by observing the motion of the planets.
The English scientist Henry Cavendish decided to find out how specific Newton’s law was. He used a six-foot wooden rod, attaching two small metal balls to each end, and suspended it from a wire. Near each ball he placed an object of immense mass, a 350-pound lead sphere. With this he showed that a gravitational force was exerted causing the smaller balls to move and the wire to twist.  
To guard against any influence of air currents, the apparatus (called a torsion balance) was enclosed in a room and observed with telescopes mounted on each side. By putting finely etched pieces of ivory on the end of each arm he measured the subtle movement.
From his observation, Cavendish had a pretty accurate measurement of what is now called the gravitational constant.  Remarkably, from this he was able to calculate the density and mass of the Earth. Eratosthenes had measured the size of the planet and now Cavendish had weighed it: 6.0 x 1024 kilograms, or about 13 trillion pounds.  

5). Young's double-slit light interference experiment showing the wave nature of light: 1803
Newton’s conclusion from his experiments with light was that it was made of particles, not waves. However, Thomas Young, an English physician and physicist, suspected otherwise.  He cut a small hole in a window shutter, covered it with a thick piece of paper, and punctured it with a tiny pinhole. Then he used a mirror to divert the thin beam of light that came shining through. 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 historic moment came when Young noticed that the light hitting the wall behind the shutter was not the total of the two streams. Instead the wall had alternating bands of light and shadow. He realized that this was due to the interference patterns that occur in wave formations. Bright bands appeared where the crests or troughs of two waves overlapped, reinforcing each other.  Dark areas marked the locations where a crest from one wave lined up with a trough from the other, neutralizing each other.

[Show rainbow of Light through a prism]
4) Newton showing the decomposition of light: 1665
     Before studying gravity, Newton turned his attention towards light. For 2000 years, a belief created by Aristotle was that white light was the purest form of light. It was thought that any other color must be some altered form. But Sir Isaac had other ideas.
What Newton did was direct a beam of sunlight through a glass prism and showed that it decomposed into the spectrum of light: red, orange, yellow, green, blue, indigo, violet, and the gradations in between. Rainbows had always been observed, but people discarded them to be nothing but interesting aberrations. Newton proved that the fundamental components of a simple beam of white light were a more complex array of colors.

[Illustration of an electron]
3) Millikan's oil-drop experiment measuring the charge of a single electron: 1909
In the late nineteenth century, British physicist J. J. Thomson had shown that electricity consisted of negatively charged particles called electrons. Yet there was no one who knew the value of their charge, until 1909 when American scientist Robert Millikan created a simple device.

In a perfume atomizer he sprayed tiny drops of oil into the 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, altering the voltage on the plates could control the speed of its descent. He was astonished to find that when the electrical force matched the force of gravity a droplet would hover in midair “like a brilliant star on a black background.” Varying the voltage in one drop after another Millikan concluded that the smallest of these charges was none other than that of a single electron.

 [Picture of Leaning Tower with balls falling off of it]
2) Galileo's experiment on the equality of falling objects: early 1600s.
    The story is now legendary, how Galileo dropped two different weights from the top of the Leaning Tower of Pisa and showed them landing at the same time. Before Galileo’s simple demonstration everyone in Europe believed Aristotle, that heavier objects fell faster than lighter ones.
This was the beginning of Galileo’s trouble with authorities. Despite losing his job as a mathematics professor at the University of Pisa for establishing a new fact, Galileo became famous throughout the continent.
Galileo, and Newton after him, established the basic principles of modern science: First, that experience – not authority – should be the measuring stick of scientific investigation and, second, that mathematical formulas can best depict the laws of nature.  Although they admitted that the location helped to make it unforgettable, Apollo astronauts dropped a feather and a weight on the moon, again demonstrating Galileo’s equality of falling bodies. This was possibly the “most watched experiment” of all time.

[Show the light experiment with electrons]
1) Young's double-slit experiment applied to the interference of single electrons: early 1900s
In the early days of quantum physics, thought experiments were used to establish some fundamental features. Scientists theorized that contradictory qualities are part of subatomic particles.
They contemplated that by using Young’s double-slit process (experiment 5 above) on a beam of electrons instead of light, the particles would be split in two streams that would interfere with each other, leaving the same kind of light and dark striped pattern as was cast by light. This would show that particles were also waves. Some called them wavicles.
In Richard Feynman's discussion on this experiment, he remarked that “It contains everything you need to know about quantum mechanics. We choose to examine a phenomenon which is impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery.”

According to an article in Physics World, it wasn't until 1961 that someone carried out the experiment in the real world. Since then, particle interference has been demonstrated with neutrons, atoms, and molecules as large as carbon-60 and carbon-70.  

 The fact that the conclusion to the top experiment of all time could be so ambiguous shows how very limited our understanding of physical reality really is. The idea that particles have a contradictory makeup proves how far we have yet to go in grasping the true nature of reality. As Werner Heisenberg wrote, “What we observe is not nature itself, but nature exposed to our method of questioning.”

References
Crease, R. P. (2002, September). The Most Beautiful Experiment. Physics World Magazine. http://physicsweb.org/article/world/15/9/2
Feynman, R. (1963). The Feynman Lecture on Physics. Reading, Mass: Addison-Wesley.
Galilei, G. (1953). Dialogue on the Great World Systems, ed. by Giorgio de Santillana. Chicago: University of Chicago Press.
Heisenberg, W. (1958). Physics and Philosophy. New York: Harper & Rowe.
Johnson, G. (2002, September 24). Here They Are, Science's 10 Most Beautiful Experiments, New York Science Times. http://www.nytimes.com/2002/09/24/science
Young, T. (1802). On the theory of light and colors (The 1801 Bakerian Lecture) Philosophical Transactions of the Royal Society of London.