Two balls and an idea - the most beautiful experiments in the history of science

The most beautiful experiments in the history of science, according to a poll by the magazine "Physics World"

George Johnson, New York Times

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First published on 30/10/2002

At the end of the 16th century, before Galileo's experiment in Pisa, everyone knew that heavy objects fall faster than light objects. That's what Aristotle said

Whether it is the fission of subatomic particles in particle accelerators, the decoding of the genome or the analysis of the vibrations of a distant planet, the experiments that capture the public's attention sometimes cost millions of dollars and produce enormous amounts of information, which are processed for months by supercomputers. Some of the world's research groups have already grown to the size of small commercial companies.

But at its core, science is founded on the clear thought of a single person who is faced with some mystery. When Robert Kries, a member of the Department of Philosophy at the University of the State of New York and House Historian of the Brookhaven National Laboratory, recently asked physicists to choose the most beautiful experiment of all time, it became clear that the ten winning experiments were mostly the achievements of individual scientists, with at most a few assistants involved.

Newton's prism. The colors that make up white light

Most of the experiments - whose rating was published last month in the journal "Physics World" - were conducted on simple desks, and none of them required more computational power than a slide rule or calculator could provide.

What these experiments have in common is that they embody that elusive quality called "beauty" by scientists. This is beauty in the classical sense: the logical simplicity of the experimental device, as well as the logical simplicity of the analysis, recall in their cleanliness the outlines of a Greek structure. Confusion and ambiguity are put aside for a moment, and something new about the forces of nature is revealed in all its clarity.

The list in "Physical World" was ranked according to popularity, and first place was won by an experiment that confirmed the quantum theory in the physical world. Science is an enterprise of accumulated information - this is part of its beauty. Below is the list of the winning experiments, in chronological order with additional explanations: Measuring the circumference of the earth by Artosthenes (rating: 7). At noon on the longest day of the year, in the Egyptian city now called Aswan, the sun hangs in the center of the sky: objects do not cast shadows, and the sun's rays shine in a straight line into a water well. Artosthenes, the librarian at the Library of Alexandria in the third century BC, learned this fact from reading, and realized that he had the information he needed to estimate the circumference of the Earth. On that day and at that hour he measured the shadows in Alexandria, and discovered that the sun's rays there hit the earth at a certain tilt, deviating from the perfect vertical by about seven degrees.

The rest was pure geometry. Assuming that the earth is round, if the two cities are seven degrees apart, this distance constitutes seven parts of the 360 ​​degrees of the complete circle - that is, roughly one-fiftieth. According to the travel time between the two cities, Artosthenes estimated that the distance between them is about 5,000 stadia, and hence the circumference of the earth is 50 times greater, that is, about 250 thousand stadia. Scholars disagree on the question of the length of a Greek stadium, so it is impossible to estimate how precise Artosthenes was. But according to one of the calculations, he was only wrong by 5%.

Galileo's experiment with falling objects (rating: 2). At the end of the 16th century, everyone knew that heavy objects fall faster than light objects. After all, so said Aristotle. The fact that a scholar from ancient Greece was still considered an authority testified to how much science had deteriorated in the Middle Ages.

Galileo Galilei, who held the chair of mathematics at the University of Pisa, was bold enough to question popular belief. The story has become part of scientific folklore: Galileo dropped two objects of different weights from the Tower of Pisa, showing that they land on the earth at the same time. His challenge to Aristotle may have occurred to him at his workplace, but he proved the importance of the position that holds that nature - and not human authority, however important - is the supreme arbiter in matters of science.

Galileo's experiments with balls rolling on inclined planes (rating: 8). Galileo continued to refine his ideas about objects in motion. He took a board 12 cubits long and half a cubit wide (about 5 meters by 20 centimeters) and carved a groove in it along its length, as straight and smooth as possible. He tilted the board and rolled brass balls down it. He measured the time it took them to reach the bottom using a water clock - a large vessel in which the water was emptied into a glass cup through a thin tube. At the end of each trial, Galileo weighed the water that flowed out - a measure of the time that had passed - and compared the amount of water to the distance traveled by the ball.

Aristotle would have predicted that the speed of a rolling ball would remain constant: if you double the duration of the movement, the distance the ball travels also doubles. Galileo showed that the distance is proportional to the square of the duration of the movement: when you double the duration of the movement, the ball travels a distance 4 times greater than the original distance. This is because the force of gravity causes a constant acceleration in the ball's speed of movement.

Newton's experiment to split sunlight using a prism (rating: 4). Isaac Newton was born the year Galileo died. He graduated from Trinity College in Cambridge in 1665 and then shut himself up in his house for a few years while things raged outside. He had no trouble finding occupations for himself.
The popular explanation in those days was that white light is light in its purest form (again, according to Aristotle) ​​and that colored light is light that has been changed in some way. To test this hypothesis, Newton passed a beam of sunlight through a prism, and showed that the beam split into a rainbow of colors that appeared on the wall. The rainbow was of course a familiar phenomenon, but until then it was considered nothing more than a beautiful curiosity. In fact, Newton deduced, these colors - red, yellow, green, blue, indigo, purple and the intermediate shades - were the basic colors from which white light was composed. What seemed simple on the surface, turned out on closer inspection to be beautiful in its complexity.

Cavendish's torsion bar experiment (rating: 6). Another contribution of Newton was his theory of gravitation, which claims that the force of attraction between two objects increases proportionally to the mass of the objects, and decreases proportionally to the square of the distance between them. But what is the absolute strength of gravity?

At the end of the 18th century, the English scientist Henry Cavendish decided to answer this question. He took a two meter long wooden pole and attached small metal balls to its ends, like a dumbbell. He hung the pole on a metal wire. Two lead balls weighing 160 kg each, placed nearby, exerted enough gravitational force to pull the two small balls, causing the dumbbell to rotate and the metal wire on which it was suspended to twist. To measure the degree of displacement of the rod, Cavendish placed it in a box. On the ends of the rod he placed two pieces of ivory on which thin lines were cut. He placed two more such dishes on the edge of the box. Cavendish checked the degree of change in the direction the grooves were facing, and from this he deduced the degree of displacement of the rod.
To avoid the influence of air currents, the device (known as torsion balances) was placed in a closed room, and was observed through two telescopes fixed on the walls of the room.
The result was an impressive estimate of the accuracy of the parameter known as the gravitational constant, and from this Cavendish was able to calculate the density and mass of the Earth. Artosthenes measured the circumference of the sphere; Cavendish considered him. The weight of the earth, he deduced, is six trillion trillion kilograms, that is, the number of kilograms whose numerical expression is 6 followed by 24 zeros.

The most beautiful experiments in the history of science. Second article: from the demonstration of the wave properties of light to the discovery of the atomic nucleus


Foucault's Pendulum. The earth moves on its axis

Yang's light interference experiment (rating: 5). Newton was not right at all. Relying on various arguments, he convinced the scientific community that light consists of particles and not waves. In 1803, Thomas Young, an English physician and physicist, put this theory to the test. He cut a hole in a window shutter, covered it with a piece of thick paper pierced with a tiny pin and used a mirror to deflect the thin beam of light that passed through the hole. He then took "a piece of parchment, the width of which is about the thirtieth part of an inch" (0.085 centimeter) and placed it inside the beam of light, so that it crossed the beam in two; Half of the light passed from the right of the card and the other half passed from the left. The result was a shadow made up of alternating light and dark strips - a phenomenon that could be explained by the fact that the two rays behaved like intersecting waves. If the rays were not waves, a completely shaded section should have appeared between them.

Atomic nucleus. The "raisin cake model" has been disproved

Bright bands appeared when two wave peaks overlapped each other and reinforced each other's strength; Dark bands appeared where a high overlapped a low, so they canceled each other out.

The experiment has been repeated many times over the years using parchment and two holes to split the light beam. The double slit experiments, as they were called, were the standard method for determining wave motion - a fact that became especially important a century later, with the establishment of quantum theory.

Foucault's Pendulum (Rating: 10). Last year, when scientists placed a pendulum over the South Pole and tested its movement, they recreated a famous experiment first performed in Paris in 1851. To a 67 meter long steel cable, the French scientist Jean-Bernard-Leon Foucault attached an iron ball weighing 28 kg. He hung the cable from the dome of the Pantheon and started the movement of the pendulum. To test the nature of the movement, he attached a stylus to the bottom of the ball and spread a pile of soft sand on the floor.
The crowd watched mesmerized by the pendulum, which inexplicably seemed to swing in a circle, leaving slightly different traces with each swing. In fact, it was the floor of the Pantheon that moved slowly, and thus Foucault showed, more convincingly than ever before, that the Earth moves on its axis. In Paris, the pendulum, which moves clockwise, completed a complete circle every 30 hours; In the southern hemisphere it would circle in the opposite direction to the clockwise direction, and on the equator it would not circle at all. At the South Pole, as the scientists confirmed last year, the time required to complete the circle is 24 hours.

Milliken's Oil Drops Experiment (Rating: 3). Since ancient times, scientists have studied electricity - an intangible essence that descends from the sky in the form of lightning, but can also be easily produced by running a brush through the hair. In 1897 (in an experiment that could easily be included in the current list) the British physicist JJ Thomson proved that electricity consists of negatively charged particles: electrons. In 1909, the American scientist Robert Milliken was able to measure the charge of these particles.

Using a perfume spray Millikan sprayed tiny drops of oil into a transparent chamber. In the upper and lower part of the cell, metal plates were assembled that were connected to the battery, so that one of the plates had a positive charge and the other had a negative charge. Since each drop picked up a tiny charge of static electricity when passing through the air, it was possible to control the speed of its fall by changing the electrical voltage of the plates, which exerted on the oil drop an attractive force similar to the force of a magnet (when the electric force was equal to the force of gravity, the drop was suspended in the air motionless – "like a star shining on a black background").

Milliken watched drop by drop, varying the electrical voltage and noting the effect. After many repetitions he concluded that the charge takes on a series of fixed values. The lowest value was nothing but the charge size of a single electron.

The discovery of the nucleus by Rutherford (rating: 9). When Ernest Rutherford conducted experiments with radioactive radiation at the University of Manchester in 1911, the prevailing belief was that atoms were large, soft lumps of positive electric charge, within which electrons were embedded - what was called the "raisin cake" model. But when he and his assistants fired positively charged particles, called alpha particles, at a thin gold foil, they were surprised to find that a tiny percentage of them were repelled. It was as if bullets fired at jelly were ricocheted back.
Rutherford concluded that in fact the atoms are not soft at all. Most of the mass, it seems, was concentrated in a tiny core, now known as the nucleus, with the electrons orbiting around it. With corrections originating in quantum theory, this image of the atom remains intact to this day.

Application of Young's double-slit experiment in the interference of single electrons (rating: 1). Neither Newton nor Young was completely right about the nature of light. Light is not made of particles, but it cannot be described as a pure wave either. In the first five years of the 20th century, Max Planck and later Albert Einstein showed that light is emitted and absorbed in "packets" called photons. But other experiments continued to confirm that light has wavy properties.

It was quantum theory, developed over the following decades, that made it possible to reconcile the seeming contradiction: photons and other subatomic particles - electrons, protons, etc. - exhibit two properties that complement each other; These particles are actually, as one physicist put it, "wavicles".
To explain the idea to themselves and others, physicists often used a thought experiment in which they reproduced Young's double-slit experiment with an electron beam instead of a light beam. According to the laws of quantum mechanics, the stream of particles is expected to split into two, and the smaller streams are expected to cross each other, so that the same pattern of dark and light bands is obtained that is cast by a light beam. The particles are expected to behave like waves.

According to an accompanying article in the "World of Physics", by the editor of the journal Peter Rogers, it was only in 1961 that someone (Klaus Johansson from the University of Tübingen) managed to carry out the experiment in practice. At this point, no one was surprised by the result, and the report, like most reports, was unwittingly absorbed into the history of science.

Physics Today

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