great physics experiments

32 physics experiments that changed the world

From the discovery of gravity to the first mission to defend Earth from an asteroid, here are the most important physics experiments that changed the world.

Physics experiments have changed the world irrevocably, altering our reality and enabling us to take gigantic leaps in technology. From ancient times to now, here's a look at some of the greatest physics experiments of all time.

Conservation of energy

Energy conservation — the idea that energy cannot be created or destroyed, only transformed — is one of the most important laws of physics. James Prescott Joule demonstrated this rule, the first law of thermodynamics , when he filled a large container with water and fixed a paddle wheel inside it. The wheel was held in place by an axle with a string around it and then looped over a pulley and attached to a weight, which, when dropped, caused the wheel to spin. By sloshing the water with the wheel, Joule demonstrated that the heat energy gained by the water from the wheel's movement was equal to the potential energy lost by dropping the weight.

Measurement of the electron's charge

As the fundamental carriers of electric charge, electrons carry the smallest amount of electricity possible. But the particles are truly tiny, with a mass 1,838 times smaller than the already-minuscule proton.

So how could you measure the charge on something so small? Physicist Robert Millikan's answer was to drop electrically charged oil drops through the plates of a capacitor and adjust the voltage of the capacitor until the electric field it emitted produced a force on some of the drops that balanced out gravity — thus suspending them in the air. Repeating the experiment for different voltages revealed that, no matter the size of the drops, the total charge it carried was a multiple of a base number. Millikan had found the fundamental charge of the electron.

 "Gold foil experiment" revealing the structure of the atom

Once thought to be indivisible, the atom was slowly divided and split by a series of experiments during the late 19th and early 20th centuries. These included J.J. Thomson's 1897 discovery of the electron and James Chadwick's 1932 identification of the neutron. But perhaps the most famous of these experiments was Hans Geiger and Ernest Marsden's " gold foil experiment ." Under the direction of Ernest Rutherford, the students fired positively charged alpha particles at a thin sheet of gold foil. To their surprise, the particles passed through, revealing that atoms consisted of a positively charged nucleus separated by a significant empty space by their orbiting electrons.

Nuclear chain reaction

By the mid-20th century, scientists were aware of the basic structure of the atom and that, according to Einstein, matter and energy were different forms of the same thing. This set the stage for the wartime work of Enrico Fermi, who in 1942 demonstrated that atoms could be split to release enormous quantities of energy.

While working at the University of Chicago with an experimental setup he called an "atomic pile," Fermi demonstrated the first-ever controlled nuclear fission reaction. Fermi fired neutrons at the unstable isotope uranium-235, causing it to split and release more neutrons in a growing chain reaction. The experiment paved the way for the development of nuclear reactors and was used by J. Robert Oppenheimer and the Manhattan Project to build the first atomic bombs.

Wave-particle duality

One of the most famous experiments in physics is also one that illustrates, with disturbing simplicity, the bizarreness of the quantum world. The experiment consisted of two slits, through which electrons would travel to create an interference pattern on a screen, like waves. Scientists were stunned when they placed a detector near the screen and found that its presence caused the electrons to switch their behavior to act instead as particles.

First performed by Thomas Young to demonstrate the wave nature of light, the experiment was later used by physicists in the 20th century to show that all particles, including photons , were both waves and particles at the same time — and they acted more like particles when they were being measured directly.

Splitting of white light into colors

White light is a mixture of all the colors of the rainbow, but before 1672, the composite nature of light was completely unknown. Isaac Newton determined this by using a prism that bent light of different wavelengths, or colors, by different amounts, decomposing white light into its composite colors. The result was one of the most famous experiments in scientific history and a discovery that, alongside other contributions by Newton, gave birth to the modern field of optics.

Discovery of gravity

In perhaps the most widely repeated story in all of science, Newton is said to have chanced upon the theory of gravity while contemplating under the shade of an apple tree. According to the legend, when an apple fell and struck him on the head, he supposedly yelled "Eureka!" as he realized that the same force that brought the apple tumbling to Earth also kept the moon in orbit around our planet and Earth circling the sun. That force, of course, would become known as gravity .

The story is slightly embellished, however. According to Newton's own account, the apple did not strike him on the head, and there's no record of what he said or if he said anything, at the moment of discovery. Nonetheless, the realization led Newton to develop his theory of gravity in 1687, which was updated by Einstein's theory of general relativity 228 years later.

Blackbody radiation

By the turn of the 20th century, many physicists — having advanced theories that explained gravity, mechanics, thermodynamics and the behavior of electromagnetic fields — were confident that they had conquered the vast majority of their field. But one troubling source of doubt remained: Theories predicted the existence of a "blackbody" — an object capable of absorbing and then remitting all incident radiation. The problem was that physicists couldn't find it.

In fact, data from experiments conducted with close approximations of black bodies — a box with a single hole whose inside walls are black — revealed that significantly less energy was emitted from blackbodies than classical theories led scientists to believe, especially at shorter wavelengths. The contradiction between experiment and theory became known as the "ultraviolet catastrophe."

The discovery prompted Max Planck to propose that the energy emitted by blackbodies wasn't continuous but rather split into discrete integer chunks called quanta. His radical proposal catalyzed the development of quantum mechanics , whose bizarre rules are completely unintuitive to observers living in the macroscopic world.

Einstein and the eclipse

Following its publication in 1915, Einstein's groundbreaking theory of general relativity briefly remained just that — a theory. Then, in 1919, astronomer Sir Arthur Eddington devised and completed stunning proof using that year's total solar eclipse .

Key to Einstein's theory was the notion that space — and, therefore, the path that light would follow through it — was warped by powerful gravitational forces. So, as the moon's shadow passed in front of the sun, Eddington recorded the position of nearby stars from his vantage point on the island of Principe in the Gulf of Guinea. By comparing these positions to those he had recorded at night without the sun in the sky, Eddington observed that they had been shifted slightly by the sun's gravity, completing his stunning proof of Einstein's theory.

Higgs boson

In 1964, Peter Higgs suggested that matter gets its mass from a field that permeates all of space, imparting particles with mass through their interactions with a particle known as the Higgs boson .

To search for the boson, thousands of particle physicists planned, constructed and fired up the Large Hadron Collider . In 2012, after trillions upon trillions of collisions in which two protons are smashed together at near light speed, the physicists finally spotted the telltale signature of the boson.

Weighing the world

Although he's perhaps best known for his discovery of hydrogen, 18th-century physicist Henry Cavendish's most ingenious experiment accurately estimated the weight of our entire planet. Using a special piece of equipment known as a torsion balance (two rods with one smaller and one larger pair of lead balls attached to the end), Cavendish measured the minuscule force of gravitational attraction between the masses. Then, by measuring the weight of one of the small balls, he measured the gravitational force between it and Earth, giving him an easy formula for calculating our planet's density and — therefore, its weight — that remains accurate to this day.

Conservation of mass

Much like energy, matter in our universe is finite and cannot be created or destroyed, only rearranged. In 1789, to arrive at this startling conclusion, French chemist Antoine Lavoisier placed a burning candle inside a sealed glass jar. After the candle had burned and melted into a puddle of wax, Lavoisier weighed the jar and its contents, finding that it had not changed

Leaning Tower of Pisa experiment

Greek philosopher Aristotle believed that objects fall at different rates because the force acting upon them was stronger for heavier objects — a claim that went unchallenged for more than a millennium.

Then came the Italian polymath Galileo Galilei, who corrected Aristotle's false claim by showing that two objects with different masses fall at exactly the same rate. Some claim Galileo's famous experiment was conducted by dropping two spheres from the Leaning Tower of Pisa, but others say this part of the story is apocryphal. Nonetheless, the experiment was perhaps most famously demonstrated by Apollo 15 astronaut David Scott, who, while dropping a feather and a hammer on the moon, showed that without air, the two objects fell at the same speed.

Detection of gravitational waves

If gravity warps space-time as Einstein predicted, then the collision of two extremely dense objects, such as neutron stars or black holes , should also create detectable shock waves in space that could reveal physics unseen by light. The problem is that these gravitational waves are tiny, often the size of a few thousandths of a proton or neutron, so detecting them requires an extremely sensitive experiment.

Enter LIGO, the Laser Interferometer Gravitational-Wave Observatory. The L-shaped detector has two 2.5-mile-long (4 km) arms containing two identical laser beams. When a gravitational wave laps at our cosmic shores, the laser in one arm is compressed and the other expands, alerting scientists to the wave's presence. In 2015, LIGO achieved its task, making the first-ever direct detection of gravitational waves and opening up an entirely new window to the cosmos.

Destruction of heliocentrism

The idea that Earth orbits the sun goes back to the fifth century B.C. to Greek philosophers Hicetas and Philolaus. Nonetheless, Claudius Ptolemy's belief that Earth was the center of the universe later took root and dominated scientific thought for more than a millennium.

Then came Nicolaus Copernicus, who proposed that Earth did, in fact, revolve around the sun and not the other way around. Concrete evidence for this was later offered by Galileo, who in 1610 peered through his telescope to observe the planet Venus moving through distinct phases — proof that it, too, orbited the sun. Galileo's discovery did not win him any friends with the Catholic Church, which tried him for heresy for his unorthodox proposal.

Foucault's pendulum

First used by French physicist Jean Bernard Léon Foucault in 1851, the famous pendulum consisted of a brass bob containing sand and suspended by a cable from the ceiling. As it swung back and forth, the angle of the line traced out by the sand changed subtly over time — clear evidence that some unknown rotation was causing it to shift. This rotation was the spinning of Earth on its axis.

Discovery of the electron

In the 19th century, physicists found that by creating a vacuum inside a glass tube and sending electricity through it, they could make the tube give off a fluorescent glow. But exactly what caused this effect, called a cathode ray, was unclear.

Then, in 1897, physicist J.J. Thomson discovered that by applying a magnetic field to the rays inside the tube, he could control the direction in which they traveled. This revelation showed Thomson that the charge within the tube came from tiny particles 1,000 times smaller than hydrogen atoms. The tiny electron had finally been found.

Deflection of an asteroid

In 2022, NASA scientists hit an astronomical "bull's-eye" by intentionally steering the 1,210-pound (550 kilograms), $314 million Double Asteroid Redirection Test (DART) spacecraft into the asteroid Dimorphos just 56 feet (17 meters) from its center. The test was designed to see if a small spacecraft propelled along a planned trajectory could, if given enough lead time, redirect an asteroid from a potentially catastrophic impact with Earth.

DART was a smashing success . The probe's original goal was to change the orbit of Dimorphos around its larger partner — the 2,560-foot-wide (780 m) asteroid Didymos — by at least 73 seconds, but the spacecraft actually altered Dimorphos' orbit by a stunning 32 minutes. NASA hailed the collision as a watershed moment for planetary defense, marking the first time that humans proved capable of diverting Armageddon, and without any assistance from Bruce Willis.

Faraday induction

In 1831, Michael Faraday, the self-taught son of a blacksmith born in rural south England, proposed the law of electromagnetic induction. The law was the result of three experiments by Faraday, the most notable of which involved the movement of a magnet inside a coil made by wrapping a wire around a paper cylinder. As the magnet moved inside the cylinder, it induced an electric current through the coil — proving that electric and magnetic fields were inextricably linked and paving the way for electric generators and devices.

Measurement of the speed of light

Light is the fastest thing in our universe, which makes measuring its speed a unique challenge. In 1676, Danish astronomer Ole Roemer chanced upon the first estimate for light's propagation while studying Io, Jupiter's innermost moon. By timing the eclipses of Io by Jupiter, Roemer was hoping to find the moon's orbital period.

What he noticed instead was that, as Earth's orbit moved closer to Jupiter, the time intervals between successive eclipses became shorter. Roemer's crucial insight was that this was due to a finite speed of light, which he roughly calculated based on Earth's orbit. Other methods later refined the measurement of light's speed, eventually arriving at its current value of 2.98 × 10^8 meters per second (about 186,282 miles per second).

Disproof of the "luminiferous ether"

Most waves, such as sound waves and water waves, require a medium to travel through. In the 19th century, physicists thought the same rule applied to light, too, with electromagnetic waves traveling through a ubiquitous medium dubbed the "luminiferous ether."

Albert Michelson and Edward W. Morley set out to prove this conjecture with a remarkably ingenious hypothesis: As the sun moves through the ether, it should displace some of the strange substance, meaning light should travel detectably faster when it moves with the ether wind than against it. They set up an interferometer experiment that used mirrors to split light beams along two opposing directions before bouncing them back with distant mirrors. If the light beams returned at different times, then the ether was real.

But the light beams inside their interferometer did not vary. Michelson and Morley concluded that their experiment had failed and moved on to other projects. But the result — which had conclusively disproved the ether theory — was later used by Einstein in his theory of special relativity to correctly state that light's speed through a fixed medium does not change, even if its source is moving.

Discovery of radioactivity

In 1897, while working in a converted shed with her husband Pierre, Marie Curie began to investigate the source of a strange new type of radiation emitted from the elements thorium and uranium. Marie Curie discovered that the radiation these elements emitted did not depend on any other factors, such as their temperature or molecular structure, but changed purely based on their quantities. While grinding up an even more radioactive substance known as pitchblende, she also discovered that it consisted of two elements that she dubbed radium and polonium.

Curie's work revealed the nature of radioactivity, a truly random property of atoms that comes from their internal structure. Curie won the Nobel Prize (twice) for her discoveries — making her the first woman to do so — and later trained doctors to use X-rays to image broken bones and bullet wounds. She died of aplastic pernicious anemia, a disease caused by radiation exposure, in 1934.

Expansion of the universe

While using the 100-inch Hooker telescope in California to study the light glimmering from distant galaxies in 1929, Edwin Hubble made a surprising observation: The light from the distant galaxies appeared to be shifted toward the red end of the spectrum — an indication that they were receding from Earth and each other. The farther away a galaxy was, the faster it was moving away.

Hubble's observation became a crucial piece of evidence for the Big Bang theory of our universe. Yet precise measurements for galaxies' recession, known as the Hubble constant, still confound scientists to this day .

Put simply, the universe is indeed expanding, but depending on where cosmologists look, it's doing so at different rates. In the past, the two best experiments to measure the expansion rate were the European Space Agency 's Planck satellite and the Hubble Space Telescope . The two observatories, each of which used a different method to measure the expansion rate, arrived at different results. These conflicting measurements have led to what some call a "cosmology crisis" that could reveal new physics or even replace the standard model of cosmology.

Ignition of nuclear fusion

In 2022, scientists at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory in California used the world's most powerful laser to achieve something physicists have been dreaming about for nearly a century: the ignition of a pellet of fuel by nuclear fusion .

The demonstration marked the first time that the energy going out of the plasma in the nuclear reactor's fiery core exceeded the energy beamed in by the laser, and has been a rallying call for fusion scientists that the distant goal of near-limitless and clean power is, in fact, achievable.

However, scientists have cautioned that the energy from the plasma exceeds only that from the lasers, and not from the energy from the whole reactor. Additionally, the laser-confinement method used by the NIF reactor, built to test thermonuclear explosions for bomb development, will be difficult to scale up.

Measurement of Earth's circumference

By roughly 500 B.C., most ancient Greeks believed the world was round — citing evidence provided by Aristotle and guided by a suggestion from Pythagoras, who believed a sphere was the most aesthetically pleasing shape for our planet.

Then, around 245 B.C., Eratosthenes of Cyrene thought of a way to make the measurement directly. Eratosthenes hired a team of bematists (professional surveyors who measured distances by walking in equal-length steps called stadia) to walk from Syene to Alexandria. They found that the distance between the two cities was roughly 5,000 stadia.

Eratosthenes then visited a well in Syene that had been reported to have an interesting property: At noon on the summer solstice each year, the sun illuminated the well's bottom without casting any shadows. Eratosthenes went to Alexandria during the solstice, stuck a pole in the ground and measured the shadow from it to be about one-fiftieth of a complete circle. Pairing this with his measurement of the distance between the two cities, he determined that Earth's circumference was about 250,000 stadia, or 24,497 miles (39,424 km). Earth is now known to measure 24,901 miles (40,074 km) around the equator, making the ancient Greeks' measurements remarkably accurate.

Discovery of black holes

The acceptance of Einstein's theory of general relativity led to some startling predictions about our universe and the nature of reality. In 1915, Karl Schwarzschild's solutions to Einstein's field equations predicted that it was possible for mass to be compressed into such a small radius that it would collapse into a gravitational singularity from which not even light could escape — a black hole.

Schwarzschild's solution remained speculation until 1971, when Paul Murdin and Louise Webster used NASA's Uhuru X-ray Explorer Satellite to identify a bright X-ray source in the constellation Cygnus that they correctly contended was a black hole.

More conclusive evidence came in 2015, when the LIGO experiment detected gravitational waves from two of the colliding cosmic monsters. Then, in 2019, the Event Horizon Telescope captured the first image of the accretion disk of superheated matter surrounding the supermassive black hole at the center of the galaxy M87.

Discovery of X-rays

While testing whether the radiation produced by cathode rays could escape through glass in 1895, German physicist Wilhelm Conrad Röntgen saw that the radiation could not only do so, but it could also zip through very thick objects, leaving a shadow on a lead screen placed behind them. He quickly realized the medical potential of these rays — later known as X-rays — for imaging skeletons and organs. His observations gave birth to the field of radiology, enabling doctors to safely and noninvasively scan for tumors, broken bones and organ failure.

The Bell test

In 1964, physicist John Stewart Bell proposed a test to prove that quantum entanglement — the weird instantaneous connection between two far-apart particles that Einstein objected to as "spooky action at a distance" — was required by quantum theory.

The test has taken many experimental forms since Bell first proposed it, but the findings remain the same: Despite what our intuition tells us, what happens in one part of the universe can instantaneously affect what happens in another, provided the objects in each region are entangled.

Detection of the quark

In 1968, experiments at the Stanford Linear Accelerator Center found that electrons and their lepton cousins, muons, were scattering from protons in a distinct way that could only be explained by the protons being composed of smaller components. These findings matched predictions by physicist Murray Gell-Mann, who dubbed them "quarks" after a line in James Joyce's "Finnegans Wake."

Archimedes' naked leap from his bathtub

First recorded in the first century B.C. by Roman architect Vitruvius, Archimedes' discovery of buoyancy is one of the most famous stories in science. The prompting for Archimedes' finding came from King Hieron of Syracuse, who suspected that a pure-gold crown a blacksmith made for him actually contained silver. To get an answer, Hieron enlisted Archimedes' help.

The problem stumped Archimedes, but not long after, as the story goes, he filled up a bathtub with water and noticed that the water spilled out as he got in. This caused him to realize that the water displaced by his body was equal to his weight — and because gold weighed more than silver, he had found a method for judging the authenticity of the crown. "Eureka!" ("I've got it!") Archimedes is said to have cried, leaping from his bathtub to announce his discovery to the king.

Deepest and most detailed photo of the universe

In 2022, the James Webb Space Telescope unveiled the deepest and most detailed picture of the universe ever taken . Called "Webb's First Deep Field," the image captures light as it appeared when our universe was just a few hundred million years old, right when galaxies began to form and light from the first stars started flickering.

The image contains an overwhelmingly dense collection of galaxies, the light from which, on its way to us, was warped by the gravitational pull of a galaxy cluster. This process, known as gravitational lensing, brings the fainter light into focus. Despite the dizzying number of galaxies in view, the image represents just a tiny sliver of sky — the speck of sky blocked out by a grain of sand held on the tip of a finger at arm's length.

OSIRIS-REx asteroid-sampling mission

In 2023, NASA's OSIRIS-REx spacecraft came hurtling back through Earth's atmosphere after a years-long journey to Bennu, a " potentially hazardous asteroid " with a 1-in-2,700 chance of smashing cataclysmically into Earth — the highest odds of any identified space object.

The goal of the mission was to see whether the building blocks for life on Earth came from outer space. OSIRIS-REx circled the asteroid for 22 months to search for a landing spot, touching down to collect a 2-ounce (60 grams) sample from Bennu's surface that could contain the extraterrestrial precursors to life on our planet. Scientists have already found many surprising details that have the potential to rewrite the history of our solar system .

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Ben Turner is a U.K. based staff writer at Live Science. He covers physics and astronomy, among other topics like tech and climate change. He graduated from University College London with a degree in particle physics before training as a journalist. When he's not writing, Ben enjoys reading literature, playing the guitar and embarrassing himself with chess.

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by Chris Woodford . Last updated: January 6, 2023.

Photo: There are always new theories to test and experiments to try. Even when we've completely nailed how Earth works, there's still the rest of the Universe to explore! Fourier telescope experiment photo by courtesy of NASA .

1: Galileo demonstrates that objects fall at the same speed (1589)

Photo: Galileo proved that different things fall at the same speed.

2: Isaac Newton splits white light into colors (1672)

Artwork: A glass prism splits white light into a spectrum. Nature recreates Newton's famous experiment whenever you see a rainbow!

3: Henry Cavendish weighs the world (1798)

Artwork: Henry Cavendish's experiment seen from above. 1) Two small balls, connected by a stick, are suspended by a thread so they're free to rotate. 2) The balls are attracted by two much larger (more massive) balls, fixed in place. 3) A light beam shines from the side at a mirror (green), mounted so it moves with the small balls. The beam is reflected back onto a measuring scale. 4) As the two sets of balls attract, the mirror pivots, shifting the reflected beam along the scale, so allowing the movement to be measured.

4: Thomas Young proves light is a wave... or does he? (1803)

Artwork: Thomas Young's famous double-slit experiment proved that light behaved like a wave—at least, some of the time. Left: A laser (1) produces coherent (regular, in-step) light (2) that passes through a pair of slits (3) onto a screen (4). If Newton were completely correct, we'd expect to see a single bright area on the screen and darkness either side. What we actually see is shown on the right. Light appears to ripple out in waves from the two slits (5), producing a distinctive interference pattern of light and dark areas (6).

5: James Prescott Joule demonstrates the conservation of energy (1840)

Artwork: The "Mechanical Equivalent of Heat"—James Prescott Joule's famous experiment proving the law now known as the conservation of energy.

6: Hippolyte Fizeau measures the speed of light (1851)

Artwork: How Fizeau measured the speed of light.

7: Robert Millikan measures the charge on the electron (1909)

Artwork: How Millikan measured the charge on the electron. 1) Oil drops (yellow) are squirted into the experimental apparatus, which has a large positive plate (blue) on top and a large negative plate (red) beneath. 2) X rays (green) are fired in. 3) The X rays give the oil drops a negative electrical charge. 4) The negatively charged drops can be made to "float" in between the two plates so their weight (red) is exactly balanced by the upward electrical pull of the positive plate (blue). When these two forces are equal, we can easily calculate the charge on the drops, which is always a whole number multiple of the basic charge on the electron.

8: Ernest Rutherford (and associates) split the atom (1897–1932)

Artwork: Transmutation: When Rutherford fired alpha particles (helium nuclei) at nitrogen, he produced oxygen. As he later wrote: "We must conclude that the nitrogen atom is disintegrated under the intense forces developed in a close collision with a swift alpha particle, and that the hydrogen atom which is liberated formed a constituent part of the nitrogen nucleus." In other words, he had split one atom apart to make another one.

Artwork: In Rutherford's gold-foil experiment (also known as the Geiger-Marsden experiment), atoms in a sheet of gold foil (1) allow positively charged alpha particles to pass through them (2) as long as the particles are traveling clear of the nucleus. Any particles fired at the nucleus are deflected by its positive charge (3). Fired at exactly the right angle, they will bounce right back! While this experiment is not splitting any atoms, as such, it was a key part of the decades-long effort to understand what atoms are made of—and in that sense, it did help physicists to "split" (venture inside) the atom.

9: Enrico Fermi demonstrates the nuclear chain reaction (1942)

Artwork: The nuclear chain reaction that turns uranium-235 into uranium-236 with a huge release of energy.

10: Rosalind Franklin photographs DNA with X rays (1953)

Artwork: The double-helix structure of DNA. Photographed with X rays, these intertwined curves appear as an X shape. Studying the X pattern in one of Franklin's photos was an important clue that tipped off Crick and Watson about the double helix.

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Don't want to read our articles try listening instead, find out more, on this website.

• Six Easy Pieces by Richard Feynman. Basic Books, 2011. This book isn't half as "easy" as the title suggests, but it does contain interesting introductions to some of the topics covered here, including the conservation of energy, the double-slit experiment, and quantum theory.
• The Oxford Handbook of the History of Physics by Jed Z. Buchwald and Robert Fox (eds). Oxford University Press, 2013/2017. A collection of twenty nine scholarly essays charting the history of physics from Galileo's gravity to the age of silicon chips.
• Great Experiments in Physics: Firsthand Accounts from Galileo to Einstein Edited by Maurice Shamos. Dover, 1959/1987. This is one of my favorite science books, ever. It's a great compilation of some classic physics experiments (including four of those listed here—the experiments by Henry Cavendish, Thomas Young, James Joule, and Robert Millikan) written by the experimenters themselves. A rare opportunity to read firsthand accounts of first-rate science!

Text copyright © Chris Woodford 2012, 2023. All rights reserved. Full copyright notice and terms of use .

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Top 10 Beautiful Physics Experiments

The list below shows the top 10 most frequently mentioned experiments by readers of  Physics World .

Top 10 beautiful experiments:

• Young's double-slit experiment applied to the interference of single electrons
• Galileo's experiment on falling bodies (1600s)
• Millikan's oil-drop experiment (1910s)
• Newton's decomposition of sunlight with a prism (1665-1666)
• Young's light-interference experiment (1801)
• Cavendish's torsion-bar experiment (1798)
• Eratosthenes' measurement of the Earth's circumference (3rd century BC)
• Galileo's experiments with rolling balls down inclined planes (1600s)
• Rutherford's discovery of the nucleus (1911)
• Foucault's pendulum (1851)

Others experiments that were cited included:

• Archimedes' experiment on hydrostatics
• Roemer's observations of the speed of light
• Joule's paddle-wheel heat experiments
• Reynolds's pipe flow experiment
• Mach & Salcher's acoustic shock wave
• Michelson-Morley measurement of the null effect of the ether
• Röntgen's detection of Maxwell's displacement current
• Oersted's discovery of electromagnetism
• The Braggs' X-ray diffraction of salt crystals
• Eddington's measurement of the bending of starlight
• Stern-Gerlach demonstration of space quantization
• Schrödinger's cat thought experiment
• Trinity test of nuclear chain reaction
• Wu et al.'s measurement of parity violation
• Goldhaber's study of neutrino helicity
• Feynman dipping an O-ring in water

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The Top 10 Greatest Physics Experiments Of All Time

Experiments form the backbone of science. It is through experiments that many theories in science have been discovered, proved and evolved into fundamental facts. While it is true that most of the science we have today is theoretical, it was experiments along histories that led to major milestones in science, especially in Physics.

Physics tuition in Singapore will enhance student’s interest when they learn about experiments that led to major milestones in Physics.  Here are the top 10 greatest physics experiments of all time that have shed light on some of the mysteries that surround us.

1. Galileo Galilei’s Experiment on Speed of Falling Objects

Before Galileo, Aristotle had argued that heavy objects fall at a faster rate than lighter objects. But Galileo who is famed for his work on gravity, motion and light proved that objects fall at the same speed irrespective of their weight. He demonstrated this with a stone and feather experiment.

2. James Prescott Joule‘s Energy Conservation Experiment

This experiment conducted in 1840 demonstrated that conservation of energy is nothing but a conversion of energy from one form to another. He found that Energy lost = energy gained is another way and you can’t create or destroy energy, but you can change it from one form into another.

3. Isaac Newton’s Discovery of Light Spectrum

Isaac Newton placed a triangle glass (prism) along his window and split sunlight into many colors, this experiment done in 1672 is a landmark revelation that led onto many other interesting and essential discoveries about the light spectrum and its properties.

4. Henry Cavendish’s Earth Density Experiment

Demonstrated in 1798, this popular experiment with only 4 balls and 2 sticks achieved the impossible task of weighing the world via the measurement of Earth’s density.

5. Hippolyte Fizeau’s Experiment on Light Speed

In 1851, Fizeau managed to measure the speed at which light travels using a simple gear wheel and mirrors. Thou it is approximately 5% off the actual speed, it’s amazing at that time and has built a foundation for Léon Foucault whom improved the experiment and more accurately calculated the speed of light. Most of the mysteries of physics are easily cracked with simple experiments just as Fizeau’s.

6.Robert Millikan’s Measurement of the Charge of Electron

In 1909, American scientist Millikan found that the measure of an electron’s charge could be evaluated by using metal plates, transparent chamber, and a few oil drops. By measuring the velocity between power off and on, he discovered the charge on the electron and won himself the Nobel Prize in Physics.

7. Enrico Fermi’s Nuclear Chain Reaction Experiment

In 1942, Enrico Fermi fired an uncharged neutron to Uranium 235 to form Uranium 236, the additional neutrons generated went on to hit other Uranium 235, causing a nuclear chain reaction, which has unearthed potentially dangerous and at the same time beneficial revelations about nuclear energy . E=MC^2 is the famous formula responsible for this chain reaction.

8. Rutherford’s Gold Foil Experiment

In 1909, Rutherford with the aid of Geiger and Marsden, discovered the atomic structure as we know it today with a core nucleus and electrons around it by firing positively charged particles at a sheet of gold foil.

9. Rosalind Franklin DNA Radiographs

Franklin’s photo of X-ray diffraction had thrown light on immense and critical information regarding DNA structure using a similar concept to shadow puppetry.

10. Foucault’s Pendulum Experiment

This experiment done in 1851 in Paris demonstrated the all-important revolving movement of earth on its own axis. Researchers have since confirmed about 24 hours as the oscillation period. As you can see that what we have taken for granted about various elements around us has actually been discovered through several trial and errors by great physicists.  Being tutored in JC Physics tuition will give students a great opportunity to learn in-depth all they need to know about physics and more, so they shine in the subject and score high grades.

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The Top 10 Science Experiments of All Time

These seminal experiments changed our understanding of the universe and ourselves..

Every day, we conduct science experiments, posing an “if” with a “then” and seeing what shakes out. Maybe it’s just taking a slightly different route on our commute home or heating that burrito for a few seconds longer in the microwave. Or it could be trying one more variation of that gene, or wondering what kind of code would best fit a given problem. Ultimately, this striving, questioning spirit is at the root of our ability to discover anything at all. A willingness to experiment has helped us delve deeper into the nature of reality through the pursuit we call science. 

A select batch of these science experiments has stood the test of time in showcasing our species at its inquiring, intelligent best. Whether elegant or crude, and often with a touch of serendipity, these singular efforts have delivered insights that changed our view of ourselves or the universe. 

Here are nine such successful endeavors — plus a glorious failure — that could be hailed as the top science experiments of all time.

Eratosthenes Measures the World

Experimental result: The first recorded measurement of Earth’s circumference 

When: end of the third century B.C.

Just how big is our world? Of the many answers from ancient cultures, a stunningly accurate value calculated by Eratosthenes has echoed down the ages. Born around 276 B.C. in Cyrene, a Greek settlement on the coast of modern-day Libya, Eratosthenes became a voracious scholar — a trait that brought him both critics and admirers. The haters nicknamed him Beta, after the second letter of the Greek alphabet. University of Puget Sound physics professor James Evans explains the Classical-style burn: “Eratosthenes moved so often from one field to another that his contemporaries thought of him as only second-best in each of them.” Those who instead celebrated the multitalented Eratosthenes dubbed him Pentathlos, after the five-event athletic competition.

That mental dexterity landed the scholar a gig as chief librarian at the famous library in Alexandria, Egypt. It was there that he conducted his famous experiment. He had heard of a well in Syene, a Nile River city to the south (modern-day Aswan), where the noon sun shone straight down, casting no shadows, on the date of the Northern Hemisphere’s summer solstice. Intrigued, Eratosthenes measured the shadow cast by a vertical stick in Alexandria on this same day and time. He determined the angle of the sun’s light there to be 7.2 degrees, or 1/50th of a circle’s 360 degrees. 

Knowing — as many educated Greeks did — Earth was spherical, Eratosthenes fathomed that if he knew the distance between the two cities, he could multiply that figure by 50 and gauge Earth’s curvature, and hence its total circumference. Supplied with that information, Eratosthenes deduced Earth’s circumference as 250,000 stades, a Hellenistic unit of length equaling roughly 600 feet. The span equates to about 28,500 miles, well within the ballpark of the correct figure of 24,900 miles. 

Eratosthenes’ motive for getting Earth’s size right was his keenness for geography, a field whose name he coined. Fittingly, modernity has bestowed upon him one more nickname: father of geography. Not bad for a guy once dismissed as second-rate.

William Harvey Takes the Pulse of Nature

Experimental result: The discovery of blood circulation

When: Theory published in 1628

Boy, was Galen wrong. 

The Greek physician-cum-philosopher proposed a model of blood flow in the second century that, despite being full of whoppers, prevailed for nearly 1,500 years. Among its claims: The liver constantly makes new blood from food we eat; blood flows throughout the body in two separate streams, one infused (via the lungs) with “vital spirits” from air; and the blood that tissues soak up never returns to the heart. 

Overturning all this dogma took a series of often gruesome experiments. 

High-born in England in 1578, William Harvey rose to become royal physician to King James I, affording him the time and means to pursue his greatest interest: anatomy. He first hacked away (literally, in some cases) at the Galenic model by exsanguinating — draining the blood from — test critters, including sheep and pigs. Harvey realized that if Galen were right, an impossible volume of blood, exceeding the animals’ size, would have to pump through the heart every hour. 

To drive this point home, Harvey sliced open live animals in public, demonstrating their puny blood supplies. He also constricted blood flow into a snake’s exposed heart by finger-pinching a main vein. The heart shrunk and paled; when pierced, it poured forth little blood. By contrast, choking off the main exiting artery swelled the heart. Through studies of the slow heart beats of reptiles and animals near death, he discerned the heart’s contractions, and deduced that it pumped blood through the body in a circuit.

According to Andrew Gregory, a professor of history and philosophy of science at University College London, this was no easy deduction on Harvey’s part. “If you look at a heart beating normally in its normal surroundings, it is very difficult to work out what is actually happening,” he says. 

Experiments with willing people, which involved temporarily blocking blood flow in and out of limbs, further bore out Harvey’s revolutionary conception of blood circulation. He published the full theory in a 1628 book, De Motu Cordis [The Motion of the Heart]. His evidence-based approach transformed medical science, and he’s recognized today as the father of modern medicine and physiology.

Gregor Mendel Cultivates Genetics

Experimental result: The fundamental rules of genetic inheritance 

When: 1855-1863 

A child, to varying degrees, resembles a parent, whether it’s a passing resemblance or a full-blown mini-me. Why? 

The profound mystery behind the inheritance of physical traits began to unravel a century and a half ago, thanks to Gregor Mendel. Born in 1822 in what is now the Czech Republic, Mendel showed a knack for the physical sciences, though his farming family had little money for formal education. Following the advice of a professor, he joined the Augustinian order, a monastic group that emphasized research and learning, in 1843. 

Ensconced at a monastery in Brno, the shy Gregor quickly began spending time in the garden. Fuchsias in particular grabbed his attention, their daintiness hinting at an underlying grand design. “The fuchsias probably gave him the idea for the famous experiments,” says Sander Gliboff, who researches the history of biology at Indiana University Bloomington. “He had been crossing different varieties, trying to get new colors or combinations of colors, and he got repeatable results that suggested some law of heredity at work.”

These laws became clear with his cultivation of pea plants. Using paintbrushes, Mendel dabbed pollen from one to another, precisely pairing thousands of plants with certain traits over a stretch of about seven years. He meticulously documented how matching yellow peas and green peas, for instance, always yielded a yellow plant. Yet mating these yellow offspring together produced a generation where a quarter of the peas gleamed green again. Ratios like these led to Mendel’s coining of the terms dominant (the yellow color, in this case) and recessive for what we now call genes, and which Mendel referred to as “factors.” 

He was ahead of his time. His studies received scant attention in their day, but decades later, when other scientists discovered and replicated Mendel’s experiments, they came to be regarded as a breakthrough. 

“The genius in Mendel’s experiments was his way of formulating simple hypotheses that explain a few things very well, instead of tackling all the complexities of heredity at once,” says Gliboff. “His brilliance was in putting it all together into a project that he could actually do.”

Isaac Newton Eyes Optics

Experimental result: The nature of color and light

When: 1665-1666

Before he was that Isaac Newton — scientist extraordinaire and inventor of the laws of motion, calculus and universal gravitation (plus a crimefighter to boot) — plain ol’ Isaac found himself with time to kill. To escape a devastating outbreak of plague in his college town of Cambridge, Newton holed up at his boyhood home in the English countryside. There, he tinkered with a prism he picked up at a local fair — a “child’s plaything,” according to Patricia Fara, fellow of Clare College, Cambridge. 

Let sunlight pass through a prism and a rainbow, or spectrum, of colors splays out. In Newton’s time, prevailing thinking held that light takes on the color from the medium it transits, like sunlight through stained glass. Unconvinced, Newton set up a prism experiment that proved color is instead an inherent property of light itself. This revolutionary insight established the field of optics, fundamental to modern science and technology. 

Newton deftly executed the delicate experiment: He bored a hole in a window shutter, allowing a single beam of sunlight to pass through two prisms. By blocking some of the resulting colors from reaching the second prism, Newton showed that different colors refracted, or bent, differently through a prism. He then singled out a color from the first prism and passed it alone through the second prism; when the color came out unchanged, it proved the prism didn’t affect the color of the ray. The medium did not matter. Color was tied up, somehow, with light itself. 

Partly owing to the ad hoc, homemade nature of Newton’s experimental setup, plus his incomplete descriptions in a seminal 1672 paper, his contemporaries initially struggled to replicate the results. “It’s a really, really technically difficult experiment to carry out,” says Fara. “But once you have seen it, it’s incredibly convincing.” 

In making his name, Newton certainly displayed a flair for experimentation, occasionally delving into the self-as-subject variety. One time, he stared at the sun so long he nearly went blind. Another, he wormed a long, thick needle under his eyelid, pressing on the back of his eyeball to gauge how it affected his vision. Although he had plenty of misses in his career — forays into occultism, dabbling in biblical numerology — Newton’s hits ensured his lasting fame.

Michelson and Morley Whiff on Ether

Experimental result: The way light moves

Say “hey!” and the sound waves travel through a medium (air) to reach your listener’s ears. Ocean waves, too, move through their own medium: water. Light waves are a special case, however. In a vacuum, with all media such as air and water removed, light somehow still gets from here to there. How can that be? 

The answer, according to the physics en vogue in the late 19th century, was an invisible, ubiquitous medium delightfully dubbed the “luminiferous ether.” Working together at what is now Case Western Reserve University in Ohio, Albert Michelson and Edward W. Morley set out to prove this ether’s existence. What followed is arguably the most famous failed experiment in history. 

The scientists’ hypothesis was thus: As Earth orbits the sun, it constantly plows through ether, generating an ether wind. When the path of a light beam travels in the same direction as the wind, the light should move a bit faster compared with sailing against the wind. 

To measure the effect, miniscule though it would have to be, Michelson had just the thing. In the early 1880s, he had invented a type of interferometer, an instrument that brings sources of light together to create an interference pattern, like when ripples on a pond intermingle. A Michelson interferometer beams light through a one-way mirror. The light splits in two, and the resulting beams travel at right angles to each other. After some distance, they reflect off mirrors back toward a central meeting point. If the light beams arrive at different times, due to some sort of unequal displacement during their journeys (say, from the ether wind), they create a distinctive interference pattern. 

The researchers protected their delicate interferometer setup from vibrations by placing it atop a solid sandstone slab, floating almost friction-free in a trough of mercury and further isolated in a campus building’s basement. Michelson and Morley slowly rotated the slab, expecting to see interference patterns as the light beams synced in and out with the ether’s direction. 

Instead, nothing. Light’s speed did not vary. 

Neither researcher fully grasped the significance of their null result. Chalking it up to experimental error, they moved on to other projects. (Fruitfully so: In 1907, Michelson became the first American to win a Nobel Prize, for optical instrument-based investigations.) But the huge dent Michelson and Morley unintentionally kicked into ether theory set off a chain of further experimentation and theorizing that led to Albert Einstein’s 1905 breakthrough new paradigm of light, special relativity.

Marie Curie’s Work Matters

Experimental result: Defining radioactivity 

Few women are represented in the annals of legendary scientific experiments, reflecting their historical exclusion from the discipline. Marie Sklodowska broke this mold. 

Born in 1867 in Warsaw, she immigrated to Paris at age 24 for the chance to further study math and physics. There, she met and married physicist Pierre Curie, a close intellectual partner who helped her revolutionary ideas gain a foothold within the male-dominated field. “If it wasn’t for Pierre, Marie would never have been accepted by the scientific community,” says Marilyn B. Ogilvie, professor emeritus in the history of science at the University of Oklahoma. “Nonetheless, the basic hypotheses — those that guided the future course of investigation into the nature of radioactivity — were hers.”

The Curies worked together mostly out of a converted shed on the college campus where Pierre worked. For her doctoral thesis in 1897, Marie began investigating a newfangled kind of radiation, similar to X-rays and discovered just a year earlier. Using an instrument called an electrometer, built by Pierre and his brother, Marie measured the mysterious rays emitted by thorium and uranium. Regardless of the elements’ mineralogical makeup — a yellow crystal or a black powder, in uranium’s case — radiation rates depended solely on the amount of the element present. 

From this observation, Marie deduced that the emission of radiation had nothing to do with a substance’s molecular arrangements. Instead, radioactivity — a term she coined — was an inherent property of individual atoms, emanating from their internal structure. Up until this point, scientists had thought atoms elementary, indivisible entities. Marie had cracked the door open to understanding matter at a more fundamental, subatomic level. 

Curie was the first woman to win a Nobel Prize, in 1903, and one of a very select few people to earn a second Nobel, in 1911 (for her later discoveries of the elements radium and polonium). 

“In her life and work,” says Ogilvie, “she became a role model for young women who wanted a career in science.”

Ivan Pavlov Salivates at the Idea

Experimental result: The discovery of conditioned reflexes

When: 1890s-1900s

Russian physiologist Ivan Pavlov scooped up a Nobel Prize in 1904 for his work with dogs, investigating how saliva and stomach juices digest food. While his scientific legacy will always be tied to doggie drool, it is the operations of the mind — canine, human and otherwise — for which Pavlov remains celebrated today.

Gauging gastric secretions was no picnic. Pavlov and his students collected the fluids that canine digestive organs produced, with a tube suspended from some pooches’ mouths to capture saliva. Come feeding time, the researchers began noticing that dogs who were experienced in the trials would start drooling into the tubes before they’d even tasted a morsel. Like numerous other bodily functions, the generation of saliva was considered a reflex at the time, an unconscious action only occurring in the presence of food. But Pavlov’s dogs had learned to associate the appearance of an experimenter with meals, meaning the canines’ experience had conditioned their physical responses. 

“Up until Pavlov’s work, reflexes were considered fixed or hardwired and not changeable,” says Catharine Rankin, a psychology professor at the University of British Columbia and president of the Pavlovian Society. “His work showed that they could change as a result of experience.” 

Pavlov and his team then taught the dogs to associate food with neutral stimuli as varied as buzzers, metronomes, rotating objects, black squares, whistles, lamp flashes and electric shocks. Pavlov never did ring a bell, however; credit an early mistranslation of the Russian word for buzzer for that enduring myth. 

The findings formed the basis for the concept of classical, or Pavlovian, conditioning. It extends to essentially any learning about stimuli, even if reflexive responses are not involved. “Pavlovian conditioning is happening to us all of the time,” says W. Jeffrey Wilson of Albion College, fellow officer of the Pavlovian Society. “Our brains are constantly connecting things we experience together.” In fact, trying to “un-wire” these conditioned responses is the strategy behind modern treatments for post-traumatic stress disorder, as well as addiction.

Robert Millikan Gets a Charge

Experimental result: The precise value of a single electron’s charge

By most measures, Robert Millikan had done well for himself. Born in 1868 in a small town in Illinois, he went on to earn degrees from Oberlin College and Columbia University. He studied physics with European luminaries in Germany. He then joined the University of Chicago’s physics department, and even penned some successful textbooks. 

But his colleagues were doing far more. The turn of the 20th century was a heady time for physics: In the span of just over a decade, the world was introduced to quantum physics, special relativity and the electron — the first evidence that atoms had divisible parts. By 1908, Millikan found himself pushing 40 without a significant discovery to his name. 

The electron, though, offered an opportunity. Researchers had struggled with whether the particle represented a fundamental unit of electric charge, the same in all cases. It was a critical determination for further developing particle physics. With nothing to lose, Millikan gave it a go. 

In his lab at the University of Chicago, he began working with containers of thick water vapor, called cloud chambers, and varying the strength of an electric field within them. Clouds of water droplets formed around charged atoms and molecules before descending due to gravity. By adjusting the strength of the electric field, he could slow down or even halt a single droplet’s fall, countering gravity with electricity. Find the precise strength where they balanced, and — assuming it did so consistently — that would reveal the charge’s value. 

When it turned out water evaporated too quickly, Millikan and his students — the often-unsung heroes of science — switched to a longer-lasting substance: oil, sprayed into the chamber by a drugstore perfume atomizer. 

The increasingly sophisticated oil-drop experiments eventually determined that the electron did indeed represent a unit of charge. They estimated its value to within whiskers of the currently accepted charge of one electron (1.602 x 10-19 coulombs). It was a coup for particle physics, as well as Millikan. 

“There’s no question that it was a brilliant experiment,” says Caltech physicist David Goodstein. “Millikan’s result proved beyond reasonable doubt that the electron existed and was quantized with a definite charge. All of the discoveries of particle physics follow from that.”

Young, Davisson and Germer See Particles Do the Wave

Experimental result: The wavelike nature of light and electrons 

When: 1801 and 1927, respectively 

Light: particle or wave? Having long wrestled with this seeming either/or, many physicists settled on particle after Isaac Newton’s tour de force through optics. But a rudimentary, yet powerful, demonstration by fellow Englishman Thomas Young shattered this convention. 

Young’s interests covered everything from Egyptology (he helped decode the Rosetta Stone) to medicine and optics. To probe light’s essence, Young devised an experiment in 1801. He cut two thin slits into an opaque object, let sunlight stream through them and watched how the beams cast a series of bright and dark fringes on a screen beyond. Young reasoned that this pattern emerged from light wavily spreading outward, like ripples across a pond, with crests and troughs from different light waves amplifying and canceling each other. 

Although contemporary physicists initially rebuffed Young’s findings, rampant rerunning of these so-called double-slit experiments established that the particles of light really do move like waves. “Double-slit experiments have become so compelling [because] they are relatively easy to conduct,” says David Kaiser, a professor of physics and of the history of science at MIT. “There is an unusually large ratio, in this case, between the relative simplicity and accessibility of the experimental design and the deep conceptual significance of the results.”

More than a century later, a related experiment by Clinton Davisson and Lester Germer showed the depth of this significance. At what is now called Nokia Bell Labs in New Jersey, the physicists ricocheted electron particles off a nickel crystal. The scattered electrons interacted to produce a pattern only possible if the particles also acted like waves. Subsequent double slit-style experiments with electrons proved that particles with matter and undulating energy (light) can each act like both particles and waves. The paradoxical idea lies at the heart of quantum physics, which at the time was just beginning to explain the behavior of matter at a fundamental level. 

“What these experiments show, at their root, is that the stuff of the world, be it radiation or seemingly solid matter, has some irreducible, unavoidable wavelike characteristics,” says Kaiser. “No matter how surprising or counterintuitive that may seem, physicists must take that essential ‘waviness’ into account.”

Robert Paine Stresses Starfish

Experimental result: The disproportionate impact of keystone species on ecosystems

When: Initially presented in a 1966 paper

Just like the purple starfish he crowbarred off rocks and chucked into the Pacific Ocean, Bob Paine threw conventional wisdom right out the window. 

By the 1960s, ecologists had come to agree that habitats thrived primarily through diversity. The common practice of observing these interacting webs of creatures great and small suggested as much. Paine took a different approach. 

Curious what would happen if he intervened in an environment, Paine ran his starfish-banishing experiments in tidal pools along and off the rugged coast of Washington state. The removal of this single species, it turned out, could destabilize a whole ecosystem. Unchecked, the starfish’s barnacle prey went wild — only to then be devoured by marauding mussels. These shellfish, in turn, started crowding out the limpets and algal species. The eventual result: a food web in tatters, with only mussel-dominated pools left behind. 

Paine dubbed the starfish a keystone species, after the necessary center stone that locks an arch into place. A revelatory concept, it meant that all species do not contribute equally in a given ecosystem. Paine’s discovery had a major influence on conservation, overturning the practice of narrowly preserving an individual species for the sake of it, versus an ecosystem-based management strategy.

“His influence was absolutely transformative,” says Oregon State University’s Jane Lubchenco, a marine ecologist. She and her husband, fellow OSU professor Bruce Menge, met 50 years ago as graduate students in Paine’s lab at the University of Washington. Lubchenco, the administrator of the National Oceanic Atmospheric Administration from 2009 to 2013, saw over the years the impact that Paine’s keystone species concept had on policies related to fisheries management.

Lubchenco and Menge credit Paine’s inquisitiveness and dogged personality for changing their field. “A thing that made him so charismatic was almost a childlike enthusiasm for ideas,” says Menge. “Curiosity drove him to start the experiment, and then he got these spectacular results.”

Paine died in 2016. His later work had begun exploring the profound implications of humans as a hyper-keystone species, altering the global ecosystem through climate change and unchecked predation.

Adam Hadhazy is based in New Jersey. His work has also appeared in New Scientist and Popular Science , among other publications. This story originally appeared in print as "10 Experiments That Changed Everything"

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10 Physics Experiments That Changed Our View Of the Universe

No one can question the impact of science on human civilization, and the importance of experimentation in science is equally undeniable. Some experiments confirm what we already know, others suggest a mechanism by which observed phenomena are driven.

For the latter type of experiment think of ancient Greek polymath Archimedes in the bathtub realizing that the displacement of water was directly related to the volume of an object placed in it, which legend suggests led to him running down the street naked yelling "Eureka!" —  something we now know probably didn't happen.

Most scientific research is based on investigating "known unknowns" — scientists observe something, develop a hypothesis to be tested, and then design experiments to test this. But other experiments have a more profound effect on our understanding, suggesting things we had no idea about — the "unknown unknowns."

Throughout the history of science, there have been experiments in all the major disciplines that have delivered paradigm-shifting or even status quo-shattering results.

But when it comes to our understanding of the Universe arguably no field of science has delivered more results that fundamentally shifted our understanding of the universe than physics — encompassing astronomy, perhaps the earliest science, particle physics, nuclear physics, cosmology, and quantum mechanics. Some experiments are performed physically, while others are performed hypothetically in some of history's greatest minds.

These physics experiments fundamentally changed how we view the universe and our place within it.

The Earth moves from the center of the Universe

The idea that the Earth orbits the sun along with the rest of the planets may not seem particularly controversial, but the fact that we do not occupy a unique perspective or privileged position in the universe caused major waves in the 1600s when it finally began to gain traction.

Though the concept of a spherical Earth revolving around a "central fire" had planted the seeds of heliocentrism as early as the 5th Century BC via the musings of philosophers Philolaus and Hicetas, something extended upon by Aristarchus of Samos two centuries later, from the 2nd Century AD scientific thought had been dominated by the geocentric, or Earth-centered, theory of Claudius Ptolemy of Alexandria .

This would be the case for almost 1,400 years until the publication of Nicolaus Copernicus's De revolutionibus orbium coelestium libri VI or "Six Books Concerning the Revolutions of the Heavenly Orbs" in 1543, which put heliocentrism back on the table.

It was Italian natural philosopher, astronomer, and mathematician Galileo Galilei that would make the experimental steps that saw the so-called Copernican model of a sun-centric solar system eventually accepted as the accurate version of space at a local scale.

In 1610, using his telescope Galileo observed the planet Venus , discovering that it has phases just like the moon.

Galileo reasoned that these phases could only be explained by Venus going around the sun, upon occasion passing behind and beyond our star rather than revolving around the Earth.

The concept that the Earth was not the center of the cosmos would enrage the church which believed it contradicted scripture. This led to the inquisition process being brought against Galileo resulting in him being gagged against speaking on or writing about heliocentrism.

The nature of color: Netwon splits light

Though Sir Issac Newton's laws of motion and his contributions to the theory of gravity are widely regarded as his crowning achievements, the great passion of the author of Principia Mathematica was optics.

In the early 17th Century, when optics was growing as a field in physics with the development of instruments like the microscope, Newton decided to investigate the nature of light, in turn discovering how color arises.

The experiment devised by Newton to do this was devilishly simple. The physicist pushed a small pinhole through his blinds to allow a small beam of sunlight through. He found that when refracted by a prism this light changed into an oblong made up of different colors. Newton found that no matter the shape or size of the hole he cut and thus the shape of the beam of sunlight, the refracted light remains an oblong block of the same colors in the same order.

Even more surprisingly, he found that if he introduced a second black he could change the rainbow back into white light.

This showed white light from the sun was made of a miscellany of different colors. Delving deeper still, Newton found that red or blue light, when refracted by a prism, remained unchanged.

Perhaps the most important discovery this experiment yielded was the fact that the angle of the refraction of light depended on its color, the first hint that colors of light have their own frequency and wavelength.

To the heart of the atom: The Geiger-Marsden experiment

The concept of an atom — the point at which matter cannon longer be cut — dates back to the ancient Greeks, with the word itself derived from the Greek word "atomos" meaning "indivisible." Until 1897, scientists believed that atoms had no internal structure and were the smallest units of matter. That was before the discovery of a small negatively charged particle — the electron — by Joseph John Thomson.

In 1904, J.J Thomspon suggested that these particles were embedded in a positively charged substance much like fruit dispersed in plum pudding in his appropriately named plum-pudding model of the atom.

This model was overturned by the Geiger-Marsden experiment , also known as the gold foil experiment or the α-particle scattering experiments, pioneered by Ernest Rutherford and conducted by his protégés, Ernest Marsden and Hans Geiger.

Firing α-particles — which we now know are identical to a helium-4 nucleus — emitted by a radioactive source at a thin sheet of gold foil Rutherford reasoned that if the plum-pudding model of the atom was correct these traveling particles would experience the tiniest of deflections. This is due to the fact that an α-particle is about 7,000 times more massive than an electron.

The 1911 experiments showed that occasionally α-particles experienced a large deflection. While only one in 20,000 alpha particles had been deflected 45° or more this was enough to spark a major rethink of the atom and unveiled the presence of the atomic nucleus. 

Rutherford compared the results to firing a 15-inch shell at a sheet of tissue paper and having it bounce back directly at you!

This revealed that the majority of the matter in an atom was concentrated at its center. Rutherford proposed a model of the atom with electrons orbiting a massive positively charged nucleus.

This model in time would be overturned, but it represented a vital step in discovering the proton and the neutron and unveiling atomic structure.

The Twin Paradox: Time is relative (so is space)

For Newton, the concept of space was fairly simple. A stage upon which the events of the Universe simply unfold. But, over the course of the first two decades of the 20th Century, Albert Einstein would shatter the notion of prosaic space, showing space itself to be a player in the events of the Universe, both directing the action and being influenced by other actors such as mass. This in itself would have been revolutionary, but Einstein didn't stop there, he showed time and space to be a single entity with the 4th dimension of time just as subject to change as space depending on the circumstance of the observer.

While Einstein became a master of the Gedankenexperiment — or thought experiment  — to develop the theory of special and then general relativity, there is one thought experiment above all others that perhaps best exemplifies Einstein's groundbreaking approach to time, the so-called "twin paradox."

The twin paradox expresses the idea that "moving clocks run slow" and the concept of time dilation. It imagines twin sisters — Terra and Astra — the latter of whom blasts off from Earth in a rocket headed for a distant star system. Terra waits on Earth and from her reference frame, she will see Astra's moving clock running slow. 

But, here's where the paradox element is introduced.

The paradox element

In Astra's reference frame, it isn't her clock that is moving, it's Terra's. That means she sees Terra's clock running slower than hers. So the question is, who is right? When the sisters meet up again upon Astra's return to Earth after many years, who is older?

The sisters discover that Terra has aged while Astra has retained her youth, and the reason for this is one of the key aspects of special relativity, it only applies to non-inertial reference frames — reference frames that don't accelerate. During Astra's trip through the cosmos, there are various times when she has to accelerate, which also includes changing directions.

The difference in age between Terra and Astra depends on factors such as her speed, how long she was away, and how many times she had to change speeds or directions.

The effect of time dilation is no longer restricted to thought experiments. Physicists have measured its effect on short-lived called Muons. When created by cosmic rays hitting the upper atmosphere these particles should exist for just 2.2 microseconds. Even when factoring in time dilation and the incredible velocity of muons–-0.98c or 98% the speed of light–-very few of these particles should survive long enough to strike the surface of our planet. But thanks to time-dilation, just like Astra retaining her youth, many of these particles do survive long enough to reach the surface of the planet.

What is light? The double-slit experiment

The splitting of light as it passed through a prism was the beginning of our investigation into this fundamental aspect of reality. For decades the argument raged amongst physicists as to whether light was a particle or wave. The double-slit experiment proved light is neither a particle, nor a wave, but has properties of both.

The double-slit experiment begins with monochromatic light — light of one wavelength and thus color — shining through two slits with a width separated by a distance similar to its wavelength. 

As the wave passes through both slits, it splits into two new waves — just like water waves do as they encounter a rock. These two waves then interfere with each other, where a peak meets a trough, they will cancel each other out — so-called destructive interference. However, where a peak meets a peak the waves are reinforced — constructive interference — and the points with the brightest light. 

On a second wall behind the screen, the light creates a stripy pattern, called an interference pattern. This demonstrates the wave nature of light, but there is more to this experiment.

If the light sent through the slits is reduced in intensity to one photon at a time, we see a particle-like distribution building on the screen. But, as the particles pile up, an interference pattern begins to build up like the photons are interfering with themselves. 

This demonstrated that while a photon was detected as having the properties of a particle, interference unique to a wave appeared passing through the double-slit, thus revealing that the photon has properties of a particle and a wave.

Particle wave daulity in matter: The double-slit experiment part 2

Physicists weren't done with the double-slit experiment. It had already revealed the particle-wave duality of light, but researchers were determined to run the experiment with another particle , replacing light with electrons — tiny negatively charged fundamental particles of matter. 

Using an electron gun to fire particles through the double-slit portion to a fluorescent screen or another type of particle detector, the electrons seem to appear randomly.

As more electrons come through the slits, an interference pattern — bands of dark "hits" and light misses — develops just as we see with the photons implying that the electrons are traveling just like photons do — like waves.

This interference pattern disappears if the experiment is run again, but this time with one of the slits closed, which leads to a pile-up of hits on the screen just like we would expect with bullets.

Re-rerunning the experiment with both slits open and the electrons dripping through one at a time we find that the interference pattern begins to emerge again. This implies that like the photons the electrons are interfering with themselves as they pass through the double slits.

The consequence of this is we were forced to abandon the classic idea of a particle possessing a single defined trajectory through space. The particle can be considered passing through each slit causing constructive and destructive interference. It also shows that matter — like light — exhibits particle-wave duality .

Quantum Entanglement: Investigating spooky action at a distance

Any scientific phenomenon that stunned Albert Einstein must be revolutionary. The concept of entanglement is the idea that two particles can be linked in such a way that changing one instantly changes the other. But, what troubled Einstein was the fact that this change would happen instantaneously even if the particles are at opposite ends of the Universe.

This challenges the ideas in physics of local realism — the concepts that the cause of a physical change must be local and that the properties of objects are real and exist in our physical universe independent of our minds. These challenges led Einstein to describe entanglement as "spooky action at a distance" and resulted in him spending the last years of his life devising thought experiments that would show the theory of quantum physics was incomplete with hidden variables explaining the nature of entanglement.

A physical experiment would finally validate the non-local nature of entanglement. In the 1960s physicist, John Bell devised a test called Bell's Inequality to hunt for hidden variables.

The aim was to test three assumptions; locality, realism, and freedom of choice — the idea that physicists can make measurements freely without the influence of hidden variables. Experiments to test Bell's Inequality have shown that when particles are entangled, the outcomes of measurements are more statistically correlated than would be expected in non-quantum systems described by classical physics.

Most physicists believe that entanglement violates either the first or second principle of Bell's Inequality. What is certain is that a change in an entangled particle causes an instantaneous change in its partner.

The cat, the box, and the poison

The rules of the subatomic world described by quantum physics are weird. Scientists would describe this as counterintuitive and perhaps one thought experiment above all others perfectly exemplifies this weirdness.

In quantum mechanics, the physics of the very small, possible states of a system is determined by wave functions that can overlap. This means that a quantum system modeled by waves can be described as existing in multiple states at one time — a  superposition  — with these states collapsing and taking a single value when measured or forced to interact with another system.

Part of what is known as the Copenhagen interpretation of quantum mechanics, Erwin Schrödinger wanted to show the flaws in this theory and inadvertently created one of the most talked-about thought experiments of all time — Schrödinger's cat.

Schrödinger suggested placing a cat in a box with a diabolical device — a vial of deadly poison that would break upon the decay of an atomic nucleus. Because the decay of an atom is a completely random process, there is no way of determining if this has happened without opening the box.

That meant that if we treat the box as a quantum system, Schrödinger's cat is in the ultimate superposition — being both dead and alive at the same time. The situation would only resolve upon the opening of the box when the wavefunction of the system collapses and the cat is found to be either dead or alive.

What is the Cosmic Microwave Background?

The cosmic microwave background (CMB) is radiation left over from an event shortly after the big bang called " the last scattering ." This was the point, around 14 billion years ago, at which the Universe had cooled enough to allow electrons to join protons to form the first atoms.

As a consequence of this, photons were no longer endlessly scattered by free electrons and were suddenly permitted to freely travel the Universe. In other words, the Universe went from opaque to transparent. Radiation from this point should have a uniform temperature and some by spread through the Universe in a highly-uniform way.

Until 1965, Bob Dicke and his Princeton University team had been diligently searching for evidence of this "cosmic fossil" frozen into the Universe. But, unbeknownst to them, another team, just 50 or so miles away in New Jersey had already detected the CMB, they just didn't know it yet.

Astronomers Arno Penzias and Robert Wilson were having problems with the Holmdel Horn Antenna – a microwave radio telescope and satellite communication system–at Bell Labs. The duo was attempting to use the sensitive instrument to search for hydrogen in the Milky Way but they were picking up the same buzz from all areas of the sky. The duo attempted to rid themselves of this "background noise" several times trying to limit everything that occurred to them that could be the cause of this static.

Diving deeper

This involved eliminating badly insulated wires and even involved crawling into the horn-shaped antenna to remove what they described as "white dielectric material"–pigeon droppings to you and me–left by roosting birds. Penzias and Wilson finally determined that the signal was not coming from Earth. It was only when communicating with Dicke at Princeton that Penzias and Wilson realized what they had found. After a brief phone conversation with the Bell Labs team, Dicke's words to his team said it all: "Well, boys, we've been scooped."

Arno Penzias and Robert Wilson would share the 1978 Nobel Prize in Physics for their discovery of the CMB with Pyotr Leonidovich Kapitsa "for his basic inventions and discoveries in the area of low-temperature physics.

We now know the CMB fills the Universe with a uniform temperature of 2.7 K–less than three degrees above absolute zero, and at one point bits effects could be seen in every living room in the U.S. According to NASA , the CMB was "responsible for a sizeable amount of static on your television set–well, before the days of cable. Turn your television to an "in-between" channel, and part of the static you'll see is the afterglow of the big bang."

The CMB revealed conclusively that the Universe had undergone a period of rapid expansion in its early history confirming the Big Bang model of cosmology beyond doubt. But the expanding Universe was yet to deliver its greatest blow to our understanding of the cosmos.

What is dark energy? The Universe is expanding... and it's accelerating

At the beginning of the 20th Century, Edwin Hubble discovered from the observed relation between distance and recession velocity of galaxies that the universe is expanding.

Until 1929 , and the publication of Hubble's short paper, "A relation between distance and radial velocity among extra-galactic nebulae," the common consensus in science was that the universe was static and unchanging. Albert Einstein had even added a factor called the cosmological constant — represented by the Greek letter Lambda — into his equations of the universe to ensure it remained static.

But, if this revelation was a surprise to the scientific community, the discovery in 1998 that this universal expansion is accelerating came as a complete shock. To see why this is, imagine giving a swing a push and then watching it gradually slow. As it is about to stop suddenly it begins to speed up again, accelerating despite no added force.

That's what the findings from astronomers that examined distant type Ia supernova — known as "standard candles" because of how their uniform light output makes them excellent distance measures — imply is happening with the Universe. Despite slowing after the initial rapid expansion of the Big Bang, the very fabric of space is again accelerating in its expansion.

This led to the introduction of " dark energy " as a placeholder for whatever force is driving this accelerating expansion. Independently confirmed since the initial supernova observations, NASA now estimates dark energy to account for 68 percent of the Universe's matter/energy content.

And the effect of dark energy is now described by the reintroduced cosmological constant  — still represented by lambda — rescued from the science dustbin with a new purpose.

Infinite Worlds: Discovering exoplanets

For as long as humanity has known that the stars are bodies just like the sun we have wondered about the planets that could orbit these distant stellar bodies and if they could potentially harbor life just as Earth does.

Yet, despite astronomy's long history and its status as arguably the first science, the discovery of the first planet outside the solar system — an extrasolar planet or exoplanet — would take until the end of the 20th century.

Two major "firsts" in terms of exoplanet discoveries both occurred in the 1990s. In January 1992 astronomers Dale Frail and Aleksander Wolszczan announced the discovery of two rocky planets and a possible third orbiting a pulsar —  PSR B1257+12  — located almost 2,000 light-years from Earth.

Pulsars are rapidly rotating neutron stars and blast out powerful radiation, meaning that the three planets around PSR B1257+12 could not possibly support life.

The discoveries will continue

In 1995, Michel Mayor and Didier Queloz discovered 51 Pegasi b  — a so-called hot Jupiter exoplanet so close to its star that it has a scorching hot surface temperature of 1,000–1,800 degrees Fahrenheit and completes an orbit in just four days.

The duo, who shared the 2019 Nobel Prize in Physics for the discovery, located the planet using a detection method called the radial velocity technique . This measures the tiny wobble that an orbiting planet causes in its host star. The tiny movement causes a slight shift in the wavelength of light emitted by the star.

This makes light from the star redder if it is tugged away, or bluer if it is tugged towards Earth, which astronomers can use to infer the presence of a planet.

Since the discovery of the first exoplanet humanity hasn't looked back. NASA's exoplanet catalog now numbers over 4,800 confirmed worlds beyond the solar system — a testament to the power of detection methods that can pinpoint the tiniest of signals.

Of these, NASA says 927 planets have been discovered using the radial velocity method. This makes it the second most successful exoplanet detection method after the transit method — which measures tiny dips in light as a planet passes across the face of its star — which has been used to find 3854 exoplanets according to NASA.

With the event of the James Webb Space Telescope (JWST) the golden age of exoplanet science has truly begun.

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July 27, 2023

The Most Surprising Discoveries in Physics

Experts weigh in on the most shocking, paradigm-shifting and delightful findings in the history of physics

By Clara Moskowitz

sakkmesterke/Getty Images

Ever since Isaac Newton and the falling apple , surprises have often pushed physics forward. Many truths about the universe we live in and the particles that make up ourselves and the world around us, as well as the forces that drive them, seemed to come out of left field when they were first discovered. For instance, scientists once thought atoms were the smallest bits of matter in existence until they split atomic nuclei to find protons and neutrons, which in turn proved to be made of even smaller fundamental particles, called quarks. And it was less than 100 years ago that researchers found out the Milky Way wasn’t the only galaxy in the cosmos but rather one of billions.

The surprises in the history of physics are far too many to comprehensively describe, but we polled a variety of physicists for some of their favorites. A few discoveries, such as the accelerating expansion of the universe , were so groundbreaking that multiple experts picked them as top choices. And many of these events occurred relatively recently, showing that the field of physics continues to astound us. Here’s a selection of physicists’ responses on the most amazing, stunning and flabbergasting findings.

Dark Energy

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One of the most shocking findings in the history of physics was the discovery of dark energy just before the turn of the millennium. None of us working in physics saw that coming! The observations that distant supernovae are dimmer than expected led to the idea that the universe is not just expanding but accelerating. These objects are very well understood, no matter how far back in time they are observed, so alternative explanations just don’t work. The name “dark energy” was given to the material that causes this acceleration. After the initial discovery, many other observations of different types confirmed this result, such as studies of the cosmic microwave background, which is the leftover light from the big bang, and studies of clusters of galaxies. The list goes on and on. We now have a standard model of cosmology in which the ordinary matter and energy that we experience in our daily lives—our body, the air we breathe, the walls around us, and all the stars and planets—add up to only 5 percent of the content of the universe. Most of the universe is “the dark side”: the universe is thought to consist of 25 percent dark matter and 70 percent dark energy. I, for one, am working to identify the nature of these mysterious components.

This discovery of dark energy in particular created a paradigm shift. The simplest explanation [for dark energy] would be a cosmological constant originally introduced by Albert Einstein as a possible term in the equations of the general theory of relativity but then abandoned by him as his “biggest blunder.” Now it seems he may have been right after all. The trouble is that the predicted value for the cosmological constant from calculations using quantum field theory produces a number that is too large by a factor of 10 120 . Editor’s Note: If the constant was this large, the universe would have expanded much, much faster than it did .] This conundrum has been known for some time, and theorists conjectured that there must be some physics that drives the number down to zero instead [to match the observed expansion history of the universe]. Now with the discovery of dark energy, however, the number must be driven down to a particular tiny value [rather than zero to explain the accelerating expansion], which is much harder to explain. This cosmological constant problem is thought by many to be the deepest unsolved problems in all of modern physics.” — Katherine Freese, University of Texas at Austin

Expanding Universe

I think the accelerating expansion of the universe has to be a strong contender. I’ve read references published around 1990 that talk confidently about how we will soon use supernovae to measure the rate at which the expansion of the universe is  decelerating and the curvature of the cosmos and how this will tell us about the ultimate fate of our universe (because closed matter-dominated universes undergo a ‘ big crunch ,’ while open ones expand forever)—very little of which applies to the dark-energy-dominated, spatially-flat cosmos that we appear to actually live in! I think this also qualifies because even with the benefit of hindsight, it still seems very surprising that the dark energy/cosmological constant has its measured value. — Tracy Slatyer, Massachusetts Institute of Technology

Charmed Quarks and Accelerating Cosmos

The most spectacular discoveries in fundamental physics since I started graduate school in 1973 have been the following: (1) The discovery in October 1974 of the J/psi particle, interpreted in terms of a new quark, the charmed quark , which gave dramatic confirmation to the then emerging Standard Model of particle physics. (2) The discovery in the late 1990s that the expansion of the universe is accelerating, apparently because of a tiny but nonzero energy density of the vacuum, upending many of our ideas about the cosmos. —Edward Witten, Institute for Advanced Study, Princeton, N.J.

Black Holes

One of the most surprising discoveries in the history of physics is Karl Schwarzschild’s black hole solution of the Einstein equation. [ Editor’s Note: Schwarzschild calculated the first exact solution to Einstein’s field equation of general relativity, and the solution predicted the existence of black holes .]

It is apocryphally said that when Einstein discovered his highly nonlinear equation, he thought an exact solution would never be found, but Schwarzschild proved him wrong only months later. Yet the structure of the solution was so surprising that many thought black holes did not exist. Einstein himself wrote in 1939 that [“the ‘Schwarzschild singularities’ do not exist in physical reality”]. It is only a century later, with the recent direct LIGO [Laser Interferometer Gravitational Wave Observatory] and EHT [Event Horizon Telescope] observations of black holes that the last shreds of disbelief have been stamped out.” — Andrew Strominger, Harvard University

It’s got to be the flexibility of spacetime. Let’s say I hop on a really fast rocket or go very close to a black hole and then return to where I started. If I go fast enough on the rocket or go close enough to the black hole, I can have only 10 minutes go by on my watch while 10,000 years go by for Earthlings. This is an experimentally verified time machine that lets you travel to the future ! — Edgar Shaghoulian, University of California, Santa Cruz

I think my favorite event in physics was the prediction of the existence of the neutrino [a subatomic particle with no charge and very little mass] because so much of our fundamental approach to physics today grew out of that moment. The neutrino prediction by Wolfgang Pauli was one of the first examples of taking energy and momentum conservation seriously—you must either explain nuclear beta decay [a common radioactive process] by violating this conservation law or by introducing a new particle. The neutrino would be the first new particle predicted that wasn’t obvious in everyday life. Today predictions for new ghostlike particles are almost a dime a dozen, but in the early part of the last century, introducing potentially unobservable particles simply wasn’t done. When Enrico Fermi introduced the interaction explaining why the neutrino was so unlikely to be observed, he predicted the first new force [the weak nuclear force] beyond the two that are obvious in everyday life (gravity and electromagnetism). Today physicists consider many new types of forces all the time, but back then that just wasn’t in the picture. The idea of unifying forces, which is so essential to physics today, grew out of the discovery of Fermi’s ‘weak force’ that the neutrino feels. One of the most amazing examples that shows quantum mechanics makes sense as a theory, because it can happen on kilometer scales, where we can really see it, comes from neutrino physics. So that moment, when Pauli predicted the neutrino, is my favorite surprise because of all the paths it led to in physics. — Janet Conrad, Massachusetts Institute of Technology

Oscillations

I would say the discovery of neutrino oscillations is up there for me. Neutrinos themselves were predicted to exist by Pauli and subsequently discovered in a great demonstration of the power of theory. But what makes neutrinos incredibly interesting little particles is the fact that they have mass and can change flavors, which requires a modification of the Standard Model of particle physics. — Sanjana Curtis, University of Chicago

Long ago two ancient Greek savants, Democritus and Leucippus, argued that matter consists of atoms, a notion that would be confirmed more than two millennia later. I recently coined the word ‘ leucippity ’ to characterize those speculative hypotheses that wait many years for widespread acceptance. My new word honors the elder of the two proponents of the atomic hypothesis, Leucippus.

Isaac Newton concluded that light consists of particles in 1672; Christiaan Huygens developed his wave theory of light six years later. Who got it right? The question lingered for two centuries until James Clerk Maxwell’s profound and leucippitous discovery that light favors Huygens’s wave theory. (Later on Einstein would have his say on this matter.) Leucippity abounds in science. Alfred Wegener’s prescient ‘geopoetry’ of drifting continents emerged as the mature science of plate tectonics half a century afterward. More recently, the discovery of a boson [the Higgs boson] first imagined by Peter Higgs and a few others in 1964 was triumphantly announced at CERN [the European laboratory for particle physics near Geneva] on July 4, 2012. Lastly, the gravitational waves produced by mergers of black-hole pairs were detected by LIGO in 2015, a full century after their existence had been proposed by Einstein. Leucippity again! —Sheldon Lee Glashow, Harvard University

Phase Transitions

In my opinion, one of the most incredible and surprising experimental findings in physics resulted from when the pioneer of helium liquefaction, Heike Onnes, performed experiments in which he cooled metals such as gold, platinum and mercury to liquid helium temperatures. On the same day that he found that the electrical resistance of mercury dropped to effectively zero at liquid helium temperatures, he also found that [using a vacuum pump] on a normal liquid helium sample caused the liquid to further cool and aggressively boil before suddenly becoming placid. This is incredible! On the same day Onnes discovered both the phase transition to a state of superconductivity in mercury and the phase transition to the state of superfluidity in helium. — Charles Brown, Yale University

Bell and Michelson-Morley

Two discoveries— Bell’s theorem and the Michelson-Morley interferometry experiment —upended our understandings of space, time and the nature of reality, so I can’t resist voting for them both.

The American Physical Society calls the Michelson-Morley experiment “ what might be regarded as the most famous failed experiment to date .” Until the experiment was performed in 1887, scientists believed that light waves propagate through a medium that scientists called the luminiferous aether. After all, sound waves propagate through air, and surfers’ waves propagate through water. But Albert Michelson and Edward Morley provided strong evidence that light is different; it needs no medium. This lack paved the path for Einstein’s special theory of relativity (nothing can travel more quickly than light, E = mc 2 [the c stands for the speed of light in a vacuum], how short an object looks depends on how quickly you’re moving relative to it, etcetera), which led to his general theory of relativity (spacetime has a shape).

Bell’s theorem [named after John Stewart Bell] revealed that quantum systems have wonky relationships with information and with each other. Ordinarily, if you know everything about a pair of systems—say, everything about a pair of people named Audrey and Baxter—then you know everything about each individual—everything about Audrey and everything about Baxter. But if Audrey and Baxter are labels of quantum particles, then you can know everything about the pair without knowing anything about the individuals. Information can be not in one particle and not in the other but sort of in the relationship between the two: the whole is greater than the sum of its parts in quantum physics. Bell’s insight paved the path for the quantum computers and networks now under construction across the world. — Nicole Yunger Halpern, University of Maryland, author of  Quantum Steampunk

Here are a few surprising discoveries that pop into my mind, in no particular order:

(1) Special relativity: the fact that the speed of light is constant, irrespective of the frame of reference.

(2) General relativity : the fact that gravity represents a curvature of spacetime.

(3) The expansion of the universe, the ensuing big bang model and the fact that the expansion is accelerating.

(4) The ‘unreasonable’ effectiveness of mathematics in formulating the fundamental laws of nature.

(5) The probabilistic nature of quantum mechanics . —Mario Livio, astrophysicist