Since 1983, the meter has been defined based on the speed of light

Explore the fundamental constants of the metric system

In 2019, the International System of Units was revised, such that all seven fundamental units are defined in terms of combinations of seven fundamental constants of the universe. These include the speed of light in a vacuum, Planck's constant, Avogadro's number, and the elementary charge of protons and electrons.

The speed of electricity may refer to the movement of electrons or energy

In the 18th century, various experiments, including one in which monks holding an iron wire were shocked, suggested that electricity traveled at infinite speed. Advancements in technology, recognizing that electric currents were moving electrons, and the development of the theory of electromagnetism showed that electric signals can travel at near light speed, despite electrons moving much slower.

How 'vibes' factor into scientists' decisions about using certain units

Although there are absolute quantities from which units can be defined, such as the speed of light and the corresponding light year for distance, the usefulness of units comes from their ability to easily communicate values, rather than how fundamental their origins are. For example, while Kelvin measures absolute temperature, Celsius is scaled to the temperatures of boiling and freezing water, making them more practical for everyday usage.

The finite speed of light means we can only perceive the world's past

When objects emit or reflect light, it takes time for the light to reach an observer and be detected. During the travel time, the object may have changed, limiting detections to describing the conditions of the object when the light was first emitted or reflected, before it was detected.

The magnetic constant sets the strength of magnetic fields

The permeability of free space, or μ₀, contributes to the speed of light in the universe—a fundamental constant. It also helps quantify how the movement of electric charges generates a magnetic field and vice versa.

The Planck time is the smallest unit of time that could ever be theoretically measured

Equal to about 5.39 × 10⁻⁴³ seconds, the unit is derived from calculating how long it would take light to travel the Planck length—the smallest theoretically measurable length. Quantum physics allows events to occur on smaller timescales, making them undetectable and their order to lose meaning.

How the extended life of rapidly decaying particles supports the special theory of relativity

Muons produced in particle accelerators are known to decay into other particles after about 2.2 microseconds on average, but those produced in the atmosphere live much longer—enough to travel thousands of meters to detectors on Earth's surface. As predicted by the mathematics of special relativity, by traveling near the speed of light, muons experience time passing dozens of times more slowly, allowing them to live long enough to reach Earth's surface before decaying.

Astronomers use a set of scaffolding methods to measure distances across the universe

Different methods are used depending on how far away an object is—from radar for nearby planets, to parallax for stars, to redshift and supernovae for distant galaxies. These tools allow scientists to chart the universe across unimaginable scales using astronomical units (AU), light-years, and parsecs.

The modern marathon’s arbitrary distance is thanks to the British royal family

In 1908, the London Games set the marathon distance to 26.2 miles to meet the British royal family’s request that the race start at Windsor Castle and end at their royal box. In 1921, that arbitrary distance became the official length for the marathon.

The general theory of relativity defines gravity as the curvature of spacetime

The breakthrough that redefined Isaac Newton's picture of gravity came when Albert Einstein realized that freely falling is indistinguishable from weightlessness, leading him to the principle of equivalence between gravity and acceleration. His prediction that gravity bends light was confirmed in 1919 during a solar eclipse.

Railroads and long-distance communication led to the development of time zones

Noon had been determined independently at each location based on when the sun was highest in the sky, but this became impractical for standardizing transportation. British star-mapping and navigation advancements established Greenwich as the prime meridian through Greenwich Mean Time, or GMT.

What is a calorie?

Today it may go without thinking that we measure food by calories...but where did this come from and what does it mean? A kilo-calorie (or, as we call it, simply "calorie") is the amount of heat energy needed to raise the temperature of one...

The Curies developed a way to measure radioactivity in an old medical dissecting room

Despite lacking proper laboratory conditions, Marie and Pierre built an ionization chamber and a piezoelectric quartz electrometer to measure the amount of charged particles released by radiation, with a sensitivity of ten-trillionth of an ampere. Substances with different emission rates indicated different elements.

Earth's circumference was estimated over 2,000 years ago using geometry and shadows

By measuring the angle of the shadow cast in Syene when no shadow was cast in Alexandria, Eratosthenes used the distance between the two cities and trigonometry to estimate the Earth's circumference to within a few percentage points of the actual value.

The mass of a cell can be measured using hollow, vibrating beams

In 2010, scientists at MIT constructed a U-shaped device that vibrates like a guitar string with an internal fluid channel. As cells and bacteria pass through the device, its rate of vibration shifts, with greater mass creating larger shifts.

The standard measurement for precious metals comes from a small town in France

The troy ounce is the standard measurement for precious metals like gold and silver and it’s heavier than a standard ounce. The term “troy” is widely believed to come from the town of Troyes in France, which was a popular trading hub for merchants all across Europe in the Middle Ages.

The invention of the barometer proved that air has weight

The instrument measures air pressure by creating a partial vacuum—a region with very low density—which serves as the basis for how suction works. Mercury-based barometers were used until 2007, when they were restricted due to mercury's toxicity.

The construction of LIGO needed to compensate for Earth's curvature

Across the 4-kilometer (2.5-mile) arms, the height of Earth's surface falls by almost a meter, preventing the lasers traveling within the arms from meeting the mirror at the end of each. Concrete slabs built beneath the arms help keep them level and provide structural support against seismic vibrations.

Radio waves are used in broadcasting, weather radar, telecommunications, and GPS

Its ability to bend around obstacles and reflect off the ionosphere—an electrified layer of Earth's upper atmosphere—allows this type of light to travel long distances without damaging living things due to its low energy. MRI scanners, WiFi, and Bluetooth technologies are also applications of this form of light.

Credit for inventing the laser was fought over for nearly three decades

Although Theodore Maiman built the first working laser in 1960, Gordon Gould coined the term earlier. Charles Townes and Arthur Schawlow of Bell Labs filed a patent after Gould came up with the idea and name, but before Gould filed his patent.

As of 2025, the longest lightning bolt in the world spanned several states

Although the average bolt measures less than 16 kilometers (10 miles), the 2017 "megaflash" spanned 829 kilometers (515 miles) from eastern Texas to Missouri. Severe thunderstorms are common in the region, where warm, humid air from the Gulf collides with cool, dry air from the north, generating atmospheric instability.

The Metropolitan Museum of Art, explained

The Metropolitan Museum of Art, founded in 1870, houses 1.5 million works spanning centuries—from ancient Egyptian artifacts to impressionist masterpieces. Its iconic Fifth Avenue building, designed by architect Richard Morris Hunt, and tranquil Cloisters location reflect its dedication to showcasing global creativity.

Why millions of telegrams are still sent every year

Telegrams enabled much faster communication across distances when they were invented in the 1800s, but they became generally obsolete after emails and cell phones were widely adopted. So why are 17 million still sent every year? A combination of factors: the legal requirement to send paper documents far and fast, the resistance to updating processes, and nostalgia. Explore more with this video.

A 101 on the science and history of meteor showers

Bits of rock, ice, and dust flying through space called meteors burn up in a spectacular display when they hit our atmosphere 60 miles up at an average of 30,000 mph. Some of these are isolated objects, but others are recurring phenomena caused by...

Taxicab geometry defines displacement as the sum of vertical and horizontal distances

In this mathematical system, pi is valued at four, and circles aren't round—they appear as diamonds with corners instead. The system simplifies pixel distances in digital imaging and helps engineers with robot navigation and chip design.

Stars appear to twinkle because turbulence in the motion of molecules in Earth's atmosphere causes light to bend and wiggle rapidly on its way to the surface—a phenomenon called scintillation.

Track the location of the Voyager probes, currently between 13 and 15 billion miles away from Earth.

Long before Major League Baseball began allowing instant replay for balls and strikes, an umpire reluctantly used the technology to determine whether a hit was a home run, a move that inadvertently ushered in a new era for the sport.

Objects can be 3D printed with light by projecting 2D computerized tomography (CT) scans onto rotating cylinders of resin, which solidify upon exposure.

The first horror movie is believed to be George Méliès' 1986 short 'The Haunted Castle,' which pioneered the use of special effects

The term 'algorithm' originated from the Latinized name of al-Khwārizmī, a 9th-century mathematician whose works introduced Arabic numerals and the decimal-based numeral system to Europe and whose treatise 'Al-Jabr' gave algebra its name.

The concept of phantom limbs, the perception that amputated limbs are still present, dates back to a French military surgeon in the 16th century.

See this overview of the firework rules across all 50 states, ranging from a complete ban on non-professional fireworks in Massachusetts to states with more lax rules like Indiana.

In 1984, Operation Smash Hit intentionally crashed a 239-ton train into a nuclear waste canister at 100 miles per hour to prove the canister's safety, which remained completely intact and lost just 0.29 of its 100 pounds of pressure.

A century of Billboard issues documenting the evolution of popular music.

Some coin denominations have ridges etched on their sides, partly to prevent counterfeiting, in a practice called 'reeding.'

The US Constitution is widely regarded as the longest-standing written national constitution still in force—see how it has influenced rights worldwide.

For centuries, Niagara Falls has been a magnet for daredevils seeking to conquer one of nature's most powerful wonders, with its 200-foot drop and water rushing at 32 feet per second with 280 tons of force.

Pure water does not conduct electricity because this flow of energy requires the movement of charged particles, and water is a neutral molecule.

While atoms are normally made of protons, neutrons, and electrons, exotic ones have been built with antimatter—alternate versions of normal matter that live for fractions of a second and are about a billion times as rare.

Mark Twain spent decades feuding with the US Postal Service, convinced its costs and regulations were bureaucratic overreach.

Clever Hans was a 1900s horse that was believed to be able to solve math equations, tell time, and read and comprehend German, until a psychology student discovered how he got his answers.

Budweiser stores yeast derived from the original 1800s strain in a secret bunker, which only the Head of Yeast Management can access—not even the CEO

'Saturday Night Fever,' the 1977 film that introduced disco to the mainstream, was based on a fabricated magazine story.

As of 2025, the most cited papers in science include a review of graphene, methods to improve image recognition, streamlining electron density calculations, and optimizing protein studies.

Sony set the standard for cylinder-shaped batteries when it repurposed manufacturing equipment used to make rolls of magnetic recording tape.

The word 'jazz' evolved from 'jasm,' 19th-century American slang for 'energy'

The most expensive ad of all time was for Chanel No. 5 perfume and cost roughly $33M to produce in 2004—about $57M in today's dollars.

The first steps to rebuilding your credit score include getting to know exactly what's on your credit report and figuring out how far you are from the '700 club,' or 'good score' range.

Bone marrow (soft tissue found inside certain bones) generates roughly 500 billion new blood cells each day