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

Photons do not experience distance or the passage of time

According to special relativity, the speed of light is the cosmic speed limit, and the faster an object moves through space, the less it moves through time. As a result, particles of light only move through space and have no velocity through time, perceiving all locations they travel through simultaneously.

Light appears to slow down as it travels through different materials

As electromagnetic waves interact with particles in a transparent medium, particles vibrate and generate their own electromagnetic waves. The interference of the original and generated waves, which are slightly delayed, results in ensemble light with an effective slower speed but unchanged color.

Space-time changes its shape and our perceptions of reality to maintain light's speed

To preserve the speed of light for all observers, the universe sacrifices the uniformity of space and time through the phenomena of time dilation and length contraction. This means that individuals may disagree on measurements of time and space based on their motion or location in a gravitational field.

The scattering of light in the early universe prevents seeing the beginning of time

A "cosmic fog" of elementary particles in the early universe prevented light from traveling far before being absorbed. Only 380,000 years after the big bang, when the first atoms formed, was light free to travel and reach telescopes for study.

Watch the motion of an individual photon using a high-speed camera

Although light travels over 3 million football fields per second, cameras that capture trillions of frames per second can observe individual particles of light as they interact with atoms in materials they travel through. Such observations can be used to determine the speed of light in different materials.

Even in an infinite universe, its limited age prevents a sky full of starlight

Olbers' paradox asks why the sky would ever appear dark if the universe is full of stars in all directions emitting light everywhere. Light from the farthest stars requires more time to reach our skies than the universe's age.

Why it takes tens of thousands of years for light from the sun's core to reach its surface

Photons take about eight minutes to travel through empty space to Earth after leaving the sun's surface. However, traveling from the core to the surface involves these particles of light undergoing a random walk—scattering off charged particles deep in the sun like a pinball—causing a trip that is more than 200 times shorter to last as long as 170,000 years.

The wave nature of matter can produce phenomena like negative time and retrocausality

Within quantum mechanics, matter simultaneously exists as both fixed particles and extended waves, allowing it to emerge across barriers—just as sound travels through walls—before it fully enters the obstacle. The resulting measurements can show that particles spent negative time within a barrier, with the future occurring before the past.

Astronomers use a star's color to determine how fast it's moving

According to the Doppler Effect, light from objects moving away from Earth shifts toward the red end of the electromagnetic spectrum, with the severity of the shift correlating to recession speed. Due to the universe's expansion, light from the most distant galaxies can be moved from ultraviolet to infrared.

Observations of Jupiter's moons first demonstrated that light was not instantaneous

In 1676, Danish astronomer Ole Rømer noticed that eclipses of the moons occurred several minutes later when Earth and Jupiter were farther apart, reasoning that light needed more time to travel the greater distance. Lacking knowledge of Earth's orbital diameter, he was unable to calculate the speed of light.

Delays in the decay of unstable particles provide evidence for time dilation

Physicists used the muon half-life measured in labs to predict how many should reach detectors on Earth's surface after their production in collisions between atmospheric molecules and cosmic rays. Higher detection rates were consistent with a longer half-life due to slower movement through time.

The reflection of light within a material is the basis for fiber optics

As light travels from one material to another, it can be absorbed, reflected, or transmitted through, depending on the properties of the two materials. Fiber optic cables are designed to reflect light internally, minimizing data loss as information is sent at high speeds through flashes that represent zeros and ones.

Discoveries in electromagnetism became the catalyst for overturning Isaac Newton

To maintain the consistency of experimental results involving electricity, magnetism, and light, time and space had to cease being absolute and could change in value to keep the speed of light constant. This cosmic speed limit forced Einstein to revise the existing model of gravity.

Optical clocks track time by measuring electron transitions involving visible light

The frequency of visible light is significantly greater than that of microwaves used in cesium atomic clocks. This makes optical clocks about 100 times more accurate, losing at most one second across the entire age of the universe.

Light echoes are expanding shells of reflected starlight

Just as sound waves reveal the location of obstacles by reflecting off them, pulses of light emitted by stars illuminate rings of interstellar dust, giving the appearance of ejected material as the light travels outward.

Variations in the energy of light force the use of multiple telescopes

Just as Earth's atmosphere allows the sun's visible light to pass through while blocking other harmful radiation, telescope mirrors and other optics can only reflect, absorb, or transmit certain types of light. Detectors also utilize semiconducting materials that react to specific energies of light, thereby preventing a one-telescope-fits-all solution.

Supercollider particles lag behind light by just 10 feet after 14 minutes of travel

The circular shape of the Large Hadron Collider allows particles to be repeatedly accelerated and redirected via electromagnetic fields rather than building equivalent energy in a linear chamber hundreds of miles long. The latter would require 5 trillion D batteries to achieve equivalent near-light speeds.

Massive objects can significantly bend light to produce gravitational lensing

The theory of general relativity posits that mass and energy bend space and slow down the passage of time. One consequence is that massive objects, such as black holes, curve spacetime to such an extent that light from background sources can loop around them, producing multiple images or ring-like patterns.

Dark matter's limited force interactions make its detection difficult

Despite making up the vast majority of matter in the universe, dark matter can only be directed indirectly via its influence on other objects through gravity, the weakest of the fundamental forces. This means direct detection of dark matter may only be possible with future, more sensitive instruments.

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

The 1875 Metre Convention established the unit as the length between two lines inscribed on the surface of bars made by the International Bureau of Weights and Measures. Advancements in the measurement of light redefined it as the distance light travels in 1/299,792,458 of a second.

Proteins with limited light sensitivity prevent humans from seeing all types of light

Specialized photoreceptor cells in the retina called cones each possess photopigments that activate when exposed to three ranges of visible light. Other forms of electromagnetic radiation outside the activation range do not generate signals for processing in the brain, thereby limiting average human vision to approximately 10 million colors.

Earth's smooth, continuous motion as it orbits the sun prevents detectable motion

Although humans are sensitive to acceleration, Earth's nearly circular orbit around the sun and its slow spin relative to its size produce negligibly small, minimally changing accelerations. However, the changing seasons and the day-night cycle are evidence of Earth's movement through space, as verified by satellites and telescopes.

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.

Humans experience the world subjectively. What does that mean for scientific research?

When you close your eyes, do you see static or colorful patterns or an "intrinsic grey" known as Eigengrau in German? This variation of our internal experiences is just the tip of the iceberg, one that scientists are increasingly studying and...

Learn why modern scientific patterns have been described as theories rather than laws

After confirming that the accuracy of some laws of physics is limited to certain domains, science became more hesitant to categorize identified mathematical relationships, such as quantum mechanics, as unchanging laws. Instead, these recent, complex frameworks are designated as theories to acknowledge that they are conditional and may be part of a larger, more complete future framework.

Differences in seismic wave speeds let scientists determine an earthquake’s location

Because primary waves are faster than secondary waves, seismic stations will detect longer gaps between these waves’ arrivals the farther an earthquake is. Calculating the distance to the earthquake from three stations allows its origin to be triangulated.

After NASA's Voyager 1 captured Earth as a single pixel as it left the solar system in 1990, astronomer Carl Sagan used the image to reflect on humanity's fragility and the responsibility to protect 'the only world we've ever known.'

According to the physics of black holes, our 4D reality may be a hologram—a projection of information coded on a 2D surface on the edge of the universe.

Scientists can determine Earth's past climate conditions by analyzing air trapped in microscopic bubbles within ice cores—columns of compressed snow layers that build up over hundreds of thousands of years.

All rainbows are 'raincircles' caused by water droplets separating white sunlight into a spectrum of colors, but Earth's surface blocks the bottom half of the circle from our line of sight.

Most of the observable universe's volume consists of voids and supervoids—enormous regions of space with little to no galaxies, stars, or light.

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

The Mona Lisa's celebrity status and repeated restorations have obscured what made the painting so revolutionary for its time

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

Visualize the path of every solar eclipse from 1900 to 2099 and see what each looks like when observed from different locations on Earth.

Chaos theory—informally known as the butterfly effect—is the reason why weather forecasts are presented with probabilities rather than certainties.

How fusion—which releases energy when two lighter atoms combine to form a heavier atom—differs from fission

Radioactive elements can introduce software errors because the ionizing radiation they produce can detach electrons from atoms, and computer code relies on the distribution of electrons.

Learn more about the history of fireworks, which dates back to an accidental invention during an alchemist's quest for information about immortality.

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.

When caught in the mouths of cardinal fish, seed shrimp release glowing, bioluminescent chemicals, which alert predators of the cardinal fish, prompting the shrimp's ejection.

Explore fun facts from the USPS, including unique post office buildings, a stamp that traveled past Pluto, and more.

Apeirophobia is an overwhelming fear of living forever, perhaps stemming from the brain's inability to conceptualize an infinite life.

The deadliest aspect of volcanic eruptions is often not lava, but pyroclastic flows—avalanches of ash, gas, and rock that can move more than half the speed of sound.

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.

The first line of 'immortal' human cells, which can divide indefinitely and are used in scientific research, was nonconsensually taken from a Black woman.

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.

The 2026 Ebola outbreak is hampered by a lack of vaccine against the species of the virus, heightening the need for rapid tests to aid in disease control.

Corals are themselves not coral reefs—the term refers to the tiny marine invertebrates that build the rigid reef.

For centuries, it was common to sleep in two shifts: a few hours at the beginning of the night, and a longer period beginning around midnight