Because they move at the cosmic speed limit through space, photons—particles of light—do not 'perceive' distance or the passage of time.

According to the special theory of relativity, the faster objects move through space, the less they move through time, and therefore age more slowly.

For the speed of light to be constant for all observers, space and time cannot be absolute, universal dimensions that are identical for everyone.

Individuals moving at different speeds may disagree on the order of observed events because each experiences the passage of time differently, according to special relativity.

Discoveries in electromagnetism, which suggested that light traveled at a single speed, became the catalyst for overturning Isaac Newton's view that all speeds are relative.

The underlying fabric of the universe—spacetime—changes shape in the presence of mass and energy, which affects measurements of space and time.

Although moving through space faster than the speed of light is forbidden by special relativity, dark energy causes about 97% of galaxies to seem to do so.

Visualize how the order of events changes, according to special relativity, when the perspective is changed between observers who move relative to one another.

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

Tachyons are theoretical particles that travel through space faster than light

By discarding absolute space and time, special relativity introduces paradoxes—scenarios in which different, equally valid observers obtain conflicting measurements—such as the twin and ladder paradoxes.

Because some models of the universe do not incorporate time, it may be an emergent property born from our limited perception of the universe, rather than a fundamental property of reality.

Photons are both particles of light and messengers of the electromagnetic force

Light takes approximately eight minutes to travel from the sun's surface to Earth, but it takes thousands of years to reach the star's surface from its core—a distance more than 200 times shorter.

Although the color of light changes due to motion between a source and an observer—the Doppler effect—relativistic motion further alters it due to time dilation.

A spaceship traveling near the speed of light would experience relativistic aberration—its field of view will seemingly narrow into the direction of travel.

Units involving light, such as light-seconds and light-years, describe the distances traveled by light across that amount of time.

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

While special relativity describes motion along flat spacetime—the underlying fabric of the universe—general relativity extends this theory to explain motion through curved spacetime.

It is impossible for a telescope to ever see the Big Bang and the beginning of time because light in the early universe was trapped in a dense 'cosmic fog' of elementary particles.

The Planck time, equal to 5.39 × 10⁻⁴³ seconds or less than a millionth of a trillionth of a trillionth of a trillionth of a second, is the smallest unit of time that could ever be theoretically measured.

Time seems to stop for objects at the event horizon of black holes

The hypothetical particle that travels faster than light

Time disappears when encompassing all quantum configurations across the universe

Observations of colliding neutron stars by the Laser Interferometer Gravitational-Wave Observatory in 2017 confirmed that gravity travels at the speed of light.

As a consequence of special relativity, time travel cannot change history because all events in the past, present, and future are locked in place.

According to the equivalence principle, objects in a closed space cannot tell the difference between whether they are accelerating or experiencing a uniform gravitational field.

Light appears to slow down as it travels through different materials

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

Although associated with his theories of relativity, Albert Einstein initially viewed the concept of spacetime, which originated with his teacher, Hermann Minkowski, negatively.

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

Quantum mechanics and general relativity—two of the most successful explanations of the physics seen across the universe—are logically incompatible with one another because they use contradictory models for space and time.

Particles produced in cosmic-ray showers that would normally decay long before reaching Earth's surface are observed by ground-based detectors, as predicted by special relativity.

Almost three centuries before Albert Einstein presented the special theory of relativity, Galileo Galilei devised a principle of relativity involving uniform motion.

In the 'block universe' model—a consequence of the special theory of relativity—the past, present, and future exist simultaneously, suggesting free will is an illusion.

Watch some effects of special relativity become increasingly more apparent in 'A Slower Speed of Light,' a video game prototype from the MIT Game Lab.

Within quantum physics, the order of events across time may not follow causality

General relativity describes a singularity as the final moment in time

Albert Einstein's theory of gravity arose from what he described as 'the happiest thought of his life': realizing that a person who fell off the roof of their house would not feel their own weight as they fell.

One model of the universe expresses all constants using only time

What Stephen Hawking said about time travel

Is time travel really possible?

Massive objects can significantly bend light to produce gravitational lensing

Within Einstein's picture of the universe, gravity is technically not a force because forces cause accelerations that deviate objects from their natural path, and moving solely due to gravity is natural.

Scientists showed that the relativistic effect of time dilation was real after comparing atomic clocks flown around the world with stationary ones on Earth's surface.

Despite the visibility of lasers in space battles within science fiction, they are impossible to see in outer space unless they are pointed directly at an observer.

The cosmic microwave background radiation is the oldest light in the universe

Experimenting with space-time

Within quantum mechanics, the Heisenberg uncertainty principle limits how precisely one can simultaneously know the values of pairs of complementary properties for an object, such as position and momentum.

As the universe expands, the color of light shifts as it travels through space

Explaining pulsars, the cosmic clocks and lighthouses of the universe