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.

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

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

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

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.

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

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.

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

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.

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.

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.

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.

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.

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

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

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

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

By changing one's frame of reference in special relativity, electric forces can be reinterpreted as magnetic forces, highlighting how the two together constitute electromagnetism.

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

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.

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

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

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.

Relative motion between a source and an observer shifts the color of observed light

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.

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.

General relativity describes a singularity as the final moment in time

Is time travel really possible?

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

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

If a source of music traveled past someone at twice the speed of sound, assuming sufficiently sensitive hearing and powerful equipment, they would hear the music backward.

The speed of electricity may refer to the movement of electrons, which is on the order of a fraction of a millimeter per second, or energy, which is near the speed of light.

Tachyons are theoretical particles that travel through space faster than light

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

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.

The arrow of time—the concept that time only moves in the forward direction—is thought to result from the universe progressing toward greater disorder, or entropy.

Experimenting with space-time

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

Time disappears when encompassing all quantum configurations across the universe

Play a puzzle game where beating levels requires taking advantage of the effects of special relativity, including time dilation and length contraction.

Earth's core is about 2.5 years younger than its surface according to relativity

One model of the universe expresses all constants using only time

Philosophers have debated the nature of time since at least the 6th century B.C.E.

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.

Scientists validated Albert Einstein's theory of gravity by measuring the apparent positions of stars via their starlight as it traveled near the sun on its way to Earth during a total solar eclipse.

Branching timelines are allowed and resolve grandfather paradoxes in physics

Five potential ways to achieve time travel

In 1676, Danish astronomer Ole Rømer determined that light did not travel instantaneously by observing a delay in the appearance of solar eclipses on Jupiter when it was farther from Earth.

What Stephen Hawking said about time travel

Time crystals are systems of atoms organized into periodic patterns of motion