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.

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

By analyzing the color of starlight, astronomers can determine how fast stars within a galaxy are moving toward or away from Earth, revealing the expansion rate of the universe.

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

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

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

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

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.

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

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.

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.

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

A sound's pitch will seem to change if the listener or the source moves toward or away from the other because motion changes the time it takes each successive sound wave to reach the listener.

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

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.

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.

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.

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.

Through gravitational redshift, light loses energy as it leaves a massive object

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

Light appears to slow down as it travels through different materials

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.

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.

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

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.

A breakdown of the experimental test results supporting the general theory of relativity

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

Gold has a yellowish appearance instead of the silvery coloration of most other metals because of special relativity, which causes its electrons to absorb blue light.

Massive objects can significantly bend light to produce gravitational lensing

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

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

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.

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

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.

Play a puzzle game where the speed of light is reduced to everyday speeds

General relativity describes a singularity as the final moment in time

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

A nuclear reactor's characteristic blue light is produced from superluminal particles

Tachyons are theoretical particles that travel through space faster than light

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

Rayleigh scattering of light gives the atmosphere its blue skies and red sunsets

Experimenting with space-time

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

Gravitational waves—ripples in spacetime that result from the motion of objects—were detected a century after Albert Einstein predicted their existence upon developing his theory of gravity.

GPS satellites are programmed to account for their faster passage of time in space

Gravity Probe B verified that rotating objects drag spacetime

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.

View the electromagnetic spectrum and the properties of different types of light

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.

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.