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

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.

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.

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.

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

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.

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

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

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

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

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.

Massive objects can significantly bend light to produce gravitational lensing

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

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

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.

Experimenting with space-time

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

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.

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

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.

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

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

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.

Light echoes are expanding shells of reflected starlight

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

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.

The mathematics of general relativity does not rule out the existence of wormholes

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.

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

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

Explore a visualization of curved spacetime under general relativity

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

View a gallery of Einstein rings

An imperfection 1/50th the width of a human hair blurred Hubble's initial images

General relativity describes a singularity as the final moment in time

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.

Tachyons are theoretical particles that travel through space faster than light

Wormholes, though unstable, would offer a theoretical shortcut through spacetime

Light appears to slow down as it travels through different materials

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.

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.

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

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

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

Gravity Probe B verified that rotating objects drag 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.

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.

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.

Spikes of light around stars are artifacts of light bending around the mirror supports