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.

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

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

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

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.

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.

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

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

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.

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

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

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.

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.

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

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.

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.

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.

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.

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

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.

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.

Einstein created a mathematical term for anti-gravity to keep the universe static

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.

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.

During his 'miraculous year,' Albert Einstein published four groundbreaking papers, including the postulates of special relativity.

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

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

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

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.

Uncertainties in measurements make finding gravity's Newtonian limit unclear

Experimenting with space-time

One of Einstein's papers challenged quantum theory but helped confirm entanglement

Known for his theories of relativity, Albert Einstein's Nobel Prize in physics was for his discovery of the photoelectric effect, which makes digital cameras possible.

To develop general relativity, Einstein relied on his college friend, Marcel Grossman

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.

Light has been modeled as a wave and a stream of particles throughout history

Light appears to slow down as it travels through different materials

General relativity describes a singularity as the final moment in time

How neutron stars serve as laboratories for testing Einstein's theory of gravity

Classical mechanics describes the motion of and forces on everyday objects

Tachyons are theoretical particles that travel through space faster than light

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.

Mathematician David Hilbert developed general relativity equations before Einstein

Unlike other metals, mercury is a liquid at room temperature because of special relativity, which affects the spatial distribution of electrons in the atoms.

Science is rarely a linear progression of understanding

Nearly 50 years before the first observational evidence, a soldier fighting on the Eastern Front in World War I found a mathematical solution to Einstein's equations that suggested the existence of black holes.

Explore the fundamental constants of the metric system

Explore Albert Einstein's Zürich notebook and review his notes on relativity

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.