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

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.

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.

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

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

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.

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.

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

By discarding absolute space and time, special relativity introduces apparent paradoxes—scenarios in which different, equally valid observers obtain conflicting measurements.

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

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.

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

Mathematician David Hilbert developed general relativity equations before Einstein

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.

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

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

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

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.

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

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

Experimenting with space-time

Based on Newton's laws of motion, classical mechanics describes the motion of and forces on everyday objects.

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

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

Uncertainties in measurements make finding gravity's Newtonian limit unclear

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

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.

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

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.

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.

General relativity describes a singularity as the final moment in time

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.

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.

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

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.

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.

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

Quantum electrodynamics is considered the first true quantum field theory, which applied quantum mechanics to the electromagnetic radiation field.

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

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.

Although often described as incompatible, one can derive classical physics equations from quantum mechanics.

One model of the universe expresses all constants using only time

Newton's laws describe how objects resist acceleration and respond to forces

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

Gravity Probe B verified that rotating objects drag spacetime

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

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.