Light appears to slow as it travels through transparent materials due to interactions with the charged particles in the atoms of the material.

Light appears to slow down as it travels through different materials

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

Particles moving faster than the speed of light give nuclear reactors an eerie blue glow.

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.

Electrical energy transfers at near light speed, even though the electrons carrying the energy in circuits only move a fraction of a millimeter per second.

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

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

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

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

Massive objects can significantly bend light to produce gravitational lensing

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

Gold has a yellowish appearance rather than the silvery color of most metals because of special relativity, which causes gold's electrons to absorb blue light.

The reflection of light within a material is the basis for fiber optics

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

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

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

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.

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 takes approximately eight minutes to travel from the sun's surface to Earth, but it takes thousands of years to reach the star's surface from its core—a distance more than 200 times shorter.

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

Light waves can interfere and cancel each other out

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.

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.

Light echoes are expanding shells of reflected starlight

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.

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

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

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

Objects can be made invisible through refraction, in which light from behind an object is bent around it, making only the background visible.

The double-slit experiment is observational evidence for the wave-particle duality

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.

Lasers can be used to reduce the motion of atoms because particles of light—photons—can collide with particles of matter, as billiard balls do, to slow them down.

With a core consisting of a glass fiber roughly the width of human hair, fiber-optic cables transmit digital information via flashes of light that reflect within the glass.

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.

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

You can measure the speed of light with a microwave oven and chocolate, cheese, or other uniformly thick foods.

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

Walk around Relativity Park—a virtual theme park where you can experience relativistic effects by slowing down the speed of light.

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

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

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.

Some materials can absorb low-energy light and emit high-energy radiation

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

View an animation of how certain chemicals fluoresce—that is, emit visible light after having absorbed higher-energy light, such as ultraviolet radiation.

Variations in the energy of light force the use of multiple telescopes

Stars appear to twinkle because turbulence in the motion of molecules in Earth's atmosphere causes light to bend and wiggle rapidly on its way to the surface—a phenomenon called scintillation.

Astronomical models suggest dark matter must be 'cold,' or slow-moving

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