Particles produced in cosmic ray showers that would normally decay long before they reach Earth's surface are observed in ground-based detectors because they experience time more slowly when moving near the speed of light—a phenomenon predicted by special relativity.

Delays in the decay of unstable particles provide evidence for time dilation

Physicists used the muon half-life measured in labs to predict how many should reach detectors on Earth's surface after their production in collisions between atmospheric molecules and cosmic rays. Higher detection rates were consistent with a longer half-life due to slower movement through time.

In 2017, a hidden chamber was found in the Great Pyramid of Giza by measuring particles created in the collisions between cosmic rays and atmospheric particles.

Scientists have determined the properties of cosmic rays by analyzing their trajectories in bubble chambers—containers filled with a vapor that clings to charged particles.

Antimatter was discovered by examining debris from collisions between atmospheric gases and cosmic rays—high-energy matter particles from space.

Photons do not experience distance or the passage of time.

If particle accelerators did create black holes by packing tremendous energy into a small volume, then trillions would be created in Earth's atmosphere daily by cosmic rays.

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

Cosmic rays are thought to be accelerated to near the speed of light by repeatedly bouncing across shockwaves produced by high-energy cosmic events, such as supernovae.

Learn how to build your own cloud chamber to detect cosmic rays—particles from space that move near the speed of light—at home.

Some cosmic rays possess energies as high as 100,000,000 teraelectron volts—more than 10 million times the energy of particles accelerated in the Large Hadron Collider.

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

Cosmic rays are evidence of incredibly powerful astronomical events—solar cosmic rays provide information about eruptions and flares on the sun's surface, while galactic cosmic rays shed light on dying stars.

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.

Antimatter can be created from dark lightning—high-energy light emitted by deflected electrons after they are accelerated to near the speed of light by thunderstorms.

Einstein’s theory of special relativity

Special relativity, formulated by Einstein, describes how space and time are linked for objects moving at constant speeds. It states that the speed of light is constant for all observers, leading to time dilation (moving clocks run slower), length contraction (moving objects shrink), and mass-energy equivalence (E = mc²). This video breaks down how motion and time are fundamentally related.

Particle accelerators detect subatomic particles using millions of sensors

Calorimeters measure the energy of particles. Tracking devices can measure momentum and charge from the trajectory of particles, which is determined by recording tiny electrical signals. Time-of-flight detectors provide particle velocities directly and particle mass indirectly, giving physicists a suite of properties to compare with those of known matter.

At the Large Hadron Collider, protons are accelerated so close to the speed of light that they would lag behind a laser by just 10 feet after 14 minutes of travel.

Cosmic rays can be detected using cloud chambers—containers where alcohol vapor condenses around trails of ions created by these high-speed particles from space.

Cosmogenic nuclide dating reveals how long a surface has been exposed to air by measuring the abundance of rare isotopes produced when cosmic rays interact with minerals.

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

When objects emit or reflect light, it takes time for the light to reach an observer and be detected. During the travel time, the object may have changed, limiting detections to describing the conditions of the object when the light was first emitted or reflected, before it was detected.

Particle interactions follow rules related to specific quantum properties

Like a business whose purchased inventory matches sold inventory, physicists identified specific patterns across decades of particle accelerator data, which became new conservation laws for elementary particles. These laws help identify which particle decays are expected or forbidden.

Scientists can map Earth's shape and structure by measuring the ratios of certain particles arriving from different directions in space, as these ratios change with the density of the material particles travel through.

According to the special theory of relativity, because the passage of time depends on how quickly objects move through space, people will not agree on the order of events they observe if they are moving at different speeds.

After photons—particles of light—neutrinos are the most abundant particles in the universe.

Tachyons are theoretical particles that travel through space faster than light

The equations of special relativity require these objects to possess imaginary mass, and they would travel faster the more energy is taken away from them. The existence of tachyons would necessitate an unknown form of matter and lead to paradoxes involving information that can hypothetically travel backward in time.

It may be possible to create dark matter in particle accelerators

In high-energy collisions, exotic forms of matter can be produced by directly converting energy into mass according to *E = mc*². Even though a dark matter particle would not be directly observable, its creation would result in unaccounted-for mass, which can be compared to potential dark matter candidates for indirect detection.

Special relativity shows how motion at high speeds reshapes time, space, and energy

Einstein’s 1905 special theory of relativity revealed that the speed of light is absolute in the universe, forcing time and space to vary with one's speed. This idea led to the famous equation E=mc2, which shows that mass and energy are interchangeable.

While atoms are normally made of protons, neutrons, and electrons, exotic ones have been built with antimatter—alternate versions of normal matter that live for fractions of a second.

A beam of neutrinos can be created by firing a beam of high-energy protons at a target to produce unstable particles that decay into neutrinos.

In 1997, the Compton Gamma Ray Observatory identified 'positron geysers'—fountains of antimatter particles—emerging from near the center of the Milky Way.

Particle accelerators propel matter toward small targets at high speed

Devices like cathode-ray tube televisions were once basic particle accelerators, but modern ones, such as the Large Hadron Collider, use massive magnetic fields and circular tunnels to achieve energies high enough to recreate the conditions just after the big bang.

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.

Earth's magnetic field, which deflects charged particles, helps protect us from cosmic rays, but the sensitivity of these particles to magnetic fields also makes it difficult for astronomers to determine their sources.

Leptons do not experience the strong nuclear force that binds quarks together

Particles like electrons can exist in isolation, enabling them to move more freely. Within circuits, this allows electrons to carry energy across wire surfaces quickly. Contrastingly, light created in the sun's core is delayed by a million years on its way to Earth due to collisions with free electrons.

At CERN's Antimatter Factory, researchers create and guide antiprotons and antielectrons together to form antihydrogen, and all these antiparticles are analyzed to help scientists study the behavior of antimatter.

Antimatter's rarity across the cosmos is one of the biggest problems in modern physics because the high-energy environment of the early universe should have produced equal amounts of matter and antimatter.

Understanding time dilation

The faster you go, the slower time ticks for you. And the stronger the gravitational field acting on you, the slower time ticks for you. These phenomena – collectively called time dilation – were first explained by Albert Einstein’s theories of general and special relativity. In this 11-minute video, beloved astrophysicist Neil deGrasse Tyson explains them to comedian Chuck Nice.

Light appears to slow down as it travels through different materials

As electromagnetic waves interact with particles in a transparent medium, particles vibrate and generate their own electromagnetic waves. The interference of the original and generated waves, which are slightly delayed, results in ensemble light with an effective slower speed but unchanged color.

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

Within quantum mechanics, matter simultaneously exists as both fixed particles and extended waves, allowing it to emerge across barriers—just as sound travels through walls—before it fully enters the obstacle. The resulting measurements can show that particles spent negative time within a barrier, with the future occurring before the past.

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

Theories for what constitutes dark matter have ranged from slow, planet-sized objects to light subatomic particles that travel at high speeds. However, fast-moving candidates would quickly spread across the universe, preventing the gradual gathering of visible matter into the galaxies and galaxy clusters seen today through gravity.

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

The Planck time, equal to 5.39 × 10⁻⁴³ seconds or less than a millionth of a trillionth of a trillionth of a trillionth of a second, is the smallest unit of time that could ever be theoretically measured.

The hypothetical particle that travels faster than light

Astrophysicists inspired by science fiction developed the theory of the tachyon—a Greek word for "fast"—which is a field or particle hypothetically able to travel faster than light. Although no experiments have confirmed any part of it, it's a fun theory to consider.

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 and could appear to change based on one's perspective.

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.

Explaining pulsars, the cosmic clocks and lighthouses of the universe

As of 2023, over 3,000 of these rapidly rotating neutron stars have been discovered, which contain magnetic fields along which energized particles accelerate and emit light, including X-rays, gamma rays, visible light, and, most commonly, radio waves. The fastest of these, PSR J1748–2446ad, rotates 716 times per second, meaning its surface equator moves at about 24% the speed of light.

Particle detection has evolved from cloud and bubble chambers to digital 3D displays

As invisible particles passed through cloud and bubble chambers, they disrupted supersaturated vapor and superheated liquid, respectively, creating trails that served as proxies for cosmic rays. The transition from photographed events to digital sensors enhanced data collection and accuracy.

Dark energy causes 97% of the galaxies to move faster than light

Although special relativity prevents objects from moving faster than the speed of light, the expansion of the universe, which increases with distance between objects, produces observed superluminal motion. This does not violate the theory because galaxies are moving with the expanding space, rather than through it.

View a ranking of the masses of fundamental particles, from the massless photon to the top quark, and a timeline of their discoveries.

Visualizing Einstein's special relativity

Before Einstein developed his theory of general relativity, he worked out what is known as special relativity—or why time is experienced different for different observers. The breakthrough rests on his theory that light travels at a constant...