COSMIC RAYS
The universe is full of mysteries, and some of its most fascinating secrets arrive on Earth from outer space. Every second, energetic particles from distant regions of the cosmos travel towards our planet, striking its atmosphere from all directions. These invisible visitors are known as cosmic rays.
But here is an interesting twist: cosmic rays are not actually rays of light! They are mainly high-energy particles travelling at enormous speeds, some remarkably close to the speed of light. Their discovery opened up an exciting new field of scientific research and continues to help scientists investigate the mysteries of the universe.
WHAT EXACTLY ARE COSMIC RAYS?
Cosmic rays are high-energy particles that travel through space. Most of them are atomic nuclei, particularly protons, while others are heavier atomic nuclei and electrons. They originate from various sources, including the Sun, supernova remnants and other energetic events in the universe.
When these particles reach Earth, they encounter our planet's atmosphere. Some collide with atoms and molecules in the air, producing new particles that continue travelling towards the ground.
Scientists classify cosmic rays into two main categories: primary and secondary cosmic rays.
Primary cosmic rays are the energetic particles that arrive at Earth from space. They may have travelled enormous distances before reaching our planet.
Secondary cosmic rays are particles produced when primary cosmic rays collide with atoms in Earth's atmosphere. These collisions can create a variety of new particles, including muons, electrons, photons and other subatomic particles.
Thus, a single cosmic particle entering the atmosphere can set off a chain reaction that produces numerous other particles.
THE GREAT COSMIC SHOWER
Imagine a tiny particle travelling through space at an incredible speed. As it enters Earth's atmosphere, it crashes into the nucleus of an air atom. The collision releases enormous energy and produces a collection of new particles.
These particles may collide with other atmospheric atoms, generating still more particles. The process continues, creating a cascade that spreads through the atmosphere.
This phenomenon is called an extensive air shower, also known as a cosmic-ray air shower. When the shower is initiated by an exceptionally energetic cosmic ray, it can spread over a vast area.
The French physicist Pierre Auger played a pioneering role in identifying these extensive air showers in 1938. His work helped scientists understand how a single high-energy particle could produce a large shower of secondary particles.
By studying the particles that reach the ground, scientists can estimate the energy and investigate the properties of the original cosmic ray that started the shower.
It is rather like solving a scientific puzzle: researchers examine the scattered pieces to understand what happened at the beginning!
THE BALLOON FLIGHT THAT REVEALED A COSMIC MYSTERY
The story of cosmic rays took a major turn in 1912, when Austrian physicist Victor Francis Hess conducted a series of daring balloon flights.
At that time, scientists knew that electrically charged radiation could be detected in the atmosphere. They suspected that much of this radiation came from radioactive materials in the Earth's crust.
Hess decided to investigate whether the radiation changed with altitude. During his balloon flights, he carried instruments designed to measure ionising radiation as he rose higher above the ground.
To his surprise, he discovered that the radiation increased significantly at high altitudes. If the radiation came mainly from the Earth, its intensity should have decreased as he moved farther away from the ground.
His observations suggested that a powerful source of radiation existed beyond Earth.
Hess's experiments provided convincing evidence that energetic radiation was arriving from space. This discovery transformed scientists' understanding of the universe and earned him the Nobel Prize in Physics in 1936.
WAS THE SUN RESPONSIBLE?
One obvious question arose: could the Sun be the source of this mysterious radiation?
Hess investigated the question by observing radiation during a solar eclipse. Even when the Moon blocked the Sun's visible disc, he found no significant reduction in the radiation he measured.
This supported the idea that the radiation was not coming primarily from the Sun's visible surface.
However, the experiment did not prove that the Sun never produces cosmic rays. We now know that the Sun can accelerate energetic particles, particularly during solar eruptions. Hess's findings instead helped establish that cosmic radiation has important sources beyond the Sun.
Scientists have since discovered that cosmic rays come from a variety of astronomical environments, making them valuable messengers from the wider universe.
WHO GAVE THEM THE NAME 'COSMIC RAYS?
Although Victor Hess discovered evidence of this radiation, scientists still did not fully understand its nature.
In 1925, physicist Robert Andrews Millikan popularised the term cosmic rays. At the time, there was considerable debate about whether the radiation consisted of energetic particles or electromagnetic waves.
Further investigations eventually demonstrated that most cosmic rays reaching Earth are charged particles rather than ordinary light rays. Their name, however, remained unchanged.
This is a wonderful example of how scientific knowledge develops. An observation may lead to a name, but continued research can reveal that the phenomenon is more complicated than scientists first imagined.
WHY DO SCIENTISTS STUDY COSMIC RAYS?
Cosmic rays are more than mysterious visitors from space. They provide valuable information about some of the most energetic processes in the universe.
- Understanding distant cosmic events: Cosmic rays can originate in powerful environments such as supernova remnants, where exploding stars leave behind expanding clouds of material.
- Studying particle physics: When cosmic rays collide with atmospheric atoms, they produce particles and interactions that help scientists investigate the fundamental laws of nature.
- Exploring extreme energies: Some cosmic rays carry extraordinarily high energies, far beyond those of particles produced in many ordinary physical processes.
- Investigating space weather: Energetic particles from the Sun can affect satellites, astronauts and technological systems, making their study important for space exploration.
- Monitoring radiation exposure: Cosmic radiation is relevant to aviation and human spaceflight because exposure generally increases at high altitudes and beyond Earth's protective atmosphere.
Earth's atmosphere and magnetic field provide substantial protection against many incoming particles, although some secondary particles still reach the surface. Astronauts travelling beyond Earth's magnetic protection face greater radiation challenges.
DID YOU KNOW?
- They travel incredibly fast: Many cosmic-ray particles move at speeds extremely close to the speed of light.
- They arrive from every direction: Cosmic rays reach Earth from all parts of the sky, although their paths can be altered by magnetic fields.
- They create particle cascades: A single energetic cosmic ray can trigger an air shower containing enormous numbers of secondary particles.
- They helped reveal new particles: Studies of cosmic rays contributed to the discovery of several subatomic particles before modern particle accelerators became widely available.
- They can be extremely energetic: The highest-energy cosmic rays detected have energies comparable to the kinetic energy of a fast-moving tennis ball, concentrated in a single subatomic particle.
- They are not all from distant galaxies: Some energetic particles originate from the Sun and other sources within our own galaxy.
THE COSMIC MYSTERY CONTINUES
More than a century after Victor Hess's pioneering balloon flights, cosmic rays continue to challenge and inspire scientists. Modern observatories, particle detectors and space-based instruments help researchers investigate where these particles originate, how they acquire such enormous energies and what they can reveal about the universe.
Every cosmic ray that reaches Earth carries clues about the powerful processes that occur far beyond our planet. By studying the particles and the showers they create, scientists continue to piece together the story of the cosmos.
The next time you look up at the night sky, remember that the universe is not merely something we see. It is also sending invisible messengers towards us, carrying secrets from the farthest reaches of space.
