Antimatter

Antimatter is a special kind of material made from tiny parts called antiparticles. These antiparticles are like mirror images of the particles that make up everything around us, but they have some differences, like opposite electric charge.

Antimatter can be found in nature, such as in collisions of cosmic rays and certain types of radioactive decay. Scientists can also create antiparticles using big machines called particle accelerators, but only very tiny amounts have been made so far.

When a particle meets its matching antiparticle, they disappear in a flash and release energy. This energy can show up as light or other kinds of radiation. Even though we can make tiny bits of antimatter, it is very hard and expensive to collect a large amount.

Antimatter is used in helpful ways, like in positron emission tomography scans that doctors use to see inside the body, and in certain types of radiation therapy to treat diseases. Scientists are still trying to figure out why there is so much more normal matter in the universe than antimatter.

Definitions

Antimatter particles look like normal particles but have the opposite charge. For example, an antiproton is negatively charged, and an antielectron, called a positron, is positively charged. Neutrons have no charge, but their parts, called quarks, do. Normal protons and neutrons have a baryon number of +1, while antimatter versions have a baryon number of –1. Normal electrons have a lepton number of +1, while positrons have a lepton number of –1. When a particle meets its antimatter partner, they both turn into energy.

History

Conceptual

The idea of matter with reversed properties appeared in old theories. In the 1880s, a scientist named William Hicks talked about matter with negative gravity. Around the same time, Karl Pearson suggested the existence of "squirts" and sinks in a flow called the aether, with squirts being normal matter and sinks being its opposite.

The word antimatter was first used in 1898 by Arthur Schuster. He guessed that there might be antiatoms and whole systems made of antimatter, but his ideas were just ideas and not serious theories.

The modern idea of antimatter started in 1928 with a paper by Paul Dirac. He showed that his equations suggested the possibility of particles called antielectrons. Later, in 1930, J. Robert Oppenheimer pointed out that these equations also suggested the existence of a positively charged electron, now called a positron. These were discovered in 1932 by Carl D. Anderson.

Particle discoveries

After the positron was found in 1932, it took 22 years to find another antimatter particle. In 1955, a team discovered the antiproton, and in 1956, another team found the antineutron. Both teams used a machine called the Bevatron in Berkeley, California.

Antigalaxies

Finding antimatter led some to wonder if there could be whole systems made of it, like antiplanets, antistars, and antigalaxies. Some even thought antiplanets might have intelligent life. One idea was that our universe might have split from an antimatter universe at the beginning of time, which could explain why there is more matter than antimatter. By the 1970s, the idea of antigalaxies was mostly given up.

Notation

One way to show an antiparticle is by putting a line above the symbol of the particle. For example, a proton is written as p, and its antiparticle, the antiproton, is written as p̄. A proton is made from u u d quarks, so an antiproton is made from u u d antiquarks.

We can also tell particles apart by their electric charge. The electron is written as e⁻ and its antiparticle, the positron, is written as e⁺. It’s important to keep these two ways of showing particles separate.

Properties

Antimatter behaves like regular matter when it comes to gravity. Scientists dropped antimatter and it fell down just like regular matter. They did this test using very cold antimatter atoms and strong magnets to keep them safe. Most of the antimatter fell out the bottom, showing it acts like normal matter.

Besides having opposite charges, antimatter and regular matter have the same properties. This means they weigh the same and glow the same way in stars. In 2016, an experiment showed that antimatter atoms behave just like regular atoms.

Origin and asymmetry

See also: Baryogenesis

Most things on Earth are made of regular matter, not antimatter. If there were places in space with lots of antimatter, we might see special kinds of light called gamma rays where the antimatter meets regular matter.

Tiny bits of antimatter are made in space when high-energy particles called cosmic rays hit Earth’s air or other matter. These bits disappear quickly when they touch regular matter. Scientists can find signs of antimatter by looking for gamma rays made when particles called positrons meet regular matter.

Some space tools have found a big cloud of antimatter near the center of our galaxy. Scientists think this cloud might be made when normal matter gets very hot and turns into pairs of particles and antiparticles.

Scientists are also looking far away to see if some galaxies might be made of antimatter instead of regular matter. These galaxies would look just like normal ones, so it’s hard to tell them apart. By watching for special signals in space light, scientists hope to find out if such galaxies exist.

Natural production

Main article: Positron emission

Positrons, a type of antimatter, are made naturally when some radioactive materials break apart. They are also made when very energetic light hits other materials. Other antimatter particles, called antineutrinos, are created when some radioactive materials change. Many kinds of antimatter are made when cosmic rays—high-energy particles from space—hit other matter.

In 2011, scientists found that positrons are made above thunderstorm clouds by powerful flashes of gamma rays. Scientists have also found antiprotons, another type of antimatter, in special areas of space around Earth called the Van Allen Belts. They think similar areas might exist around other planets like Jupiter and Saturn.

When temperatures are very high, such as in the early universe or around black holes and neutron stars, antimatter can be created along with normal matter. Scientists have found positrons and a few antiprotons in cosmic rays, which are particles from space that reach Earth. Researchers are also looking for larger pieces of antimatter, like antihelium, which could hint at the existence of entire stars made of antimatter.

Artificial production

Positrons

Main article: Positron

In 2008, scientists made lots of positrons using a strong laser and a piece of gold. The laser pushed electrons through the gold, creating both normal matter and antimatter. This made more positrons than ever before in a lab. In 2023, they found an even better way to make positrons using a big machine called a particle accelerator.

Antiprotons, antineutrons, and antinuclei

Main articles: Antiproton and Antineutron

Scientists discovered antiprotons in 1955. These are like normal protons but with the opposite charge. Soon after, they found antineutrons, which are like neutrons but also have the opposite charge. They have also made tiny pieces of antimatter called antinuclei, made from antiprotons and antineutrons stuck together.

Antihydrogen atoms

Main article: Antihydrogen

In 1995, scientists created the first antihydrogen atoms, which are like normal hydrogen but made of antimatter. They made these atoms by mixing antiprotons and positrons. Over time, they found better ways to make colder antihydrogen atoms, which are easier to study. In 2010, they trapped some antihydrogen atoms for a short time, which was an important step in learning more about antimatter.

Antihelium

Antihelium-3 nuclei were first seen in the 1970s in experiments with protons hitting other atoms. In 2011, scientists made antihelium-4 nuclei, which are like normal helium but made of antimatter.

Preservation

Antimatter can’t be kept in normal containers because it would disappear when it touches ordinary matter. Scientists use special traps with magnetic and electric fields to hold antimatter particles. These traps can keep antimatter safe for a long time, but it’s still very hard to store.

Cost

Making antimatter is very expensive. It costs much more to create even a tiny bit of it than to make normal matter. This is because it’s very hard to produce and we don’t have many ways to make it. Some scientists are looking for cheaper ways to find antimatter in space, like in the Earth’s atmosphere or around planets like Jupiter.

Uses

Medical

We can make tiny bits of antimatter using special machines. These bits help doctors see inside the body without surgery. This is called positron emission tomography, or PET for short. It uses a type of antimatter called positrons.

Fuel

If we could collect enough antimatter, it could be used as a powerful fuel for spaceships traveling far into space. Antimatter has more energy than normal fuels, so a spaceship using it could travel faster and farther.

Weapons

Some people have thought about using antimatter to make very powerful explosions. But making enough antimatter for this is very hard, and it may never be possible. During the Cold War, some scientists studied how antimatter might work in explosions.