Antimatter

In modern physics, 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 with 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 a lot of energy. This energy can show up as light or other forms of radiation. This process is very important for understanding how energy and mass are related. Even though we can make tiny bits of antimatter, we have never been able to collect a large amount because it is very hard and expensive to produce and handle.

Antimatter is used in some 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, which remains one of the big mysteries in physics today.

Definitions

Antimatter particles have the same charge as normal particles but with the opposite sign. 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. Similarly, 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. The same idea works when looking at the tiny parts that make up particles. A proton is made from u u d quarks, so an antiproton is made from u u d antiquarks.

Another way to tell particles apart is by their electric charge. The electron is written as e⁻ and its antiparticle, the positron, is written as e⁺. It’s important not to mix these two ways of showing particles.

Properties

Antimatter behaves the same way as regular matter when it comes to gravity. When scientists dropped antimatter, it fell down just like regular matter. They tested this using a special setup with 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, supporting our understanding of how the universe works.

Origin and asymmetry

See also: Baryogenesis

Most things we see 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 and regular matter meet.

Tiny bits of antimatter are made everywhere 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 certain particles called positrons meet regular matter. These gamma rays have a special amount of energy, showing they came from antimatter.

Some space tools have found a big cloud of antimatter near the center of our galaxy. This cloud doesn’t look perfectly round and seems to match places where big stars or black holes are. 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 certain radioactive materials break apart and when very energetic light interacts with other materials. Other antimatter particles, called antineutrinos, are also created when some radioactive materials change. Many kinds of antimatter are produced when cosmic rays—high-energy particles from space—collide with 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, and 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. These antimatter particles have different energy levels than normal matter, suggesting they are made in different ways. 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 found a way to make lots of positrons using a powerful laser and a piece of gold. They used the laser to push electrons through the gold, which created both normal matter and antimatter. This made more positrons than ever before in a lab. Later, in 2023, researchers made an even better way to create positrons using a big machine called a particle accelerator.

Antiprotons, antineutrons, and antinuclei

Main articles: Antiproton and Antineutron

Scientists first discovered antiprotons in 1955. These are like normal protons but with the opposite charge. Not long after, they found antineutrons, which are similar to neutrons but also have the opposite charge. They have also made tiny pieces of antimatter called antinuclei, which are 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 the years, they have found better ways to make colder antihydrogen atoms, which are easier to study. In 2010, they even trapped some antihydrogen atoms for a short time, which was a big 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. Instead, scientists use special traps with magnetic and electric fields to hold onto 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 a lot 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

When we use special machines, we can create tiny bits of antimatter. These bits help doctors see inside the body without needing surgery. This is called positron emission tomography, or PET for short. It works by using 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 in very powerful explosions. But making enough antimatter for this is very hard, and it may never be possible. During a time called the Cold War, some scientists studied how antimatter might work in explosions.