Antihydrogen

Scientists study a special gas.

It is like the gas we know. But it is made of tiny parts that are opposite. This gas is called antihydrogen.

It is very hard to make. People make it in big machines. It is very rare in our world. If it touches normal matter, it disappears. It turns into a flash of light. Scientists want to learn why it is so rare. It is a big mystery of space.

Antihydrogen is a special kind of gas.

It is the opposite of normal hydrogen. A normal hydrogen atom has a proton and an electron. Antihydrogen is made of an antiproton and a positron. A positron is an opposite version of an electron.

Scientists make antihydrogen in large machines called particle accelerators. In 2010, a team called ALPHA trapped these atoms. They held them for 1,000 seconds. This was a big step for science.

Scientists want to see if antihydrogen acts like normal hydrogen. They use lasers to study it. In 2016, they measured how the atoms change. They found the results were the same as normal hydrogen. This supports a rule called CPT symmetry. This rule says matter and antimatter should act the same.

Antihydrogen is very hard to keep. If it touches normal matter, it will annihilate. This means the two parts hit each other and vanish. They turn into a flash of light and energy. Scientists hope to learn why our universe has so much matter and so little antimatter.

Antihydrogen is a very special kind of gas.

It is the antimatter version of the hydrogen we find everywhere. A normal hydrogen atom uses a proton and an electron. In contrast, antihydrogen uses an antiproton and a positron. A positron is just an opposite version of an electron. Scientists study these atoms to solve a huge mystery. They want to know why our universe has so much matter but very little antimatter.

Making antihydrogen is a very careful way of working. Scientists use huge machines called particle accelerators to create them. In 1995, a team led by Walter Oelert made the first antihydrogen at CERN. They shot antiprotons at clusters of xenon to make pairs of positrons. This method made very hot atoms that were hard to study. Later, CERN built the Antiproton Decelerator to make cooler atoms. This helps scientists test how matter and antimatter act.

Keeping these atoms still is a very hard job. Most antihydrogen atoms are too hot and hit the walls of their containers. When they touch normal matter, they undergo annihilation. This means the parts hit each other and vanish into energy. The positron hits an electron to make gamma rays. The antiproton hits a proton or neutron to make tiny particles called pions. These pions then turn into other things like muons and neutrinos.

Special teams have worked hard to trap these atoms for study. The ATHENA group produced antihydrogen in 2002. Later, the ALPHA team at CERN made a big breakthrough. In 2010, they trapped 38 antihydrogen atoms for a sixth of a second. By June 2011, they held 309 atoms for up to 1,000 seconds. They used magnetic fields to hold the atoms in place. This allows scientists to look at them without them disappearing.

Scientists use lasers to see how these atoms behave. In 2016, the ALPHA team used a laser to study energy levels. They looked at a change called the 1s–2s transition. This is a way the atom moves between its lowest energy states. The results were the same as normal hydrogen. This supports a rule called CPT symmetry. This rule says matter and antimatter should have the same mass and properties.

Antihydrogen is the antimatter counterpart to the hydrogen atom.

While common hydrogen is made of a proton and an electron, antihydrogen consists of an antiproton and a positron. A positron is the antimatter version of an electron. Scientists study these atoms to understand a major mystery called the baryon asymmetry problem. This problem asks why the observable universe contains so much more matter than antimatter.

When antihydrogen touches ordinary matter, the particles undergo a process called annihilation. This occurs because the particles are opposites. The positron annihilates with an electron to produce gamma rays. The antiproton is made of antiquarks. These antiquarks combine with quarks in protons or neutrons. This reaction produces high-energy particles called pions. These pions quickly decay into muons, neutrinos, positrons, and electrons. To prevent this, scientists must keep antihydrogen in a perfect vacuum.

Creating antihydrogen requires massive machines called particle accelerators. In 1995, a team led by Walter Oelert produced the first antihydrogen at CERN. They used a method involving antiprotons shot at xenon clusters. This process produced electron-positron pairs. However, the antiprotons only captured positrons with a very low probability. This resulted in "hot" or highly energetic atoms. These atoms were difficult to study because they moved too fast. To solve this, CERN built the Antiproton Decelerator (AD). This machine helps produce the low-energy antihydrogen needed for precision tests.

Different research groups have achieved different milestones in producing and holding these atoms. The ATHENA collaboration produced antihydrogen in 2002. Later, the ATRAP collaboration also worked on synthesis. By 2004, millions of antihydrogen atoms had been created. Most of these atoms were at temperatures of a few thousand kelvins. Because they were so hot, they often hit the walls of their containers and annihilated. The ALPHA team eventually found a way to trap these neutral atoms using magnetic fields. In 2010, they held 38 atoms for one-sixth of a second. By June 2011, they trapped 309 atoms for up to 1,000 seconds.

Scientists use lasers to study the internal structure of these atoms. In 2016, the ALPHA experiment measured the 1s–2s transition. This is the movement of a positron between the two lowest energy levels. Because a single-photon transition is prohibited by quantum selection rules, scientists used a special technique. They illuminated the space with a laser tuned to half the calculated transition frequency. This allowed for two-photon absorption to move the positron to the 2s level.

This laser experiment helped test a rule called CPT symmetry. This theorem predicts that antihydrogen should have the same mass and magnetic moment as hydrogen. It also predicts that excited antihydrogen should glow the same color as regular hydrogen. The ALPHA team found that the transition results were identical to hydrogen within their experimental resolution. This supports the idea of matter-antimatter symmetry. The team measured this at a precision of 200 parts per trillion (ppt). Their results showed a 58% drop in detected events during resonance runs compared to other tests.

Understanding antihydrogen also helps us understand gravity. The CPT theorem suggests antihydrogen should be attracted to matter by gravity. It should experience the same force as ordinary hydrogen. Some theories suggest the possibility of "antigravity" or repulsive gravity. However, this has not been empirically proven. New experiments like AEgIS aim to study this further. In 2018, AEgIS created a pulsed source of antihydrogen using a charge exchange reaction. This could allow scientists to use an atomic interferometer to measure gravitational behavior more precisely.