What Is Antimatter?

Antimatter has been a popular concept in science fiction. But contrary to popular belief, antimatter has been observed and even created as early as 1932 and was theorized to exist years earlier.

What is Antimatter?

Antimatter are particles (antiparticles) that have the same mass as its counterpart but carry the opposite charge and quantum spin. Spin can be compared to how the Earth rotates on its axis while orbiting the Sun.

The antiparticle counter part of the electron is the positron. It carries a positive charge rather than a negative one. The proton's version is the anti-proton and it carries a positive charge.

There is also the antineutron despite the neutron having a neutral charge. This antineutron is made up of antiquarks (the antiparticle of the quark).

A Brief History of Antimatter

In 1929, two scientists in two different experiments observed a particle that exhibited a peculiar behavior. They saw what they believed to be an electron that travelled as if it had a positive charge. Electrons are negatively charged. The two physicists, Dmitri Skobeltsyn and Chung-Yao Chao, disregarded the anomaly and never pursued it afterwards.

Three years later, in 1932, Carl D. Anderson who was studying cosmic rays at the time noticed the same thing in his experiments. Believing this to be an electron but with a positive charge, he successfully created a positron by shooting gamma rays into other materials.

Anderson was awarded the Nobel Prize for Physics in 1936 for this discovery.

This validated Paul Dirac's theory from 1928 on the existence of positrons. At the time, he called them anti-electrons. It was Anderson who named them positrons; short for positive electrons.

The antiproton was discovered in 1955 by physicists Emilio Segrè and Owen Chamberlain while the antineutron by Bruce Cork in 1956.

Studying The Behavior of Antimatter

In particle physics, particles are categorized by the following:

Electrical charge
Spin
Mass
Lifetime of the particle

Antimatter, just like matter, is stable. It has the same properties of matter except for its spin and charge. But since antimatter is destroyed when it comes into contact with matter, it exists for a very brief period. Just enough to observe it but not long enough to study it.

Video: What is Antimatter?

In 1995, CERN created the first anti-hydrogen atom (CERN created nine anti-hydrogen atoms). It is comprised of one anti-proton and one positron. The experiment was validated when Fermilab also managed to create them in their own experiment.

But these atoms were too energetic for it to be studied in depth. It was only in 2002 when CERN's ATHENA experiment announced that they have created the first "cold" anti-hydrogen.

Another problem was maintaining the stability of the atom. Since antimatter annihilates when it comes into contact with matter, it was difficult at the time to successfully contain it for a long period.

In 2010, CERN's ALPHA experiment (the successor to ATHENA which disbanded in 2005), announced holding 38 antihydrogen atoms for about a sixth of a second. This was the first time that neutral antimatter had been trapped.

Five months later, on 26 April 2011, ALPHA announced that they had trapped 309 antihydrogen atoms, some for as long as 1,000 seconds (about 17 minutes). This was longer than neutral antimatter had ever been trapped before.

Trapping antimatter such as anti-hydrogen requires a magnetic trap. The magnetic field inside the trap is adjusted to hold the anti-particle in place.

Video: Measuring and Trapping Antimatter

Annihilation

When matter and antimatter collide or touch, they destroy each other and release energy. This is called annihilation. The interaction between the two does not result in a cancellation of each other but in a burst of energy.

This is because only the charge and spin are opposite the other. The energy carried by both antimatter and matter are positive.

Why Does Antimatter Exist?

Einstein's famous equation, E=MC² states a direct correlation with mass and energy. Scientists found out that creating mass (or matter) from energy produces both particle and its corresponding antiparticle.

This means that for every particle in the universe, a corresponding anti-particle was also produced. There is an equivalent number of antimatter in the Universe as there is matter. This symmetry between the two particles albeit simple is, in reality, a mystery.

When the Big Bang occured, the release of energy should create both matter and antimatter in equal amounts. But because of annihilation, all these particles would be destroyed because of annihilation. And yet, the universe is wholly made up of matter. And that the presence of anti-matter is rare.

This anomaly is called CP Violation (Charge Conjugation - Parity Violation). Charge is the electrical charge of the particle while parity is the spatial representation of the particle (like a mirror image). This short video explains CP violation:

Video: CP Violation

A more exotic theory for CP violation is CPT violation where T is time and that it is proposed that aside from charge and parity, antimatter also moves backwards in time.

Energy Released From Annihilation

Going back to the equation E=MC²; E is Energy, M is Mass while C is the constant speed of light. In this case C is squared which is equivalent to 8.987551787 x10¹⁶ m/sec. Given the equation, the unit of energy is in joules.

This means that 1 gram of mass can release 89.9 terajoules. This is equivalent to the energy released by 21.5 kilotons of TNT or igniting 568,000 US gallons of automotive gasoline. When one gram of antimatter interacts with matter, the resulting explosion would be three times more powerful than the atomic bomb dropped in Hiroshima during World War 2.

The most powerful bomb ever detonated was the Tsar Bomba by the Russians in 1961. The energy released by the bomb was supposed to be 420 petajoules but reduced since it would also incinerate the plane and pilot carrying the bomb.

In relation to the 1 gram antimatter postulate which yields 89.9 terajoules, the Tsar Bomba's energy yield is only 0.089 terajoules.

Movies and books have often dealt with the principle of an antimatter bomb. It is a solid theory but in reality would be hard to create. The most common (and easiest) antiparticle to be produced is the positron. One positron weighs around 9.1 x 10^-28 grams. To make one gram of positrons, about 1.1 x10 ²² are needed.

At the current technology, it would take two billion years to just create half a gram of antimatter.

Practical Applications for Antimatter

With all the studies conducted with antimatter, some real world applications have already benefited from it.

PET scans or Positron Emission Tomography is an imaging technique used to create 3 dimensional images of the body. It uses positrons that are emitted by a radionuclide. The gamma rays emitted by the positrons are detected by the system and is used to construct the image.

There are also studies being made in treating cancer tumors with anti-protons. Currently, there are proton therapy treatments being experimented with cancer.

Positrons can also be used to study materials (similar to PET scans).

Other Questions About Antimatter

Physicists at CERN recently conducted a Google+ Hangout answering questions about antimatter.

Video: Hangout With CERN: Antimatter

Quantum Day

08 January 2013

What Is Antimatter?