The search for antimatter galaxies begins

The search for antimatter galaxies begins

8th May 2011 0 Comments

Image credit: G.T. Jones, Birmingham University/Fermi National Accelerator Laboratory

The penultimate Space Shuttle mission is due to launch on the 16th May, transporting a device known as the Alpha Magnetic Spectrometer, or AMS-02, to the International Space Station. The AMS-02 is designed to identify cosmic rays, high energy particles that originate from space. Although most cosmic rays are composed of ordinary matter, the leader of the AMS project, Nobel laureate Professor Samuel Ting, hopes that it will discover dark matter, strange matter and antimatter. Ting would like to prove that isolated regions of the universe are composed entirely of antimatter and if this is the case then there could be anti galaxies and even anti life.

The term antimatter was coined by German-British physicist Arthur Schuster in 1898, although his ideas were purely speculative. English physicist Paul Dirac first showed that antimatter must exist in 1928 when he combined Einstein's theory of special relativity with quantum mechanics in order to describe the motion of electrons. His results revealed that every particle has a corresponding anti particle with an opposite charge. Matter and antimatter annihilate each other upon contact and the entire rest mass of the particles is converted to kinetic energy using Einstein's famous equation . If just one kilogram of matter collided with one kilogram of antimatter, the resulting explosion would be equivalent to that of over forty million tonnes of TNT.

Within four years of Dirac's prediction, American physicist Carl Anderson discovered anti electrons, which he named positrons, from tracks produced by cosmic rays inside of a cloud chamber. A magnetic field was placed across the chamber, deflecting the path of charged particles and revealing their energy and charge. In the picture above, electrons can be seen to curve to the left and positrons to the right.

Anti protons were discovered by American physicists Emilio Segre and Owen Chamberlain in 1955. These were created inside specially designed particle accelerators. The anti neutron was discovered five years later by American physicist Bruce Cork and the first antimatter nuclei were simultaneously discovered by teams of physicists working in Switzerland and New York in 1965.

Two years later, Soviet physicist Andrei Sakharov showed that almost equal amounts of matter and antimatter were created in the big bang. Most annihilated each other within the first millionth of a second, creating the first light. It is assumed that there must have been more matter than antimatter since the universe around us appears to be made entirely of matter. It is possible that regions of antimatter exist but they must have remained isolated from matter since their creation.

Antimatter galaxies would be indistinguishable from matter galaxies but we know that there cannot be any close by. This is because they would produce bright gamma ray emissions from annihilations between matter and antimatter at the boundary between the two. If antimatter life forms existed then they would never be able to visit us. They would have to communicate using electromagnetic radiation since photons have no charge and can propagate through a vacuum.

Schuster and Dirac had both considered whether anti galaxies could exist, but real speculation began in 1995 when a team of German and Italian physicists created nine anti hydrogen atoms, proving that antimatter particles can come together to form larger structures.

The AMS project was proposed by Ting later that year and a prototype AMS-01 was launched in 1998. Over five hundred scientists from sixteen different countries have since worked on AMS-02 with funding primarily provided by NASA and the US Department of Energy. AMS-02 is comparable to Anderson's cloud chamber experiment since it identifies cosmic rays by measuring their deflection within a powerful magnetic field. The main difference is that AMS-02 has access to far more cosmic rays with a wider range of energies than would be detected on Earth.

AMS-02 is so powerful that if it does not detect anything as massive as an anti helium nucleus during its lifetime then Ting says scientists can conclude that there are no antimatter galaxies within about a billion parsecs, this is equivalent to about three billion light years.

Ting hopes that AMS-02 will also identify cosmic rays composed of neutralinos, the leading candidate for dark matter, and strange matter, a hypothetical substance containing strange, as well as the more common up and down quarks. However, the best discoveries are likely to be those that cannot be predicted. Ting states that "the most exciting objective of AMS is to probe the unknown, to search for phenomena that exist in Nature but yet we have not the tools or the imagination to find".

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