The search for antimatter galaxies

8th May 2011

Photograph of particle tracks in a bubble chamber.

Particle tracks in a bubble chamber. Image credit: CERN/CC-A.

The penultimate Space Shuttle mission is due to launch on the 16th May, and will transport 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 thought to be 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 also like to prove that isolated regions of the universe are composed entirely of antimatter. If this is the case, then there could be antimatter galaxies, and even antimatter life[1].

1. A brief history of antimatter

British physicist Arthur Schuster coined the term 'antimatter' in 1898, although his ideas were purely speculative[2a]. British physicist Paul Dirac first showed that antimatter must exist in 1928, when he combined Albert Einstein's theory of special relativity[3] with quantum mechanics, in order to describe the motion of electrons[4a]. His results revealed that every particle has a corresponding antimatter partner, with an opposite spin and charge.

Matter and antimatter annihilate each other on contact, and when this happens the entire rest mass of each particle is converted to kinetic energy, in accordance with special relativity. If just 1 kg of matter collided with 1 kg of antimatter, the resulting explosion would be equivalent to that of over 40 million tonnes of TNT.

Within four years of Dirac's prediction, American physicist Carl David Anderson discovered antielectrons, which he named positrons. Anderson did this by studying the tracks that cosmic rays produce in cloud chambers. Cloud chambers are surrounded by a magnetic field, which deflects the path of charged particles. The energy and charge of a particle can be determined from the shape of the track[5].

Italian physicist Emilio Segre and American physicist Owen Chamberlain discovered antiprotons in 1955[6]. These were created inside of specially designed particle accelerators. The antineutron was discovered one year later, by Bruce Cork and colleagues[7], and the first antimatter nuclei were simultaneously discovered by teams of physicists working in Switzerland[8] and the United States[9] in 1965.

In 1967, Russian physicist Andrei Sakharov showed that almost equal amounts of matter and antimatter were created in the big bang[10]. Most annihilated each other within the first millionth of a second, creating the first light. It's assumed that there must have been more matter than antimatter, since the universe around us appears to be made entirely of matter. It's possible, however, that regions of antimatter exist that 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 states. If antimatter life forms exist, then they would never be able to visit us. They would be able to communicate with us using electromagnetic radiation, however, since photons have no charge and can propagate through a vacuum.

Schuster[2b] and Dirac[4b] had both considered whether antimatter solar systems could exist, but real speculation began in 1995, when a team of German and Italian physicists created nine antihydrogen atoms, proving that antimatter particles can come together to form larger structures[11].

2. The AMS project

The AMS project was proposed by Ting later that year, and a prototype, AMS-01, was launched in 1998. Over 600 scientists from 16 different countries have since worked on AMS-02, with funding primarily provided by NASA and the US Department of Energy[12].

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.

Ting says that AMS-02 is so powerful that if it doesn't detect anything as massive as an antihelium nucleus during its lifetime, then there are probably no antimatter galaxies within about a billion parsecs. This is equivalent to about 3 billion light-years, a volume that contains hundreds of millions of galaxies[13].

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 quarks. The best discoveries, however, are likely to be those that cannot be predicted.

Ting states:

"the most exciting objective of AMS is to probe the unknown; to search for phenomena that exist in nature that we have not yet imagined nor had the tools to discover"[14].

UPDATE: As of 2016, the AMS-02 is still operating and has recorded over 70 billion particle events, the data are still being analysed.

3. References

  1. NASA, 'In Search of Antimatter Galaxies', last accessed 15-02-16.

  2. (a, b) Schuster, A., 1898, 'Potential Matter - A Holiday Dream', Nature, 58, pp.367.

  3. Einstein, A., 1905, 'On the electrodynamics of moving bodies', Annalen der Physik, 17, pp.891-921, reprinted in in 'The Principle of Relativity: A Collection of Original Memoirs on the Special and General Theory of Relativity', 1920, Courier Corporation.

  4. (a, b) Dirac, P. A. M., 1928, 'The quantum theory of the electron', Proceedings of the Royal Society of London, Series A, 117, pp.610-624.

  5. Anderson, C. D., 1933, 'The positive electron', Physical Review, 43, pp.491-498.

  6. Chamberlain, O., Segrè, E., Wiegand, C., and Ypsilantis, T., 1955, 'Observation of antiprotons', Physical Review, 100, pp.947.

  7. Cork, B., Lambertson, G. R., Piccioni, O., and Wenzel, W. A., 1956, 'Antineutrons produced from antiprotons in charge-exchange collisions', Physical Review, 104, pp.1193.

  8. Massam, T., et al, 1965, 'Experimental observation of antideuteron production', Il Nuovo Cimento, 63, pp.10-14.

  9. Dorfan, D. E., et al, 1965, 'Observation of Antideuterons', Physical Review Letters, 14, pp.1003.

  10. Sakharov, A. D., 1967, 'Violation of CP invariance, C asymmetry, and baryon asymmetry of the universe', JETP Letters, 5, pp.24-27.

  11. CERN, 'First antiatoms produced: antihydrogen, at CERN', last accessed 15-02-16.

  12. NASA, 'Mission control CERN: Inside the AMS-02 command center', last accessed 15-02-16.

  13. Atlas of The Universe, 'The Universe within 2 billion Light Years', last accessed 15-02-16.

  14. AMS 02, 'The Science', last accessed 15-02-16.

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