Antimatter


In particle physics, antimatter is a material composed of antiparticles, which have the same mass as particles of ordinary matter, but opposite charges, lepton numbers, and baryon numbers. Collisions between particles and antiparticles lead to the annihilation of both, giving rise to variable proportions of intense photons (gamma rays), neutrinos, and less massive particle–antiparticle pairs. The total consequence of annihilation is a release of energy available for work, proportional to the total matter and antimatter mass, in accord with the mass–energy equivalence equation, E = mc2.

Antiparticles bind with each other to form antimatter, just as ordinary particles bind to form normal matter. For example, a positron (the antiparticle of the electron) and an antiproton (the antiparticle of the proton) can form an antihydrogen atom. Physical principles indicate that complex antimatter atomic nuclei are possible, as well as anti-atoms corresponding to the known chemical elements. Studies of cosmic rays have identified both positrons and antiprotons, presumably produced by collisions between particles of ordinary matter. Satellite-based searches of cosmic rays for antideuteron and antihelium particles have yielded nothing.

There is considerable speculation as to why the observable universe is composed almost entirely of ordinary matter, as opposed to an even mixture of matter and antimatter. This asymmetry of matter and antimatter in the visible universe is one of the great unsolved problems in physics. The process by which this inequality between particles and antiparticles developed is called baryogenesis.

Antimatter in the form of anti-atoms is one of the most difficult materials to produce. Antimatter in the form of individual anti-particles, however, is commonly produced by particle accelerators and in some types of radioactive decay. The nuclei of antihelium have been artificially produced with difficulty. These are the most complex anti-nuclei so far observed.




History

The idea of negative matter appears in past theories of matter that have now been abandoned. Using the once popular vortex theory of gravity, the possibility of matter with negative gravity was discussed by William Hicks in the 1880s. Between the 1880s and the 1890s, Karl Pearson proposed the existence of "squirts" and sinks of the flow of aether. The squirts represented normal matter and the sinks represented negative matter. Pearson's theory required a fourth dimension for the aether to flow from and into.

The term antimatter was first used by Arthur Schuster in two rather whimsical letters to Nature in 1898, in which he coined the term. He hypothesized antiatoms, as well as whole antimatter solar systems, and discussed the possibility of matter and antimatter annihilating each other. Schuster's ideas were not a serious theoretical proposal, merely speculation, and like the previous ideas, differed from the modern concept of antimatter in that it possessed negative gravity.

The modern theory of antimatter began in 1928, with a paper by Paul Dirac. Dirac realised that his relativistic version of the Schrödinger wave equation for electrons predicted the possibility of antielectrons. These were discovered by Carl D. Anderson in 1932 and named positrons (a portmanteau of "positive electron"). Although Dirac did not himself use the term antimatter, its use follows on naturally enough from antielectrons, antiprotons, etc. A complete periodic table of antimatter was envisaged by Charles Janet in 1929.

The Feynman–Stueckelberg interpretation states that antimatter and antiparticles are regular particles traveling backward in time.

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In the News ...





First Solid Sign that Matter Doesn't Behave Like Antimatter   Live Science - February 27, 2017
One of the biggest mysteries in physics is why there's matter in the universe at all. This week, a group of physicists at the world's largest atom smasher, the Large Hadron Collider, might be closer to an answer: They found that particles in the same family as the protons and neutrons that make up familiar objects behave in a slightly different way from their antimatter counterparts. While matter and antimatter have all of the same properties, antimatter particles carry charges that are the opposite of those in matter.

In a block of iron, for example, the protons are positively charged and the electrons are negatively charged. A block of antimatter iron would have negatively charged antiprotons and positively charged anti-electrons (known as positrons). If matter and antimatter come in contact, they annihilate each other and turn into photons (or occasionally, a few lightweight particles such as neutrinos). Other than that, a piece of matter and antimatter should behave in the same way, and even look the same - a phenomenon called charge-parity (CP) symmetry. Besides the identical behavior, CP symmetry also implies that the amount of matter and antimatter that was formed at the Big Bang, some 13.7 billion years ago, should have been equal. Clearly it was not, because if that were the case, then all the matter and antimatter in the universe would have been annihilated at the start, and even humans wouldn't be here.




Bizarre Antimatter Emits Same Light As Regular Matter   Live Science - December 21, 2016

For the first time, physicists have shown that atoms of antimatter appear to give off the same kind of light that atoms of regular matter do when illuminated with lasers, a new study finds. More precise measurements of this emitted light could unearth clues that might finally help solve the mystery of why there is so much less antimatter than normal matter in the universe, researchers say. For every particle of normal matter, there is an antimatter counterpart with the same mass but the opposite electrical charge. The antiparticles of the electron and proton, for instance, are the positron and antiproton, respectively.




Laser helps unlock antimatter secrets   BBC - December 19, 2016

Scientists at Cern have found a new way to unlock the secrets of antimatter. In a major technological advance, physicists shone a laser on trapped anti-atoms to detect whether they behaved any differently to atoms. The work could shed light on one of the enduring mysteries about antimatter. Although the Big Bang produced matter and antimatter in equal amounts, today, the Universe overwhelmingly consists of matter - and current theories cannot explain why. Antimatter is incredibly difficult to produce and then capture and hold on to - not least because it gets annihilated on contact with ordinary matter.




Search Escalates for Key to Why Matter Exists   Scientific American - October 23, 2013

Physicists have completed a new round of searches for the answer to why matter dominates antimatter. But the radioactive decay that would solve the puzzle evades them




Underground Experiment Asks Why We're Not Antimatter   Live Science - May 29, 2013

A new experiment buried deep underground in a South Dakota mine aims to detect rare particle decays that could explain the mystery of antimatter. Scientists don't know why the universe is made of matter and not antimatter, but they hope to find differences in the way these two types of stuff behave that could explain the discrepancy. Antimatter particles have the same mass as their normal-matter counterparts, but opposite charge and spin. The South Dakota effort, called the Majorana Demonstrator, aims to observe a theorized-but-never-seen process called neutrinoless double beta decay.




Antimatter belt around Earth discovered by Pamela spacecraft   BBC - August 7, 2011

A thin band of antimatter particles called antiprotons enveloping the Earth has been spotted for the first time. The find, described in Astrophysical Journal Letters, confirms theoretical work that predicted the Earth's magnetic field could trap antimatter. The team says a small number of antiprotons lie between the Van Allen belts of trapped "normal" matter. The researchers say there may be enough to implement a scheme using antimatter to fuel future spacecraft.




Fundamental matter-antimatter symmetry confirmed   PhysOrg - July 28, 2011

International collaboration including MPQ scientists sets a new value for the antiproton mass relative to the electron with unprecedented precision.




Physicists Weigh Antimatter with Amazing Accuracy   Live Science - July 28, 2011

A new measurement provides the most accurate weight yet of antimatter, revealing the mass of the antiproton (the proton's antiparticle) down to one part in a billion, researchers announced today (July 28). To give a sense of just how accurate their measurement was, researcher Masaki Hori said: "Imagine measuring the weight of the Eiffel Tower. The accuracy we've achieved here is roughly equivalent to making that measurement to within less than the weight of a sparrow perched on top. Next time it will be a feather."




Could the Big Bang have been a quick conversion of antimatter into matter?   PhysOrg - July 19, 2011

Suppose at some point the universe ceases to expand, and instead begins collapsing in on itself (as in the Big Crunch scenario), and eventually becomes a supermassive black hole. The black hole’s extreme mass produces an extremely strong gravitational field. Through a gravitational version of the so-called Schwinger mechanism, this gravitational field converts virtual particle-antiparticle pairs from the surrounding quantum vacuum into real particle-antiparticle pairs. If the black hole is made from matter (antimatter), it could violently repel billions and billions of antiparticles (particles) out into space in a fraction of a second, creating an ejection event that would look quite similar to a Big Bang.




  Thunderstorms hurling antimatter into space caught by Fermi   PhysOrg - January 11, 2011

Scientists using NASA's Fermi Gamma-ray Space Telescope have detected beams of antimatter produced above thunderstorms on Earth, a phenomenon never seen before.




Invention Traps Mysterious Antimatter   Live Science - December 7, 2010

The trouble with studying antimatter is keeping it around without letting the odd substance come in contact with regular matter – because if that happens, the two will destroy each other in an explosive annihilation. Now researchers at the European Organization for Nuclear Research (CERN) in Geneva have unveiled a new trap that they say can store a significant amount of antihydrogen atoms.




Why We Exist: Matter Wins Battle Over Antimatter   Space.com - May 20, 2010

The seemingly inescapable fact that matter and antimatter particles destroy each other on contact has long puzzled physicists wondering how life, the universe or anything else can exist at all. But new results from a particle accelerator experiment suggest that matter does seem to win in the end. The experiment has shown a small - but significant - 1 percent difference between the amount of matter and antimatter produced, which could hint at how our matter-dominated existence came about. The current theory, known as the Standard Model of particle physics, has predicted some violation of matter-antimatter symmetry, but not enough to explain how our universe arose consisting mostly of matter with barely a trace of antimatter.




New clue to anti-matter mystery   BBC - May 19, 2010

A US-based physics experiment has found a clue as to why the world around us is composed of normal matter and not its shadowy opposite: anti-matter. Anti-matter is rare today; it can be produced in "atom smashers", in nuclear reactions or by cosmic rays. But physicists think the Big Bang should have produced equal amounts of matter and its opposite. New results from the DZero experiment at Fermilab in Illinois provide a clue to what happened to all the anti-matter.




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