- 2376 - ANTI-MATTER - Why does it exist? Present theory suggests that if particles outnumbered antiparticles in the Big Bang by as little as one part in 100 million, then the present universe could be explained by those extra particles that were not annihilated by an antiparticle counterpart.
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--------------------- 2376 - ANTI-MATTER - Why does it exist?
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- Antimatter is matter that is composed of the antiparticles of those that constitute normal matter. If a particle and its antiparticle come in contact with each other, the two annihilate and produce a burst of energy, which results in the production of other particles and antiparticles or electromagnetic radiation.
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- In these reactions, rest mass is not conserved, although energy (E=mc²) is conserved.
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- In 1928 Paul Dirac developed a relativistic equation for the electron, now known as the Dirac equation. Curiously, the equation was found to have negative energy solutions in addition to the normal positive ones.
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- This presented a problem, as electrons tend toward the lowest possible energy level; energies of negative infinity are nonsensical. As a way of getting around this, Dirac proposed that the vacuum can be considered a "sea" of negative energy, the Dirac sea. Any electrons would therefore have to sit on top of the sea.
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- Thinking further, Dirac found that a "hole" in the sea would have a positive charge. At first he thought that this was the , but the hole should have the same mass as the electron. The existence of this particle, the positron, was confirmed experimentally in 1932 by Carl D. Anderson.
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- Today's standard model shows that every particle has an antiparticle, for which each additive quantum number has the negative of the value it has for the normal matter particle.
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- The sign reversal applies only to quantum numbers (properties) which are additive, such as charge, but not to mass, for example. The positron has the opposite charge but the same mass as the electron. An atom of anti-hydrogen is composed of a negatively-charged antiproton being orbited by a positively-charged positron .
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- Scientists in 1995 succeeded in producing anti-atoms of hydrogen, and also anti-deuteron nuclei, made out of an antiproton and an antineutron, but no anti-atom more complex than anti-deuterium has been created yet.
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- In principle, anti-atoms of any element could be built from readily available sources of antiparticles. Such anti-atoms would have exactly the same properties as their normal-matter counterparts. The production of anti-elements in bulk quantities seems unlikely to ever become achievable, however.
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- Positrons and antiprotons can individually be stored in a device called a Penning trap, which uses a combination of magnetic field and electric fields to hold charged particles in a vacuum. Two international collaborations, used these devices to store thousands of slowly moving anti-hydrogen atoms in 2002.
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- Science is working to confine the anti-atoms in an inhomogenous magnetic field (one cannot use electric fields since the antiatoms are neutral) and interrogate them with lasers. If the anti-atoms have too much kinetic energy they will be able to escape the magnetic trap, and it is therefore essential that the anti-atoms be produced with as little energy as possible.
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- Antimatter/matter reactions have practical applications in medical imaging, such as positron emission tomography (PET). In some kinds of beta decay, a nuclide loses surplus positive charge by emitting a positron (in the same event, a proton becomes a neutron, and neutrinos are also given off).
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- Nuclides with surplus positive charge are easily made in a cyclotron and are widely generated for medical use. Antiparticles are created everywhere in the universe where high-energy particle collisions take place, such as in the center of our galaxy, but none has been detected that is residual from the Big Bang, as most normal matter is.
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-The unequal distribution between matter and antimatter in the universe has long been a mystery. The solution likely lies in the violation of CP-symmetry by the laws of nature.
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- In antimatter-matter collisions, the entire rest mass of the particles is converted to energy. The energy per unit mass is about 10 orders of magnitude greater than chemical energy, and about 2 orders of magnitude greater than nuclear energy that can be liberated today using chemical reactions or nuclear fission/fusion.
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- The reaction of 1 kg of antimatter with 1 kg of matter would produce 1.8×1017 J (180 petajoules) of energy (by the equation E=mc²). In contrast, burning a kilogram of gasoline produces 4.2×107 Joules and nuclear fusion of a kilogram of hydrogen would produce 2.6×1015 Joules.
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- Not all of that energy can be utilized by any realistic technology, because as much as 50% of energy produced in reactions between nucleons and anti-nucleons is carried away by neutrinos, so, for all intents and purposes, it can be considered lost.
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- The scarcity of antimatter means that it is not readily available to be used as fuel, although it could be used in antimatter catalyzed nuclear pulse propulsion. Generating a single antiproton is immensely difficult and requires particle accelerators and vast amounts of energy, millions of times more than is released after it is annihilated with ordinary matter, due to inefficiencies in the process.
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- Known methods of producing antimatter from energy also produce an equal amount of normal matter, so the theoretical limit is that half of the input energy is converted to antimatter. Counterbalancing this, when antimatter annihilates with ordinary matter, energy equal to twice the mass of the antimatter is liberated, so energy storage in the form of antimatter could (in theory) be 100% efficient.
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- Antimatter production is currently very limited, but has been growing at a nearly geometric rate since the discovery of the first antiproton in 1955. The current antimatter production rate is between 1 and 10 nanograms per year, and this is expected to increase dramatically with new facilities at CERN and Fermilab.
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- With current technology, it is considered possible to attain antimatter for US $25 million per gram by optimizing the collision and collection parameters (given current electricity generation costs).
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- Antimatter production costs, in mass production, are almost linearly tied in with electricity costs, so economical pure-antimatter thrust applications are unlikely to come online without the advent of such technologies as deuterium-tritium fusion power.
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- Since the energy density is vastly higher than these other forms, the thrust to weight equation used in antimatter rocketry and spacecraft would be very different. In fact, the energy in a few grams of antimatter is enough to transport an unmanned spacecraft to Mars in about a month.
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- In contrast the Mars Global Surveyor took eleven months to reach Mars. It is hoped that antimatter could be used as fuel for interplanetary travel or possibly interstellar travel, but it is also feared that if humanity ever gets the capabilities to do so, there could be the construction of antimatter weapons.
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- It is now thought that symmetry was broken in the early universe when charge and parity symmetry was violated (CP-violation). Standard Big Bang cosmology tells us that the universe initially contained equal amounts of matter and antimatter: however particles and antiparticles evolved slightly differently.
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- It was found that a particular heavy unstable particle, which is its own antiparticle, decays slightly more often to positrons (e+) than to electrons (e-). How this accounts for the preponderance of matter over antimatter has not been completely explained. The Standard Model of particle physics does have a way of accommodating a difference between the evolution of matter and antimatter, but it falls short of explaining the net excess of matter in the universe by about 10 orders of magnitude.
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- In 1932 Carl Anderson observed this new particle experimentally and it was named the "positron." This was the first known example of antimatter. In 1955 the antiproton was produced at the Berkeley Bevatron.
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- In 1995 scientists created the first anti-hydrogen atom at the CERN research facility in Europe by combining the anti-proton with a positron (the normal hydrogen atom consists of one proton and one electron). But when these antihydrogen atoms are produced, they are traveling at nearly the speed of light and don't last too long (40 nanoseconds is typical).
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- There is no real difference between particles and antiparticles in particle physics theories. They are equivalent. Most theoreticians believe that at the time of the Big Bang antiparticles and particles were created in almost equal numbers. But why, then, is antimatter so rare today?
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- Present theory suggests that if particles outnumbered antiparticles in the Big Bang by as little as one part in 100 million, then the present universe could be explained by those extra particles that were not annihilated by an antiparticle counterpart.
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- Other theories suggest that even if identical amounts of antimatter and matter were created in the Big Bang, the physics of antimatter and matter are slightly different. This hypothesized difference would favor residual matter after all original antimatter had been annihilated.
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- Astronomers have discovered evidence for antimatter near the center of our Milky Way galaxy by observing photons with an energy of 511 keV, which is the energy created when a positron and an electron collide and annihilate.
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- On Earth all antimatter that exists is counted in individual atoms. Low energy positrons are routinely used in a medical imaging technique called Positron Emission Tomography as well as studies of important materials used in electronics circuits. These positrons are the result of the natural decay of radioactive isotopes.
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- While useful in medical and materials research applications, there are not enough of these anti-electrons to provide a useful form of rocket fuel. High-energy antimatter particles are only produced in relatively large numbers at a few of the world's largest particle accelerators. The current worldwide production rate of antimatter is on the order of 1 to 10 nanograms (billionths of a gram!) per year.
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- May 22, 2019.
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