Antimatter
76 quotes
Biography
In modern physics, antimatter is defined as matter composed of the antiparticles of the corresponding particles in "ordinary" matter, and can be thought of as matter with reversed charges and parity, or going backward in time. Antimatter occurs in natural processes like cosmic ray collisions and some types of radioactive decay.
"The interpretation of these tracks as due to protons, or other heavier nuclei, is ruled out on the basis of range and curvature. Protons or heavier nuclei of the observed curvatures could not have ranges as great as those observed. The specific-ionization is close to that for an electron of the same curvature, hence indicating a positively-charged particle comparable in mass and magnitude of charge with an electron."
"The annihilation of positrons with electrons from biological tissues constitutes the basis of Positron Emission Tomography (PET)... widely used in ... [S]ubstances called radiotracers and radiopharmaceuticals are injected into the patient. These are chemical compounds in which one or more atoms have been replaced by short-lived, positron-emitting, radioisotope of elements that are abundant in the body, like Carbon-11... ... Oxygen-15... and Fluor-18... the latter... for the localization and monitoring of tumors... Since these isotopes are short-lived... they must be produced just before being injected... To do this, the corresponding [common] elements are bombarded with protons... from a small accelerator. ...[I]nside the PET scanner ...a series of detector rings ...record the gamma radiation emitted when the positrons are annihilated inside the body. ...[T]he recorded signals are used to make a series of slices that combine to for a 3-D image. ...[T]hey allow doctors to assess the condition of organs and tissues as they can monitor blood flow and many bodily and metabolic processes, including neuronal transmission."
"If the Standard Model describes the world successfully, how can there be physics beyond it, such as supersymmetry? There are two reasons. First, the Standard Model does not explain aspects of the study of the large-scale universe, cosmology. For example, the Standard Model cannot explain why the universe is made of matter and not antimatter, nor can it explain what constitutes the of the universe. Supersymmetry suggests explanations for both of these mysteries. Second, the boundaries of physics have been changing. Now scientists ask not only how the world works (which the Standard Model answers) but why it works that way (which the Standard Model cannot answer). Einstein asked "why" earlier in the twentieth century, but only in the past decade or so have the "why" questions become normal scientific research in particle physics rather than philosophical afterthoughts."
"In later years, the advent of a new elementary particle would scarcely ruffle the intellectual sensibilities of the world's physicists; in 1932, Anderson's announcement of the ran into a wall of resistance. If the had resolved many long-standing difficulties of nuclear theory, the positron seemed to complicate matters. It is said that Neils Bohr dismissed Anderson's finding out of hand, and when in the fall of 1932 Millikan discussed the positron in a lecture at the Cavendish, various members of the audience suggested that Anderson had doubtless become tangled in some fundamental interpretive error. But not all of Rutherford's physicists were prepared to ignore Anderson's claims, especially not the resident Cavendish expert on s, Patrick M. S. Blackett."
"The Doctor: Here on Zeta Minor is the boundary between existence as you know it and the other universe which you just don't understand. From the beginning of time it has existed side by side with the known universe. Each is the antithesis of the other. You call it "nothing", a word to cover ignorance. And centuries ago scientists invented another word for it. "Antimatter", they called it. And you, by coming here, have crossed the boundary into that other universe to plunder it. Dangerous."
"The new quantum mechanics, when applied to the problem of the structure of the atom with point-charge electrons, does not give results in agreement with experiment. The discrepancies consist of "duplexity" phenomena, the observed number of stationary states for an electron in an atom being twice the number given by the theory. ...It appears that the simplest Hamiltonian for a point-charge electron satisfying the requirements of both relativity and the general transformation theory leads to an explanation of all duplexity phenomena without further assumption.<!--p. 610-->"
"The wave equation... refers equally well to an electron with charge e as to one with charge -e. If one considers for definiteness the limiting case of large quantum numbers one would find that some of the solutions of the wave equation are wave packets moving in the way a particle of charge -e would move on the classical theory, while others are wave packets moving in the way a particle of charge e would move classically. ...the electron suddenly changing its charge from -e to e ...has not been observed. The true relativity wave equation should thus be such that its solutions split up into two non-combining sets, referring respectively to the charge -e and the charge e. ...The resulting theory is therefore still only an approximation, but it appears to be good enough to account for all the duplexity phenomena without arbitrary assumptions.<!--p. 612-->"
"On August 2, 1932, during the course of photographing cosmic-ray tracks produced in a vertical Wilson chamber (magnetic field of 15,000 gauss) designed in the summer of 1930 by Professor R. A. Millikan and the writer, the tracks... seemed... interpretable only on the basis of the existence in this case of a particle carrying a positive charge but having a mass of the same order of magnitude as that normally possessed by a free negative electron."
"In the course of the next few weeks other photographs were obtained which could be interpreted logically only on the positive-electron basis, and a brief report was then published with due reserve in interpretation in view of the importance and striking nature of the announcement."
"[O]ur equations allow of two kinds of motion for an electron, only one of which corresponds to what we are familiar with. The other corresponds to electrons with a very peculiar motion such that the faster they move, the less energy they have, and one must put energy into them to bring them to rest."
"[W]e find from the theory that if we disturb the electron, we may cause a transition from a positive-energy state of motion to a negative-energy one, so that, even if.. all.. electrons in the world.. started.. in positive-energy states, after a time some... would be in negative-energy states. ...[B]ehaviour of these states in an electromagnetic field shows that they correspond to the motion of an electron with a positive charge ...a . One might... assume that electrons in negative-energy states are just positrons, but ...observed positrons ...do not have negative energies."
"We make use of the exclusion principle of Pauli... there can be only one electron in any state of motion. We... make the assumptions that in the world as we know it, nearly all the states of negative energy for the electrons are occupied... any unoccupied negative-energy state, being a departure from uniformity, is observable and is just a ."
"An unoccupied negative-energy state, or hole... will have a positive energy, since it is a place where there is a shortage of negative energy. A hole is... just like an ordinary particle, and its identification with the ... the most reasonable way of getting over the difficulty of... negative energies..."
"On this view the positron is just a mirror-image of the electron, having exactly the same mass and opposite charge. This has already been roughly confirmed by experiment. The positron should also have similar spin properties to the electron, but this has not yet been confirmed..."
"[W]e should expect an ordinary electron, with positive energy, to be able to drop into... and fill up this hole, the energy being liberated in the form of . This would mean... an electron and a positron annihilate one another. The converse... creation of an electron and a positron from electromagnetic radiation, should also be able to take place. Such... appear to have been found experimentally, and are... being more closely investigated..."
"[I]t is probable that negative protons can exist, since as far as the theory is yet definite, there is a complete and perfect symmetry between positive and negative electric charge, and if this symmetry is really fundamental in nature, it must be possible to reverse the charge on any kind of particle. ...[N]egative protons would... be much harder to produce... since a much larger energy would be required, corresponding to... larger mass."
"We must regard it rather as an accident that the Earth (and presumably the whole solar system), contains a preponderance of negative electrons and positive positrons. It is quite possible that for some of the stars it is the other way about... built up mainly of s and negative protons. ...[T]here may be half the stars of each kind. The two... would both show exactly the same spectra... there would be no way of distinguishing them..."
"It seems probable that the interactions between elementary particles can be completely described by symmetry properties and s and by dimensionless numbers representing interaction strengths. Similarly, we might expect that the elementary particles, as quanta of these interactions, may be described in the same in terms... At the present... however, our description... must also include the , and in some cases, the magnetic moment, although in principle these are probably derivable from interaction strengths and symmetries. ...Symmetries usually result in conservation laws. ...Invariance under space inversion results in ...the conservation of parity. Let us also consider invariance under time reversal, and invariance under charge conjugation, the change of particles to antiparticles.<!--pp. 9-10-->"
"Invariance of interactions with respect to space inversion restricts observables to those which do not differentiate between a left-handed and a right-handed coordinate system. Time reversal invariance allows only observables which do not depend on the direction of time, and invariance under charge conjugation restricts observables to those which remain unchanged when all particles are changed to antiparticles.<!--p. 10-->"
"Consider a motion picture of a fundamental process, perhaps an elementary particle interaction in the presence of electric and s... If the interactions are invariant under space inversion, it will not be possible... to determine if the film has been reversed in the projector or [equivalently] projected by reflection in a mirror. If... invariant under time reversal, and if entropy is not changed in the process, it will not be possible to tell if the film is run backwards, while if... invariant under charge conjugation, it will not be possible to state whether the picture is that of our universe, or an anti-universe where every particle is replaced by its antiparticle.<!--p. 10-->"
"It appears that the strong interactions and electromagnetic interactions are invariant with respect to C, P, and T separately, while the weak interactions do not conserve P or C. All experimental results are consistent with the assumption the T invariance holds true for all interactions; consequently, from the CPT theorem, weak interactions must be invariant under CP. One could not, then, determine if the photographed scene were a scene of particles viewed normally, or a scene of antiparticles projected in a mirror.<!--p. 11-->"
"The theory of the expanding universe, which presupposes a superdense initial state of matter, apparently excludes the possibility of macroscopic separation of matter from antimatter; it must therefore be assumed that there are no antimatter bodies in nature, i.e., the universe is asymmetrical with respect to the number of particles and antiparticles (С asymmetry). In particular, the absence of antibaryons and the proposed absence of baryonic neutrinos implies a nonzero baryon charge (baryonic asymmetry)."
"We wish to point out a possible explanation of С asymmetry in the hot model of the expanding universe... by making use of effects of CP invariance violation... To explain , we propose in addition an approximate character for the baryon conservation law."
"We can visualize that neutral spinless maximons (or photons) are produced at t < 0 from contracting matter having an excess of antiquarks, that they pass "one through the other" at the instant t = 0 when the density is infinite, and decay with an excess of quarks when t > 0, realizing total of the universe. All the phenomena at t < 0 are assumed in this hypothesis to be CPT reflections of the phenomena at t > 0."
"After Dirac's publication of the electron wave equation in 1928, many people took up its study.<!--p. 52-->"