Richard Feynman on Time The Puzzle of Antimatter: Positron moving backwards in time The most fundamental laws of nature are time reversal invariant in the sense that our physical theories allow description of the fundamental reactions and processes in terms of the time reversed order. Undoubtedly one of the most influential physicists of the 20th century, Richard Feynman introduced the idea that we treat antimatter as matter travelling backwards in time. No doubt the idea of motion backward in time makes a grievous assault on common sense. The world just does not seem to operate that way, as our ever-aging bodies testify. However, to a particle physicist raised on a diet of Feynman diagrams, motion backward in time is not all that disturbing. All fundamental particle interactions work backward as well as forward and, with rare exceptions, do not distinguish between directions of time [The rare exceptions occur in so-called CP violating interactions involving short-lived particles called K0 and B mesons, which are thought to have different probabilities in time-reversed directions. These can be ignored in the current discussion since they play only a very indirect role in the structure of normal matter]. Feynman used the idea of motion backward in time when he invented his famous diagrams in the late 1940s. Dirac had developed his fully-relativistic quantum theory of the electron in 1928, and discovered that it contained negative energy solutions. These solutions were identified as anti-electrons or positrons. Positrons were observed as predicted in 1932. Following Stückelberg [Stückelberg, E. C. G. 1942. La méchanique du point matériel en théorie de la relativité. Helv. Phys. Acta 15, pp. 23-37.] and Wheeler, Feynman re-interpreted positrons as electrons moving backward in time [Feynman, R. P. 1948. Spacetime Approach to Non-relativistic Quantum Mechanics. Rev. Mod. Phys. 20, pp. 367-387. Feynman, R. P. 1949a. The Theory of Positrons.Phys. Rev. 76, pp. 749-759. Feynman, R. P. 1949b. Spacetime Approach to Quantum Electrodynamics. Phys. Rev. 76, pp. 769-789. Feynman, Richard P. 1965b. The development of the space-time view of quantum electrodynamics. Nobel Lectures Physics 1963-1970. New York: Elsevier, 1992].. Feynmans idea grew out of his earlier work at Princeton as a graduate student of John Wheeler. Together they had developed a theory of electromagnetic waves involving solutions of Maxwells equations that travel both ways in time, the so called retarded and advanced waves. The advanced waves travelled backward in time, that is, they arrived at the detector before they left their source. Despite their presence as valid solutions to Maxwells equations, advanced waves had been previously ignored by less bold thinkers Feynman noted that whether you say you have a particle moving forward in time with negative energy, or its antiparticle moving backward in time with positive energy, is really quite arbitrary at the fundamental level. Energy conservation and the other laws of physics remain intact. By reversing the charges and momenta of the backward particles, charge and momentum conservation are unaffected. By a logical extension of Feynmans consistent paths, the same rules can be applied to calculating what can or cannot be done in time travel. Nick Herbert ( American physicist and author, best known for his book Quantum Reality) wrote in a personal communication: Killing your grandmother before your mother was born would create an inconsistent history so is forbidden. What you would experience if you tried to kill your grandmother would be strange quantum forces that magically prevent this action from happening no matter how hard you tried. These quantum forces would be similar to the Pauli Exclusion Principle for the electrons that hold matter together. The Temporal Consistency Principle is a kind of Pauli Principle acting through time rather than space. If we were able to ask Feynman what he thought about time travel, he would not necessarily be a strong advocate. Despite his colourful personal style, he was a very careful physicist and although his idea sometimes sounded wild, they were very carefully reasoned. The idea of antimatter being ordinary matter travelling backwards through time does not in itself provide a way to build a time machine, even hypothetically. He would have wanted to see the time machine idea worked out in detail before he was convinced. Feynman, and earlier Stueckelberg, proposed an interpretation of the positron as an electron moving backward in time, reinterpreting the negative-energy solutions of the Dirac equation. Electrons moving backward in time would have a positive electric charge. Wheeler invoked this concept to explain the identical properties shared by all electrons, suggesting that they are all the same electron with a complex, self-intersecting worldline. Yoichiro Nambu later applied it to all production and annihilation of particle-antiparticle pairs, stating that the eventual creation and annihilation of pairs that may occur now and then is no creation or annihilation, but only a change of direction of moving particles, from past to future, or from future to past. The backwards in time point of view is nowadays accepted as completely equivalent to other pictures, but it doesnt have anything to do with the macroscopic terms cause and effect, which do not appear in a microscopic physical description. The backwards-moving electron when viewed with time moving forwards appears the same as an ordinary electron, except that it is attracted to normal electrons - we say it has a positive charge. For this reason its called a positron. This phenomena is general. Every particle in Nature has an amplitude to move backwards in time, and therefore has an anti-particle. (Richard Feynman, 1985) Clocks going Backward? What did this really mean? On his diagram it meant that if you made an electron go back in time by drawing it downward on his diagram, then it would have the same status and reactions as a positron moving upward on the diagram. Did this mean that electrons could really go back in time? Dont Worry! In fact he imagined the possibility that perhaps there were only one electron in the world zig-zaging back and forth in time. An electron moving backwards in time would carry negative energy whereas it would with respect to our ordinary time sense have positive charge and positive energy. (Earman, 1967, 1976). Using the wave structure of an electron, it can be seen that you need never worry about time travel. We can see this by reversing the time variable in the equations of the positron and seeing what happens. To change the time in the equations you need to understand the equations. So, more has to be said about the equations of the wave structure of the electron and positron. Both consist of an inward wave and an outward wave. The waves are written: Electron = Outward wave + Inward wave [with + rotation at center] = Amplitude{expo(iwt - kr) + expo(-iwt + kr)} Positron = Outward wave + Inward wave [with - rotation at center] = Amplitude{expo(iwt - kr) + expo(-iwt + kr)} The symbols mean: t=time, r=radius, w = frequency, k= wavelength constant, expo = exponential function This is simply a mathematical truth caused by the fact that if you substitute a negative time into the spherical wave equations, then this changes the phase of the standing waves to be equal and opposite, which corresponds to antimatter. Further, notice what Richard Feynman says about photons, which are treated as particles in QED, and thus by Richard Feynmans logic there should also be anti-photons, whereas the WSM (Wave Structure of Matter ) is clear on this point - there are anti-electrons (positrons) which are opposite phase Spherical Standing Waves, but there are no separate photon particles, thus no anti-photons! Notice what happens if you put a minus (-) sign in front of the time in the equations of an electron. This is equivalent, mathematically speaking, to making the electron move backward in time (Feynman was a amazing mathematician). But notice what it does physically (Some algebra is needed here. ). You have merely changed the electron equation into the positron equation. Nothing has happened to the laboratory clock! (Explanations Milo Wolff (physicist, Astronomer MIT, Aerospace Corporation) And what about photons? Photons look exactly the same in all respects when they travel backwards in time, so they are their own anti-particles. You see how clever we are at making an exception part of the rule! (Richard Feynman, 1985) While it may be clever, it is hard philosophy, and it has led to a very confused modern physics, and has led to the self fulfilling belief that we can never correctly describe and understand Reality, as Feynman and Heisenberg argued; The more you see how strangely Nature behaves, the harder it is to make a model that explains how even the simplest phenomena actually work. So theoretical physics has given up on that. (Richard Feynman, 1985) Light and matter are both single entities, and the apparent duality arises in the limitations of our language. (Werner Heisenberg, 1930) In fact Nature (Reality) must be logical to explain how we have evolved a logical aspect to our minds that allows us to create mathematical physics which correctly describes so many phenomena. Source: Feynman, P.R. (1949), “The Theory of Positrons”, Physical Review, 76: 749–459. Wheeler, J.A., and Feynman, R.P. (1945), “Interaction with the Absorber as the Mechanism of Radiation,” Reviews of Modern Physics, 17: 157–181. Quantum Theory Looks at Time Travel Daniel M. Greenberger (Department of Physics,City College of the City University of New York , NY 10031, USA) Karl Svozil (Institute of Theoretical Physics, Vienna University of Technology, Wiedner Hauptstraße 8-10/136, A-1040 Vienna, Austria) arxiv.org/pdf/quant-ph/0506027.pdf quantummatter/ Nonlocality With Time Reversibility Excerpts from The Unconscious Quantum: Metaphysics in Modern Physics and Cosmology. Victor J. Stenger 1995. Buffalo NY: Prometheus Books, pp. 145-155.colorado.edu/philosophy/vstenger/Quantum/localepr.html Image 1: Richard Fenman - a timeless. Credit: Caltec Image 2: A Feynman diagram showing the radiation of a gluon when an electron and positron are annihilated. Time runs left to right in this Feynman diagram of electron-positron annihilation. When interpreted to include retrocausality, the electron (marked e-) was not destroyed, instead becoming the positron (e+) and moving backward in time.
Posted on: Sun, 14 Sep 2014 17:09:47 +0000
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