Roger Penrose's Arrows Of Time (1) 1. Subjective - We can't remember the future 2. Radiation - Non converging electromagnetic waves 3. Quantum - Heads and tails don't merge 4. Neutral Kaon - Disintegration rate rules out reversed time 5. Black Hole - No white holes 6. Universe - Collapsing universe would not reverse time 7. Entropy - Total entropy does not decrease Other arrows of time 1. Biological aging - Stephen J. Gould 2. Cause precedes effect (finite transmission time) Review of various viewpoints on time Aristotle (384-322 BCE) Time is basic to world structure. Every change requires a cause. Everything that is in motion must be moved by something. (2:1) Archimedes (287?-212 BCE) Time flow is not an intrinsic feature of the ultimate basis of things. Author of first important treatise on statics laws of equilibrium (2:1) Saint Augustine (354--430) Remarks that time is at once familiar and deeply mysterious. "What is time?" he asks. "If nobody asks me, I know; but if I were desirous to explain it to one that should ask me, plainly I know not." Confessions, Book XI.14. Augustine, p. 239 (1912). Galileo Galilei (1564-1642) First to introduce time in dynamical considerations. Used his heart pulse as a time standard. (L. Rosenfeld, 3:188) Isaac Newton (1642-1727) His mechanics dealt with ideal planets with no frictional, electrodynamic, or tidal, braking effects. ...the time variable does not appear explicitly in the mathematical formulation of the fundamental laws of physics. Indirectly, it is also associated with the fact that the laws of classical mechanics are reversible and do not distinguish between past and future. (2:20,21) Newton assumed all forces acted at finite distances in an infinitely small time. This was indenspensible for the proof of his third law of equal action and reaction. (L. Brillouin, 4:107) Old classical mechanics assumed an absolute time, that corresponded to the idea that actions could be propagated at any distance instantaneously (meaning infinite velocity). (L. Brillouin, 4:108) Of course the LAW OF INERTIA, which was also formulated by Galileo and Newton, was thought by them to be an idealization, a limiting law which was introduced only in order to get a simple situation. Surely they thought that actual motions are irreversible, as are all observable motions. Newton was very much concerned about whether the orbits which he had calculated on the basis of the LAW OF INERTIA were adequate approximations to the actual planetary motions, which he thought were retarded by friction. (L. Rosenfeld) (3:188) "We must...believe that there exists an even flow of time." (Newton, as quoted by Gold) (3:188) But at the same time he unwittingly introduced a paradox, by the fact that the laws of dynamics which he formulated turned out to be reversible in time. That was an unintended accident in his analysis. But this did not worry people much because they still had the notion of causality, implying a succession in time, namely that the effect follows the cause. Yet even that possible basis for keeping an irreversibility of the direction in the physical laws was undermined by Newton himself, who introduced the force of gravity as an instantaneous action at a distance. Surely he was aware of the paradoxical character of this assumption, but he still insisted that it was the correct description of the actual law of gravitation. (L. Rosenfeld) (3:188-189) Joseph Lagrange (1736-1813) By regarding physical time as a fourth dimension of space, Lagrange all but eliminated time from dynamical theory. (2:3) Nicolas Carnot (1796-1832) Then in subsequent developments, when people began to analyze and formulate the laws of thermodynamics, the idealization of quasistatic phenomena was introduced by Carnot and his followers. This removed from causality the reference to time. Causality...became completely timeless. (L. Rosenfeld) (3:189) Rudolf Clausius (1822-1888) Introduced observable irreversibility as a part of the second law of thermodynamics. (L. Rosenfeld) (3:189) [In this time frame people apparently began believing that Newton's approximations" of nature were "Laws" of nature and that nature was thus reversible. [rsf] Physics was inconsistent: Newton's laws of motion were symmetrical, and the entropy law was asymmetrical to the direction of time. This difficulty was solved thanks to the illuminating work of many talented scientists,... (I. Szumilewicz) (4:182) Ludwig Boltzmann (1844-1906) But the problem did not become acute until the statistical approach to the thermodynamic laws was begun by Boltzmann, because previously no one was faced with the comparison between the reversible character of the elementary law and the irreversibility which had to be explained on its basis. Boltzmann, of course, actually introduced a new law in order to produce the irreversibility, namely a statistical element governed by a parabolic equation. It is only when the statistical element is combined with the mechanical laws that we get the irreversibilty of the second law. (L. Rosenfeld) (3:189) Boltzmann believed that the two directions of time are indistinguishable. (2:6) It is quite obvious that the Boltzmann equation, far from being a consequence of the laws of classical mechanics, is inconsistent with them. (Bergmann) (3:191) Albert Einstein (1879-1955) What is new in Einstein's relativity is the complete separation between past and future. ... Einstein assumed time-symmetry in his theories, but this assumption was superimposed and not needed. It simply does not play any role, because of the complete distinction between past and future. (L. Brillouin) (4:108) The Ritz and Einstein Agreement to Disagree (1909) The electromagnetic arrow was the subject of a lively discussion by Einstein and Ritz in 1909. Einstein (1909) argued that the retarded and advanced descriptions of radiation processes occurring in any finite region are equivalent, since the equations of wave propagation are symmetric with regard to time, but that the auxiliary conditions giving the precise circumstances of emission and absorption are very different. In the retarded description it is sufficient if all the macroscopic sources are known whereas in the advanced description all the absorption processes must be known, but unlike the former they must be fully specified in microscopic detail. In practice, we do not have this information concerning the absorption processes and so we are obliged to use the retarded description. On the other hand, Ritz (1909) asserted that only the retarded waves have any physical significance, since advanced waves are not experimentally observed. The initial conditions characterizing the source (or sources) of the radiation are the causes of its transmission and consequently are responsible for the special role played by retarded waves. [According to Brillouin (1964), Ritz was the first to make this point.] (L. Brillouin) (2:339-340) See The Ritz-Einstein Agreement to Disagree on this web site. Also see: [PDF] Chapter 2 - The Time Arrow of Radiation - of H. D. Zeh's book, The Physical Basis of the Direction of Time, Prelim. 5th edn. (Mar-05): www.time-direction.de Lewis (1930) [Winford Lewis (1878-1943)?] Claimed that nearly everywhere in physics and chemistry the ideas of unidirectional time and unidirectional causality have been purged. These ideas have been used to support some false doctrine, for example, that the universe is actually running down. Predecessor of absorber theory.(2:7-8) No satisfactory quantum electrodynamics could be developed until the retarded and advanced potentials were used simultaneously and symmetrically. (2:8-9) Wheeler & Feynman J. A. Wheeler and R.P. Feynman attempted to derive the ordinary irreversibility of radiation from the time-reversibility of Maxwell's equations. They argued that the observed properties of an electric charge, that it radiates energy and suffers damping of its motion can be explained in terms of an "absorber theory of radiation". They used the Schwarzschild- Tetrode-Fokker equation for a flat space and found that they had to postulate both "advanced" (future) and "retarded" (past) fields; the divisions of time seemed to be "inextricably mixed". ...Wheeler and Feynman assumed that (a) time-asymmetry is initially present; and (b) persists on a purely statistical basis. J.E. Hogarth(5) showed that assumption (b) is inconsistent with any realistic absorber theory of radiation. He and D.W. Sciama argued that the Wheeler-Feynman theory of radiation holds in the Steady-State cosmologies of Hoyle and Narlikar, but not in general, in the Einstein-de Sitter models. (C.T.K. Chari) (4:216) The dependence of the electromagnetic arrow of time on the thermodynamic arrow was a feature of the absorber theory formulated by Wheeler and Feynman (1945). In an attempt to produce a theory of charged elementary particles which avoided the difficulties that had beset previous theories of their interaction with electromagnetic fields, they introduced the hypothesis that every photon has an absorber as well as an emitter. In their theory an accelerated charged particle emits radiation equally into the past and future. In other words, retarded and advanced waves are generated symmetrically. If the radiation is confined to an opaque enclosure, so that all of it is absorbed, the waves striking the walls will cause the charged particles therein to radiate likewise into both the past and the future. Wheeler and Feynman showed that if the enclosure is fully opaque, the advanced waves emitted by the walls will just cancel those from the source particle and only the retarded waves will be left. ... Moreover, since all attempts to produce a quantum-mechanical version of the absorber theory lead to the same difficulties as previous theories of the interactions of charged particles with the electromagnetic field, there is no strong argument in its favour and in fact its original proponents have abandoned it. (2:341-342) Rabbi Heschel wrote that the Sabbath is like a temple in time. Levinson elaborates the idea. "The Temple is to space as the Sabbath is to time."(7) References (1) Tony Rothman, Discover, Feb (1987). (2) G. J. Whitrow, The Natural Philosophy of time, 2nd. ed. Clarendon Press, Oxford, (1984). (3) Gold, T., The Nature of time, Cornell Univ. Press, (1967). (4) J. Zeman, Time in Science and Philosophy, (1971). (5) J.E. Hogarth, "Cosmological considerations of the absorber theory of radiation", Proc. Roy. Soc. London, A, Vol. 267, 365-383, (1962). (6) Encyclopedia Americana, Vol. 18, 502, (1960). (7) A.J. Hershel, The Sabbath, p. 1; Levinson, The Temple and the World, p. 298. (From James Carroll's Constantine's Sword: The Church and the Jews, Houghton Mifflin Company, Boston - New York, 2001.) Recommended reading/viewing The Arrow of Electromagnetic Time and Generalized Absorber Theory - John.G. Cramer.     Reprint of an article published in Foundations of Physics 13, 887 (1983). Time's Arrow: Particles cannot go back to the future - CERN Bulletin 47.98; 16 November 1998 [Link updated 26 Mar 2010 and 09 Jul 2013.] A Walk through Time The Evolution of Time Measurement through the Ages - U.S. National Institute of Standards and Technology (NIST)