Abstract
A number of scholars of time, including Stephen Hawking, have cited three natural phenomena that exhibit time asymmetry and which most have chosen to call "arrows of time." These are 1) the cosmologic arrow based on the expansion of the universe, 2) the psychobiologic arrow based on the growth of complexity, organization, and information, and 3) the thermodynamic arrow based on the tendency in nature to progress to greater states of disorder as measured physically by entropy. Considering the last of these, the time asymmetry expressed by this arrow is in dirct contrast to the time symmetry or reversibility that in principle characterizes almost all of the interactions in the microscopic world described by physical theory, except for the decay of two particles known as the neutral K meson and the neutral B meson. This almost universal symmetry at the microscopic level leads to the fundamental question first addressed by Ludwig Boltzmann using statistical mechanics late in the 19th Century and in recent times most notably by Ilya Prigogine: how is it that almost all of the individual events in the microscopic realm are amenable to a time symmetric description, yet these events somehow aggregate to yield a macroscopic world characterized by time asymmetry? In this paper I point out that there are a number of interactions described by quantum electrodynamics, interactions that underlie and activate virtually all of earthly nature, which more cogently illustrate the nature of the thermodynamic arrow than focusing on the usual statistical arguments alone. A large proportion of these interactions involve the emission of photons as a result of excited atomic and molecular energy states decaying to their ground states. Once this radiation is emitted, it is an irreversible event in the sense that the chance that the radiation could return and re-excite the atom or molecule is extremely small. Furthermore, if, for example, there is a cascade of two or more photons in the decay of an excited state through intermediate energy states, the chance that the photons could return, and in the right temporal order, is even smaller. In addition there are electromagnetic interactions arising from atomic and molecular collisions that also result in the emission of electromagnetic radiation (in physics called bremsstrahlung radiation). This radiation is emitted whenever an electrically charged particle is accelerated. Since acceleration is the rate of change of velocity and velocity involves both speed and direction, any time a charged particle undergoes a change in direction, as is the case in a collision, such radiation is emitted. However, this radiation is not only emitted in the case of charged particles, but also when two electrically neutral molecules collide. This is because during the collision there is a mutual distortion of their electric charge distributions, which induces emission of radiation. The essential point is that effectively the radiation from both the emissions from excited atoms and molecules and from the collisions is ultimately lost in the medium, even though in principle, according to electromagnetic theory, any single one of these events is reversible. Thus, in accord with the usual statistical arguments, the aggregate effect of these electromagnetic phenomena at the quantum level occurring all around us is irreversible at the macroscopic level. I suggest that, if only for pedagogic reasons, this is a conceptually valid and useful way of understanding the nature of the thermodynamic arrow.This holds true in any relativistic frame of reference where electromagnetism plays a similar role in the physical phenomena of that frame. Finally, I must emphasize that the foregoing remarks are qualitative and I make no claim that they are philosophically rigorous or that new physics is involved.