Reverse Time by Sarah E. Pate
Posted on Friday 17 December 2004 to unknown
OK, so I'm up to page 68 of Leonardo's Notebooks, only another 1497 to go.It is impossible that the eye should project from itself, by visual rays, the visual virtue, since, as soon as it opens, that front portion [of the eye] which would give rise to this emanation would have to go forth to the object and this it could not do without time.
And this being so, it could not travel so high as the sun in a month's time when the eye wanted to see it. And if it could reach the sun it would necessarily follow that it should perpetually remain in a continuous line from the eye to the sun and should always diverge in such a way as to form between the sun and the eye the base and the apex of a pyramid. This being the case, if the eye consisted of a million worlds, it would not prevent its being consumed in the projection of its virtue; and if this virtue would have to travel through the air as perfumes do, the winds would bent it and carry it into another place. But we do [in fact] see the mass of the sun with the same rapidity as [an object] at the distance of a braccio, and the power of sight is not disturbed by the blowing of the winds nor by any other accident.Here Leonardo appears to be arguing against the notion that light travels at a finite speed. Furthermore, he proposes that it only travels in straight lines and only one direction, i.e. from the source of light to the receiver (the eye).
Like Dirac's equation, Maxwell's wave equation for light also has two solutions, the so-called "retarded solution" that describes a wave traveling forward in time and the "advanced solution" that describes a light wave traveling backward in time. Both of these waves travel at the same speed-the speed of light in vacuum-- but in opposite temporal directions. The retarded wave travels in the normal direction -- from past to future -- while the advanced wave goes the other way -- from the future into the past...Here is Richard Feynman's own account of this intriguing theory (from ?Surely You?re Joking Mr. Feynman? scroll down to "Monster Minds"), which he thought after some reflection must be in flawed in some way.
Ordinary light waves are called "retarded" because you always receive them after they are sent; advanced waves, on the other hand, are always received before they are sent... Although advanced waves are permitted by Maxwell's equations, no advanced waves have ever shown up in any experiment. All light waves that we know about seem to be of the retarded variety. For more than a hundred years physicists have wondered about how to interpret the mysterious advanced-wave solution to Maxwell's equations. Because of the lack of experimental evidence, most are content to ignore the advanced solution, dismissing it as an option that nature simply chose not to exercise...
In 1945, John Wheeler and his graduate student Richard Feynman, then at Princeton University, proposed a novel way of looking at light that gives the backward-in-time solutions of Maxwell's equations equal status with the forward-in-time solutions. John Wheeler, author of a seven-pound paperback book on gravity, is presently director of a fundamental physics research center in Austin, Texas. Richard Feynman, who received the Nobel prize for developing an improved quantum theory of electromagnetic radiation, is remembered in some circles as "the Groucho Marx of physics" for his humorous and irreverent behavior.
The Wheeler-Feynman model, called the "absorber theory of radiation," makes electromagnetism a two-way street as far as the time dimension is concerned. They base their time-symmetric theory on the assumption that every light wave emitted by an atom must be absorbed by another atom and that these two events, light emission plus light absorption, should be considered as a single inseparable process.
In conventional radiation theory, an atom emits a wave of light without regard to the light's eventual absorption. As the light wave leaves its present atom, traveling in a particular direction, the atom recoils in the opposite direction... [the] theory explains this atomic recoil as the reaction of the emitted wave back on the atom. As you jump from a boat onto the dock, the boat will recoil for a similar reason, responding to your backward push... In conventional radiation theory, light from atom A travels at a speed of 186,000 mps to atom B which passively absorbs the light. Atom A recoils because the emitted light pushes back on it (self-interaction) as the light escapes from its parent atom. Atom B recoils because it absorbs the incoming light wave's momentum, like a baseball player being pushed back as he catches a fast ball.
According to the Wheeler-Feynman theory, the situation is more complicated-- radiation occurs in two steps.
First atom A emits, without recoiling (no self-interaction), a half-sized (retarded) wave that travels forward in time at a speed of 186,000 mps to the absorber atom B. Atom B recoils as it takes up this light's momentum. Then, stimulated by its recoil motion, atom B emits a half-sized (advanced) wave that travels backward in time at a speed of 186,000 mps to atom A. Atom A recoils as it takes up this advanced wave's momentum.
The timing of the emission and absorption events guarantees that at any moment the half-sized advanced wave sent back in time from the absorber always finds itself at the same position in space as the half-sized retarded wave sent forward in time from the emitter. Thus the two waves fuse together to form a single full-sized wave which appears to have been sent from the emitter and received by the absorber.
Because of this exact superposition of advanced and retarded waves, the Wheeler-Feynman model produces the same apparent wave motion as conventional radiation theory. The two theories give the same result but propose radically different models of what is actually happening. Conventional radiation theory is a simple matter of cause and effect. Wheeler-Feynman theory involves a handshaking procedure much like data exchanged between two computers in which a data transfer initiated by one computer is not completed until the exchange has been acknowledged by a message sent back from the second computer.
...in order for light to be emitted it must be connected to some future absorber by a two-way retarded-advanced wave handshake process. Because of the need for the presence of absorbers, the absorber theory predicts that if there are none in a particular direction in space, then light will refuse to shine in that particular direction!
If absorber theory is correct, your flashlight would go out whenever you shine it up into the night sky in the direction of an "antiemission locus" a region of space entirely devoid of absorbers of light in the frequency range of your flashlight. In certain other directions, containing only a few absorber atoms, your flashlight would dim, but not go out. Only in those directions in which light was totally absorbed would the flashlight be able to shine at its full brightness.
When we stand in the dark and look at a star a hundred light years away, not only have the retarded light waves from the star been traveling for a hundred years to reach our eyes, but the advanced waves generated by absorption processes within our eyes have reached a hundred years into the past, completing the transaction that permitted the star to shine in our direction.This means that the star must have "known", in a sense, that you were going to be standing there before it bothered to send that photon on its way all those hundreds (or even billions) of years ago. In other words our notion of the passage of time is an illusion, that past and future exist simultaneously as coordinates on the fourth axis of Space-Time.