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Light in Reverse Gear II

by John G. Cramer

Alternate View Column AV-08
Published in the August-1985 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 2/1/85 and is copyrighted © 1985, John G. Cramer. All rights reserved.
No part may be reproduced in any form without the explicit permission of the author.


    The "four-wave mixer", a laser technique for reversing the motion direction of light waves so that they can be turned around and returned to their point of origin was the subject of my last Alternate View column (ANALOG, June-1985). In this AV column I want to go one step further by examining a hypothetical kind of time-reversed light wave which should actually go backward in time. As we shall see, such backward waves could be used to send information from the present to the past, resulting in a sort of "poor man's" time travel. Recent science fiction novels like Jim Hogan's Thrice in Time and Greg Benford's Timescape have been based on backwards-in-time communication and the situations that might arise from it. In both books a world disaster is averted by sending a message to the past. In Timescape a side-effect of the signaling prevents Lee Harvey Oswald from assassinating President Kennedy, irreversibly altering the course of events after 1963.

    How could there be waves that move backward in time? In T. H. White's wonderful fantasy novel The Once and Future King the wizard Merlin changes Wart, the young King Arthur, into an ant and sends him exploring in an ant hill. Over the entrance of the ant hill Wart comes upon a large sign which reads (in ant language) "EVERYTHING NOT FORBIDDEN IS COMPULSORY!". That, as it turns out, is an excellent description of the way things work in ant hills.

    Some of the most prominent practitioners of particle physics have been impelled by a long series of successful predictions and discoveries to adopt a similarly ant-like philosophy concerning new phenomena: "If a trusted theory describes some new phenomenon of nature and there is no sound physical reason why the phenomenon is forbidden to exist, then its existence is compulsory". The time-reversed light seems to be "compulsory" in the sense of this dictum. Yet it has never been observed.

    These backward light waves are called "advanced" waves as distinguished from normal "retarded" light waves. The advanced/retarded distinction hinges on whether the light arrives at some distant place before or after it was sent. The existence of both kinds of waves is implied by Maxwell's Equations, the four amazing little equations which contain descriptions of all electric and magnetic phenomena. Maxwell's Equations can be rearranged into a "wave equation" which paints a mathematical picture of the way that light works. The wave equation can be neatly solved, and one such solution (the retarded solution) describes the behavior and properties of ordinary light: it travels through space with a speed of 299.79 million meters per second, energy must be supplied to make it (so we say that it carries positive energy), and it requires a positive time to travel a given distance. For example, when ordinary "retarded" light travels one light-year it arrives one year later than it started.

    However, there is also a second solution of the wave equation, one which is usually discarded by practicing physicists. It is the advanced solution. It describes a kind of light which also travels at 299.79 million meters per second but which carries negative energy and travels a given distance in a negative time. In producing such waves we would gain energy instead of losing it. An advanced-wave flashlight would charge its battery as it produced the beam. An opaque object absorbing the advanced-wave light would lose energy and probably become colder. And advanced-wave light would arrive at a point one light-year away a year before it started.

    Suppose that we could make a transmitter and antenna arrangement which broadcast advanced radio waves. Since these waves carry negative energy, the production of the advanced waves would cause the apparatus to gain energy. We could connect our transmitter to the electric power grid and collect money from light company for the electric energy we were adding to the grid. If we pointed the antenna at the Sun, we could literally "suck energy" from it, since the antenna would produce energy and the negative energy waves striking the Sun would cool it slightly, reducing its net energy by the same amount. Or we could point the antenna at empty space and not worry about how the accumulating energy debt might be paid.

    When advanced-wave light travels from point A to point B it arrives at point B earlier than the time it left point A. Shortly after World War II, when radar was still new, a pulsed radar beam was first bounced off the Moon and reflected back to Earth. Measuring the round trip time of the radar pulse (about 2.5 seconds) became a very precise way of determining the Earth-Moon distance. If the same measurement were done with advanced radar waves the reflection from the Moon's surface would arrive back at the Earth 2.5 seconds before the pulse was transmitted.

    From there, it isn't much of a trick to lengthen the interval with automatic repeaters which bounce the advanced waves many times, lengthening the look-ahead time from seconds to minutes or hours or even days. A computer could be hooked up to broadcast ASCII-coded advance-wave messages to the past and to receive and decode them when received. Such messages could be used in any number of schemes for fun, profit, or military preparedness. The reader who is interested in possible applications is referred to Isaac Asimov's pseudo-science-fact articles in the Astounding SF's of the 1950's concerning "thiothimoline", a kind of soluble organic crystal with the unique property that it dissolved slightly before water was added.

    Unfortunately for time-reversed signaling, no one has ever been able to produce any advanced-wave light. It is a seemingly valid solution of Maxwell's Equations, but for some reason Nature always gives us retarded light instead. How do we know that for sure? One reason is that if negative-energy light could easily be made we should see atoms spontaneously gaining energy as they kick out advanced photons. Within convincingly low limits this never happens. Sensitive experiments have also been designed using radio telescope techniques to look directly for the effects of advanced radio waves, with negative results which set very low upper limits on their possible production. There is also experimental evidence that negative-energy advanced neutrinos and advanced gravity waves are similarly absent from our universe. According to the "EVERYTHING NOT FORBIDDEN IS COMPULSORY" dictum, if there is no physical reason why advanced waves must be absent then they should be present.

    Contemporary physics does not seem to be able to tell us why there are no advanced waves. The best that can be done at the moment is to explain their absence by invoking the Principle of Causality. Briefly stated, causality is the principle that a cause must precede any and all of its effects in time sequence. Causality has the status of a natural law because no violation of causality has ever been observed in a physical measurement. And signalling with advanced waves could easily be used to produce a violation of the causality principle. But this argument is essentially circular: We don't see advanced waves because they would violate causality, and causality is OK because we can't make advanced waves. From this it should be clear that causality is not the reason that advanced waves are absent in the universe; it is only an alternative way of saying that they are absent.

    This is not to say that there are not theories which seek to explain the absence of advanced waves. Probably the most famous of these was originated by Nobel laureate Richard P. Feynman and his PhD thesis supervisor John A. Wheeler and is called Wheeler-Feynman (W-F) absorber theory. Briefly, it asserts that advanced waves are produced on an equal basis with retarded waves, but when the retarded wave is absorbed (sometime in the future) a cancellation process takes place which erases all traces of the advanced waves and their "advanced" effects. According to the W-F theory it is always the retarded wave (not the advanced one) which is absorbed because there is always more absorption in the future than in the past due to the 2nd law of thermodynamics (see my AV column of 6/85 for a discussion of the 2nd Law). However, it has been demonstrated that this connection doesn't work in an expanding universe.

    Other theories have attempted to tie the dominance of retarded-wave light to the expansion of the universe, but this approach has been shown to be invalid. I must modestly admit to being the originator of another theory which explains the dominance of retarded waves. It uses the singularity present at the Big Bang as a reflector, bouncing and cancelling any advanced waves which come from the future and try to penetrate it and leaving only retarded waves in the universe. In my approach light goes forward in time for the same reason that light goes forward from a spotlight: because there is a reflector behind it which bounces anything going in the wrong direction. A side effect of my reflector theory is that the universe must be open or critical (not closed) or there will be singularities in both time directions and both kinds of waves would be equally suppressed.

    But such explanations not withstanding, the physics behind our inability to make advanced waves, the so-called Electromagnetic Arrow of Time, remains one of the unsolved problems of contemporary physics. Perhaps, if we had a deeper understanding of why advanced waves cannot be made, we might arrange for an altered set of physical conditions in which they could be made. But there is a price. The principle of causality, frustrating as it is, insulates us from having to understand and solve the kind of causal-loop time travel paradoxes with which SF readers are quite familiar. But if advanced waves were to put us in the position of being Causality Busters, then our grip on reality would become more tenuous. The past could never be considered over and done with, because anyone with the proper hardware could send messages back in time and alter what has already happened. A world in which even the past cannot be depended upon to remain fixed sounds like an unstable and disorienting environment in which to live. Perhaps Nature knew what She was doing in banning advanced waves from our universe.|


Advanced Waves:
J. A. Wheeler and R. P. Feynman, Reviews of Modern Physics 17, 157 (1945),
and Reviews of Modern Physics 21, 425 (1949).

P. C. W. Davies, The Physics of Time Asymmetry, Chapter 5, Univ. of California Press, Berkeley, CA (1977).

John G. Cramer, Physical Review D22, 362 (1980).

Big Bang Reflector:
John G. Cramer, Foundations of Physics 13, 887 (1983).

SF Novels by John Cramer:  my two hard SF novels, Twistor and Einstein's Bridge, are newly released as eBooks by Book View Cafe and are available at : .

AV Columns Online: Electronic reprints of about 177 "The Alternate View" columns by John G. Cramer, previously published in Analog , are available online at:

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