Thursday, February 01, 2001
Beat the Clock
Julian Barbour's theory on time ["From Here to Eternity," December] seems to take us back to determinism, implying a stacked deck of events in which we seem to experience life and make decisions but in fact do not. What does this say about free will or consciousness itself?

Mark Bentley
Seattle, Washington

The inability to marry quantum theory and relativity gives way to the insight that one or both theories are flawed in some respect, however small the flaws may be. Nonetheless, to state that time does not exist in order to explain the unification of physical theory is self-invalidating, the study of physics being an attempt to explain the causality of all things. If all things past, present, and future already exist, then there is no such thing as cause and effect.

J.G. Stanley
Penrose, Colorado

Tim Folger refers to me, an emeritus professor aged 78, as "a young American physicist." Maybe he really accepts Julian Barbour's thesis that time is an illusion. Even when the Wheeler-DeWitt equation was first written down (1965), I was 42, an age by which theoretical physicists are regarded as already over the hill.

Is time more of an illusion than space or anything else? I still believe what I told Barbour in Spain: "Time is what a clock measures, nothing more"— but also nothing less. Solutions of the Wheeler-DeWitt equation can be interpreted if one understands that they describe correlations between the objects that make up the universe, e.g., ticking clocks and moving planets. One does not need to grind an ax about time.

It is fine to have a novel viewpoint, but the acid test is whether it suggests a new experiment or explains a new observation. Folger leads the reader to believe that the Wheeler-DeWitt equation, which Barbour holds in unjustifiably high esteem, describes the entire universe. This is not true. The equation merely provides a framework, like relativity and quantum theory themselves do.

Bryce DeWitt
Department of Physics
University of Texas at Austin

Julian Barbour responds: One part of my theory not covered in Tim Folger's article concerns the role of quantum probabilities. I do conceive all possible Nows as existing together in a timeless landscape (Platonia). Each Now is just like one possible configuration of the particles in a molecule, only vastly larger. The Wheeler-DeWitt equation (in the simplest interpretation) is just like Schrödinger's time-independent equation for the stationary states of atoms and molecules. This equation gives the different probabilities for all their possible different configurations. These probabilities never change. They are eternal.

Although all Nows are present in Platonia, they have very different probabilities. In my scheme, there is no cause and effect that acts from past to future. But there is a timeless, rational explanation of these probabilities. This is just like atomic and molecular physics. In principle, this makes the scheme predictive, since we should always find ourselves in Nows that have high probabilities (and distinctive properties that we can look for to test the theory). If my idea is correct, the great issue of determinism and free will must be transformed. The probabilities are fixed deterministically, but we do not know which Nows we shall find ourselves in.

In answer to Bryce DeWitt, for whom I have great respect, I would counter that he still has not answered the question I have put to him several times: What is a clock? I believe I have found an answer, and it makes me think he should estimate his own equation higher than he does. I believe it will eventually predict certain characteristic properties of the universe, just as Newton showed that all matter gravitates, although he knew nothing of the makeup of matter. However, I agree with Bryce about the acid test, and my theory will remain speculation until it gains support from observation.

Due to a production error, the RADARSAT images of sea ice on page 19 in November's R&D; were labeled "August 1, 2000" and "September 1, 2000." The dates should have read "November 30, 1996," and "December 7, 1996."

On page 36 of the December issue ("The Chemistry of Plastic Artifacts"), the nitrate molecule was given as NO2, which is nitrite. Nitrate is NO3.

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