Saturday, September 09, 2006

Michael Lockwood: The Labyrinth of Time

Michael Lockwood's The Labyrinth of Time starts out as an explanation of the physical structure of time, but morphs gradually into a treatise on anything Lockwood is interested in that can be remotely connected to time. Lockwood's interests include time travel, quantum electrodynamics, black holes, and quantum gravity. His exposition is better in areas that are clearly physics, and worse on things like the meaning of causality and time travel.

Lockwood starts by presenting two basic ways of thinking about time. In one, the past is fixed and the future is mere possibility turning into the unchanging past as we move past it. The other says that both the past and future are fixed, our point of view is called the present, and as time passes, we get to watch a narrow slice of the fixed 4-dimensional reality pass by. Most of physics works equally well whichever direction time moves, and Lockwood wants to explain why the past and the future seem different to us. We remember the past and not the future. In the end, this paradox wasn't cleared up for me.

Lockwood does present some good new visualizations that clarify space-time and various "problems" with reasoning about the speed of light. His presentation of simultaneity, using a moving train car with two different observers was remarkably clear, as was his explanation of the twin paradox using two unaccelerated paths through a hypothetical cylindrically connected space-time. The presentations of the physics of black holes (and how to use a black hole as a power source) were also good, but I didn't get much out of his approaches to string theory or quantum gravity. I think part of it was that the presentations were more hurried nearer the end of the book; he did better when he took the time to explain the basic concepts clearly first before getting to the advanced ideas. I suspect that except for people steeped in these later areas, his explanations will come up short.

Lockwood kept coming back to time travel paradoxes, without ultimately resolving the issue. Apparently the standard interpretations of physics say that paradoxes aren't allowed, and the open question is what mechanism the universe will use to prevent them. Apparently, if you don't believe in many worlds, you are forced to believe that there is only one past and a single future that will eventualy unfold. This doesn't allow causal loops (even though the fundamental equations seems to countenance them) much less time travellers acting in the past to prevent their known futures from (re-)occuring. I think the underlying theories force you to accept a single past even if you don't go for many worlds. So, whether the past becomes fixed as we journey with the moving present, or our viewpoint moves across an already fixed landscape, you're stuck.

I don't see how these arguments would convince anyone that the universe would act to prevent closed time-like loops. Until we see events conspiring to prevent causal loops, I'm happier reconciling the claim that physics allows time travel by accepting that space-time may be a causal spiral than expecting the universe to allow time travel while conspiring subtly to prevent someone from killing her grandfather. But as Norm Hardy pointed out, some of the phyisicists who seem to understand the equations say that the equations force you to believe in an invariant past. And in the end, Einstein's ability to predict, based on the equations, a lot of what we now believe and that experiments have confirmed, says that that's a strong argument.

Peter McCluskey also wrote about Labyrinth.

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