books book reviews

books about time

reviewed by T. Nelson

score+3

The Oxford Handbook of Philosophy of Time
Craig Callender, ed.

Oxford University Press, 2011, reprinted 2014, 689 pages

T he fact that time is a physical phenomenon about which virtually nothing is known has not stopped people from arguing about it for centuries. I was actually expecting this book to be a history of what earlier philosophers like Aristotle, Husserl, Hans Reichenbach, Hermann Weyl, William James, Martin Heidegger, and Henri Bergson, who I first heard of in Heidegger's Being and Time (reviewed here) thought of the subject. Bergson, the author of Matter and Memory, analogized time to a reel of audio tape, which can speed up and slow down for the individual.

Instead what we have here is a collection of articles by modern-day analytic philosophers discussing their recent work. The moderns write less evocatively than the earlier philosophers; their focus is less on time itself than on how to talk about it. How, for example, can we analyze propositions like “There are three kings named Edward” or “Queen Anne is dead”? Is the latter really a statement about Queen Anne, or about the rotation of the Earth in the intervening years? Is it a statement about the present or a representation of the fact that she died in the past, in 1714 to be exact—and does 1714 really still exist?

These questions may sound silly, but they are important philosophical issues, and anyone wishing to say something about time must be familiar with them or risk being thought uninformed. There's also a lot of discussion about how to represent propositions with time value into the abstractions of modal logic. This notation, which originated with C.I. Lewis, handles propositions and logical relationships fairly well, but, as many articles show, fails to adequately convey the richness of natural language when discussing past or future events.

Another theme is the question of presentism vs. eternalism, the former being the idea that only things happening in the present are real, and the latter being that everything that ever happened or will happen is equally real, as if they are frames on a film projector, and thus, perhaps, preordained. Much work here still remains to be done, not least of which is to incorporate into these discussions some knowledge about what time actually is, as opposed to arguing in a relative vacuum, as philosophers are sometimes wont to do.

In the section on metaphysics, it is explained that A-theory is the idea that time runs from the past to the present and then to the future, while B-theory is the idea that time runs from earlier times to later times. These are the sorts of things that philosophers argue about these days, and the fierceness of the disagreements here may explain why being a philosopher can be such a dangerous job, in more ways than one.

Douglas Kutash points out that we cannot actually influence the future, only the present; yet we perceive asymmetry in that we fear death more than birth and we are happier about a headache in the past than in the future.

The last section is from the physicists.

Most physicists seem to subscribe to the idea, attributed to Boltzmann, that the direction or ‘arrow’ of time is related to an increase in entropy. Boltzmann's idea was very profound: in any given region of space, he said, whichever way entropy is increasing over time, the biological mechanisms that create our memories must proceed likewise, because they are based on the same thermodynamic principles. Therefore, time will always be perceived to be moving in whichever direction entropy is increasing, and time's so-called arrow always seems to point in the same direction: toward increasing entropy.

The Second Law of Thermodynamics (dS/dt≥0) is probably the most highly regarded law in physics. Eddington said it held the ‘supreme position’ among the laws of nature. Einstein said it was the only theory that would never be overthrown (cited in Zeh, 5th ed., p.6).

For this law to provide us with a non-circular explanation of time, however, we have to explain it in terms of something orthogonal to time. This seems to me to be too big a task for entropy, which is purely a measure of disorder. To fully understand time in this context, we would seem to need to invoke some other feature, such that time appears as a natural consequence. Some possibilities are explored in the last section.

Like fish unable to understand water, we have great difficulty conceptually extricating ourselves from time. Perhaps this is what Huw Price is getting at when he says that the modal asymmetries we use in physics exhibit an intrinsic past-to-future orientation. He writes “it requires that we be careful about what we mean by the state of a physical system—careful that we don't simply take for granted a conception that cements in place ... time-asymmetric modal categories” [p.309].

Clearly, thinking about time without invoking time is not easy—it requires a deep understanding not only of psychophysics but also special and general relativity, and Minkowski spacetime in particular, which several authors discuss, and which is assumed to be understood by the reader, at least in general terms. There are few equations here, unless you count the chapters on modal logic and Yuri Balashov's chapter on persistence, which he has since expanded in his 2010 book Persistence and Spacetime, or the chapters by John Earman, who talks about the electromagnetic arrow of time, and Steven Savitt, who discusses the special theory of relativity.

Some of the philosophers, perhaps understandably, seem to have an uncertain grasp of this latter topic. We can't get too excited these days when someone flatly states that SR is false and GR is on thin ice. Others confuse their readers by adding random shes, sprinkling their prose with PC sugar, as if we were still living in the 1990s (a cultural spatio-temporal anomaly deserving a scientific explanation, to be sure), but others, such as the physicists, who seem to be less caught up in popular social fads, write in a more mature style.

This divergence of opinions and styles makes the book aggravating as well as entertaining: I am rather proud of myself for not having ripped this book to shreds, at least not yet; but in a few places I found myself asking ‘Why, oh why, can't philosophers go back to writing incredibly long sentences using nonsensical made-up words-with-lots-of-hyphens like they did in the good old days?’

This, alas, is a question of metaphilosophy, which is outside the subject of the book. The book is manifestly philosophical, not scientific, so it would even be unrealistic to expect answers about time. When they're successful, philosophers help us think more clearly about the subject and give us a consistent vocabulary. Mostly what you look for in a book like this is some intelligent discussion of the philosophical issues. Huw Price in “The Flow Of Time” is a great example, and there are some others.

But if you're looking for answers to scientific questions, you need to read books by H. D. Zeh and others, or else head over to the lab. In science, unlike in philosophy, we get no credit for asking a question, no matter how profound—only for answering it credibly; even so, if someone did ask it first, it's polite to cite them. Hence this book has a lot of value, even to scientists.

If I had any answers, I might just come out at this point and say what time is, but that would deprive people of the enjoyment of figuring it out for themselves. That would be cruel: they're having too much fun with this. But if I had to guess, I'd suspect we're going to be rather surprised when we find the answer.

apr 06, 2015

score+5

The Physical Basis of the Direction of Time, 5th ed.
H. Dieter Zeh

Springer, 2007, 231 pages

H ere physicist Dieter Zeh investigates some phenomena that may be responsible for the direction of time. What these processes—entropy, radiation, quantum decoherence, and gravitation—have in common is that they are irreversible. For example, a star may illuminate billions of cubic light years. For all the light to go in reverse, collect at one point, and then undergo reverse fusion at the exact same point it was produced is so improbable that, despite the fact that our physical laws are symmetric to time, it never happens. Symmetric laws produce asymmetric results. This, say modern physicists, is the arrow of time.

Quantum mechanical phenomena, such as radioactive decay, also create an arrow of time. Zeh subscribes to the idea that a conscious observer is required for decoherence. He defends Everett's interpretation by saying that decoherence leads to many quasi-classical branches of the world, all of which exist in a superposed state—what we might call a Schrödinger's cat universe—not, as later theorists thought, as a many-worlds scenario. The superposed observer states, says Zeh, must be taken into account because they are components of the global wave function.

Another possibility is the expansion of the universe. It's often speculated that if time is caused by expansion of space, time would run backwards as we approached the Big Crunch. We can reasonably question whether our memories would run in reverse (in which case we'd still think the universe was expanding), but what about cosmic inflation? We have no physics that could ever account for a reverse inflation of space.

Likewise with black holes. Zeh builds on Jacob Bekenstein's proposal for a unification of thermodynamics and spacetime geometrical laws, saying that black holes are drastically asymmetric under time reversal, since they contain only a future horizon and a future singularity. The nonexistence of white holes is strong evidence for time's arrow because they would be inconsistent with an arrow of time that is valid everywhere in the external region.

Well, that's all very interesting, you might say, but what does any of this tell us about time? We already know things never happen in reverse. Surely radioactive decay and black holes are just ordinary phenomena moving through time. But what does that mean? So far, physics has no answer. There are some ideas, such as superspace, in which a three-geometry (3)G(t) is thought of as a carrier of information on physical time, but even Einstein, who described time as a dimension fused with space, recognized that a complete understanding of time will also require understanding the human mind.

Although this book is short, it's not for the faint-hearted: the reader is expected to understand the equations without explanation, and the text is dense with ideas and opinions about various theories. The reader is also expected to be conversant with electrodynamics, black hole physics, QM, GR, and QFT. If you're fuzzy on these topics, I recommend the book on decoherence by Joos et al..

You may also wish to read a good gravitation text like Ohanian and Ruffini or Misner, Thorne, and Wheeler (which Zeh cites numerous times). And, if you have any time left over, read The Oxford Handbook (reviewed at left) to get an idea why Zeh picked these particular examples. That'll keep you busy for a long time. But if the Big Crunch theory is right, you may have already done it.

apr 20, 2015