What is Time?

Last winter, I taught Introduction to Philosophy for the first time since the 1980s, so I was curious to find out what questions most interested the students. I was surprised to find that many students were keen to know what time is and whether time travel is possible. I had fun putting together a class on the topic and have since read a couple of very interesting new books on time, one by a physicist and the other by a biologist. I thank that biologists’ conclusions about time are more plausible than some of the physicists’.

Various distinguished philosophers and scientists have argued that time is an illusion. Parmenides claimed that reality is eternal and timeless because change requires that something both is and is not. John McTaggart argued that problems about past, present, and future make time incoherent. Kurt Gödel thought that relativity theory showed time is unreal. Julian Barbour argued that reality is timeless because duration is indefinable and redundant. I don’t find any of these abstract arguments convincing, although I do see the appeal: if time is unreal I don’t need to worry about income tax deadlines.

The distinguished physicist Lee Smolin has a new book, Time Reborn, that addresses the reality of time in two stages. He first considers a series of reasons why physicists have thought that time is unreal, and gives his own reasons why he thinks time does in fact exist. The first reason he gives for doubting time is the Newtonian view that nothing happens except the law-determined rearrangement of particles, so the future is determined by the past. But I don’t see why determinism is a problem for the reality of time, because Newton’s law’s of motion all refer to time via concepts such as velocity and acceleration. Smolin’s second reason concern’s Einstein’s relativity theory which takes time as just another dimension of space. But time is very different from spatial dimensions which are reversible: if you can go forward, you can go back; if you can go up, you can down; if you can go left, you can go right. But there are no known cases where anyone has ever gone backward in time rather than forward. That is why I find time travel implausible, not because there are any good reasons for it being impossible, but rather because there is no evidence for it being actual.

Smolin’s attempt to rescue time from physics rests on some very dubious ideas. He endorses Leibniz’s principle of sufficient reason that requires there to be an answer for all questions about the universe, including why physical constants such as the gravitational constant have the values they do. But Leibniz’s principle belongs to theology rather than science or naturalistic metaphysics, for it may well be that some things are just true by chance, as I argued in an earlier blog post. Smolin tries to explain the values of physical constants and the form that laws currently take by a hypothesis of cosmological natural selection, which advocates that universes reproduce by the creation of new ones inside black holes. The postulation of countless universes makes this one of the least parsimonious hypotheses in the history of science. In contrast, Steinhardt and Turok’s book Endless Universe presents a much simpler account of how our current universe began around 14 billion years as the latest in an eternal series of cycles of expansion and collapse. This cyclic theory clearly assumes that time is real, without Smolin’s dubious claims about sufficient reason and cosmological selection.

There is no reason why physics should get the only say about the reality of time. I highly recommend Til Roenneberg’s recent book: Internal Time: Chronotypes, Social Jet Lag, and Why We’re So Tired. It describes fascinating studies about people’s biological clocks, including explanations of why my undergraduate students find early morning classes much more difficult than I do, and of why early risers like me are unusual in suffering less jet lag going from North America to Europe rather than the other way. I recently learned that non-human animals also have sophisticated representations of time. For example, rats can learn to respond to stimuli at different time intervals, thanks to special neurons that keep track of time, and macaque monkeys have neurons that time stamp events. Unlike some physicists and philosophers, biologists and psychologists need have no doubts about the reality of time.

But what is the nature of the time that exists? Smolin makes a good case for the view of Leibniz and Mach that time is a system of relations among events, not an absolute container for events. Oddly, however, he concludes that space is an illusion, rather than an analogous system of relations among objects. I think that both space and time are real, in the relational sense, because that supposition is part of the best explanation of a vast range of physical, biological, and psychological facts.