Wednesday, April 10, 2013

The flow of time

Famously, D. C. Williams ridiculed the deep-seated intuition (at the heart of Bergson's thought, of course) that there is a flow of time, asking how fast time is flowing, if it's flowing. Some wits have tried to respond: "Always at a second per second." But there is a much better and less trivial answer. And interestingly it is an answer that has a home in the B-theory of time.

The Twin Paradox suggests that we should distinguish the internal time of an individual from something like the generally shared external time of the human community. Thinking about time travel suggests a similar distinction, as Lewis has noted. But once we have a distinction between internal and external time, then we can give a non-trivial answer to Williams' question. The flow of time is measured in terms of external units of time per internal units of time. If external time is defined by the shared life of the human community (that's one among a number of options—we should probably understand "external time" in a context-sensitive way), normally time flows at one external second per second. But if I were to engage in travel at relativistic speeds, it could be that in a month of internal time, eight years of external time would elapse. And if I were to engage in gradual backwards time travel, then I would have a negative rate of time's flow: maybe I would be moving at −1 external century per internal second. (In non-gradual time travel, the rate would be undefined.)

The distinction between internal and external time fits best with the B-theory. So the notion of time's flow, surprisingly, seems to have its home in the B-theory.


Dagmara Lizlovs said...

"we should distinguish the internal time of an individual from something like the generally shared external time of the human community. "

Regarding internal time. I think that the rate of flow of internal time is some form of non-linear function. When you're a kid time flows so terribly slow. Remember how long it took to get to Christmas from the first day in December when you were a kid. That was forever. Remember when you were out for June, July, August on summer vacation from school? That time seemed forever. As you got older internal time sped up. Then with each successive decade like on some logarithmic scale, time speeds up. The acceleration begins when you hit 30, and accelerates logarithmically thereafter. This is most noticeable when you hit 40. Once you get past 50, it is really bad. There is the strange variation. Time during the work day slows down, unless there is deadline, and speeds up over the weekend. Then again, if you are a deer hunter sitting in your stand waiting for that buck to show up, time will actually stop.

Michael Gonzalez said...

I honestly don't think that A-theorists who speak of the "flow of time" are referring to the subjective feeling that things take a "long time" or the difference between one's inertial reference frame and that of some larger community (which implies that we are actually relativizing our sense of the "flow" to what's happening with others, and I don't think that's what anyone is doing; at least not consciously). It seems to me that "flow of time" was always meant as "temporal becoming". We perceive that things come into being and then go out of being; that events transpire from their beginnings to their climaxes, and then pass out of existence.

Perhaps "flow" is an ill-chosen word, if it conjures up the need for "speed of flow", since that's just categorically different from what A-theorists are talking about. Or so it seems to me.

Alexander R Pruss said...

Maybe, but if so then Williams is completely off-base in thinking this has to do with a perception of time being like a river. For when a river flows past us, water doesn't come in and out of being.

Anonymous said...

I'm not sure I know how to make sense of the claim that there is external time on the B-theory. If you wouldn't mind, can you either clarify that, or direct me to somewhere where you have (or another has) clarified that?


Dagmara Lizlovs said...

Here I go again. I'm about to start another weekend which in internal time will last about four seconds.