Time and Change

Is time washing over you?

Posted Apr 01, 2012

Time and Change

Is time flowing past you right now?

Things change. We all get older. Yesterdays’ steamed rice becomes today’s potential food poisoning source. Some people think that it seems as though time is flowing over and around us. Certainly a necessary precondition for change is the existence of time. Things change by being one way at one time, and a different way at another time. That much is pretty much agreed wisdom. That is where the agreement ends. Philosophers are deeply divided about the nature of time itself, and divided over whether time really passes.

How could anyone claim that time does not pass? Surely it’s obvious. That all depends on what you mean by the phrase “temporal passage. No one wants to deny that you and I and my steamed rice exist at different times. So no one wants to deny that different times exist.  If all you mean by the claim that time passes, is that there exists a temporal dimension composed of temporal instants, then you probably won’t get too much argument. Philosophers who think there really is temporal passage mean something more than that.

Philosophers who think that temporal passage is a real phenomenon think that time itself is in some way dynamical. They think that when we use words like “past” “present” and “future” these words pick out special features of reality. What does that mean? It means they think that some particular time—call it t—is present for just a moment. That time, t, goes from being future, to being present, to being past. Time passes because the present moves. Presentness is a bit like a wave on this view. It washes over different temporal instants. When the wave catches an instant, that instant is the present—the now—and when the wave passes over that instant it becomes the past. Instants that have yet to receive the wave are in the future.

So it is now the case that Monday 2 April 2012 (at a particular moment) is present, and that 2014 is future, and 1920 past. But it was the case that some instant in 1920 was present and that some instant in Monday 2 April 2012 was future.

Different philosophers model the movement of the present differently. Some think that the present moves by new instants of reality being added to the sum total of what already exists. The present is then the newest instant that has “just” been added, and the past is everything that is already there. So the present is a bit like a wave that washes new material onto the shore. The latest material added is the present, and the existing sand dune is the past. The future is, as yet, unreal.

Other philosophers think that only the present moment exist, and that any moment that was present but is now past has ceased to exist and any moment that is future will at some point come to exist, but does not yet exist. On this view there is just the wave of presentness. Which events are the wave change, but only the events on the wave exist.

Regardless of how the existence of a moving present is modelled, on this view of time our world is constantly changing. It is not just that there is change within the world because at different times different events are occurring. The world itself is changing as different times become the present, and then the past. Defenders of this view claim that this is why it seems to us that time flows: it seems as though the future is coming towards us, becoming present, and then receding into the past.

Opponents of this view reject outright the idea that time passes. Such philosophers tend to conceptualise time as being just one more dimension. We have all of the spatial dimensions (let’s suppose there are three, apologies to string theorists) and then we have a fourth dimension: time.  Just as we can spread out grains of rice along the bench in space, we can imagine spreading out grains of rice along the temporal dimension, that is, in time. Just as space does not pass or flow, neither does time. How do we explain change then?

Well space does not flow or pass, but things look different at different spatial locations. China looks different to Sydney, which looks different to Mars. That’s not because space passes, but because what is located in China is different to what is located in Sydney. So too for time. Rice is located yesterday, and rice is located today. Yesterday’s rice is fresh. Today’s rice is not. The change in the rice is not due to time passing; though it is (in part) due to the fact that one lot of rice is located at one point on the temporal dimension and the other lot of rice is located elsewhere on the temporal dimension.

That, however, does not require that there is any moment that is the “real” present that moves about. To understand the rice going off, we don’t need to think that yesterday was, at one point, washed over by the wave of presentness at which point today was the future, and then today became the present and yesterday became the past as the present moved. According to these philosophers, there is no objective “present” that moves. The present is just the place at which you happen to be located.

What does that mean? Well suppose you say “I am here” when you are in Sydney. What you say is true, and by “here” you refer to some location in Sydney. Suppose a Martian says the same thing on Mars. Then he refers to some location on Mars.  Both speakers say something true when they say “I am here” even though they are both in different locations. Philosophers who reject the existence of temporal passage think that is how words like “now” and “present” work. They don’t pick out some special location in time, just like “here” does not pick out some special location in space. They just refer to whichever time the speaker happens to be located at. So the present does not move. Every individual is located in the present, regardless of where along the temporal dimension they happen to be located.

Of course, this is consistent with things changing in the world. Things don’t change because the present moves. Things change because different events happen at different times. On this view the world itself, considered as a whole, is a big static entity. There is change within the entity in virtue of different things happening in different parts of it. But the entity as a whole does not change. This is to be contrasted with the view offered by friends of temporal passage, who think the world as a whole changes as the wave of presentness moves about.

Debate continues to rage. So you should ask yourself – is the wave of temporal passage now washing over me?