Now?

Factual Questions
1

When exactly is now? What is the future exactly as it relates to the present and the past?

2

Nothing really to add – I just love that quote. I don’t know that there is a simple answer to your questions. Maybe Chronos will be along soon to address this.

3

Terry Pratchett’s latest work looks into this thorny problem.

4

Well, this is really more philosophy than physics (don’t be surprised if it gets moved to GD), but I’ll give it my best shot. From any event[sup]*[/sup] A, the future of that event is defined to be the set of all events which can be influenced by A. What this means practically is that a photon, or something slower, can get to any point in my future. Similarly, the past of an event is the set of all events which can be influenced by A. This is often represented by something called a spacetime diagram, as such:


        ^ t
        |
        |
  \  future   /
   \    |    /
    \   |   /
  e  \  |  /  e
  l   \ | /   l
  s    \|/    s     x
<-e-----A-----e----->
  w    /|\    w
  h   / | \   h
  e  /  |  \  e
  n /   |   \ n
   /    |    \
  /    past   \
        |

The X axis is used to represent all three spatial axes, since it’s hard to draw a four-dimensional diagram. The diagonal lines represent the path of a photon, so everything in the upper wedge is future of A, and everything in the lower edge is the past of A. Since, in 3+1 dimensions, these regions would be shaped something like a cone, and their boundaries are the paths of light, they’re called light cones. Notice also, by the way, the other regions, labelled “elsewhen”. These areas are neither past, present, nor future of A. The “present” of A isn’t anything other than the event A itself.

Now we get to the problem of consciousness. We can define the event A to be the location of your brain, at the moment when the nerve impulses travelled through it corresponding to you thinking “now”. The event itself, then, was “now”, and it is part of the boundary between the past of that event and the future of that event.
[sub]*An event is like a point, in three dimensions, but with a time specified: It’s a particular position in space, at a particular moment of time.[/sub]