Past, present, and future
The space-time region surrounding a given space-time point can be                         separated into three regions, as shown in Fig. 17–3. In one                         region we have space-like intervals, and in two regions, time-like                         intervals. Physically, these three regions into which space-time around                         a given point is divided have an interesting physical relationship to                         that point: a physical object or a signal can get from a point in                         region $2$ to the event $O$ by moving along at a speed less than the                         speed of light. Therefore events in this region can affect the                         point $O$, can have an influence on it from the past. In fact, of                         course, an object at $P$ on the negative $t$-axis is precisely in the                         “past” with respect to $O$; it is the same space-point as $O$, only                         earlier. What happened there then, affects $O$ now. (Unfortunately, that                         is the way life is.) Another object at $Q$ can get to $O$ by moving with                         a certain speed less than $c$, so if this object were in a space ship                         and moving, it would be, again, the past of the same space-point. That                         is, in another coordinate system, the axis of time might go through both                         $O$ and $Q$. So all points of region $2$ are in the “past” of $O$, and                         anything that happens in this region can affect $O$. Therefore                         region $2$ is sometimes called the affective past, or affecting past; it is the locus of all events which can affect                         point $O$ in any way. Region $3$, on the other hand, is a region which we can affect                         from $O$, we can “hit” things by shooting “bullets” out at                         speeds less than $c$. So this is the world whose future can be affected                         by us, and we may call that the affective future. Now the interesting thing about all the rest of space-time,                         i.e., region $1$, is that we can neither affect it now from $O$,                         nor can it affect us now at $O$, because nothing can go faster                         than the speed of light. Of course, what happens at $R$ can                         affect us later; that is, if the sun is exploding “right now,”                         it takes eight minutes before we know about it, and it cannot possibly                         affect us before then. What we mean by “right now” is a mysterious thing which we cannot                         define and we cannot affect, but it can affect us later, or we could                         have affected it if we had done something far enough in the past. When                         we look at the star Alpha Centauri, we see it as it was four years ago;                         we might wonder what it is like “now.” “Now” means at the same time                         from our special coordinate system. We can only see Alpha Centauri by                         the light that has come from our past, up to four years ago, but we do                         not know what it is doing “now”; it will take four years before what                         it is doing “now” can affect us. Alpha Centauri “now” is an idea or                         concept of our mind; it is not something that is really definable                         physically at the moment, because we have to wait to observe it; we                         cannot even define it right “now.” Furthermore, the “now” depends on                         the coordinate system. If, for example, Alpha Centauri were moving, an                         observer there would not agree with us because he would put his axes at                         an angle, and his “now” would be a different time. We have                         already talked about the fact that simultaneity is not a unique thing. There are fortune tellers, or people who tell us                         they can know the future, and there are many wonderful stories about the                         man who suddenly discovers that he has knowledge about the affective                         future. Well, there are lots of paradoxes                         produced by that because if we know something is going to happen, then                         we can make sure we will avoid it by doing the right thing at the right                         time, and so on. But actually there is no fortune teller who can even                         tell us the present!  There is no one who can tell us what is                         really happening right now, at any reasonable distance, because that is                         unobservable. We might ask ourselves this question, which we leave to                         the student to try to answer: Would any paradox be produced if it were                         suddenly to become possible to know things that are in the space-like                         intervals of region $1$?