Spacetime and the common "Now"

A spacetime diagram (time on y-axis and space along the x-axis) explains how space squishes and time dilates when objects move both through space and time. Any object that does not move along x-axis, moves only through time. An object that moves through space and time both, will sketch a sloped line (slope cannot be less than 1). The diagram is smartly scaled such that "1 second interval" on y-axis measures the same as "3.0 x 10^8 m" on the x-axis; only light passing in space time would sketch a slope of 1. Since the time is on the y-axis, slopes more that 1 represent velocities less than the velocity of light. It follows that all moving objects will have slopes greater than or equal to 1.

Sal Khan, the narrator has done an excellent job in explaining how a moving object draws the x-axis closer to the y-axis (squishing of space and dilation of time at same time). A corollary to this is, for an object that moves with speed of light the x-axis will merge with the y-axis and the time will not flow for that moving object.

Measuring events in space time:

Take a step further and rotate this trajectory with slope 1 (sketched by light) around the y-axis. We get a cone - it's called a light cone.

Light cones are used to detect events in spacetime. Imagine yourself not moving in space. You're on the y-axis and there is a light cone around you (like you inside a cone). As time passes, you and the light cone (in which you are) move in the direction of time. Any event outside the cone is not visible to you until the moving cone encompasses it.

The most bizarre thing is what follows here:

If there are 2 objects in side this cone and they aren't moving at all or moving such that they aren't moving relative to one another they have the same present ("now"). You could cut a horizontal slice through them - this means their clocks will be synchronized and they have a common present. If one object moves relative to another, then the one's present is the other's past or future (depending on the direction of relative motion). Imagine a swinging pendulum in a huge light cone and a very distant object at the very edge of our universe. Every time the pendulum swings back and forth the distant object connects with pendulum in its past and future; depending on how far the two are, these past and future could be centuries in the past and centuries in the future.

It makes you wonder if the past, present and future were locked inside a block universe.